Soil-Water Relationships

Soil-Water Relationships The soil is composed of three major parts: air, water, and solids (Figure 1). The solid component forms the framework of the soil and consists of mineral and organic matter. The mineral fraction is made up of sand, silt, and clay particles. The proportion of the soil occupied by water and air is referred to as the pore volume. The pore volume is generally constant for a given soil layer but may be altered by tillage and compaction. The ratio of air to water stored in the pores changes as water is added to or lost from the soil. Water is added by rainfall or irrigation, as shown in Figure 2. Water is lost through surface runoff, evaporation (direct loss from the soil to the atmosphere), transpiration (losses from plant tissue), and either percolation (seepage into lower layers) or drainage. The pore volume is actually a reservoir for holding water. Not all of the water in the reservoir is available for plant use. Figure 3 represents a "wet" (saturated) soil immediately after a large rainfall. Note that all of the pores are filled with water. Gravity will pull some of this water down through the soil below the crop's root zone. The water that is redistributed below the root zone due to the force of gravity is gravitational water. In general, gravitational water is not available to plants, especially in sandy soils, because the redistribution process occurs quickly (in two days or less).

Figure 2. Source and fate of water added to a soil system

Saturated (wet) soil. All pores (light areas) are filled with water. The dark areas represent soil solids.

Water distribution in a soil at field capacity. Capillary water (lightly shaded areas ) in soil pores is available to plants. Field capacity represents the upper limit of plant-available water.

Water distribution in a soil at thw wilting point. This water is held tightly in thin films around soil particles and is unavailable to plants. The wilting point represents the lower limit of plant-available water.

Plant-available water, PAW, is the volume of water stored in the soil reservoir that can be used by plants. It is the difference between the volume of water stored when the soil is at field capacity and the volume still remaining when the soil reaches the permanent wilting point (the lower limit), as shown in Figure 6.

Figure 6. Relationship between plant-available water and water distribution in the soil.

Soil-Water and Plant Stress As a plant extracts water from the soil, the amount of PAW remaining in the soil decreases. The amount of PAW removed since the last irrigation or rainfall is the depletion volume. Irrigation scheduling decisions are often based on the assumption that crop yield or quality will not be reduced as long as the amount of water used by the crop does not exceed the allowable depletion volume. The allowable depletion of PAW depends on the soil and the crop. For example, consider corn growing in a sandy loam soil three days after a soaking rain. Even though enough PAW may be avai1able for good plant growth, the plant may wilt during the day when potential evapotranspiration (PET) is high. Evapotranspiration is the process by which water is lost from the soil to the atmosphere by evaporation from the soil surface and by the transpiration process of plants growing in the soil. Potential evapotranspiration is the maximum amount of water that could be lost through this process under a given set of atmospheric conditions, assuming that the crop covers the entire soil surface and that the amount of water present in the soil does not limit the process. Potential evapotranspiration is controlled by atmospheric conditions and is higher during the day. Plants must extract water from the soil that is next to the roots. As the zone around the root begins to dry, water must move through the soil toward the root (Figure 7). Daytime wilting occurs because PET is high and the plant takes up water faster than the water can be replaced.

Figure 7. As the plant extracts water, the soil immediately adjacent to the roots (light areas) dries. If the rate of water movement from moist zones is less than the PET, the plant temporarily wilts.

At night when PET decreases to near zero, water steadily moves from the wetter soil to the drier zone around the roots. The plant recovers turgor and wilting ceases (Figure 8). This process of wilting during the day and recovering at night is referred to as temporary wiltzng. Proper irrigation scheduling reduces the length of time a crop is temporarily wilted.

Figure 8. At night when the PET is low, the plant recovers from wilting as water moves from moist zones (dark areas) to eliminate the dry zones around the roots.

Figure 9. The relationship between water distribution in the soil and the concept of irrigation scheduling when 50 percent of the PAW has been depleted.

