SOIL The Exciting World Beneath Our Feet. Physical properties of soil Texture Structure Particle density Bulk density Pore space Water relations Plasticity Soil tilth is the term used in publications aimed at the general population to refer to the physical condition of the soil.
Soil texture The relative proportions, by weight, of the particle size fractions (sand, silt and clay) that are < 2mm in equivalent spherical diameter. Also called particle size distribution. Soil texture is determined by sedimentation from suspension. The rate of sedimentation of particles is related to their density, diameter and shape. The diameters are calculated from falling times, assuming that the particles are spheres hence, equivalent spherical diameters The size fractions are: Sand from 2 to 0.05 mm in diameter individual particles can be seen with the naked eye and felt by the fingers Silt from 0.05 to 0.002 mm in diameter not visible by naked eye, but visible under the light microscope. Not felt individually by fingers, but will scratch fingernails and grit between the teeth. Clay - < 0.002 mm (2 microns) visible with the electron microscope If I was allowed to know only one physical property of a soil, I would want to know its texture.
Soil texture triangle Twelve soil textural classes are recognized, based on the percentages of sand, silt and clay present (see figure). Sands: particles act individually (do not stick together). Soils dominated by them: have large pores, low water holding capacity, drain rapidly, and have low fertility. Silts: may form aggregates. Soils dominated by them: have medium size pores, high water holding capacity, drain at moderate rate and have moderate fertility. Clays: have a high surface area per unit weight. Soils dominated by them: form aggregates, have small pores, high water holding capacity. If dominated by phyllosilicate clay minerals, they drain slowly and are sticky, plastic, and tend to be fertile; if dominated by oxide clay minerals, they drain more rapidly and tend to be non-sticky, non-plastic and of low fertility. Soil texture is not easily altered; it can change over long periods in intense chemical weathering environments.
Soil structure Soil structure refers to the grouping of soil particles into aggregates ( peds to the soil classification people). It refers to the spatial organization of the soil particles, and controls the size distribution of pores and the nature of the pore spaces. Types of soil structure A) Structureless single grain: particles act individually - massive: soil act as one irregular mass B) Spheroidal rounded granular aggregates < 10 mm diameter (also known as crumb structure) C) Blocky angular: sharp edges and corners - subangular: rounded edges and corners D) Prism-like columnar: rounded tops -- prismatic: flat tops E) Platy - have horizontal structures (rare) Soil texture quite strongly influences the soil structure, but not vice-versa.
Factors influenced by soil structure moisture holding capacity movement of air and water into and through soils plant availability of soil water root penetration soil microorganism activity heat transfer ease of cultivation seed germination and more
Management of soil structure Soil structure, the spatial arrangement of the soil particles, is changing on an almost continuous basis. Changes for the worse are very easy to accomplish, whereas changes for the better require effort and diligence. For example, compaction is easy to cause and makes a soil more dense, thereby decreasing the amount of pore space and the size of individual pore spaces. The soil becomes harder for roots, water and air to penetrate, harder to cultivate, and less productive. Reducing the bulk density, and retaining the lower bulk density, requires good management and persistence.
Factors affecting soil structure Climate related: wetting and drying freezing and thawing Biological activity: plant roots microorganism excretions soil animals insects, worms, etc. organic residues Management: cultivation (not too much, or when soil is too wet) crop rotation liming fertilizers (when necessary for fertility reasons)
Other physical parameters Particle density: Silicate dominated soils have particle densities between 2.65 and 2.70 gcm -3. The silicate and carbonate minerals all have densities in or close to this range. Particle density does not change, except with major production of iron oxides by extreme weathering. Bulk density: The weight per unit volume of the whole soil, including the pore space. Normal ranges: sand dominated = 1.2-1.8 gcm -3 ; clays and silts = 1.0-1.5 gcm -3. Bulk density is constantly changing. Porosity: The percent of the soil volume that is pore space. Subject to almost constant change. Macropores: Drain under the force of gravity. Micropores: Retain water against the force of gravity. Water content: normally presented on a dry weight basis the weight of water as a percentage of the dry soil weight. Percent saturation: The percent of the pore space that is occupied by water.
Soil Water : Water acts as a solvent and as a carrier of plant nutrients: to the plants and in the plants. Plants transpire a large amount of water: through their leaves to the atmosphere. Soil water helps in temperature regulation: both in the plants and the soil surface zone. Water must be available where and when the plant needs it. Insufficient water is one of the major factors that limit crop production.
