Soil Chemistry Key Terms
Key Terms in Soil Chemistry Buffering capacity - this is the soil's ability to resist changes in soil ph. Soils with a high buffering capacity require a great deal of amendment to alter ph. This is good if the soil already has the ph you want, but it can be a problem if you try to change the ph - for instance, if you want to make the soil more acid in order to grow acid-loving plants. Normally, soils high in clay or organic matter have high buffering capacities, which means the ph is difficult to change. Calcareous soils also often have high buffering capacities because lime effectively neutralises acid - a great deal of acidification may be necessary to eliminate the lime before you can achieve a significant drop in ph. Conversely, in lime-free soils, a small acid treatment can drop ph significantly. Soils can also resist upward changes in ph, depending on their composition. Because buffering capacity determines how much amendment it will take to change ph (and how much it will cost), this is an important characteristic for growers. Alluvial - Alluvial parent material is transported and deposited by water, and may be found particularly in valley basins, forming an Alluvium soil. Alluvial material may consist of broken rocky matter, or sediment, formed from material that has been carried in suspension by a river or stream and dropped as the velocity of the current decreases. River plains and deltas are made entirely of alluvial deposits, but smaller pockets can be found in the beds of upland streams. Cation Exchange Capacity (CEC) - The CEC measures the extent to which soil can hold and exchange plant nutrient cations. The ability of soil to hold positively charged nutrients so that they are not leached and lost from soil is important to maintaining soil fertility. Clay and organic matter have a negative charge. They allow the soil to hold these nutrient cations due to the attraction of charges. Soils with high clay or organic matter content will have a higher CEC. Sandy soils tend to have a lower CEC.
Base saturation - this is the portion of the CEC (Cation Exchange Capacity) accounted for by the basic ions (calcium, magnesium, potassium, and sodium) and expressed as a percentage of the CEC. It is also known as the base saturation percentage. A soil saturated with calcium and magnesium is considered normal and fertile. If a soil has more than 15% exchangeable sodium, it is an alkali soil. If a soil is base-saturated - which means that there is a high percentage of exchangeable hydrogen - it will be more acidic. Marling - the process of adding marl (which is traditionally a calcareous clay soil) to light sandy soil to improve its texture and fertility and to increase its water-holding capacity. Field Capacity this is the amount of water remaining in the soil a few days after having been wetted and after free drainage has ceased. At field capacity, water fills the micropores and mesopores, while air fills the macropores. At field capacity, the water and air balance of the soil is considered to be ideal for crop growth. Macropores are large soil pores, usually between aggregates, that are generally greater than 0.08 mm in diameter. Macropores drain freely by gravity and allow easy movement of water and air. They provide habitat for soil organisms and plant roots can grow into them. With diameters less than 0.08 mm, micropores are small soil pores usually found within structural aggregates. Suction is required to remove water from micropores. Capillary rise - If a piece of tissue is dipped in water, the water is sucked upward by the tissue. The same process happens with a groundwater table and the soil above it. The groundwater can be sucked upward by the soil through micropores. This process is called capillary rise. It occurs due to the cohesion (stickiness) between water molecules, and between water molecules and surrounding soil particles.
In fine-textured soil (clay), the upward movement of water is slow but covers a long distance. On the other hand, in coarse textured soil (sand), the upward movement of the water is quick but covers only a short distance. Permanent Wilting Point - This is the water content of a soil when most plants (corn, wheat, sunflowers) growing in that soil wilt and fail to recover their turgor upon rewetting. Temporary Wilting Point This refers to wilting that occurs in hot weather when the rate of transpiration exceeds the rate at which a plant can absorb moisture from the soil. The plant recovers when the temperature falls. Available Water Capacity - The total available water (holding) capacity is the portion of water that can be absorbed by plant roots. By definition, it is the amount of water available, stored, or released between field capacity and the permanent wilting point water contents. Sometimes referred to as Available Water Content. The soil types with higher total available water content are generally more conducive to high biomass productivity because they can supply adequate moisture to plants during times when rainfall does not occur. Sandy soils are more prone to drought and will quickly (within a few days) be depleted of their available water when transpiration rates are high. For example, for a plant growing in fine sand with most of its roots in the top 30 cm of soil, there is less than one inch of readily available water. A plant transpiring at the rate of 0.25 inches per day will thus start showing stress symptoms within four days if no rainfall occurs. Shallow rooted crops have limited access to the available soil water, and so shallow rooted crops on sandy soils are particularly vulnerable to drought periods. Irrigation may be needed and is generally beneficial on soils with low available water capacity.
