growth depends on two ’s Physical important natural resources — Characteristics soil and . Soil provides the mechanical support and nutri- There are many factors that ent reservoir necessary for plant determine the physical characteris- growth. Water is essential for plant tics of soil. These include , life processes. Effective manage- , bulk density, and soil ment of these resources for crop porosity. They all affect the interac- production requires the producer to tion between soil, water, and air. understand relationships between Soil Composition. A unit soil, water, and . of soil is a combination of solid Knowledge about available soil material, composed of and water and soil texture can influence organic matter, and open space, the decision-making process, such called pores. By volume, most as determining what crops to plant are roughly 50 percent solids and and when to irrigate. This publica- 50 percent pore space. tion provides general information The mineral matter makes up on the physical characteristics of about 45 to 47 percent of the total soil, soil and water interactions, and soil volume. This mineral matter how plants use water, particularly consists of small of either as these topics relate to irrigated , , or . Soil, agriculture. However, the informa- Organic matter is made up tion is pertinent to -fed agri- of decaying plant and animal Water, and cultural production as well. More substances and is distributed in in-depth information on other and among the mineral particles. Plant related topics is available in other Organic matter can account for up K-State Research and Extension to about 5 percent of the overall publications, including MF2389 soil makeup by volume, but many Relationships What is ET?; L935 Important agricultural soils have less than 1 Agricultural Soil Properties; and percent organic matter. Danny H. Rogers L934 Agricultural Crop Water Use. The pores, spaces that occur Professor, Extension Irrigation Engineer, The basic soil, water, and plant between the mineral particles, are Biological and Agricultural Engineering relationships are important to agri- important because they store air Jonathan Aguilar cultural producers, but especially to and water in the soil. Assistant Professor, Water Resources irrigation users that desire to use Figure 1 shows the approximate Engineer, Southwest Research Extension best management practices such relationship between the substances Center as irrigation scheduling. Irrigation in the soil composition, with the scheduling determines when and Isaya Kisekka pore space shown split between air Assistant Professor, Irrigation Research how much water needs to be added Engineer, Southwest Research Extension to a crop’s root zone to promote Center optimum yields. One climatic- or evapotranspiration (ET)-based Mineral 47% Philip L. Barnes irrigation scheduling option is the Associate Professor, Water Quality, Organic 3% Air 25% Biological and Agricultural Engineering KanSched program (see reference Water 25% list for program availability). Freddie R. Lamm Professor, Irrigation Research Engineer, Northwest Research and Extension Center Figure 1. Typical soil composition by volume.

