University of Tennessee Instructor Copy. Soil Management in Residential 6Gardens and Landscapes NATALIE BUMGARNER

Transcription

1 Soil Management in Residential 6Gardens and Landscapes NATALIE BUMGARNER Assistant Professor & Extension Specialist University of Tennessee This chapter includes content from the chapter on soil science written by Hugh Savoy in the previous TEMG handbook.

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3 CONTENTS INTRODUCTION 6-5 BASICS OF SOIL QUALITY, PRODUCTIVITY, AND MANAGEMENT 6-5 Elements of Soil Quality Soil Productivity, Quality, and Management Links The Central Role of Organic Matter MANAGING PHYSICAL PROPERTIES OF SOIL 6-8 Methods for Adding Organic Matter Cover Crops and Green Manures Crop Choices General Practices Manure Compost Other Soil Conditioners Ditches or Drainage Tiles Practices that Protect the Soil Surface and Minimize Erosion Tillage Practices MANAGING CHEMICAL PROPERTIES OF SOIL (SOIL FERTILITY) 6-14 Soil Sampling and Using Soil Reports Sampling Location Sampling Procedure Sample Handling and Mailing Understanding the Soil Test Report Managing Soil ph Increasing Soil ph Reducing Soil ph Managing Soil Fertility for Plant Nutrition Types of Fertilizers Chemical Fertilizers Organic Fertilizers USDA Organic Fertilizers Fertilizer Description

4 Fertilizer Formulations Granular Blended Fertilizer Granular Homogenized Fertilizer Water Soluble Fertilizer Weed-and-Feed Fertilizer Liquid Fertilizer Starter Fertilizer Fertilizer Spikes Slow-release or Timed-release Fertilizer Fertilizer Application Methods and Terms Broadcast Sidedress Topdress Banding Foliar Fertilization Using a Soil Test to Address Fertilization Choosing a Fertilizer Describing Soil Organic Matter Content MANAGING BIOLOGICAL PROPERTIES OF SOIL 6-25 Roles of Common Soil Organisms Bacteria Fungi Soil Fauna (Animals) Supporting Diverse Soil Biology Supplying Organic Material Maintaining Appropriate Physical Conditions and Habitat Cropping, Tillage, and Pest Management Practices SUMMARY

5 SOURCE: Natalie Bumgarner, University of Tennessee INTRODUCTION Chapter 5 presents the basic physical, chemical, and biological properties of soil. These factors all contribute to the vital roles that soil plays in our environment and in plant productivity. This chapter focuses on ways that gardeners and homeowners can affect soil quality and productivity to support sustainable lawns and gardens. Specific practices for the management of soil physical, chemical, and biological properties are presented. It is important to realize that optimum soil quality and productivity are achieved by management strategies that integrate a range of practices. BASICS OF SOIL QUALITY, PRODUCTIVITY, AND MANAGEMENT Methods for sustaining and improving soil systems have been widely studied. However, the complexity of soil can be a challenge for homeowners and residential growers because much of the research has focused on commercial agriculture. This section introduces basic concepts and practical management steps that are applicable to homeowners and gardeners. Elements of Soil Quality Soil quality is a description of the ability of soil to meet our current needs in supporting plant growth while also meeting the broader, longterm needs of sustaining our ecosystem into the future. (See Chapter 4.) It is essentially a description of soil health and fitness for use. Soil functions can be used as indicators of soil quality: supporting biological diversity and activity in soil regulating the flow of water (and dissolved materials) serving as a filter, buffer, and degradation location for organic and inorganic materials Buckwheat (Fagopyrum esculentum) just emerging as a summer cover crop to add organic matter and reduce weed growth in a vegetable garden after harvesting a spring cool-season vegetable crop 6-5

6 Te nn es op y In se e Soil productivity refers to the capacity of the soil to produce plants. It is important to understand that the productivity of soil involves much more than the nutrients it contains. The physical, SOURCE: USDA-NRCS U ni ve r si ty of Soil quality and productivity depend on the proper physical, chemical, and biological functioning of soil. st Soil Productivity, Quality, and Management Links or C The boldfaced words in this list of roles represent general areas of soil function. Specific indicators based on measurable physical, chemical, and biological properties can be used to assess soil quality. Table 1 lists these indicators and describes the soil functions to which they allude. This table is based on the Natural Resources Conservation Service soil quality assessment matrix in the companion materials. ct providing support and physical stability for plants, animals, and structures. chemical, and biological factors of soil presented in Table 1 all affect soil productivity. It can be useful to think of soil quality as the cause and soil productivity as the effect. Soils that function well and fulfill the five crucial roles of biological diversity, water and solute flow, filtering, nutrient cycling and storage, and stability are generally productive in terms of plant growth. Soil productivity is of vital importance in agriculture because it is linked with present and future crop yields and farm profitability. In residential and consumer horticulture, soil productivity is important for successful home food production, maintaining aesthetically pleasing turf and ornamentals in the landscape, and addressing a range of environmental stewardship issues. (See Chapter 4.) This chapter outlines specific practices and tools that farmers and gardeners alike can employ to improve soil quality and productivity. Further discussions on enhancing stewardship in specific areas of the landscape and garden are covered in later chapters on ornamental and edible crops. ru serving as both a reservoir and cycling area for nutrients and carbon 6-6

7 SOIL QUALITY INDICATOR DIVERSITY- BIOLOGICAL TABLE 1 Selected Soil Quality Indicators and Related Areas of Soil Function The column on the left lists specific, measureable indicators of soil quality. The other five columns relate to the five critical functions of soil. The Xs mark the soil functions to which each soil quality indicator corresponds. WATER AND SOLUTE FLOW (of dissolved ions in soil water) FILTERING, BUFFERING, AND DEGRADATION OF ORGANIC AND INORGANIC MATERIALS NUTRIENT AND CARBON CYCLING AND STORAGE STABILITY AND SOURCE OF PHYSICAL SUPPORT Stability of soil aggregates X X X X Water-holding capacity X X X Bulk density (weight/volume) X X X X Earthworms (presence) X X X X X Water infiltration X X Organic matter X X X X X Soil nitrate X X X X Soil enzymes X X Soil ph X X X X Soil structure X X X X X Soil crusts X 6-7

