Understanding Soil’s Physical Properties:
Understanding soil's physical properties and its relationship to soil moisture will help you make better soil management decisions. Soil texture and structure greatly influence water infiltration, permeability, and water-holding capacity Jeff Ball, 2001 (1).
The mineral and organic matter fractions of the soil are the solids and serve as the storehouse and exchange sites for plant nutrients and other chemicals. They are important from a fertility and environmental standpoint. They are these fractions, along with cultural practices that influence other physical properties and processes Daniels et al. 2006 PDF (2).
A desirable surface soil in good condition for plant growth contains approximately 50% solid material and 50% pore space (Figure 1). The solid material is composed of mineral material and organic matter. Mineral material comprises 45% to 48% of the total volume of a typical Mid-Atlantic soil. About 2% to 5% of the volume is made up of organic matter, which may contain both plant and animal residues in varying stages of decay or decomposition (active and passive fractions). Under ideal moisture conditions for growing plants, the remaining 50% soil pore space would contain approximately equal amounts of air (25%) and water (25%) Daniels et al.,2006 PDF (2).
Figure 1: In an ideal soil, Air and Water fill the pore space and compose about 50 percent of the volume, Organic Matter accounts for about 1-5 percent of the soil volume and Mineral Matter accounts for the remaining 45-49 percent (USDA, NIFA, 2015).
Soil texture is an important soil characteristic that drives crop production and field management. Soil texture refers to the composition of the soil in terms of the proportion of small, medium, and large mineral particles in a specific soil mass Jeff Ball, 2001 (1). These particles vary widely in size and consist of three categories - sand, silt and clay, also called “separates”.
Figure 2: A coarse soil is a sand or sandy loam; a medium soil is a loam, silt loam, or silt; and a fine soil is a sandy clay, silty clay, or clay.
Texture directly affects plant growth and soil management as described in Table 1. Combination of each separate in the right percentage will lead to optimal soil health. A desirable soil is a loam with enough sand to drain well yet with enough clay and silt to hold moisture. Loam is an even mixture of the three main types of soil (40% Sand, 40% Silt and 20% Clay) plus humus, providing good aggregation with a large number of pores that hold water against gravity Blog posted on 7 April 2011. Agverra.Com. (3).
Table 1: Comparison of Fine-Textured (Clayey) Soil and Coarse-Textured (Sandy) Soil*
Soil Texture Influences Water and Nutrient Retention:
Defined by the fineness or coarseness of soil particle size, soil texture has an important role in nutrient management because, along with soil structure, it influences the soil’s capacities and limitations, including: soil aggregation, water holding capacity, nutrient retention and supply, drainage, and nutrient leaching. For instance, finer textured soils have greater ability to store soil nutrients Daniels et al.,2006 PDF (2).
Why Soil Texture Determination is So Important:
1. Directly Associated with Soil Organic Matter (SOM): The decomposition rate of SOM will vary by the different soil textures (sand, silt, and clay). Many studies have shown that SOM associated with the silt- or clay-size fractions has a lower SOM turnover than sand-size fractions that are more susceptible to decomposition, and thus a higher SOM turnover rate Overstreet et al., 2015 (4).
2. Affects Cation Exchange Capacity and Nutrient Retention: Soil texture shows how we should apply mobile nutrients, such as K, N and to a lesser extent, S. For example, soils with a light texture and low cation exchange capacity (CEC) are more susceptible to leaching and should be managed by applying smaller quantities of nutrients more frequently Dairy Australia. 2007-2015 (5). Since a soil's CEC (Figure 3) comes from the clay and organic matter present, it can be estimated from soil texture and color. The most common soil cations (including their chemical symbol and charge) are: calcium (Ca++), magnesium (Mg++), potassium (K+), ammonium (NH4+), hydrogen (H+) and sodium (Na+). The total number of cations a soil can hold or its total negative charge is the soil's cation exchange capacity. The higher the CEC (clay and OM humus), the higher the negative charge and the more cations that can be held.
Figure 3: Copyright 2008: Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Clay soils with a high CEC can retain large amounts of cations against leaching. Sandy soils with a low CEC retain smaller quantities of cations, and this has important implications when planning a fertilizer program. In soils with a low CEC, consideration should be given to splitting applications of K and S fertilizers. Table 2 below relates soil texture to the CEC Dairy Australia. 2007-2015 (5).