Plant Factors Three plant factors must be considered in developing a sound irrigation schedule: the crop's effective root depth, its moisture use rate, and its sensitivity to drought stress (that is, the amount that crop yield or quality is reduced by drought stress). Effective Root Depth Rooting depth is the depth of the soil reservoir that the plant can reach to get PAW. Crop roots do not extract water uniformly from the entire root zone. Thus,the effective root depth is that portion of the root zone where the crop extracts the majority of its water. Effective root depth is determined by both crop and soil properties. Plant Influence on Effective Root Depth. Different species of plants have different potential rooting depths. The potential rooting depth is the maximum rooting depth of a crop when grown in a moist soil with no barriers or restrictions that inhibit root elongation. Potential rooting depths of most agricultural crops important in North Carolina range from about 2 to 5 feet. For example, the potential rooting depth of corn is about 4 feet. Water uptake by a specific crop is closely related to its root distribution in the soil. About 70 percent of a plant's roots are found in the upper half of the crop's maximum rooting depth. Deeper roots can extract moisture to keep the plant alive, but they do not extract suffficient water to maintain optimum growth. When adequate moisture is present, water uptake by the crop is about the same as its root distribution. Thus, about 70 percent of the water used by the crop comes from the upper half of the root zone (Figure 10). This zone is the effective root depth.

Figure 10. The amount of water extracted by plants is influenced by the distribution of the root in the soil.

Soil Influence on Effective Root Depth. The maximum rooting depth of crops in North Carolina is usually less than their potential rooting depth and is restricted by soil chemical or physical barriers. North Carolina subsoils have a pH of about 4.5 to 5.0, which presents a chemical barrier to root growth, as shown in Figure 11. Liming practices rarely improve soil pH below the 2-foot depth. Shallow soils (Carolina slate belt soils) or soils with compacted tillage pans (coastal plain soils) are examples of soils with physical barriers that restrict root penetration below the plow depth (usually less than 12 inches unless subsoiling is practiced). Thus, for example, while corn has a potential rooting depth of 4 feet, when grown under North Carolina conditions, its maximum rooting depth is about 2 feet. Maximum rooting depths for several crops under North Carolina conditions are given in Table 2.

Figure 11. Soil properties that influence the plant's rooting depth.

The effective root depth is the depth that should be used to compute the volume of PAW in the soil reservoir. The effective root depth for a mature root zone is estimated to be one-half the maximum rooting depth listed in Table 2. For example, under North Carolina conditions corn has a maximum rooting depth of 2 feet; thus, the maximum effective root depth is estimated to be 1 foot. Effective root depth is further influenced by the stage of crop development. Effective root depths for most aops inaease as top growth inaeases until the reproductive stage is reached. After this time, effective root depth remains fairly constant. Maximum rooting depth and effective rooting depth as a function of corn development are shown in Figure 12.

Figure 12. Corn rooting depth in North Carolina during various stages of development. Irrigation scheduling should be based on effective root depth rather than maximum rooting depth.

Crop Water Use Rate

Crop Water Use Rate Often, irrigation scheduling requires an estimate of the rate at which PAW is being extracted. A "checkbook" approach is often used to keep a daily accounting of water additions and removal. Traveling irrigation systems usually require several days to complete one irrigation cycle. Soil-water measurements should be used to schedule irrigation for these systems, but continued PAW extraction during the irrigation cycle must also be estimated so that the last part of the field does not get too dry. In the above situations, the crop's water use rate must be estimated. Estimates of the water use rate for most crops are available from county Extension Service or Soil Conservation Service offices. As with rooting depth, water use rate is a function of the crop's stage of development, as shown in Figure 13. For example, corn uses water three times as fast during the pollination period (65 to 75 days after planting, 0.25 inch per day) as during the knee-high stage (35 to 40 days after planting, 0.08 inch per day).

Figure 13. Corn daily water use as influenced by stage of development. Irrigation scheduling decisions should be adjusted to reflect changes in crop water consumption during the growing season.