8 The Central Role of Organic Matter The management of organic matter is at the heart of soil quality and stewardship. In fact, organic matter levels in the soil can affect all aspects of soil quality, as shown in Table 1. Therefore, many of the practices for managing soil quality and productivity described in the following sections pertain to organic matter. Increasing organic matter levels in soil can improve available water-holding capacity improve nutrient-holding capacity add plant nutrients and increase their availability to plants promote soil structure reduce bulk density and compaction reduce soil crusts and increase water infiltration increase biological activity, biodiversity, and nutrient cycling in soil make clayey soils easier to work by hand. MANAGING PHYSICAL PROPERTIES OF SOIL The physical properties of soil can be costly and challenging to change, and when change is initiated the process can be slow. Although improving soil quality over time is beneficial in any residential area, keep in mind that soil productivity improvements may be limited in the first few years after sustainable management practices have been initiated. The full benefits of soil management are usually realized over years rather than weeks or months. Also, some site and soil quality factors may limit productivity even under long-term, high-quality management efforts. For those reasons, site selection is a key factor for both the immediate and long-term health and growth of plants. Amendments and cultural practices that can improve soil physical properties include adding ditches or tiles; minimizing runoff and erosion; maintaining soil cover and minimizing tillage; reducing compaction; and increasing organic matter through the use of composts, green manures, and plant residues. Methods for Adding Organic Matter COVER CROPS AND GREEN MANURES Cover crops are plants grown for their contribution to soil, water, and plant relationships, and also their potential benefit to management of pests, pathogens, and weeds. (See photo on facing page.) The term cover crop refers to any living plant that acts as a ground cover. Cover crops become green manures when they are killed (cut) and left on the soil surface or tilled into the soil. Microbial action enables them to add organic matter and nutrients to the soil. These crops can benefit the soil both as a living cover crop and in their decomposition as green manures. Cover crop benefits include enhancing soil structure, reducing compaction, and increasing water infiltration through the addition of organic matter and actions of the cover crop s roots preventing soil crusting and increasing infiltration by protecting soil surfaces and adding organic matter reducing soil erosion (especially in winter when the soil would otherwise be bare) enhancing nitrogen cycling by reducing leaching (that is, holding nitrogen in plants instead of allowing it to move with water through the soil during winter) potentially reducing weed growth potentially suppressing insects and diseases producing nitrogen through fixation (in the case of legumes). Crop Choices Both legumes and nonlegumes are used as green manure cover crops. Commonly used crops are 6-8

9 SOURCE: Natalie Bumgarner, University of Tennessee listed in Table 2. Legumes (such as peas, beans, clover, vetch, and alfalfa) are valuable because they have root nodules that contain nitrogenfixing bacteria. (See Chapter 5.) The ability of legumes to fix nitrogen means that some of the nitrogen added to the soil after the plant decays is a net addition to the nitrogen level because it came from atmospheric nitrogen. Legumes may also have deep roots that improve soil drainage and bring up nutrients from the subsoil to levels where shallow-rooted plants can use them. The nonlegumes used as green manure cover crops are mostly grasses that would be grain crops if grown to maturity. They are grown because they are economical, easily established, and can produce large amounts of organic material quickly. Examples include rye, oats, and wheat. Buckwheat is an example of a cover crop that is neither a grass nor a legume. It does not produce as much biomass as some of the other cover crops, but it can be used as a summer cover crop when warm temperatures or relatively short periods of time between cropping may not allow the use of other species. Other nonlegumes that add biomass include rapeseed and radish. These crops can increase organic matter and also assist with disease management because some have biofumigation potential when incorporated into garden soil. Forage radishes can reduce compaction and improve infiltration because of their large root structure. General Practices Cover crops and green manures are often used between annual crops or before perennial crops. For annual crops, green manure cover crops are commonly planted in the late summer or early fall (about six weeks before a hard frost) and plowed under a few weeks before spring planting. They provide organic matter and nutrients after they have been incorporated and have decomposed, and they also offer physical protection from erosion over the winter. Winter covers are useful to gardeners trying to improve their soils, especially those who are unable to compost enough material to apply over large areas. In addition, summer cover crops may be used over the main growing season in a rotation system designed to control plant pests and weeds or to improve soils. Cover crops may be left to decay on the soil surface or they may be chopped or mowed A raised bed in spring that was seeded the previous fall with a mixture of cereal (rye) and legume (vetch and pea) cover crops. 6-9

10 Crimson clover (Trifolium incarnatum) Hairy vetch (Vicia villosa) White clover (Trifolium repens) Wheat (Triticum aestivum) TABLE 2 Green Manure and Cover Crop Species CROP TYPE COMMON SOWING TIME INCORPORATION TIME Oats (Avena sativa) Cereal rye (Secale cereale) Buckwheat (Fagopyrum esculentum) Annual ryegrass (Lolium multiflorum) Rape (Brassica napus) Forage radish (Raphanus sativus) Legume Fall or Spring Fall or Spring Legume Fall or Spring Fall or Spring Legume Year-round Varies; often used as a perennial living mulch Nonlegume grain Fall Spring Nonlegume grain Spring or fall Spring, fall, or summer Nonlegume grain Fall Spring Nonlegume Late spring or summer Summer or fall Nonlegume grass Fall Spring Nonlegume Spring or fall Summer or fall Nonlegume Fall Spring Examples of Cover Crop Roles in the Home Garden Different cover crops can provide varied benefits to the gardener. For example, grain or grass crops can be good nonhost crops for many soil-borne diseases and nematodes because they are monocots. Crops such as cereal and annual rye provide quick cover in the fall to protect soils over the winter and produce large amounts of organic matter to till in the spring. Legumes are often mixed with these grain crops to provide plant material with a higher nitrogen content. The nitrogen in the plant material reduces the potential for the microbes that decompose the incorporated cover crops to compete with plants for nutrients during decomposition (called immobilization). Some cover crops are not grasses or legumes. An example is forage or tillage radishes that are commonly sown as winter covers. These cover crops have large tap roots and can capture and store soil nitrogen, and they have also been shown to relieve soil compaction. They are in the same family as several common garden vegetables (such as broccoli, cauliflower, collards, kale, and turnips), so be aware of this relationship as rotations are planned. 6-10

