Soil itself is not essential to surface of the earth that serves as a a growing plant. Soil serves as a natural medium for the growth of storage reservoir for nutrients and land plants. (ii) The unconsolidated water needed for plant growth. mineral or organic matter on the These soil properties are essential to surface of the earth that has been sub- crop production. jected to and shows effects of genetic Crop production potential is and environmental factors of: climate greatly influenced by the phys- (including water and temperature ical and chemical properties of effects), and macro- and microorgan- soils. These same properties also isms, conditioned by relief, acting on influence related activities such parent material over a period of time. as tillage, erosion, drainage, and A product, soil differs from the mate- irrigation. Important agronomic rial from which it is derived in many soil properties include the soil physical, chemical, biological, and water-holding capacity, infiltration morphological properties and charac- rate, aggregation, temperature, teristics. organic matter content, and nutri- As a material medium for plant ent availability. growth, a given volume of soil is This publication focuses on composed of three parts: solids, physical characteristics of soil that air, and water, as illustrated in influence how soil retains water Figure 1. The solid portion can be Important and water availability for crop pro- further divided into mineral and duction. Extensive information on organic matter, while the air and soils and their suitability for various water portion of the soil is called Agricultural applications are available in county the pore volume. Under normal soil surveys. These are available at farming practices, the ratio of solid Soil http://soils.usda.gov/survey. to pores remains constant and for The definitions of soil from the a given environmental condition, Properties Soil Science Society of America is: the organic matter remains stable. soil: (i) the unconsolidated mineral Organic matter can be altered or organic material on the immediate by changing conditions such as Danny H. Rogers Professor, Extension Irrigation Engineer, Biological and Agricultural Engineering

Jonathan Aguilar Assistant Professor, Water Resources Engineer, Southwest Research Extension Center The solid portion of soil is either Mineral or Minerals Isaya Kisekka Assistant Professor, Irrigation Research Organic Matter Organic Engineer, Southwest Research Extension Air Center Water Philip L. Barnes Associate Professor, Water Quality, Water and Air ll pore space, Biological and Agricultural Engineering As one increases, the other decreases Freddie R. Lamm Professor, Irrigation Research Engineer, Northwest Research and Extension Center Figure 1. Soil composition components. Kansas State University Agricultural Experiment Station and Cooperative Extension Service erosion or cultural practices. Most 100 10 soils have an organic matter con- 90 tent of less than 1 percent to about 20 5 percent by volume. Organic 80 matter consists of decaying plant 30 70 and animal material, largely clay percent silt 40 microbes and insects. Although it 60 percent clay is only a small fraction of the total 50 50 soil volume, organic matter’s role silty clay 60 in serving as the primary stor- sandy 40 clay silty age receptor for plant nutrients is clay loam 70 clay loam extremely important. 30 sandy clay loam 80 The balance between water and 20 loam air in the soil is important. The sandy loam silty loam 190 ratio of water to air in the pores 10 loamy silt 100 of the soil volume varies greatly; sand sand as one increases, the other must 100 90 80 70 60 50 40 30 20 10 decrease. Soils that are completely filled with water, or saturated, have percent sand all air displaced from the soil pores. Figure 2. A soil textural classification triangle. Without air, or oxygen in particular, root respiration is disrupted, which tion, most peds can be seen and are is an integral part of the plant important to maintaining soil tilth and crop productivity. Sand growth process. 2.00 – 0.05 mm Unlike soil texture, soil struc- Soil Texture ture can be affected by crop cultural Silt There are three classes of soil practices. The soil structure develops 0.05 – 0.002 mm particles — sand, silt, and clay over time as the result of root pene- tration, freezing and thawing cycles, Clay — and their relative proportions < 0.002 mm determine the soil textural class, as the presence of organic and inor- shown in Figure 2. Texture is a way ganic binding agents, and biological to describe a soil by feel. For exam- activities. Soil structure is affected FigureThe 3. relative The relative sizes sizes of soil of soilparticles particles. ple, coarse sand may feel gritty, by cultural practices that can accel- erate the loss of organic materials while a silt loam has the feel of most layer). flour. The relative sizes of the three or destroy aggregates through soil particles are shown in Figure 3, improper tillage practices. On the series level, one of the while the size classification ranges Generally, productive agricul- properties defined is the soil depth are depicted in Figure 4. The soil tural soils have a soil structure that of the various layers, which typically texture is determined using a labo- allows good root penetration and consists of three levels: A (top soil), ratory analysis that determines the allows a good exchange of air and B, and, C. Progressively downward, fractions of sand, silt, and clay. water within the soil profile. each layer is less affected by the five soil forming factors of parent mate- Soil Structure Soil Series rial, climate, topography, biota, and time. Biota refers to the biological Primary soil particles can be Soils classified in the same organisms and activities. combined into secondary arrange- series are soils that have been ments called peds, which can be formed from the same parent The layers can be distinctive of various sizes and shapes. Unlike material and are similar in profile parent material, such as when a most soil particles, which are too arrangement and characteristics, different material is overlain on an small to be seen without magnifica- except in the A horizon (upper- existing soil, such as wind-deposited loess on flood sedimentation. The 2 K-State Research and Extension — Important Agricultural Soil Properties depth of soil and the corresponding thickness of various textural layers have an important influence on irri- gation management decisions. Soil Density The density of a substance is defined as the mass per unit volume. In soils, this value is called the bulk density:

Bulk density (Pb) = Ms/Vb = mass of dry soil/volume of soil sample Bulk density is typically expressed in grams/cm3. Typical soil bulk densities range from 1.1 to 1.7 g/cm3.

Another soil density term is par- Figure 4. USDA soil classification of primary soil particles by size. ticle density, which is defined as the mass of the dry soil particles divided Soil Water tion water over a given time. The by the volume of the soil particles. infiltration rate of a soil is not con- The space between soil solids For all practical purposes, the mass stant and depends on many factors, or particles is called the pore space. of dry soil particles is the same including the surface conditions, Soil porosity describes this portion as the mass of the dry soil, as the such as residue cover and surface of the soil volume that can hold weight of air is the only difference. roughness; soil water content; and either air or water. Porosity is gen- Particle density (Pp ) = Ms/Vs slope. erally expressed as a percentage. = mass of dry soil/volume of the soil The infiltration rate for soils is particles or solids Porosity ( ϕ ) = volume of pores/volume of soil high at the initiation of infiltration Particle density is typically and decreases as the infiltration 3 The porosity of a soil can be expressed in grams/cm and the process continues. The elapsed time calculated using bulk density and ranges for typical soils is 2.6 to 2.7 between the initiation of infiltration 3 3 the typical estimate of particle den- cm . A Pp value of 2.65 g/cm is and the current time of an event is sity of 2.65g/cm3. used for most soils. called the opportunity time. Porosity (%) = (1-(Pb / Pp )) × 100 The particle density of a soil is The infiltration rate relationship typically constant, changing only Most agricultural soils have a with time is shown in Figure 5 for slightly if the organic matter con- porosity of about 50 percent, but if the various soils. As the opportu- tent changes, whereas bulk density they become compacted, porosity nity time increases, the infiltration depends on the amount of pore decreases, which limits the space rate decreases until it becomes space in a soil. Soil pore space can available for water and air and relatively constant. The infiltration be altered. In agricultural settings, restricts root development. rate for this condition is called the increasing bulk density indicates Soil infiltration and permeabil- basic infiltration rate or steady state compaction is occurring, which ity describe the movement of water infiltration rate. Soils that have could cause detrimental growing in soils. Infiltration is the move- similar intake rates are classified as conditions, such as limiting root ment of water into the soil from being from the same intake family. penetration or decreasing the soil the soil surface. The infiltration Soils with high clay content tend aeration process. rate, also called the intake rate, is a to have low intake rates, silt soils measurement of the ability of a soil have higher intake rates than clay, to absorb rainfall or applied irriga- and sands tend to have the highest intake rates. K-State Research and Extension — Important Agricultural Soil Properties 3 After water has entered a soil, 5 the term to describe the water Intake Family Curves movement within the soil profile is permeability. Soils with larger and 4 connected pore spaces are more permeable than soil with small, 3 disconnected pore spaces, even if 2.0 the porosity values are similar. The inches/hour downward movement of water 1.5 Rate, Rate, 2 within a soil profile is called perco- 1.0 lation. The measure of the rate of 0.5 0.3 water movement is called hydraulic 1 0.1 conductivity and measured in units In ltration of inches/hour. These rates can be quite variable. 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Soil Water Content Time, Hours The amount of water in a soil Figure 5. Infiltration rate curves from the Natural Resources Conservation Service. available for plant use is one of Soil permeability is grouped into classes ranging from very slow to very rapid, as the most important measurements shown in Table 1. needed for proper irrigation man- agement. While the porosity of Table 1. Soil Permeability Classes most agricultural soils is similar, Classification Infiltration Rate (in/hr) the amount of plant-available Very Slow < 0.06 water varies greatly. This is due to Slow 0.06 – 0.2 size of the pore space in a soil and Moderately Slow 0.2 – 0.6 the way water is held within the Moderate 0.6 – 2.0 pores of a soil. Figure 6 illustrates that for a given soil volume, larger Moderately Rapid 2.0 – 6.0 particle sizes are associated with Rapid 6.0 – 20.0 larger pore size. Since all soils have Very Rapid > 20.0 about the same porosity, this means smaller-sized particle soils will have smaller, but more, pore spaces. Soils can be a mixture of particle sizes and this also affects the size and number of pores. Soil geometry is also much more complex than these Illustration of the e ect of particle size on the size and Illustration