Soil water The physical structure of the soil is the dominant factor controlling soil water relations, whether the concern is infiltration, transmission, retention or water availability for plant uptake. Pore size distribution and pore interconnectivity are controlled by the spatial distribution of the soil particles (i.e. by both the soil texture and the soil structure) Try to imagine the pore space in soils (it helps to close your eyes). A balance such that about ½ of the pore space consists of macropores (which drain rapidly) and ½ consists of micropores (which retain water for plant use) is optimal. Good soil management has been described by one soil physicist as being THE PRESERVATION OF MACROPORES
Soil water potential Water is held in the soil and work must be done to remove it. Soil water potential is a measure of how strongly water is held in the soil. The strength with which the remaining water is held increases as the soil water content decreases. In other words, the water that is closest to the particle surface is the most strongly held, and the water in small pores is held more strongly than that in large pores.
Soil water The soil water table is the reference point for soil water potential. Above the water table, the soil water is being pulled down so it is under tension, or at a negative potential. Below the water table, the soil water is under positive pressure, or at a positive potential. Because most plants have their roots in soil that is above the water table, they are always working against the negative potential to extract water.
Soil water Two forces account for the retention of water by soil solids: Cohesion of water the attraction between the hydrogen atoms of one water molecule and the oxygens to each of two neighbouring water molecules. Adhesion of water the attraction between a hydrogen of a water molecule and an oxygen atom of the silicate mineral surface. These hydrogen bonds between water molecules and the mineral surface cause the minerals to be strongly hydrophillic: they attract water.
Hydrogen bonds and capillary rise The hydrogen bond accounts for the high melting and boiling points of water, and for the unusually strong capillary rise exhibited by water when in contact with hydrophillic surfaces. The height of capillary rise is inversely proportional to the radius of the capillary. Rise is greater in small capillaries than in large. Hence water is more strongly held in the small pores than in the large pores. See Demo
Soil water potential The most understandable units for soil water potential are cm of water and atmospheres. Because the soil water potential is negative above the water table, a soil water potential of -10 cm of water would be the equivalent of the downward force exerted by a column of water 10 cm high pulling downward on the water in the soil. (Drinking straw analogy). A potential of -1 atmospheres would be the equivalent of a 10 meter long column of water pulling downward. (To imagine such a force, remember the increase in the pressure on your eardrums as you dive below the water in a swimming pool, only as a sucking force.)
The soil water content vs soil water potential relationship Sponge demonstration Field capacity (FC): The water content that is retained against the force of gravity (-0.1 atm for sands; -0.5 atm for clays; average potential for most soils -0.3 atm). Macropores have drained Wilting coefficient (WC): The water content at which plants can no longer extract water (-15 atm). Hygroscopic coefficient (HC): The water content of airdry soil. Only water remaining is strongly adsorbed to particle surfaces.
Plant-oriented classification of soil water Superfluous water: the water that freely drains under the force of gravity. While present, it excludes air and, as it drains, it leaches nutrients. Available water: The amount of water held between the field capacity and the wilting coefficient. This becomes increasingly difficult for plants to access, as the remaining available water decreases. Organic matter in the soil increases the amount of available water by increasing the FC and decreasing the WC i.e. by changing the soil structure. Unavailable water: The water remaining in very small pores and as hygroscopic water.
Soil water Potential (- atm) 0 0.3 1 15 31 100 1000 10000 l l l l l l l l Physical Perspective Free water Hygroscopic water Location Macropores Micropores Adsorbed water Plant-related Perspective Superfluous Available Unavailable
Available Water vs Soil Texture Soil water vs soil texture 30 25 Water content (%) 20 15 10 5 0 Sand Sandy Loam Silt Clay Clay loam loam loam Soil texture Field capacity Wilting coefficient
Infiltration rate The rate of downward entry of water into soil is influenced by: Soil texture more rapid in coarse soil Soil structure more rapid into soil with macropores that open to the surface Structural stability unstable aggregates break down and the fragments block pores Surface condition soil crusts with their small pores inhibit infiltration Layering presence of layers of different texture decrease the rate (clay over sand, or sand over clay) Prior water content faster into drier soils NOTE: Dry organic soils tend to be hydrophobic. Water initially beads on the surface, and enters very slowly for some period of time.