11 (before seeds mature), helping to reduce erosion and retain moisture. Leaving cover crops on the surface also slows the rate of breakdown, which can prevent slow nutrient release when plants are young. If cover crops are plowed or turned under, be sure to allow time for them to decay before planting to reduce nitrogen immobilization by the breakdown of the organic matter in the soil. The timing can vary based on the season, cover crop type, and plant maturity. More mature grasses and those plowed under in cooler conditions may need a few weeks to break down so they will not compete with young plants for soil nitrogen. Legumes or summer cover crops, such as buckwheat, may not require as much time before planting because of their faster breakdown or the higher nitrogen content in their tissues. Warmer temperatures can speed breakdown. Healthy worm populations can also promote the decomposition of cover crops, as they shred organic matter into smaller pieces and begin the decomposition with their digestive enzymes. These actions hasten further decomposition by microbes. Cover crops can be used in a similar fashion before planting perennial fruit crops or landscape ornamentals. In many cases, improving the physical (and chemical) properties of soil by growing cover crops before planting can have long-term benefits for the plants that follow. MANURE Manures have long been used to enhance soil quality and productivity. In addition to providing organic matter, they also supply nutrients. However, there are serious food safety risks for fresh vegetables because of the potential for pathogens in manures. Homescale composting will not reach temperatures sufficient to kill such pathogens (150 F) and even industrial-scale compost may not be turned enough times or managed properly to ensure that the entire pile reaches these temperatures for the needed duration. If considering the use of manure outside of the vegetable garden, there are other issues to consider, such as the composition of the manure. Unlike purchased fertilizers, these materials vary in nutrient content. Fresh manure can have such high levels of nutrients that plants can be damaged. High nutrient content also means manures may present risks of rapid nutrient losses from volatilization (loss of nitrogen to the air) or leaching in subsequent rains. When manures with large amounts of hay or straw bedding are added directly to soil, the breakdown of organic matter can pull nutrients from soils (by immobilization) and slow crop growth if not properly managed. They can also contain weed seeds or herbicide residues if animals were fed from pasture forages that had been treated with certain herbicides. (See the digital companion section for information on specific products.) COMPOST Composting is essentially a process of managing the biological breakdown of organic matter to optimize the end product for use in horticulture and agriculture. The process itself is discussed in an accompanying digital companion section; this section focuses on application methods. Compost can be used to enhance the physical, chemical, and biological properties of soil. It can be incorporated into the soil before planting annual or perennial crops, or it can be applied as a sidedress around the base of plants to slowly provide organic matter and nutrients. If purchasing compost, be sure to use a source that has consistent quality and is free from weed seeds, pathogens, or herbicide residues that could damage crops. Before buying, inquire about both the process and the materials used to produce it. Residential composting uses lawn and garden plant residues and food waste to recycle nutrients within the residential system. It eliminates some of the risks inherent in purchased compost because the source of all the content is known. However, precise management is required to produce highquality compost. Because of the temperature requirements to kill pathogens, meat and dairy food waste as well as pet waste should never be used in any compost that will be used in vegetable gardens. Not all composts are equal. Whether home-composted or purchased, only stable 6-11

12 Finished compost will smell earthy and be cool or only slightly warm. The original composted materials will not be distinguishable. compost should be added to the landscape bed or garden before planting. Stable and mature are terms use to describe compost that has largely decomposed. During periods of rapid decomposition, soil microbes rapidly reproduce to decompose organic matter, which requires nitrogen. Microbes also require pore spaces be filled with a combination of water and air for optimum activity. Until the most intensive stages of decomposition have passed, nitrogen present in compost will be tied up in these processes and therefore unavailable to plants. If compost still feels hot, smells strongly of ammonia, or has distinct pieces of food or plants still apparent, it is likely not to be stable and will require more composting time. After the decomposition of most of the easily available sources of energy for microbes is complete, microbial (primarily bacterial) activity and growth has slowed and the compost is more stable and appropriate for incorporation. Mature compost should be cool, have an earthy odor, and an appearance similar to soil. (See photo below.) Other Soil Conditioners Soil conditioner is a general term for materials used to improve the physical properties of soil. Green manures, cover crops, animal manures, and composts all fall into the category of soil conditioners. Other commonly used soil conditioners include sand, peat, perlite, vermiculite, and various potting mixtures. One of the largest challenges with use of the latter materials is that a large volume is needed to affect the soil s physical properties. Although recommendations vary, adding onefifth to one-third of these materials by volume is often recommended as a soil conditioner. Small amounts of added conditioners may have no positive or even a negative effect. For instance, small amounts of sand added to a fine-textured clay soil can actually reduce porosity and increase bulk density as the small clay particles tightly enclose the larger sand particles. Costs associated with purchasing, hauling, and spreading a large amount of material can be a limitation. Also, the practice of heavily amending soils in planting beds or in planting holes may create discontinuities between the amended and the native soil that can slow root SOURCE: USDA-NRCS 6-12

13 growth. See Chapters 8, 9, and 12 for additional information on soil amendment practices in specific ornamental and edible crops. Ditches or Drainage Tiles Soil drainage can, in some instances, be improved by ditching or by installing tile drains. Detailed knowledge of the site and soils are essential for effective use. Ditching, or surface drainage, provides a way to divert water and should be used only to lower a high water table. Only water from saturated soil can flow into ditches or drain lines, so they will be effective only when the water table is above the drain line. Drainage tiles, generally perforated plastic pipes and sometimes referred to as French drains, may be installed at specific distances and depths to increase the rate of water movement through the soil profile. This type of drain is installed below the soil surface because it affects water movement in the soil profile rather than on the soil surface. Often in fine-textured soils, the many small pore spaces hold water tightly and slow the movement of water through the soil profile. In such cases, ditches and drainage tiles are unlikely to help because water will flow too slowly to these structures. If ditching and tiling are not viable options, planting on a temporary or permanent raised bed is a good option to enhance site drainage. Practices that Protect the Soil Surface and Minimize Erosion Erosion, or soil loss, can be caused by both wind and water, and covering the soil surface is essential in addressing both types of loss. Wind erosion is more common in sandy soils, but it can also affect other soil types on sites that are exposed over the winter for example, a vegetable garden tilled in the fall and left bare over the winter. Protecting soil from water erosion includes both preventing soil detachment and minimizing runoff. Raindrop impact is one of the main ways that soil particles become detached and susceptible to erosive forces. In addition, water with suspended particles moving across the surface of the soil loosens and then picks up additional soil particles, carrying them with the water. The most effective way to reduce soil detachment and runoff is to keep the soil covered and increase infiltration. The use of mulches and plant cover (such as grasses and cover crops) are probably the most effective tools to minimize erosion in a residential landscape. Surface cover reduces raindrop impact that can loosen soil particles and cause surface crusting, which then contributes to increased runoff. In addition, mulches, grasses, and other plant covers add organic matter to the soil, improving structure and aiding in water infiltration and storage. Minimizing runoff is important to enable water to enter the soil profile where it is accessible to plant roots. Reducing runoff also alleviates offsite environmental impacts. (See Chapter 4.) Lessening runoff by reducing the slope and the distance water travels on the soil surface can be accomplished by installing swales, which can take the form of soil berms that slow water movement and increase infiltration. Grassed waterways can also be used to reduce water speed to reduce erosion and increase infiltration. These two practices can be both aesthetic and functional in residential lawns, landscapes, and home food production areas. Topsoil, which is most affected by erosion, is the most productive soil and contains the highest levels of nutrients and organic matter. When topsoil has been lost, plants must depend on the physical properties of the remaining subsoil, which is generally not as good for plant growth. Thus erosion harms both current and future soil quality and productivity. Because soil formation is a slow process, a significant erosion event can remove in a matter of minutes soil that took decades or centuries to form. Furthermore, soil carried by runoff leads to sediment loading in waterways. (See Chapter 4.) Tillage Practices Tillage can be a way to incorporate organic matter, control weed pressure, provide a darker surface to increase early-season soil warming, and prepare a fine seedbed. However, it also exposes organic matter to loss by rapidly 6-13