Crop Sensitivity to Drought Stress The reduction in crop yield or quality resulting from drought stress depends on the stage of crop development. For example, corn is most susceptible to stresses caused by dry conditions at the siLicing stage (Figure 14). For a given level of stress, the yield reduction for corn would be four times greater at the silking stage than at the knee-high stage. From the yield standpoint, applying irrigation water at silking would be worth four times more than if the same amount of water was applied during the knee-high stage. Knowledge of this relationship is most useful when the irrigation capacity or water supply is limited. When water is in short supply, irrigation should be delayed or cancelled during the least susceptible crop growth stages. This water can then be reserved for use during more sensitive growth stages.

Figure 14. Corn susceptibility to drought stress as influenced by stage of development. The higher the susceptibility, the more yield reduction will result from a unit of dry stress.

The susceptibi1ity of corn to dry stress at various stages of development is shown in Figure 14. This relationship is typical for most agricultural crops irfigated in North Carolina. The most critical irrigation period typically begins just before the reproductive stage and lasts about 30 to 40 days to the end of the fruit enlargement or grain development stage. Because the root system is fully developed by the beginning of the reproductive period, irrigation amounts should be computed to replace the depleted PAW within the effective root zone (12 inches). Exceptions include tobacco and other transplanted crops where irrigation is often scheduled immediately after transplanting to ensure stand establishment. When if rigation is scheduled before the crop root system is fully developed, the amount of irrigation to apply should be based on the depleted PAW within the actual effective root depth at the time of irrigation. For example, irrigation scheduled when corn is at the knee-high stage (35 to 40 days after planting) should apply only about two-thirds as much water as an irfigation scheduled during the tasseling stage (65 days after planting) because the effective rooting depth at the knee-high stage is only two-thirds as deep (8 inches compared to 12 inches), as shown in Figure 12. For soils that have an abrupt textural change within the effective root depth, such as a loamy sand surface texture overlying a sandy clay loam, a correction may be necessary to account for the different amounts of PAW within each soil texture. For example calculations of irrigation amounts, refer to Extension Publication AG-452-4, Irrigation Scheduling to Improve Water and Energy Use Efficiencies.