Table 2: Levels of exchangeable cations (cmol (+)/kg), Source: Metson, (1961)
3. Affects Soil pH: The ideal pH range for soil is from 6.0 to 6.5 because most plant nutrients are in their most available state. If a soil test indicates a pH below 6.5, the usual recommendation is for the application of ground limestone. Table 3 indicates the number of tons per acre of ground limestone required to raise the pH of a given soil to based on the original pH, desired pH, and soil type Vossen, P. 2002 PDF (6).
Table 3: Approximate Amount of Finely Ground Limestone Needed to Raise the pH of a 7-inch Layer of Soil*
Some soils are alkaline and have a pH above 6.5. Some fertilizers (ammonium sulfate, urea, and ammonium nitrate) create an acid reaction in the soil, so they aid in lowering or maintaining a specific pH Vossen, P. 2002 PDF (6).
Table 4: Tons of sulfur needed to lower pH to 6.5*
4. Affects Bulk Density: By using the bulk density, calculations can be made to determine the porosity of the soil. Low values mean a porous soil and high values a soil with low porosity. It is generally accepted that the higher the porosity (the higher the soil moisture content), the greater the ability of soil to conduct electrical currents Jordán, 2013 (7). Loose, porous soils have lower bulk densities and greater porosities than tightly packed soils. Crop residues left on the soil surface lead to improved soil aggregation and porosity, and an increase in the number of macropores, and thus to greater infiltration rates. This compares to single clay and silt particles that are associated with smaller pores. Murphy, 2014 PDF (8).
In a study conducted in New Jersey, involving the USDA NRCS, soil bulk density was evaluated between various soil land uses and vegetative covers (Table 3). Soils with the highest plant diversity gave lowest bulk densities (most pore space), resulting in higher permeability rates. In contrast, other land uses, with lower plant diversity, gave higher bulk densities that discouraged deep root penetration and the development of macropores have the highest bulk density and lowest permeability rates.”Lamm. D., Winger. M, 2013 PPT (9).
Table 3: Bulk Density of Soils in New Jersey
5. Affects Aggregate Stability or Structure: Aggregate stability refers to soil structure resilience in response to external mechanical forces. Many consider soil aggregation to be a parameter reflecting soil health, as it depends on chemical, physical and biological factors. Aggregation is part of the humification and soil carbon building process and is essential for maintaining soil structure. The presence of these aggregates creates macropores (spaces between the aggregates) which markedly improve the infiltration of water Univ. of Hawaii at Manoa, 2015 (10).
Pore size is probably one of the most important physical features of a soil. It controls water, air movement and storage. Pores come in all sizes, although clays have predominantly small pores (micropores), and sands have large pores (macropores). Most soils are a mixture of sand, silt and clay particles, so there is a mixture of different sized soil pores. As organic matter is added, the number of macropores increases. These increases result from the increase in aggregation, decay of root channels and creation of earthworm channels Univ. of Hawaii at Manoa, 2015 (10) .
Figure 4: An ideal soil condition (i.e. loam) is one with an equal number of large and small pores.
The pore space created by these aggregates and the pore space between the aggregates is referred to as soil structure (see Fig. 5).
Figure 5: Pore space or porosity is the volume of soil voids that can be filled by water and/or air; it is inversely related to bulk density.
Soil aggregation is an important indicator of the workability of the soil. Soils that are well aggregated are said to have “good soil tilth” Univ. of Hawaii at Manoa, 2015 (10).
6. Affects Water Infiltration, Permeability, and Water-Holding Capacity: Water holding capacity describes the ability of a soil to hold water and varies by soil texture (Table 4). Soils with a high percentage of small clay particles are called “heavy or fine-textured” and are characterized by slow water infiltration and percolation, low soil aeration, and a tendency for the soil to hold moisture with great tension. Soils with a high percentage of large sand particles are called “light or coarse-textured” and are characterized by rapid water infiltration and percolation, high soil aeration, but low water holding capacity NRCS Montana, 2016 (11).
Table 4: Water holding capacity describes the ability of a soil to hold water and varies by soil texture:
Clay helps create a soil matrix of smaller pores which hold water with greater tension (Figure 6). By increasing clay content, water holding capacity increases. However, the narrow pore spacing in heavy/fine soils may reduce water availability by holding water more tightly, reducing water content and the plant's ability to extract water (wilting point increases). In contrast, a light/coarse-textured soil (e.g. sand or sandy loam) with wide pore spacing can result in poor soil aggregation with low water and nutrient storage capacity in the root zone. Sandy soil types can also be water repellent due to the build up of waxes on the surface of sand particles, restricting the rate of water infiltration into soil and resulting in greater surface water losses Sheppard, J et al. 2016 (12).