14 increasing microbial activity, and it brings buried weed seeds to the surface, triggering their germination. Tillage also reduces soil structure. While tillage can be used to aerate compacted soil, the benefit may be short-lived and may not provide long-term solutions to the problem. Tillage breaks the soil aggregates into smaller pieces, or individual soil particles, which in turn reduces the larger pores and water channels in soil created by earthworms and plant roots. Tillage affects soil organic matter content over the long term. When soil is tilled, soil aggregates that protect organic matter are broken apart. The combination of exposed organic matter and increased oxygen increases microbial decomposition of the organic matter and reduces its percentage in soil over time. For all of these reasons, the use of reduced or no-till practices is quite common in production agriculture. In fact, many raised beds may be repeatedly cropped without tilling because organic matter can be incorporated and weeds controlled by hand. These facts can seem to create a conflicting picture of how to use tillage optimally. In annual cropping areas (such as traditional vegetable gardens), tilling may occur multiple times a year. Thus, the need to till the soil for weed control and organic matter incorporation should be balanced with the drawbacks of frequent tillage in building soil organic matter and structure. On the other hand, many lawns, landscapes, or fruit production areas are rarely tilled after planting. In these areas, tillage may be used before planting to incorporate organic matter and soil amendments. The optimum balance varies with the soil and site conditions and the plants being grown. MANAGING CHEMICAL PROPERTIES OF SOIL (SOIL FERTILITY) Soil fertility refers to the ability of the soil to supply a plant with essential nutrients in adequate amounts and proportions. It is important because it directly affects crop productivity but also because it is more easily adjusted than many soil physical and biological factors. In many production systems, the management focus is often on optimizing soil fertility for crop growth. While proper fertility management is crucial, these practices are best integrated with management of soil physical and biological properties to support long-term soil quality and productivity. Soil Sampling and Using Soil Reports Soil testing is the first important step in managing soil fertility because it provides the homeowner or gardener with important information on current levels of plant nutrients in the soil. Routine testing is essential whenever the soil is disturbed, materials are added to the soil, or plants are continuously grown in the area. Carefully collected soil samples may be sent to the UT Extension Soil, Plant and Pest Center laboratory for analysis, as described in a later section of this chapter. The most important thing to remember when collecting soil samples is that test results are only as good as the sampling methods used. SAMPLING LOCATION Inspect the areas under consideration for planting and determine if the soils in those areas appear to be about the same. If the areas are similar, a composite sample can be prepared. Collect 10 to 15 subsamples using the method described below. Be systematic in sampling, and consider using a sampling grid or zigzag pattern to ensure that equal soil volumes from each portion of the growing area are sampled. (See Figures 1 and 2.) If areas in the potential site have had different types of plant material or the soil appears to be quite different in texture or color, take separate samples from each area. Use composite samples only where the soils and previous management activities are similar. For instance, a soil sample from a shrub border area that will or does contain azaleas or other acid-loving plants should not be combined with a soil sample from an adjacent lawn. Also take separate samples from areas that will be used to grow vegetables or fruit. 6-14

15 From an area of 10 acres or more From a lawn area SAMPLING PROCEDURE Samples may be taken in the spring or fall, but it is more common to take fall samples so that amendments can be made and have time to take effect before the beginning of the next growing season. The vertical space within which most plant roots grow is the most important area to test. Recommended sampling depths are lawns and turf 4 inches gardens 6 inches orchards 8 to 12 inches. If using a spade to collect samples, remove an area of soil of the appropriate depth. Then clean the spade and take another thin slice of the soil with the clean spade that covers the width and depth of the hole. The middle portion of this slice is often the most accurate to use for a sample. (See Figure 3.) If using a soil probe, simply take soil cores of the appropriate depth for the crop to be produced. Be careful to exclude surface debris and plant material and place the samples in a clean, nongalvanized bucket or container. Be sure to use a container that has not been used to hold fertilizers. The equal 10 to 15 subsamples taken from a consistent area can be Driveway Tree Patio 1 mixed together. Repeat this sampling procedure as needed for areas that have been managed differently, disturbed, or have dissimilar soils. SAMPLE HANDLING AND MAILING The UT Soil, Plant, and Pest Center provides information about tests and sampling on its website. Information about sample collection and sample boxes is available at all county Extension offices. (See Figure 4.) Fees vary based on whether samples are mailed directly from the homeowner to the lab or mailed from the county office. Moist soil samples should be air dried on a paper plate before packaging to mail to the laboratory for analysis. Use a waterproof marker to record information on the sample boxes because pencil marks and some inks may fade during shipment. Samples should not be submitted in plastic bags because it can affect ph results and delay the analysis. Soil samples should be submitted with a Soil and Media Information Sheet (Form F394) found on the UT Soil, Plant and Pest website and in the companion materials. For home gardens, the soil Plus test results provide data on ph, phosphorus, potassium, calcium, magnesium zinc, manganese, iron, copper, sodium, and boron levels, and they also give lime 6 House Street FIGURE 1 (L) Example of a soil sampling pattern around an open garden or landscape area. FIGURE 2 (R) Example of a soil sampling pattern around a residence. 6-15

