Crop Insights written by Derrel Martin1, Tyler Smith1, William Kranz1, Suat Irmak1, Simon van Donk1 and John Shanahan2
Effective management of an irrigation system requires the understanding and use of the basic concepts of soil water. Without these concepts, the irrigator will not know how much water to apply or when to apply it. The goal of irrigation management is to maintain the amount of water in the soil between field capacity and the minimal allowable water balance to satisfy plant requirements. Plants can suffer in soils above field capacity because of decreased aeration and nutrient leaching. If soil water drops below a critical level, plants can experience stress. Thus, it is necessary to determine the amount of water available in the soil for plant use and the amount of water to apply when irrigating.
The 2 most important measures of soil water for managing irrigation systems are: 1) soil water content, the amount of water in the soil, and 2) soil water potential, a measure of how available this water is to plants - in other words, how hard do plants have to work to remove water from the soil?
Soil is composed of 3 major components: soil particles, air and water (see illustration below). The fractions of water and air are contained in the voids between soil particles. The ratio of the volume of pores (voids) to the total (bulk) volume of a soil is the porosity.
| Soil Water Definitions |
Soil Water - Water contained within or flowing through the soil profile. Available Soil Water - Water in the soil that does not drain and can be extracted by plants. The available water can vary from zero to the water-holding capacity of the soil. Field Capacity - The upper limit of available soil water that remains after drainage due to the effects of gravity. Wilting Point - The driest condition at which plants can extract water from the soil. The wilting point is the lower limit of available soil water. Unavailable Water - Water held by soil particles that cannot be used by plants. Some soils, such as clays, retain substantial amounts of water when soil is below the permanent wilting point. Water-Holding Capacity - Maximum amount of available water when the soil is at field capacity. It is computed as the difference between field capacity and the wilting point. Minimum Allowable Balance - The soil water content where crops begin to experience water stress. Limits vary but plants can usually use approximately 50% of the water holding capacity without experiencing stress. Gravitational Water - Water above field capacity that drains below the active root zone by the force of gravity. |
The amount of water in the soil is not the only concern in irrigation management – plants must be able to extract water from the soil. Soil water potential is an indicator or measure of the energy status of soil water relative to that of water at a standard reference3 and is often expressed as energy per unit of volume (in units of bars or centibars). The 3 major components of total soil water potential are 1) gravitational potential, 2) matric potential and 3) solute potential. The gravitational potential is due to the force of gravity pulling downward on the water in the soil. Matric potential describes the force the soil matrix places on the water by adhesion and capillarity and is known as the soil water tension. Dissolved solids (salts) in the soil water cause solute potential. The solute potential affects the availability and movement of water in soils when semi-permeable membranes (like plant roots) are present.
The higher the salt concentration in the soil solution, the more work a plant has to do to extract water from the soil. Thus, where soil salinity is appreciable, solute potential must be considered for evaluating plant water uptake.
The component that dominates the release of water from soil to plants when salts are not present is the matric potential. Several forces are involved in the retention of water by the soil matrix. The most strongly held water is adsorbed around soil particles by electrical forces. This water is held too tightly for plants to extract. Water is also held in the pores between soil particles by a combination of attractive (surface tension) and adhesive forces. The strength of the attractive force depends on the sizes of the soil pores. Large pores will freely give up pore water to plants due to the matric potential in the soil or to drainage due to the gravitational potential.
There is a corresponding matric water potential for a given amount of water in a particular soil. The magnitude of the matric potential is expressed as soil water tension. The curve representing the relationship between the tension of the soil and its volumetric water content is the soil-water release curve. The curve in Figure 1 shows that water is released (volumetric water content decreases) by the soil as the tension increases.
Soil-water release curves are often used to define the amount of water available to plants. There are 2 terms used to define the upper and lower limits of plant water availability. The upper limit, field capacity (FC), is defined as the soil water content where the drainage rate, caused by gravity, becomes negligible. Thus, the soil is holding all of the water it can without any significant loss due to drainage. The wilting point (WP), the lower limit, is the water content below which plants can no longer extract water from the soil and will not recover if the water stress is relieved. Both limits are affected by cropping practices and environmental conditions; thus, the values are not exact. The volumetric water content at FC and WP is given in Figure 1 for 6 example soil textures.
Figure 1. Example water release curves for 6 soil textures.
The water held between field capacity and the permanent wilting point is called the available soil water or the available water capacity (AWC), i.e., available for plant use. For the sandy loam soil shown in Figure 1, the volumetric water content at field capacity is 0.22, and the volumetric water content at WP is about 0.10. Thus, the available water capacity for that soil is 0.12 (0.22 - 0.10). The AWC of the soil is often expressed in units of depth of available water per unit depth of soil, i.e., inches of water per foot of soil. In the example above, the AWC is 0.12 inches of water per inch of soil (1.44 inches of water per foot of soil).
Field soils are generally at water contents between the FC and WP. A commonly used term in irrigation management is soil water depletion (SWD). Soil water depletion refers to the amount of available water that has been removed. It is very useful in irrigation management to know the depth of water required to fill a layer of soil to field capacity. This depth is equal to SWD.
Data for soil properties are available from various sources. County Soil Survey Reports and the Web Soil Survey from the USDA-NRCS normally list these data. Ranges of values for available water-holding capacity for typical soil texture classes are listed in Table 2.