Kansas State University Agricultural Experiment Station and Cooperative Extension Service and water. The amount of water percent clay, and 40 percent silt by together. Structureless soils can and air present in the pore spaces weight is classified as a clay . be either single-grained (individ- varies over time in an inverse rela- Soil Structure. Soil structure is ual unattached particles, such as a tionship. This means that for more the shape and arrangement of soil sand dune) or massive (individual water to be contained in the soil, particles into aggregates. Soil struc- particles adhered together without there has to be less air. The amount ture is an important characteristic regular cleavage, such as or of water in soil pore space is essen- used to classify soils and heavily .) Soil structure is unsta- tial to crop production and will be influences agricultural productivity ble and can change with weather further discussed in the section on and other uses, such as load-bear- conditions, biological activity, and soil . ing capacity for structures. practices. Soil Texture. The size of the The principal forms of soil Soil Bulk Density and Porosity. particles that make up the soil structure are platy, prismatic, Soil bulk density expresses the ratio determine soil texture. The tradi- columnar, blocky, and granular. of the mass weight of dry soil to tional method of determining soil These soil structure descriptions its total volume. The total volume size consists of separating indicate how the particles arrange includes both the solids and the the particles into three convenient themselves into aggregates. pore spaces. Soil bulk density is size ranges. These soil fractions or Aggregated soil types are generally important because it is an indicator separates are sand, silt, and clay. the most desirable for plant growth. of the soil’s porosity. The porosity Generally, only particles small- Soil structure terms also are used in of a soil is defined as the volume of er than 2 mm (1/12 inch) in size conjunction with descriptive words pores in a soil. A compacted soil has are categorized as soil particles. to indicate the class and grade of low porosity and thus a greater bulk Particles larger than this are cate- soil. Class refers to the size of the density. A loose soil has a greater gorized as gravel, stones, cobbles, or aggregates, while grade describes porosity and a lower bulk densi- boulders. how strongly the aggregates hold ty. Like soil structure, a soil’s bulk Sand particles range in size 100 from 2 mm to 0.05 mm. There are 10 subcategories assigned to this range 90 that include coarse, medium, and 20 fine sand. 80 Silt particles range in size from 30 70 0.05 mm down to 0.002 mm. The clay percent silt physical appearance of silt is much 40 like sand, but the characteristics are 60 percent clay more like clay. Clay particles are 50 50 less than 0.002 mm in size. silty clay 60 sandy Clay is an important soil 40 clay silty fraction because it has the most clay loam 70 clay loam influence on soil behavior such as 30 sandy clay loam water-holding capacity. Clay and 80 20 silt particles cannot be seen with loam sandy loam the naked eye. silty loam 190 10 Soil texture is determined by loamy silt 100 the mass ratios, or the percent by sand sand weight, of the three soil fractions. 100 90 80 70 60 50 40 30 20 10 The soil textural triangle, Figure 2, shows the different textural classes percent sand and the percentage by weight of each soil fraction. For example, a Figure 2. A soil textural classification triangle, showing a clay loam soil composed of 30 percent sand, 30 percent clay, and 40 percent silt. soil containing 30 percent sand, 30 2 K-State Research and Extension — Soil, Water, and Plant Relationships Table 1. Average water-holding capacity for Kansas soils, depths greater than 12 inches (NRCS National Engineering Handbook Part 652 Irrigation Guide). Percentage by Mass Fraction by Volume Available Available Soil Bulk Field Wilting Field Wilting Water Water Texture Density Capacity Point Capacity Point 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 density and porosity can be affected the water is free to drain or perco- When the soil is oven dried, all soil by weather-related factors, biologi- late due to the force of gravity. This water has been removed from the cal activities, and soil management excess water is referred to as grav- soil. The amount of water at any practices. Table 1 lists typical bulk itational water. Since this percola- soil-water content varies by soil densities for Kansas soils. tion takes time, some of this extra type. Specific water-holding capac- water could be used by plants or ities can be obtained from various Soil and Water lost to evaporation. sources; however, NRCS County Interactions is defined as the amount of water Soil Surveys are probably the most remaining in the soil after rapid extensive and readily accessible. Soil acts like a reservoir that percolation has occurred. This is not Figure 3 illustrates typical amounts holds water and nutrients plants a definite soil water point; there- of water held at the defined soil need to grow. Some soils are large fore, field capacity often is defined water content for sand, loam, and reservoirs with more holding capac- as approximately one-third atmo- silty clay loam soils. The reasons for ity that release water and nutrients sphere tension. Tension is defined the differences between soil types is easily to plants, while other soils in a following section. explained in the next sections. have limited reservoirs. The follow- ing discussion focuses on soil water Wilting point is defined as the Water content can be expressed as it relates to plant availability and soil water content at which the as inches of available water or as applying irrigation water. potential or ability of the plant root a percentage. Typical values of to absorb water is balanced by the both expressions are shown in Soil Water Content. Soil water of the soil. Most Table 1 for soils at depths greater content is the amount of water crops show significant signs of than 12 inches. Typically the top stored in the soil at a given time. stress, such as wilting to the extent soil layer has slightly higher avail- The most commonly defined soil of dying, if soil water reaches the able water-holding capacity (see water content values are saturation, wilting point, especially for extend- L935, Important Agricultural Soil field capacity, wilting point, and ed periods of time. Wilting point is Properties for more information). oven dried. At saturation, which usually approximated by a value of usually occurs immediately after How Soil Holds Water. Soil 0.15 atmospheres (bars). a heavy rainfall or an irrigation holds water in two ways: (1) as a application, all pore spaces in the Soil that has been oven dried thin film on individual soil particles soil are filled with water. When the is used as a reference point for and (2) as water stored in the pores soil is at or near saturation, some of determining soil water content. of the soil. Water stored as a thin K-State Research and Extension — Soil, Water, and Plant Relationships 3 film on individual soil particles is 7 held in place by adsorption forces. 6.28 6.61 Adsorption involves complex 6 Saturation chemical and physical reactions but in simple terms, a thin film of water 5 Saturation adheres to the outside layers of soil 4.80 4.20 4.80 particle molecules. Water stored in the pores of the soil is stored 4 Field capacity 3.66 by capillary forces. An example of 3.06 the capillary force phenomenon 3 2.52 would be to place one end of a glass Field capacity capillary tube in a pan of water. 2 Wilting point