Table for reference only: Soil Type Total Available Water, % Total Available Water, in/ft coarse sand 5 0.6 fine sand 15 1.8 loamy sand 17 2.0 sandy loam 20 2.4 sandy clay loam 16 1.9 loam 32 3.8 silt loam 35 4.2 silty clay loam 20 2.4 clay loam 18 2.2 silty clay 22 2.6 clay 20 2.4 peat 50 6.0 Total Soil Water Storage Capacity - The total soil water storage capacity refers to a situation where all of the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded. A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels have the lowest total porosity, typically around 30 to 40% by volume. Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40 to 60%. Restricted drainage conditions can cause the soil to attain its total porosity water content. When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is severely restricted. Most horticultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen diffusion to the roots. Without air-filled pores, the concentration of carbon dioxide and other gases like ethylene increase, producing toxic conditions and limiting plant growth. Root cells switch to anaerobic respiration, which is much less efficient than aerobic respiration in converting glucose molecules to ATP (adenosine triphosphate, the chemical energy within cells for metabolism and cell division). Anaerobic respiration also produces ethanol, which is toxic to plant cells. If enthanol accumulates root death begins to occur. As anaerobic (reduced) conditions develop in the soil, nitrification ceases and denitrification is enhanced. Many plants quickly yellow in response to this saturated soil state as nitrogen becomes limiting, and the plant tries to
adjust by producing more adventitious roots. Prolonged anaerobic conditions in the soil starts to reduce manganese, iron (causing phosphorus to be more soluble), sulphur (producing hydrogen sulphide), and eventually methane gases. Hydrophytic (wetland type) plants are adapted to saturated soils because they are able to obtain oxygen through other forms of plant structure adaptations (such as pneumatophores and aerenchyma). Drainable Porosity This is the pore volume of water that is removed (or added) when the water table is lowered (or raised) in response to gravity and in the absence of evaporation. Consider a soil that is saturated, with the water table at the surface. If this soil has a subsurface drainage pipe buried 1 metre or more down and it is discharging to the atmosphere at some lower elevation, the drainable porosity water content will be released to the drain until the water table is lowered to the depth of the drain. Any nutrients or pesticides dissolved or suspended in this readily drainable pore space will also be carried along with this water, either flowing to the drain or continuing downward to the water table via deep percolation if no drainage restriction exists. In large pores, nutrients that might otherwise adsorb to the soil particles (such as ammonium or phosphate) will bypass the soil because of limited time for contact and chemical reactions to occur with the soil surface area. Soils with a wide range of different pore sizes (sandy loams) or soils with mostly small sized pores are better at filtering nutrients and pesticides as they leach through the soil profile. The low available water holding capacity and high drainable porosity for sandy soils causes these soils to leach nutrients readily. It will not take much rain or irrigation (or application of liquid manure) to replenish the available soil water and to raise the soil water content to a drainable state. Applying the proper depth of irrigation to these soils will both conserve water and enhance irrigation and nutrient use efficiency. Air Filled Porosity - The proportion of a soil's volume that is filled with air at a given time; a low air-filled porosity indicates poor aeration and restricted drainage. Macroporosity/Preferential Flow As noted earlier, macropores refer to very large soil pores through which water flows primarily in response to
gravity. Macropores occur in coarse sands and gravels, soil structural cracks, or may form as the result of worm holes, other small burrowing microorganisms, decaying roots, and some tillage operations, such as subsoiling. Since water can be infiltrated quickly and flows rapidly downward in macropores, this is often called preferential flow. The significance of macropores and preferential flow is that nutrients and other dissolved and suspended substances can be rapidly transported down past the root zone without filtration through the soil. Although the magnitude of macroporosity in soils is generally small, the environmental impact may be significant because only a small concentration of nutrient, pesticide or other contaminant can damage water quality. Normally, when pesticides and contaminants filter through the soil, they are acted upon and partially broken down by bacteria and other soil organisms, or become combined with other compounds, so that very little if any reaches the ground water. However, this cannot happen if preferential flow occurs, instead the contaminants run straight into the ground water. Additional problems may occur if small particles such as silt are carried straight through to the ground water or drains, where they may cause silting. Macroporosity does have some benefits, however. In particular, it can improve the drainage of heavy soils as well as increasing the amount of soil air. Sand slitting - a technique often used on sports and recreation surfaces to improve drainage. A thin 'slice' is cut through the sward and the resulting 'slit' filled with sand.