Figure 6: Diagram showing the relative amounts of available and unavailable water in soils ranging in texture from sand to clay:
7. Affects Nitrate Nitrogen and Salt Leaching Potential: The lower water holding capacity of light/coarse textured soils leads to increased loss of nitrate due to leaching. Nitrate-N can be leached from any soil if rainfall or irrigation moves water through the root zone Lamb, J. et al 2014 (13). Also, salinity is more easily managed with light/coarse-textured soils (sandy or loamy) as opposed to heavy/fine-textured soils (silty clay, sandy clay, clay), thus facilitating easier management of soil salinity by applying low-salt irrigation water to leach salts from the soil NRCS Montana, 2016 (11).
Managing your field irrigation sprinkler system based on soil texture J. Fontela, Senninger Irrigation, Inc., 2013 (14): Water infiltrates soil’s pores at varying rates depending on texture. Sprinkler drop size, application intensity and rate should be adjusted based on the soil texture.
- Large droplets have a higher kinetic energy and can cause surface sealing and lead to erosion or inefficient irrigation. If you have dense clay soils, then water will slowly seep down to the root zones while sandy soils will quickly absorb water.
Higher application intensity also runs the risk of permanently damaging soil’s structure by rearranging its composition. In general, tighter (clayey) soils benefit from smaller droplets while looser (sandy) soils require larger droplets.
If application rates exceed soil infiltration rates, over-watering and runoff are likely to occur. Water slowly seeps down to the root zones in dense clay soils while sandy soils will quickly absorb water.
Below in Figure 7, black dots representative of basic droplet sizes and based on prevalent field soil texture – going from large, medium to small – to help give you an idea of what deflector or pad you should select for your sprinkler (J. Fontela, Senninger Irrigation, Inc., 2013).
Figure 7: Fontela, J. 2013. Reasons to Focus on Droplet Size & Soil Texture When Selecting a Sprinkler, Senninger Irrigation, Inc.
- Ball, Jeff. September, 2001. Soil and Water Relationships. Ag News and Views. Soils and Crops. The Samuel Roberts Noble Foundation, Inc. http://www.noble.org/ag/soils/soilwaterrelationships/
- Daniels, W.L, Haering, K.C. 2006. Chapter 3. Concepts of Basic Soil Science. Department of Crop and Soil Environmental Sciences, Virginia Tech
- Agverra.Com. Blog posted on 7 April 2011. Different Soil Types – Know Your Soil Type. Growth as Nature Intended
- Overstreet, L.F., North Dakota State University, DeJong-Huges J., University of Minnesota. April 20, 2015. The Importance of Soil Organic Matter in Cropping Systems of the Northern Great Plains. Tillage: University of Minnesota Extension
- DAIRY SOILS AND FERTILISER MANUAL. CHAPTER 220.127.116.11. 2007-2015. Interpreting Soil Tissue Tests. Dairy Australia. Southbank, Victoria, 3006
- Vossen, P. 2002. Changing pH in Soil. University of California Cooperative Extension. UCDavis.
- Jordán, A. August 19, 2013. What is soil structure? Soil System Sciences. European Geosciences Union (EGU) blogs.
- Murphy, B, W. 2014. Soil Organic Matter and Soil Function – Review of the Literature and Underlying Data. Effects of soil organic matter on functional soil properties. Grains Research and Development Corporation and Department of the Environment, Canberra, Australia.
- Winger, M., Lamm. D., 2013. How Can I Tell if My Soil is Healthy? USDA NRCS Idaho and North Carolina
- University of Hawaii at Manoa. 2015. Soil Texture and Soil Structure. Soil Nutrient Management at Maui - College of Tropical Agriculture and Human Resources
- NRCS Montana, 2016. Home/Newsroom/Features/Soil Texture Classification
- Sheppard, J., Hoyle, F. 2016. Fact Sheets - Water Availability 2016. Soil Quality Pty Ltd. Department of Agriculture and Food, Western Australia.
- Lamb, J., Fernandez, F., and Kaiser, D. Extension Specialists in Nutrient Management. 2014. Understanding Nitrogen in Soils. University of Minnesota Extension
- Fontela, J., May 23, 2013. Reasons to Focus on Droplet Size & Soil Texture When Selecting a Sprinkler. Senninger Irrigation, Inc.