16 FIGURE 3 Collecting a soil sample using a spade or soil probe. and fertilizer recommendations. Optional tests for organic matter and nitrate content can also be useful. Understanding the Soil Test Report A soil test report provides data on the current nutrient levels of the sample and recommendations for amending the soil to reach optimum productivity for the crop. Over a period of many years, soil laboratories have gathered and assessed information on crop nutrient needs and soil reactions to fertilizer in establishing ideal target ranges and fertilizer application recommendations. For a soil test, each nutrient tested is reported in pounds per acre and assigned a soil test category. The ratings for phosphorus and potassium are low (L), medium (M), high (H), or very high (VH). When the secondary and micronutrients are tested, they are rated as either sufficient (S) or deficient (D). Nitrogen is not usually reported because values for plant available nitrogen are difficult to report consistently and accurately. Recommendations for field crops are reported in pounds of plant nutrients and tons of agricultural limestone to apply per acre. For lawns and gardens, recommendations are generally reported in pounds of actual fertilizer grades and agricultural limestone to apply per 1,000 square feet. More information on interpreting soil test results is provided later in this chapter. Managing Soil ph The ph of soil, or more precisely the ph of the soil solution (see Chapter 5), is very important because the soil solution supplies nutrients that plants need to grow and thrive. The range of ph values is from 1 to 14. Seven is considered neutral, while values above 7 are alkaline, or basic, and those below 7 are acidic. Soils in Tennessee are generally acidic, with a ph ranging from 4 to 7. However, in some areas of the state, such as the inner basin, alkaline soils are more common. Ornamental plants generally perform better at a ph range of 5.2 to 6.5, whereas most vegetable crops do better at a ph of 6.0 to 6.5. INCREASING SOIL PH Soil ph can be low for a variety of reasons, as discussed in Chapter 5, but the most important aspect of management is maintaining the soil ph level within the appropriate range. If the soil test reports a lower-than-optimum ph, Discard outer edges of sample 6" 6-16

17 lime can be used to raise the ph and improve soil fertility. The amount of liming materials needed depends on the type of material, existing soil ph, soil type (see Table 3), and the unit of change desired to meet the ph requirement of the plants to be grown. Liming can improve soil fertility by adding calcium and magnesium raising the ph so that molybdenum (Mo), potassium (K), phosphorus (P), and sulfur (S) become more available reducing the possibility of toxicities from aluminum (Al) and manganese (Mn) enhancing beneficial microbial activity for better nitrogen fixation improving the soil structure of clay soils through the aggregating actions of calcium (Ca). For most garden and landscape plants, liming materials should be spread uniformly and mixed into the top 6 inches of soil. If large increases in soil ph are needed, lime applications can be split and applied six months apart. Limestone can be added to lawns at any time of year, but it will not have an effect on plants until it is in the soil solution. Liming in the fall enhances the movement of limestone into the soil because the winter freeze thaw cycle increases soil porosity. Also, keep in mind that it is not recommended to apply liming materials and soluble nitrogen fertilizers (especially those with NH 3 ) at the same time because of the potential for more rapid nitrogen loss to the air (volatilization) at higher ph ranges. REDUCING SOIL PH In some cases lowering the soil ph is necessary because of the soil s parent material, previous management practices, or the need to tailor soil conditions to acid-loving plants. Reducing the ph is usually accomplished by adding a sulfur source. The relationship between soil type and the amount of sulfur to be added is shown in Table 4. A number of sulfur materials can be used to lower soil ph, and elemental sulfur is common. It is inadvisable to add large amounts of aluminum sulfate because of the potential for aluminum toxicity in plants. At high ph levels, adding elemental sulfur is not highly effective. In dealing with high ph soils, it is best to use iron sulfate initially and then use elemental FIGURE 4 Mixing and preparing a soil sample to be mailed to a testing center. Thoroughly crumble and mix Remove about 1 cup to box and ship for analysis 6-17

18 TABLE 3 Amount of Limestone Needed to Raise the ph to 6.5 from Various Starting ph Levels POUNDS OF LIMESTONE TO ADD PER 1,000 SQUARE FEET BASED ON SOIL TYPE STARTING ph SAND SANDY LOAM LOAM SILT LOAM CLAY LOAM TABLE 4 Amount of Sulfur Needed to Lower the ph to 6.5 from Various Starting Levels sulfur to reduce the ph further. If soil ph is currently in the optimum range for most plants (6.0 to 6.5) but needs to be lower for blueberries or another acid-loving crop, elemental sulfur is the most commonly used material. When acidloving crops are listed on the information sheet, the soil test report will recommend appropriate materials to lower the ph at the site. Managing Soil Fertility for Plant Nutrition Nutrients are provided to plants through soil in several ways, including the weathering of soil minerals and the breakdown of organic matter. While these methods are very important, gardeners and growers often require more rapid and specific tools for managing plant nutrition. In such situations, fertilizers can be applied. Chemical fertilizers (synthetic or conventional) or organic materials can be used. Because POUNDS OF SULFUR TO ADD PER 1,000 SQUARE FEET BASED ON SOIL TYPE STARTING ph SAND LOAM CLAY fertilizers can be lost to both water and air, soil tests should always be made before applying fertilizer to determine whether additional fertility is needed. Avoiding the application of unnecessary fertilizer can save money and prevent unfavorable environmental impacts, as explained in the following section. TYPES OF FERTILIZERS Chemical Fertilizers Chemical fertilizers are made through industrial processes or mined from deposits in the earth and purified, mixed, blended, and altered for ease of handling and application. (See Table 5.) Chemical fertilizers are essentially salts that are added to the soil to provide nutrients when the salt dissolves. For instance, potassium nitrate (KNO 3 ) fertilizer will provide K + and - NO 3 ions that plants can use. (See Chapter 5.) These fertilizers react quickly in soil. Once the 6-18

19 ions in the fertilizer dissolve in the soil solution, the nutrients can be immediately taken up by plants. However, nutrients can also be held by soil particles or form other salts, limiting their availability to plants. Organic Fertilizers As used here, the word organic refers to fertilizer materials that came from previously living things. Scientifically, it means biologically fixed carbon. Organic fertilizers provide the same nutrients to plants as chemical fertilizers. However, the nutrients may become available more slowly because microbes are needed to transform the nutrients to a form that can be readily taken up by plants. While the slower release of nutrients from organic fertilizers can be a drawback, it can also reduce the risk of nutrient leaching. Commercial organic fertilizers provide guaranteed nutrient analysis just as do chemical fertilizers, so the calculations discussed below are applicable. Organic fertilizers often contain lower nutrient percentages by weight than chemical fertilizers, so a greater volume may be needed to supply the same amount of nutrients. However, the addition of organic matter to soil is often an added benefit of organic nutrient sources. USDA Organic Fertilizers In addition to the scientific definition of organic, there are also legal definitions created by the United States Department of Agriculture to guide certified organic production. USDAcertified organic fertilizers can actually overlap with both the chemical and organic definitions. For instance, composts and many of the animal byproducts (such as bone meal) may be allowable in certified organic production. Also, some fertilizers that are categorized as chemical are naturally mined rather than produced in a human-controlled chemical reaction and are appropriate for certified organic production. However, there are also chemical and organic fertilizer materials that may not be allowable in certified organic production. Table 5 provides some examples but is not inclusive. FERTILIZER DESCRIPTION Fertilizer labels are required by law to display the quantities of nutrients provided. Percentages by weight of macronutrients nitrogen (N), phosphorus (P), and potassium (K) are always TABLE 5 Examples of Chemical, Organic, and USDA Organic Fertilizers FERTILIZER TYPE EXAMPLES OF MATERIALS DESCRIPTION Chemical Organic USDA Organic Ammonium nitrate, ammonium sulfate, diammonium phosphate, calcium nitrate, and potassium nitrate Compost and dried, activated sewage sludge; Animal byproducts, such as manure, feathers, leather, and blood meal; Plant byproducts, such as kelp products, corn gluten meal, and proteins Compost, fish emulsion, bone meal, blood meal; Kelp products, rock phosphate, and greensand These fertilizer materials are usually soluble in water and more quickly available for plant uptake. Foliar or root burn can occur in plants if too much is applied. Nutrients are often released slowly as a result of soil microbial activity, so these sources generally have a low burn potential. The Organic Materials Review Institute (OMRI) can be used as a source of information about products and specific name brands that are allowed in certified organic production. However, these are required only for commercial producers. 6-19