of the e ect of a mixture of circular depictions. number of pore spaces within a xed soil volume. soil particle size on the number and size Four important terms are used of pores within a given soil volume. Figure 6. The effect of soil particle size and particle mixture on pore space. to describe the water level within a soil (Figure 7). When all the soil pores are filled with water, the soil Water retained by the soil for removal. is said to be saturated. However, after gravitational drainage is held The amount of water retained the gravitational pull on the water in place by capillary and adsorp- in the soil after gravitational drain- causes some of it to drain. While tion forces. This occurs due to the age is the field capacity of the soil. this water could be used by plants, surface tension (cohesion) of water As plants extract water, the remain- the gravitational drainage is gen- and the attraction of the water to ing water is held more tightly until erally too rapid to allow plants to the soil particle by adhesion. Some eventually the plant cannot extract capture much of this water. of the water is available to plants additional water. 4 K-State Research and Extension — Important Agricultural Soil Properties The soil water content at this point is called the wilting point. The film of water that remains around the soil particles is held by adsorp- tion forces and is called hygroscopic water. It is not available for plant use. Several of these conditions are illustrated in Figure 8. The amount of water between field capacity and wilting point is called plant available water. Aver- age water-holding capacities for various soil textural classes are shown in Tables 2 and 3. Table 2 values tend to be larger for the surface layers than for the Table Figure 7. Illustration of water levels within a soil. 3 subsoil layers. Of course, the holding capacity can vary from sample in proportion to the volume Example calculations: Deter- site to site. The effect of different of the soil sample. See Formula B. mine the gravimetric water content water holding capacities for three It is relatively easy to collect for a silt loam soil sample collected soil types is illustrated in Figure samples and measure the gravimet- from the surface 12-inch layer and 9. Even though all three soils hold ric water content of a soil, but the weighing 105 grams. After drying, over 5 inches of water at saturation, more useful value is the volumetric the sample weighs 85 grams. The the sand only holds about 1 inch of water content. The gravimetric gravimetric water content is 24% plant available water. The silty clay water content is related to the (see Formula D). loam soil has the largest amount volumetric content by the following Determine the volumetric of water in storage but has a larger relationship shown in Formula C. water content for the same sample. amount not plant available, so the Soil bulk density values are The bulk density from Table 2 for loam soil ends up with the most given in tables 2 and 3. The density a silt loam is 1.40. The volumetric plant available water. Figure 10 3 of water is 1.0 gram/cm or 62.4 water content is 34%, as shown in graphically illustrates typical field 3 pounds/ ft . Formula E. capacity and wilting point values Formula A for 11 soil textures. At field capac- (wet sample weight – dry sample weight) × 100 Gravimetric water % = ity, the available water is said to be (dry sample weight) 100 percent available in reference to plant use, while at wilting point, Formula B (volume of water in sample) × 100 the available water to plants is 0 Volumetric water % = percent available. (volume of the soil sample) Calculating Soil Water Formula C (gravimetric water % × Bulk Density) Content Volumetric water % = density of water The amount of water in a soil can be measured directly using Formula D (105 – 85) x 100 the gravimetric method, which is Gravimetric water % = = 24% simply the weight of the water in a 85 soil sample in proportion to the dry Formula E soil sample weight. See Formula A. Volumetric water % = (gravimetric water % × Bulk Density) / Density of water The soil water content by = (24 × 1.4)/ 1.0 = 34% volume is volume of water in a soil K-State Research and Extension — Important Agricultural Soil Properties 5 Table 2. Average available water capacity for Kansas soils in the 0- to 12-inch layer. Percentage by Mass Fraction by Volume Soil Bulk Density Field Wilting Available Field Wilting Available Texture Capacity Point Water Capacity Point Water Capacity Capacity Sand 1.60 8.7 3.5 5.2 0.14 0.06 0.08 Loamy Sand 1.60 11.9 4.5 7.4 0.19 0.07 0.12 Sandy Loam 1.55 15.4 5.8 9.6 0.24 0.09 0.15 Fine sandy Loam 1.50 19.5 7.5 12.0 0.29 0.11 0.18 Loam 1.45 23.6 9.2 14.4 0.34 0.13 0.21 Silt Loam 1.40 27.2 10.9 16.3 0.38 0.15 0.23 Silty Clay Loam 1.35 28.8 13.0 15.8 0.39 0.18 0.21 Sandy Clay Loam 1.40 27.0 13.5 13.5 0.38 0.19 0.19 Clay Loam 1.40 27.3 15.1 12.2 0.38 0.21 0.17 Silty Clay 1.30 28.7 18.0 10.7 0.37 0.23 0.14 Clay 1.25 29.4 20.1 9.3 0.37 0.25 0.12 Source: NRCS Kansas Irrigation Guide