Water in the tube will rise to a Water in soil (in/ft) 1.92 1.44 certain height, which depends on 1 1.02 Wilting point the diameter of the capillary tube 0.60 (Figure 4). This phenomenon can Oven dry Oven dry 7 act in any direction and is the key Sand Loam Silty clay loam to water being stored in soil pores, as illustrated in Figure 5. Figure 3. Typical soil water content within three soil textures. Soil Water Tension. The ease by which water can be extract- lasts a short time, so plants extract ed from the soil depends on the only a small portion of the water soil water tension, also known as above field capacity. Field capacity the soil water potential. These are is defined to be at approximately equivalent values, except for the one-third atmosphere pressure sign (negative vs. positive), which or approximately 0.3 bar. At this might be thought of as either a content, it is still easy for the plant push or a pull on the water. to extract water from the soil. Water being held in pores by The wilting point occurs when Figure 4. Capillary forces illustrated by the capillary storage is held in the the potential of the plant root is how far water rises in tubes of various soil at a certain tension. The same balanced by the soil water potential; sizes. is true for water held with the thus, plants are unable to absorb adsorption phenomenon. As the water beyond this tension (approx- soil dries, these tensions become imately 15 bars). As soil water larger. It is easier for a plant to approaches the wilting point, plants extract water being held at lower will exhibit increasing symptoms of Capillary tensions. water stress, such as wilting and leaf water Adsorbed water Soil particle The tensions that correspond senescence. Prolonged exposure will to the soil-water equilibrium result in plant death. As a reference, points discussed above is a good the soil water tension in an oven- Figure 5. How soil holds water. example of water tensions affect- dried soil sample is approximately ing plant water use. At saturation, 10,000 bars. capacity and the wilting point is the soil water tension is approxi- A soil water retention or soil water available to the plant. Best mately 0.001 bar. One bar tension water characteristic curve illustrates plant growth and yield for most is equivalent to 1 atmosphere of the tension relationship (Figure 6). field crops occur when the soil pressure (14.7 psi). Thus, from the These curves are slightly different water content remains in the upper above discussion, it would be easy for different types of soils due to half of the plant available soil water for a plant to extract water from different soil textures and struc- range, which is sometimes referred a saturated soil. Saturation only tures. Water between the field to as readily available soil water.

4 K-State Research and Extension — Soil, Water, and Plant Relationships 10,000 7 Saturation 1,000 Plant Gravitational 6 available water water 100 5 Field capacity 1/3 Atmosphere 15 10 4 Wilting MAD 1 point 0.3 3 15 Atmosphere 0.1 Wilting point 2 Bars of soil tension Field capacity

0.01 water available Percent Soil water tension (bars/atmospheres) tension Soil water 1 Inches of water per foot of soil per foot Inches of water 0.001 0 5 10 15 20 25 30 35 40 45 50 Oven dry % Volumetric water content 0 Figure 6. The relationship between soil Figure 7. Illustration of the relationship of soil water content terms, values, percent water content and soil water tension for available soil water, and soil tension for a silty clay loam . a loam soil type.