20 Management Practices to Promote Soil Structure by Colby J. Moorberg, Assistant Professor of Soil Science, Kansas State University Most gardeners and landscapers are familiar with soil texture the relative proportion of sand, silt, and clay particles present in soil. Soil structure, in turn, relates to how those sand, silt, and clay particles are arranged in three dimensional shapes. Both the type of structure (what shape the soil takes) and the degree to which the soil structure is developed have a huge impact on a variety of soil properties, such as aeration, infiltration, drainage, resistance to root growth, and more. Soil structure develops over time as soil particles are glued together into aggregates. At the largest scale, soil aggregates are held together by roots and fungal hyphae. At finer scales, they are held together by root hairs, fungal hyphae, and polysaccharides (long chains of sugars), and at even finer scales by plant and microbial debris, clay domains (stacks of clay layers), and humus. It is important to note that (with the exception of clay) all of these glue materials that contribute to soil structure, if disturbed, can be decomposed by microbes. Surface soils (the A horizon) are typically rich in organic matter built up over time from decomposing plant roots and residues, resulting in a granular soil structure. Strong granular structure is very desirable because it promotes aeration, allows for easy seedling emergence and root growth, and facilitates significantly higher infiltration rates. Soil structure granular soil structure in particular can be promoted by adding organic matter and gypsum, and it can be protected by avoiding soil disturbance and compaction. Addition of soil amendments such as compost, manure, mulch, and nitrogenrich plant matter like alfalfa or grass clippings promote the formation of soil humus and organic matter after they decompose. Increased soil organic matter also helps make heavy clay soils less sticky and more easily worked by hand because the humus covers the surfaces of clay minerals. Addition of gypsum, which is soluble and contains calcium (Ca 2+ ), promotes the flocculation and aggregation of clays. Existing soil structure can be protected by minimizing mechanical soil disturbance such as that caused by rotary tillage. Such disturbance destroys aggregates and in the process breaks up roots, root hairs, fungal hyphae, and polysaccharides. These changes drive decomposition and an overall decrease in soil organic matter. While these practices may temporarily improve soil tilth and seedbed preparation, in the long term soil structure breaks down and bulk density increases. Compaction should be avoided because it forces soil peds and aggregates together, eliminating pore space and increasing bulk density. It can be avoided by restricting foot or vehicular traffic to designated areas, leaving undisturbed areas for plant growth. A common concern about minimizing or eliminating tillage is losing the ability to incorporate soil amendments and plant residue. However, as with the no-till systems commonly used in production agriculture, worm populations increase after tillage is stopped. Worms feed on plant residues on the surface and incorporate the residue below ground. Any amendments added to the surface are also incorporated, eliminating the need for manual incorporation. If tillage must be used, compost or other organic matter amendments should be added to the soil surface in large amounts before tilling. This approach allows the amendments to be incorporated and also helps replace the soil organic matter lost through soil disturbance. 6-20

21 listed in the order N-P-K. For example, the designation on the label means that the fertilizer contains 10% N, 10% P, and 10% K. By convention in the chemical industry, the percentages of P and K on fertilizer labels are actually reported in terms of the phosphate (P 2 O 5 ) and potassium oxide (K 2 O) content. It is possible to convert these numbers, but many fertilizer recommendations given on soil test results are given in terms of P 2 O 5 and K 2 O to eliminate the need for conversions. If conversion is necessary, the following factors can be used to convert P 2 O 5 and K 2 O ratings to actual amounts of phosphorus and potassium: P = P 2 O 5 x 0.44 P 2 O 5 = P x 2.29 K = K 2 O x 0.83 K 2 O = K x 1.20 For example, if you needed to apply 2 pounds of P, 4.58 pounds of a P 2 O 5 fertilizer material would be needed. There are several ways to describe fertilizers in addition to using the terms chemical and organic. Three other categories relate to the number of primary macronutrients contained. Complete fertilizers refer to those containing all three of the primary macronutrients (N, P, and K). They are commonly used by homeowners because they are easy and simple to use. In fact, many soil tests recommend certain quantities of specific complete fertilizers based on soil needs. Single carrier and mixed fertilizers contain just one or two of the N, P, and K elements, respectively. They are more commonly used to address specific needs of soils or to apply a nutrient that is used in higher quantities by certain crops or at certain times of the year. FERTILIZER FORMULATIONS Granular Blended Fertilizer Dry, blended fertilizers are sold commercially and contain a physical mixture of two or more materials and two or more primary nutrients. They may contain other nutrients as well. Each particle of material can be identified in the blended fertilizer by its color or shape. Blended fertilizer may contain inert filler, such as ground limestone or rock to make the grade come out even. Manufacturers can create customized blends to suit the precise nutrient demands of most situations. Granular Homogenized Fertilizer Heating and adding ammonia to certain materials forms dry, granular fertilizers, which are uniform prills or granules. Each granule contains all the plant nutrients listed on the label so the fertilizer is homogenous for an even distribution of plant nutrients. Dry granular fertilizers may be available in some of the same grades as dry blended fertilizers. Dry granular fertilizers are generally complete fertilizers. Micronutrients may also be included in the granules so they can be more uniformly distributed in the fertilizer. Most premium or super fertilizers are granular. Water-soluble Fertilizer Dry, water-soluble fertilizers are dry materials that have been highly purified and blended with SOURCE: Natalie Bumgarner, University of Tennessee A bag of chemical fertilizer with a label showing that it contains 10% N, 10% P 2 O 5, and 10% K 2 O 6-21