Table 3. Average available water capacity for Kansas soils for depth greater than 12 inches. Percentage by Mass Fraction by Volume Soil Bulk Density Field Wilting Available Field Wilting Available Texture Capacity Point Water Capacity Point Water Capacity Capacity Sand 1.70 7.0 3.0 4.0 0.12 0.05 0.07 Loamy Sand 1.70 10.0 4.2 5.8 0.17 0.07 0.10 Sandy Loam 1.65 13.4 5.6 7.8 0.22 0.09 0.13 Fine sandy Loam 1.60 18.2 8.0 10.2 0.29 0.13 0.16 Loam 1.55 22.6 10.3 12.3 0.35 0.16 0.19 Silt Loam 1.50 26.8 12.9 13.9 0.40 0.19 0.21 Silty Clay Loam 1.45 27.6 14.5 13.1 0.40 0.21 0.19 Sandy Clay Loam 1.50 26.0 14.8 11.2 0.39 0.22 0.17 Clay Loam 1.50 26.3 16.3 10.0 0.39 0.24 0.15 Silty Clay 1.40 27.9 18.8 9.1 0.39 0.26 0.13 Clay 1.35 28.8 20.8 8.0 0.39 0.28 0.11 Source: NRCS Kansas Irrigation Guide

Since the sample represents ity can be calculated using the field mass, which is 0.15 by volume. the 12-inch soil layer, the depth of capacity value from Table 2 as The plant available portion of water in the layer can be calculated Depth of water in soil at field capacity the water content from the sample by multiplying the soil depth by = 12 inches × (38/100) / 1.0 = 4.56 is therefore 34% minus 15% or 19% the volumetric soil water content inches by volume. fraction or: The amount of plant available The plant available water depth Depth of water in the soil layer = water in the soil sample can be esti- in the 12 inch soil layer is 12 inches × (34/100) = 4.08 inches mated by subtracting the amount of 12 inches × (19/100) = 2.28inches. water in the soil at wilting point. Similarly, the depth of water in Direct measurement of soil the 12-inch soil layer at field capac- From Table 2 silt loam soil water water content by gravimetric sam- content at wilting point is 10.9% by 6 K-State Research and Extension — Important Agricultural Soil Properties Soil particle Air space in pore due Remaining lm of to gravitational capillary water adhering Gravitational water water drainage to soil particle.

Illustration of a water saturated Illustration of a soil that has had Illustration of a soil that has soil, all the pore spaces are lled gravitational water drained from reached wilting point, when with water. the pores. capillary water can no longer be extracted by plants.