the roots and distribute the water Air (-500 Bar) The dividing point between through the plant by adjusting the available soil water content and water potential, or tension, within readily available soil water content their plant cells. Water potential is Leaves (-15 Bar) is named the maximum allowable made up of several components, but depletion, or MAD soil water one of particular importance is the content. For most field crops, the osmotic or solute potential. Solute MAD level is usually defined as potential is due to the presence of about 50 percent available water. In dissolved solutes, such as sugars Stem some water-sensitive crops, such as and amino acids, in the plant cell. “Crown” vegetables and flowers, the MAD For water to move from soil, level may be less, such as 30 percent into roots, into stems, into leaves, available water. The relationship and finally into air, the water Roots (-3 Bar) between the soil water content potential must always be decreas- value, percent available soil water, ing. This is illustrated in Figure 8, Soil water (-0.3 Bar) and soil tension for a silty clay loam moving from the greater water soil is illustrated in Figure 7. potential soil (less negative) to the Figure 8. Illustration of decreasing Soil water potential is a lower water potential air (more water potential to move water from the measure of the energy status of negative). The water potential in air soil to the atmosphere through a plant. Water movement is from higher water the soil water. Water flows from a is always low as compared to plants, potential (less negative pressure reading, greater potential area to an area of so water movement is toward the expressed in bars of pressure) to lower less potential. The units of measure- air through the plants. However, water potential (more negative pressure ment are normally either bars or plants are limited in the amount of reading). Air is usually at lower water atmospheres. What can be confus- adjustment they can make. potential than a plant. ing is that soil water potentials Use of Water by Plants are negative pressures that are also if the water in the soil is at 0.3 bars expressed as tension or suction. In A plant’s root system must (around field capacity), the plant this case, water flows from greater provide a negative tension (pres- must provide at least 0.3 bars of (less negative) potential to a lesser sure) to extract water from the negative tension to pull the water (more negative) potential. Plants ground. This tension must be from the ground. At the wilting develop the tension, or potential, equivalent to the tension that holds point, the maximum negative to move water from the soil into the water in the soil. For example, tension that a plant can provide is K-State Research and Extension — Soil, Water, and Plant Relationships 5 balanced by the soil water tension. At this point, the plant can no longer extract water from the soil and will be under severe stress to the point of death. There are several factors that determine when and Reproduction where a plant uses water, and how much water a plant will use. These Vegetative factors include daily plant water growth Maturity need as influenced by climatic use Water conditions and stage of growth, plant root depth, and soil and water quality. Plant Water Need. A plant has different water needs at different stages of growth. While a plant is Stage of Growth young, it requires less water than Figure 9. Typical plant water-use curve by stage of growth. when it is in the reproductive stage. As a plant approaches maturity, its water needs drops. Curves have Percent Moisture Extraction been developed that show the daily water needs for most types of crops. Figure 9 shows a typical crop water curve. Perennial crops, such as 25% 40% alfalfa, have crop water-use curves similar to those in Figure 9, except that the crop water use is altered when the crop is cut or harvested. 25% 30% The water use would drop dramat- ically at cutting and recover with regrowth, making the water-use curve appear in a sawtooth-shaped 25% 20% pattern. Plant Root Depth. A plant’s root depth determines the depth to 25% 10%

which soil water can be extracted. Root Depth Percent A young plant with only shallow roots will not have access to soil Figure 10. Generalized crop water extraction by depth of root zone for a non-layered water deeper than its rooting depth. and unrestricted soil profile. Plants typically extract about 40 percent of their water needs from of their total root penetration. water reduces optimum plant the top quarter of their root zone, Table 2 shows the depth of root growth. For irrigation scheduling then 30 percent from the next penetration and 70 percent water purposes, the total potential plant quarter, 20 percent from the third extraction for several common field root zone is not used. Instead a quarter, and taking only 10 percent crops. Deeper portions of the root managed root zone depth of no from the deepest quarter, as illus- zone can supply a higher percent- more than 4 feet is recommended. trated in Figure 10. Therefore, age of the crop’s water needs if the Applying water to deeper depths plants will extract about 70 percent upper portion is largely depleted. subjects the irrigation to higher of their water from the top half However, reliance on use of deeper potential for deep percolation