How plants take up water What the plant needs Seems like a good place to start? It's all about the plants afterall. Plants need water. We all know that. Why do they need water? For the following reasons: Firstly, they need water in order to stand up. Some will eventually make woody tissue to help this process, but basically plants are full of pressurised water which makes them turgid. The leaves offer themselves to the sun....their stomata (pores) open....and moisture evaporates. Water is drawn upward from the roots and through the stems to replace this lost water. This process is called "evapotranspiration". The more sun, the greater the pressure to take up water. This process takes energy from the plant, and obviously requires a healthy root system and the presence of AVAILABLE water in the root zone (I'll explain the "availability" shortly). If it's not there, the plant will wilt. In cases of root disease and diseases like Fusarium, you will see whole crops crash down. Secondly, they need water to carry nutrients into themselves which are dissolved in the soil water. They can't munch on dry fertiliser. No water.....or I should say, "no passage of water into the plant"....and no nutrient uptake. If the plant can't take up water, it will become starved of nutrients. It's not so uncommon to see high nutrient soils and pale, nutrient-starved crops because of an inability of the plant to take up water. Thirdly, plants need water to photosynthesize. To summarise a fairly complex process, photosynthesis is the synthesis of sugar (energy) from light, carbon dioxide and water, with oxygen as a by-product. Take away any of those factors, and the plant won't grow. It has no energy. What else do plants need? They need oxygen, and they need it in the root zone. Like all aerobic organisms (including us), they need to respire as part of the process of utilising the sugars they created in photosynthesis, and this requires oxygen. No oxygen, and no respiration. No respiration, and no functionality. The roots can't grow....and can't take up water....and can't supply the plant with the nutrients and water that it needs. This is why we talk about a plant needing DRAINAGE. The problem in a waterlogged situation is not too much water......it's too little oxygen! Water in the soil Soil is made up of soil particles in crumb-form (peds), and pore spaces around the soil crumbs. In a well-structured soil, these crumbs are nice and stable....but in a poorly structured soil, the crumbs are unstable which often limits pore-space. The pore-spaces are necessary for holding water, and for the free gaseous exchange of oxygen and carbon dioxide between the plant roots and the soil surface (respiration process). There are three types of soil water (ie. water in the soil). Gravitational water: This is the water which is susceptible to the forces of gravity. It exists after significant rainfall, and after substantial irrigation. This is the water which fills all the pore-space, and leaves no room for oxygen and gaseous exchange. In "light" soils, this tends to drain away quickly. In heavy soils, this can take time. Capillary water: This is the water which is held with the force of SURFACE TENSION by the soil particles, and is resistent to the forces of gravity. This is the water which is present after the gravitational water has drained away, leaving spaces free for gaseous exchange. When the soil is holding it's MAXIMUM capillary water (after the gravitational water has drained), this is called FIELD CAPACITY. At this point, the plant is able to take up water easily, and has the oxygen that it needs in the root zone. Hygroscopic water: This is the water which is held so tightly (by surface tension) to the soil particles that the plant roots can't take it up. It's there.......but it's unavailable. At this stage there's generally sufficient oxygen, but there just isn't enough available water. The plant wilts, and will eventually die if it doesn't get water. When the plant wilts and is unable to recover, this is called the PERMANENT WILTING POINT. Now.......a lot happens between field capacity and permanent wilting point. Try to understand this point: The closer to the soil particle the water is held, the tighter it's held. And the further from the particle, the looser it's held. It takes little energy for the plant roots to take up the water that's far from the particle and is present at the field capacity point. By contrast, as the water is used up (or evaporates), it takes more and more energy for the plant to take up water. I often use the analogy of drinking through a straw. A short straw, ie. when a cup is 15cm away from you, is easy to use. A one-metre long straw takes a lot of energy to suck up a drink. A twenty-metre straw is impossible to use. It works much the same with plants. The more the soil dries out, the more energy the plant needs to output in order to get a decent drink. And note that the effect of increased soil salinity (due to high soil salinity, high soil-water salinity, or both) has basically the same effect as a soil drying out. Salt in the soil has as osmotic effect, and causes the water to be held more tightly around the soil particles. The higher the salinity level, the harder it is for a plant to take a drink, despite apparently sufficient moisture present.

Figure 1: a) soil water isotopic composition (δ18O) from soil cores sampled under bare soil in the plant nursery. On each date, the difference between two measurements at the same depth was less than 0.05 o/oo. Open symbols 3May; closed symbols: 10 June. b) Diagram of the disposition of soil sampling and moisture measurement.

Figure 2: Comparison between the computed plant δ18O using soil moisture and soil water δ18O profiles to the measured plant water δ18O. Each point represents one plant associated to one neutron probe access tube. open sympols: 3 May; closed symbols 10 June.

In other words, Plant Available Water (PAW) is the amount of water held in a soil between the limits of Field Capacity and Permanent Wilting Point. However, only the water near to Field Capacity may be Readily Available Water (RAW). This is particularly so for fine textured, clayey soils because a high proportion of PAW is held in small pores and as thin films and plants need to 'do more work' to extract this fraction of water from soils. RAW - Readily Available Water Not all PAW is equally available to plants. As soils dry out and PAW approaches PWP, plants will come under water-stress and wilt. It is the objective of irrigators to avoid this situation. They prefer to irrigate when the soil water content is about 50% of FC or about 100kPa. These limits, however, are set by the irrigator to suit the business enterprise. For example, if growth rates are to be restricted then the trigger for an irrigation event may be 300kPa. As the name suggests, Readily Available Water or RAW is the amount and availability of water in soils that is readily available to plants.

PAW - Plant Available Water Following rainfall, or irrigation, all the pores in soil will be filled with water; this is the Saturation Water Content (SWC). With time the water in the largest pores will drain to depth due to gravitational forces. In coarser textured, sandy and loamy soils this drainage will take place in less than a day and will, therefore, be unavailable to plants. Fine-textured, clayey soils, however, may be somewhat poorly drained and all pores may remain filled with water for several days. In these cases some of the SWC may be available for EvapoTranspiration and would need to be considered in calculations of soil water balances and irrigation scheduling. Poorly drained soils, however, are less suitable for irrigation. They are difficult to manage and may be waterlogged for times that can cause damage to plants for reasons of anaerobic root environments.