Figure 8. Illustration of gravitational and capillary held water in the soil pore spaces. pling is an accurate and reliable 7 Plant available water method to determine soil water con- tent, and the sampling can be done 6 Saturation at multiple locations within a field. However, it is not commonly used 5 as it is labor intensive and, because Readily of the sample drying requirements, 4 Field capacity available there is a time delay between col- water lection and the results. A specific 3 site also cannot be resampled since a physical sample is removed. Other 2 indirect measurement methods to Water in soil (inches per foot) determine soil water content are 1 Wilting discussed in other publications. point Oven dry More discussion on crop water 0 requirements and how plants Sand Loam Silty clay loam extract water are in publication Figure 9. Illustration of soil water levels for three soil types. L934, Agricultural Crop Water Use.

Related Information 35.0 Field capacity KanSched, an ET based irrigation 30.0 scheduling program. Available at 25.0 Gravitational water the KSRE Mobile Irrigation Wilting point Lab website: http://www.bae. 20.0 ksu.edu/mobileirrigationlab/ 15.0 Rogers, D.H. and M. Alam. 2007. 10.0 Unavailable water What is ET? An evapotranspira- 5.0 tion primer. Kansas State Uni- versity Research and Extension. Percent water by volume 0.0

Clay Irrigation Management Series, Sand Loam MF-2389 rev. 4 pp. Silt loam Clay loam Silty clay Loamy sandSandy loam Silty clay loam Rogers, D.H., P.L. Barnes, J. Fine sandy loam Sandy clay loam Aguliar, I. Kisekka, and F. Wilting point Field capacity Lamm. 2014. Soil Water Plant Relationships – An Over- Figure 10. Illustration of percentage term definitions for available water view. Kansas State University by volumetric measure and as a percentage of available water for soil textures. Research and Extension. Irriga- tion Management Series, L904 rev. 4 pp. K-State Research and Extension — Important Agricultural Soil Properties 7 Rogers, D.H., P.L. Barnes, J. Rogers, D.H., M. Alam. 1997. References Aguliar, I. Kisekka, and F. Tensiometer Use in Scheduling Soil Science Society of America, Lamm. 2014. Agricultural Crop Irrigation. Kansas State Uni- Glossary of terms: Available at Water Use. Kansas State Uni- versity Research and Extension. https://www.soils.org/publica- versity Research and Extension. Irrigation Management Series, tions/soils-glossary# Irrigation Management Series, L-796 L934 KSRE Websites: Rogers, D.H., M. Alam. 1998. Soil General Irrigation Water Measurements: An aid to www.ksre.ksu.edu/irrigate Irrigation Management. Kansas Mobile Irrigation Lab State University Research and http://www.bae.ksu.edu/mobile- Extension. Irrigation Manage- irrigationlab ment Series, L-795. Subsurface Drip Irrigation www.ksre.ksu.edu/sdi

Authors Danny H. Rogers, Jonathan Aguilar, Isaya Kisekka, Philip L. Barnes, Freddie R. Lamm

Acknowledgments: The authors would like to extend their appreciation to Dr. Loyd Stone of the KSU Department of Agronomy for his review of this publication. This research was supported in part by the Ogallala Aquifer project, a consortium between USDA Agricultural Research Service, Kansas State University, Texas AgriLife Research, Texas AgriLife Extension Service, Texas Tech University, and West Texas A&M University.

This project is funded through the Ogallala Aquifer Program Grant #602636.

Brand names appearing in this publication are for product identification purposes only. No endorsement is intended, nor is criticism implied of similar products not mentioned. Publications from Kansas State University are available at: www.ksre.ksu.edu Contents of this publication may be freely reproduced for educational purposes. All other rights reserved. In each case, credit Danny Rogers, et al, Important Agricultural Soil Properties, Kansas State University, January 2015.

Kansas State University Agricultural Experiment Station and Cooperative Extension Service L935 January 2015 K-State Research and Extension is an equal opportunity provider and employer. Issued in furtherance of Cooperative Extension Work, Acts of May 8 and June 30, 1914, as amended. Kansas State University, County Extension Councils, Extension Districts, and United States Department of Agriculture Cooperating, John D. Floros, Director.