Infiltration can be described in terms of the rate water enters the soil (i.e., the depth that water infiltrates per unit of time) or the cumulative amount of water entering the soil over time if water application continues. Cumulative infiltration is the total depth that has infiltrated after a specific water application time has elapsed (Figure 2).
Figure 2. Infiltration characteristics of a silt loam soil.
Infiltration, the entry of water into the soil, replenishes soil water. Infiltration is very important in irrigation since the goal is to supply water to the root zone to meet plant needs. In most cases, the objective is that all of the applied irrigation and rain enters the soil, thereby minimizing the amount of water that runs off the soil surface.
2 forces control infiltration. During the initial stages of water application, the capillary forces dominate water movement into the soil. Capillary forces work equally in all directions. Thus, the capillary forces pulling water into the soil are the same in the horizontal and vertical directions. As time progresses, the capillary forces diminish and gravity becomes the dominant force. Gravitational effects prevail when the soil is very wet, i.e., wetter than field capacity.
The curves shown in Figure 3 illustrate rates of infiltration with time for 3 soil textures. The curves show that initially the infiltration rate is very high and as time progresses, or more correctly, as the amount of water that has infiltrated increases, the rate of infiltration decreases. Therefore, a decay curve results with a decreasing rate of infiltration. As time continues, the infiltration rate will approach a nearly steady rate, sometimes called the basic infiltration rate.
Figure 3. Infiltration rate vs. opportunity time for 3 soil textures.
Soil texture is often the primary factor affecting the infiltration rate of soils. Coarser-textured (sandy) soils generally have higher infiltration rates than fine-textured (clay) and medium-textured (loam) soils (Table 1).
| Table 1. Basic infiltration rate by soil textural class. | ||||||||||
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Surface sealing is another factor influencing the infiltration rate. Surface sealing occurs when the shearing effect of flowing water or impact energy of large drops causes the aggregates on the soil surface to decompose into smaller aggregates and individual particles, which tend to form a thin layer with low permeability on the soil surface. It is common to find large differences between infiltration during the first irrigation event and infiltration during later irrigation events due to surface sealing especially for surface irrigation.
Conservation tillage practices that leave crop residues on the soil surface enhance infiltration. Crop residue on the surface protects the soil from the impact of water drops from sprinkler irrigation, thus reducing the formation of a surface seal. Likewise, deep tillage (chiseling) is sometimes used to enhance infiltration.
Soil water content is another factor that influences infiltration. The wetter the soil when water application begins, the lower the infiltration rate. The initial infiltration rate of a moist soil is, in general, lower than that of an identical dry soil. As time progresses, the infiltration rate of these 2 conditions will converge to the same steady-state value.
Water temperature influences infiltration rates due to the dependence of the viscosity of water on temperature. As temperature increases, the viscosity decreases - hence, the infiltration rate increases.4 As water warms, the infiltration rate can go up. An excess amount of sodium can decrease infiltration. A sodic soil, 1 with excess sodium, is extremely difficult to irrigate because infiltration is so low.
Scheduling irrigation depends on an understanding of the amount of water held in the soil at any time. Being able to manage the soil water balance can help growers avoid over- or under-applying irrigation water.
Water storage capacity in the root zone is determined by soil texture and plant growth stage. Table 2 gives the available water capacity by soil texture classifications. These values represent the amount of water in each foot of soil that is available to the plant. For most crops, including corn and soybean, 50% of the available water capacity can be used before plant stress begins. Late in the growing season, this value can increase due to full root development and a reduction in evapotranspiration.
To determine the total soil water available to the crop at a specific point in the season, it is essential to know the rooting depth of the target crop at that growth stage. By knowing the available water capacity for a given soil texture class and rooting depth, the total available water can be determined.
The example below shows how to determine the total soil water available for a corn crop grown on a sandy loam soil at a particular point in time in the growing season.
Corn is being grown, and it is in the early tassel stage of growth. Table 2 shows that the available water capacity (AWC) is 1.4 inches per foot of active root zone. Corn at early tassel has an approximate rooting depth of 2.5 feet. Multiply the rooting depth by available water capacity to determine the total available soil water:
Total available water = 2.5 x 1.4 = 3.5 inches
In Table 2, the minimum balance is given based on different soil textures. This value gives the minimum amount of water held in the soil before crop stress begins. Various crops have different minimum water balances (see footnote below table); corn and soybeans require 50% of available water capacity to maintain ET and prevent water stress. The goal of irrigation management is to maintain a soil water balance between field capacity and a minimum balance. As a result, water can be applied before plant stress occurs and without overfilling the root zone.
*Portions of this article have been adapted from the Center Pivot Irrigation Handbook, 2012, by the Biological Systems Engineering Department and Extension at the University of Nebraska-Lincoln. Lincoln, NE.
1 We gratefully acknowledge the contributions of the authors who are Professors and a Research Associate in Biological Systems Engineering at the University of Nebraska-Lincoln.
2Pioneer Agronomy Research Manager, Johnston, Iowa.
3Hillel, D. 1980. Fundamentals of soil physics. Academic Press, New York, NY.
4Duke, H.R. 1992. Water temperature fluctuations and effect on irrigation infiltration. Trans. of the ASAE 35(1):193-199.
The foregoing is provided for informational use only.