6 K-State Research and Extension — Soil, Water, and Plant Relationships Table 2. Depth of root penetration and 70 percent of their water extraction for several References common field crops. Crop Depth of Root 70% of their Water KanSched, an ET based irriga- Penetration (Feet) Extraction (Feet) tion scheduling program. Kansas State University Reasearch and Corn 4 – 6 2 – 3 Extension. Available at www. Grain Sorghum 4. 5 – 6 2 – 3 bae.ksu.edu/mobileirrigationlab/ Alfalfa 6 – 10 3 – 4 welcome-mobile-irrigation-lab Soybeans 5 – 6 2 – 3 Wheat 4 – 6 3 NRCS National Engineering Sugar Beets 5 – 6 3 Handbook, Part 652. Kansas Irrigation Guide. Available losses. The managed root depth Most Kansas crops are considered at www.nrcs.usda.gov/wps/ may be much less than 4 feet if intermediate in terms of their salt portal/nrcs/detail/ks/people/ soils have restrictive layers that tolerance. employees/?cid=nrcs142p2_033381 prevent root penetration. Some also result in restricted root Summary Related Publications: penetration. Basic knowledge of soil-plant- Rogers, D.H. and M. Alam. 2007. Soil and Water Quality. water relationships makes it possi- What is ET? An evapotranspiration Another factor on the amount of ble to better manage and conserve primer. Kansas State University soil water available to the plant irrigation water. Research and Extension. Irrigation is the soil and water quality. For Some of the important factors Management Series, MF-2389 rev. good plant growth, a soil must have to remember include: 4 pp. adequate room for water and air 1. Soil water-holding capacity Rogers, D.H., P.L. Barnes, J. movement, and for root growth. varies with soil texture. It is high A soil’s structure can be altered by Aguliar, I. Kisekka, and F. Lamm. for medium- and fine-textured 2014. Agricultural Crop Water certain soil management practices. soils but low for sandy soils. For example, excessive tillage can Use. Kansas State University break apart aggregated soil and 2. Plant roots can use only avail- Research and Extension. Irrigation excessive traffic can cause compac- able soil water, the stored Management Series, L934 tion. Both of these practices reduce water between field capacity Rogers, D.H., P.L. Barnes, J. the amount of pore space in the and permanent wilting point. Aguliar, and I. Kisekka. 2014. soil, reducing the availability of However, as a general rule, plant Important Agricultural Soil water and air, and reducing the growth and yields can be reduced Properties. Kansas State University room for root development. if soil water in the root zone Research and Extension. Irrigation remains below 50 percent of the Management Series, L935 The quality of the water is water holding capacity for a long also important to plant develop- period of time, especially during ment. Irrigation water with a high critical stages of growth. KSRE Websites: content of soluble salt is not as 3. Although plant roots may grow General Irrigation available to the plant, so greater soil www.ksre.ksu.edu/irrigate water content must be maintained to deep depths, most of the water Mobile Irrigation Lab in order to have water available to and nutrients are taken from the www.bae.ksu.edu/mobileirrigationlab the plant. Increasing salt content of upper half of the root zone. Plant the water reduces the potential to stress and yield loss can occur Subsurface Drip Irrigation move water from the soil into the even with adequate water in the www.ksre.ksu.edu/sdi roots. Some additional water would lower half of the root zone. also be needed to leach the salt 4. Poor irrigation water quality below the crop root zone to prevent can reduce the plant’s ability to salt build-up in the soil. Poor qual- take up water and can affect soil ity water can affect soil structure. structure. K-State Research and Extension — Soil, Water, and Plant Relationships 7 Acknowledgment: The authors would like to extend their apprecia- tion 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 proj- ect, 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.

Authors Danny H. Rogers, Jonathan Aguilar, Isaya Kisekka, Philip L. Barnes, Freddie R. Lamm 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 D.H. Rogers, et al., Soil, Water, and Plant Relationships, Kansas State University, December 2014.

Kansas State University Agricultural Experiment Station and Cooperative Extension Service L904 rev. December 2014 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.