22 A granular, blended fertilizer prior to application SOURCE: Natalie Bumgarner, University of Tennessee no fillers. The products are crystallized rather than prilled or granulated so the compounds dissolve more rapidly in water. Sometimes a soluble dye is added to make the fertilizer easier to see in solution. Because they are highly purified, dissolve completely, and are suitable for irrigation systems, they usually cost more than granular or blended fertilizers. Examples are some of the specialty fertilizer products sold under the Miracle Grow or Peters brands. Weed-and-Feed Fertilizer Weed-and-feed is a generic term for lawn care products containing both fertilizer and herbicide(s). They may be in granular or watersoluble forms. In most cases, the herbicide contained in a weed-and-feed product is broadleaf specific and designed not to affect desirable turfgrass species when applied according to label directions. Some of these products may also include a preemergent herbicide that will prevent the growth of annual weeds such as crabgrass. These products must be used strictly as directed on the label to be effective and safe for plants and to control environmental impacts. Liquid Fertilizer Liquid fertilizers are already dissolved in water when sold. They may contain a lower percentage of plant nutrients than dry fertilizers and are often more expensive but are convenient in small areas and for foliar fertilization. Starter Fertilizer Starter fertilizers are small amounts of fertilizers, often containing N and P, to ensure readily available nutrients for new transplants and young seeds emerging in cool soils. Generally, no more than one-third of the recommended fertilizer is applied as a starter fertilizer to prevent loss of nutrients and inefficiency while the plants are young. Fertilizer Spikes Fertilizer spikes are cakes" of fertilizer that are designed to be forced into the soil or into pots. They are easy to use around individual plants but can result in nonuniform application because nutrients are often concentrated near the spike. Slow-release or Timed-release Fertilizer Some dry fertilizers are formulated to release nitrogen slowly into the soil during the growing season. Fertilizer granules may be coated with a resin (such as Osmocote), sulfur (as in sulfur-coated urea) or a plastic polymer (as in polymer-coated urea) to reduce their solubility. Also, the fertilizer can be made from materials that are broken down slowly in the soil, making them slow-release nitrogen sources. Slow-release fertilizers are useful for turf, shrubs, fruits, and other plants that would normally need several fertilizer applications during the season. Depending on the rate of nutrient release, they may not be suitable for short-season annuals. FERTILIZER APPLICATION METHODS AND TERMS Broadcast Fertilizers are frequently broadcast (spread evenly over the soil surface) prior to planting 6-22

23 and incorporated into the soil with normal soil tillage practices. Some N, most of the P and K, and the secondary nutrients and micronutrients are often broadcast before planting annual vegetables or flowers. Sidedress Sidedressing is the application of fertilizer to one or both sides of growing plants or beside a row of plants. Crops are sidedressed after they are established and growing well. Nitrogen fertilizer is usually applied as a sidedress because it leaches rapidly through the soil. Applying N-containing fertilizers when plants are growing most rapidly helps to ensure that it will be taken up by the plant and not lost to the environment. Finished compost can also be applied as a sidedress fertilizer. Topdress Topdressing is the uniform (broadcast) application of fertilizer, usually N, over the top of solid-planted crops. For example, a solid bed of turnip greens or strawberries would be topdressed with nitrogen because sidedressing would be difficult. Turfgrass is another crop for which topdressing is common. Banding Banding describes putting all the P, and occasionally some N and K, in a band approximately 3 inches below and 3 inches to the side of a row of seeds at planting. The intent of banding is to provide readily available phosphate in soils that test low or very low in available phosphorus. Soils that are low in phosphorus can tie up fertilizer phosphorus if it is broadcast and mixed with the soil. Phosphate sources are generally lower in salts and are not as likely to burn seedlings as N and K fertilizer sources. Foliar Fertilization Foliar fertilization provides small quantities of plant nutrients to growing plants by spraying a dilute solution of fertilizer on plant leaves. This technique is most effective for correcting micronutrient deficiencies (of Fe, Mn, B, Mo, or Zn) in growing plants. Foliar fertilization is less effective with the primary and secondary nutrients simply because plants cannot take up enough through the leaves to be effective. Foliar fertilization will not compensate for poor soil fertility. USING A SOIL TEST TO ADDRESS FERTILIZATION In addition to providing instructions for adjusting soil ph, soil test reports provide recommendations for adding nutrients needed by general crop types. Annual vegetable crops have much different nutrient needs from fruit crops or landscape shrubs. Soil test reports list recommended amounts of N, P, and K needed. A couple of simple calculations are needed to determine the correct amount of fertilizer material to be added to the site. 1. (Area of the site/ area used in the recommendation) x (recommended nutrient from the soil report) = nutrient weight needed for the site 2. Nutrient weight for site/percentage of that nutrient in the fertilizer material = fertilizer weight needed for the site Here is an example: The soil test report recommends applying 2 pounds of N per 1,000 sq. ft., and the planting area is 500 sq. ft. (20 x 25 ft.). The gardener has purchased a fertilizer. First, divide the planting area size by 1,000 sq. ft. (as used in the recommendation) and then multiply by 2 (the recommended pounds of N from the soil test) to determine the pounds of N needed in the planting area. Equation 1: (500 sq. ft./1,000 sq. ft.) x 2 lb N = 1 lb N Since it is a fertilizer, 20% (or 0.2) of the weight of the fertilizer is N. So, divide the amount of N needed for the area by the amount of N in the fertilizer. Equation 2: 1 lb N/ 0.2 (20% N in fertilizer) = 5 pounds of These calculations can be used for any fertilizer material (bone meal, blood meal, fish meal, cottonseed meal, compost, and others) for which nutrient values are known. For instance, 6-23