Soil water movement saturated conditions Soil water moves in response to gradients in the soil water potential. The rate of water flow in a pore is proportional to the square of the pore radius. This means that, for a equal cross-sectional area of pores, a change of 10 times in the pore size leads to a 100 times difference in the hydraulic conductivity the ability to transmit water under saturated conditions. Typical hydraulic conductivities under saturated conditions Sands 10-2 - 10-4 cm/sec Silts - 10-5 - 10-7 cm/sec Clays - 10-8 - 10-10 cm/sec (i.e. a typical saturated sand will transmit water 1 million times faster than a typical saturated clay!)
Soil water movement partially saturated conditions Water flows much more slowly when the soil that is drier than the field capacity. Under conditions of partial saturation, the flow occurs only through the water-filled pore space i.e. through the micropores and thin films on the particle surfaces of the air-occupied pores. At field capacity, the hydraulic conductivity is on the order of 1 cm/day!
Excess water - Drainage For soils that are normally too wet, drainage may be accomplished by: 1) contouring the surface and/or constructing surface drains to direct surface water to drainage ditches; or 2) by the installation of a subsurface drainage system. Subsurface drains need to be more closely spaced and at shallower depths in clay-rich soils than in sandy soils.
Insufficient water - Irrigation Water is supplied when needed. In agricultural production, if one waits for the plants to appear wilted before irrigating, the water is being applied too late. Need to have a way of knowing when about 1/2 to 2/3 of the available water has been used, if sprinkler or flood irrigation is used. Drip irrigation is the most water-efficient means of applying irrigation. Salinization is a risk that accompanies irrigation in arid regions, where salt accumulation can be seriously problematic.
Questions, re:water???
Chemical properties and clay mineralogy Mineral surfaces Essential elements ph Clay minerals; structure and properties Cation exchange capacity (CEC) Soil fertility
Mineral surfaces Silicate, aluminosilicate and iron and aluminum oxide and hydrous oxide minerals, in widely varying amounts, constitute the major mineral constituents of soils. The nature of their surfaces is very important to their interaction with the elements that are dissolved in the pore water. Carbonates may be present in soils developed from limestone and in soils of semi-arid and arid regions. When present, they control the ph of the soil (maintain basic / alkaline conditions) which affects element solubility, but otherwise they have little interaction with elements in solution.
Essential elements Plants require a range of elements for proper growth. These must be present where and when the plants need them. Macronutrients: Elements required in large quantities From the air: carbon, hydrogen, oxygen From the soil: nitrogen, phosphorus, potassium, calcium, magnesium and sulfur Micronutrients: elements required in small quantities From the soil: iron, manganese, boron, molybdenum, copper, zinc, chlorine, cobalt and nickel
ph ph is the measure of the acidity or alkalinity of solutions. It is defined as: ph = - log 10 [H + ] A ph change of 1 unit represents a 10X change in [H + ] in solution NOTE: a lower ph indicates a higher H + concentration The ph range extends for 0 to 14. ph 7 is neutral ph <7 is acidic ph > 7 is basic (alkaline) Most soils fall within the ph 3 to ph 11 range Humid region soils: mostly ph 4 to 7+ range Arid regions soils: mostly ph 7- to 9 range
ph If I were limited to knowing only one chemical property of a soil, I would ask for its ph. WHY? 1. Basic soils are usually more fertile than acidic soils. 2. Available calcium and magnesium contents are higher. 3. Availability of phosphorus is strongly ph dependent. 4. Availability of many micronutrients is ph dependent; most have minimum concentrations below which there is deficiency and maximum concentrations above which plants exhibit toxicity symptoms 5. Al and Mn (manganese) toxicities may occur below about ph 4 this is a major problem in the humid tropics. 6. Microorganism activity (beneficial and detrimental) is ph dependent. ph is manipulated by lime addition (higher) or sulfur addition (lower).
Clay mineralogy The clay size fraction dominates the physical and chemical properties of soils to an extent that is out of proportion to the percentage present. This dominance is due to their small size, regardless of their mineralogy The aluminosilicate clay minerals are particularly effective in dominating the properties because, in addition to being of small size, they are plate shaped and as a result have a very large exposed surface area per unit weight. These plate-shaped minerals are known as layer silicates or phylosilicates. In order to fully appreciate why phyllosilicates are so important it is necessary to understand their structure at the atomic level.