Evapotranspiration and IrrigationEvapotranspiration (ET) is the combined process of plant transpiration and soil evaporation (Figure 1). Plant transpiration is the movement of moisture from the plant to the air through tiny pores in the leaves known as stomates. The water enters the plants through the roots in a liquid form and leaves the plants through the stomates in a gaseous form. Soil evaporation is the direct evaporation of water from the surface of the soil into the atmosphere.

Transport of water in plants Plants need raw materials like CO2, water and minerals for photosynthesis and for various other purposes such as making of proteins. For plants soil is the richest source of water and minerals. Roots absorb these substances and transport to the various parts of the plant. The water and minerals dissolved in it move through special tissue present in plants called xylem. Xylem consists of two kinds of elements called tracheids and vessels. Vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water conducting channels reaching all parts of the plant.

Soil Water Movement As the heavy rains over Memorial Day weekend have subsided, it may be of interest to write a few words about how water moves through soil. During long-continued heavy rains, infiltration of soil water continues under the force of gravity, carrying the water down to successively greater depths. Soil pores become filled with water, with only a small amount of free air remaining entrapped in bubbles. The soil may, for a time, become almost completely saturated with water. Downward percolation continues beyond the soil water belt into the intermediate belt, a zone too deep to be reached by plat roots. Water may ultimately reach the ground-water zone below (Fig. 1). After the rain has ceased, water continues to drain downward under the influence of gravity, but some remains held in the soil, clinging to the soil grains in thin films, by the force of capillary tension. This is the same force that causes ink to be drawn upward in a piece of blotting paper and which permits small water droplets to cling to the side of a vertical pane of glass. Films of capillary water in the soil remain held in place until gradually dissipated by evaporation or drawn into root systems. After soil has been saturated by prolonged rains and then drains until no more water moves downward under the force of gravity, the soil is said to be holding its field capacity of water. Most excess water drains out in a day’s time; usually not more that two or three days are required for gravity drainage to cease. Soil-moisture content can be stated in terms of the equivalent depth in inches of water in a given thickness of soil. At field capacity, soil-moisture content ranges from 1 to 4 inches per foot of soil, depending upon soil texture (Fig. 2). Sandy soils have low field capacity, which is rapidly reached because of the ease with which the water penetrates the large openings (macro pores). Clay soils, on the other hand, have a high field capacity, but require much longer periods to attain it because of the slow rate of water penetration due to the much smaller openings (micro pores). A comparable, but lower value of soil moisture is the wilting point, below which foliage wilts because of the inability of the plants to extract the remaining moisture (Figure 2). A few points to consider: only after heavy rainfall does the water “flow” through the soil. This is especially true in our area where evapotranspiration exceeds precipitation. During most of the growing season the water can be said to be “pulled” through the soil by capillarity. Field Capacity can be thought of as “all the water a soil can hold against the pull of gravity”. When the field capacity of a particular soil is exceeded, water begins to flow downward. One last point to consider is that available water to the plant is only the water held in the soil at tensions between field capacity and wilt point, or realistically, the water held at tensions less than wilt point (Fig. 2). The characteristic annual cycle of changes in soil moisture content deserves study because it leads to a better understanding of the principles of ground-water movement, surface runoff, and various aspects of the sculpturing of the land by running water.

ROOT Often roots are overlooked, probably because they are less visible than the rest of the plant. However, it's important to understand plant root systems because they have a pronounced effect on a plant's size and vigor, method of propagation, adaptation to soil types, and response to cultural practices and irrigation. Roots typically originate from the lower portion of a plant or cutting. They have a root cap, but lack nodes and never bear leaves or flowers directly. Their principal functions are to absorb nutrients and moisture, anchor the plant in the soil, support the stem, and store food. In some plants, they can be used for propagation.

Soil-Water Relationships