24 if an organic blood meal fertilizer with a guaranteed analysis of 12% N is used, then the value 0.12 would be used in Equation 2. Like chemical fertilizers, organic materials can be used as a broadcast fertilizer before planting or as a sidedress during growth. Keep in mind that organic fertilizers must be broken down by soil microorganisms to enable nutrients to become available to plants. These biological processes are slower and less predictable than the release of nutrients from a chemical fertilizer. So, the same amount of N may be added using an organic fertilizer source, but it will be slower to become available than from a chemical fertilizer. However, this slower release does NOT mean that more fertilizer should be added than is needed to supply the nutrients. Overuse of organic nutrient sources can be ineffective in terms of cost and can lead to leaching and runoff into surface waters. Specific fertilizer application methods for landscape shrubs, vegetative ornamentals, lawns, vegetable gardens, and tree and small fruits are described in chapters 8, 9, 10, 11, and 12, respectively. CHOOSING A FERTILIZER Many fertilizers sold for homeowners are labeled to suggest formulation for a particular plant, but this designation may be simply a marketing tool. Fertilizers should be selected based on soil test results, the nutrients provided by the fertilizer, and the plants to be grown. Generally, a higher percentage of N and lower P 2 O 5 is found in fertilizers labeled for leafy vegetables and grasses (including corn) because more vegetative growth is promoted by the higher N content. Flowers, bulbs, potatoes, tomatoes, and other heavily fruiting plants need less N because excessive vegetative growth can delay fruiting. Fruiting vegetables require high levels of K during fruit production. Fertilizers for acid-loving plants such as azaleas, camellias, and blueberries are made using ammonium forms of N Fertilizer Considerations These example calculations were based on nitrogen needs, but keep in mind that an application of fertilizer will also supply equal amounts of P 2 O 5 and K 2 O. If these values greatly exceed the recommended applications of P and K, then another fertilizer material should be selected containing lower amounts of P or K. Fortunately, fertilizer dealers usually carry fertilizer bags with just one or two of the primary nutrients. Materials such as ammonium nitrate (34-0-0), ammonium sulfate ( S), sodium nitrate (16-0-0), concentrated superphosphate (0-45-0), diammonium phosphate ( ), muriate of potash (0-0-60), and potassium nitrate ( ) can be used instead of complete fertilizers. Per pound of nutrient, the materials may be less expensive than blended or granular fertilizers, and the gardener does not pay for unnecessary products. Secondary and micronutrients are also available in small quantities, such as gypsum (for Ca and S); zinc sulfate (for Zn); fertilizer borate or household borax (for B); Epsom salts, MgSO 4, (for Mg and S); iron sulfate (for Fe and S); and several forms of chelated micronutrients. 6-24

25 because of ammonium's effect on soil acidification. Use of complete fertilizers without consideration of soil reports is an increasing problem in gardens. For instance, a homeowner may purchase a fertilizer because of its availability and ease of use. However, if the soil test indicates that the soil s phosphorus level is already sufficient, a fertilizer high in phosphate (P 2 O 5 ) will be wasteful. Furthermore, phosphorus can build up in many Tennessee soils. Therefore, with continued use of a complete fertilizer, P accumulates in the soil and can lead to poor plant uptake of other nutrients or loss of the P to the environment with erosion. Likewise, imbalances of nutrients can occur with repeated use of composts and organic fertilizers if nutrient applications are not guided by frequent soil tests. Manures are often quite high in P, so if manures or composts from manures are applied repeatedly, imbalances are likely. Describing Soil Organic Matter Content The term organic matter has been used broadly here to describe formerly living materials added to soil. In reality, there are different classifications of soil organic matter (SOM). Green manures and raw materials in compost are fresh sources of organic matter that microorganisms break down rapidly (decompose). In fact, a large percentage of the volume (approximately 80 percent) of this material will be lost during the process of decomposition or composting. The process of decomposition converts this green fraction into carbon dioxide or more stable decomposition products. A second fraction of SOM is the microbes themselves. Even though these organisms are mostly single-celled, we can find billions of them in every teaspoon of soil, so they represent a significant amount of the organic matter in soil. The third fraction is the most stable and difficult to break down and will exist for years or decades with little change. This SOM is called humus and is made up of materials that are resistant to microbial degradation. Stable SOM is highly valuable in enhancing soil properties. Many of the water- and nutrientholding impacts of organic matter are provided by SOM. However, even though this fraction is considered stable, it, too, will be slowly decomposed over time by soil microorganisms. An important question is whether SOM can be assessed and changes recommended as is done for soil nutrients. SOM can be described by laboratory tests, and this technique is used by the University of Tennessee Soil, Plant, and Pest Center. However, there are valid reasons for not providing recommendations with SOM tests. Soil texture, vegetation, climate, and many other factors including human management all impact the level of organic matter in soil. In addition, there are currently no well described minimum levels of SOM below which crops and plants suffer. Thus organic matter testing can be used as an indicator whether management practices are supporting SOM increases in soil over the long term, but recommendations for improving organic matter or target levels are not currently provided. Soil organic matter content is important to every facet of soil function and health. As SOM breaks down, it supplies food for soil microbes and provides nutrients for plants. Additions of SOM benefit the physical, chemical, and biological properties of soil. Efforts to increase organic matter content can be viewed as an ongoing process for all gardeners who seek both soil health and quality as well as plant productivity. MANAGING BIOLOGICAL PROPERTIES OF SOIL Roles of Common Soil Organisms BACTERIA Bacteria are the soil organisms generally found in the highest numbers, and they play many roles in the soil system. Often found in the region immediately surrounding roots (the rhizosphere), bacteria commonly act as a decomposer. They act on materials containing carbon and nitrogen, such as sugars, starches, fats, and proteins that are easier to decompose and act much more slowly on complex 6-25

26 Root nodules caused by nitrogen fixing bacteria (e.g., Rhizobium spp.) on alfalfa root compounds such as lignin. Bacteria can also form symbiotic relationships with plants (see Chapter 5) and can promote growth by reducing pathogen impact, increasing nutrient access, and fostering production of other growth-promoting products. Bacteria also play essential roles in conversion of nitrogen and sulfur in soils. Reactions that produce plant-available nutrients generally occur under well-aerated conditions. Thus the physical aspects of soil structure along with air and water movement in soil are essential in maintaining environments where bacteria function well in their conversion role. Bacterial reactions in soil can act to make nutrients available, but they can also work in the reverse. When large amounts of organic matter are added to soil, bacterial populations quickly increase to act as decomposers. The rapid population increase of microbes can create nitrogen demand and immobilize nutrients that would otherwise be available to plants. FUNGI Fungi and closely related organisms are the largest group of soil organisms in terms of total biomass. Fungi have a filamentous structure that enables them to grow locally, and they essentially colonize soil in search of food sources. They are crucial to decomposition processes and produce enzymes that break down more complex organic molecules. Specific enzymes are produced that will break down the many types of plant tissue, including some of the most difficult to degrade, such as lignin. Through these decay processes, fungi play a vital role in soil quality and nutrient cycling by decomposing organic matter. Also, the structure of the organisms themselves serves a vital role in enhancing soil structure, aggregation, and stability. Being nonphotosynthetic, fungi must obtain energy and nutrients from other sources. They do so not only by decomposing decaying organic material but also by forming symbiotic or parasitic relationships with living plants and animals. A well-known symbiosis was described in Chapter 5 with regard to mycorrhizal fungi. It is estimated that 70 to 80 percent of vascular plants can be mycorrhizal. Through these associations, fungi are able to provide plants with nutrients mined from soil areas inhabited by the fungal individual but not accessible to plant roots. Phosphorus, for example, is a SOURCE: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org 6-26