Soil-Water-Plant Interaction Basics
How Water Stress (Too Wet or Too Dry) Affects Crop Yield
Water is one of the main requirements crops need to grow and thrive. Irrigators need to focus on providing an optimal supply for their crops. Not too much and not too little. All farmers are interested in knowing how to minimize irrigation costs and save water. In addition, too much water will leach nitrogen, which can result in lowering yields, particularly in June when all the nitrogen is out there and the corn has not taken it in yet. Thus, it is important to know how the timing and level of water stress will affect the final yield. The following article will help you learn how soil, water and plants all work together to grow your crop and will be a good review as we are getting into the irrigation season.
How Does Soil Store Water?
Soil is a natural material consisting of solids (minerals and organic matter), liquid, gases and living organisms that occur on the land surface. The solids are particles stacked on each other, leaving open space between, much like a container of marbles. The open space (about 50% by volume on farmland), called soil pores, is occupied by liquid, gas and organisms.
Water that falls on the soil surface from rain or irrigation moves into the soil by the force of gravity, a process called infiltration. The pores between the soil particles provide a path for the water to flow in and on through the soil headed to the groundwater.
So, what causes some of the water to stay in the soil and not just be drained on through the soil by gravity like water down a sink drain?
The words that describe the forces that hold water in the soil against the force of gravity are adhesion and cohesion. Adhesion is the bonding between water and the soil particles and organic matter. Cohesion is the bonding of water to itself. These two forces in combination with the intricate maze of small pores in the soil causes water to be held in the soil, like a sponge holds water.
Pores — Open spaces between soil particles.
Infiltration — To enter, permeate or pass through soil by filtering gradually.
Adhesion — The force created by the bonding between two substances, i.e. water and the soil particles or organic matter.
Cohesion — The force that is created by the bonding of a substance to itself, i.e. water molecules bonding to other water molecules.
The important difference between the sink drain and the pores in the soil is the size of the “pipe.” As you can imagine, the inside of the drainpipe would be wet after water runs through it because of water being held to the pipe by adhesion and then a thin film of additional water held by cohesion. However, because of its large size, most of the water just runs through.
In the pores in the soil, water is also held by adhesion to the soil particles, and organic matter plus cohesion holds a thin film of water that, in the small pores of the soil, almost fills the entire space.
The soil texture or particle size is the biggest influence on how much water the soil will hold, because it determines the pore size. Soils are classified based on the sizes of the particles making up the soil:
- Clay (fine): <0.002 mm
- Silt (medium): 0.002-0.05 mm
- Sand (coarse): 0.05-1.0 mm
For perspective, an average medium thickness human hair is about 0.06 millimeters.
Soil organic matter is another important factor for storing water in soil. Soil organic matter can fill in the pores, allowing more water storage. Therefore, increasing soil organic matter will lead to more soil water storage.
Saturation (SAT) — The soil-water content when all the open space between particles is filled with water.
Field Capacity (FC) — The soil-water content of saturated soil after it is allowed to drain for two to three days. Several factors influence how fast the water will drain, so an average is usually used.
Permanent wilting point (PWP) — The soil-water content where plants wilt during the day and do not recover overnight. The level can vary based on the plant and the climate, so an average or book value is usually used.
Plant-available water-holding capacity, available water capacity, or simply available water — The volume of water in soil between field capacity and permanent wilting point. The value will differ based on soil type.
Deep percolation — The action of water draining below the soil depth where the crop roots can have access.
How Much Soil Water is Available to the Plant?
For crop production, we are interested in the amount of water available for plant use, called plant-available water. This is the amount of water held in the soil against gravity, but not so tightly that plants can’t draw it in.
When rainfall or irrigation fills all the pores in the soil, the soil is saturated. In the first few days, part of the water will drain rapidly on through the soil — called deep percolate — because the forces discussed above can’t hold all the water. The drainage slows as the forces on the remaining water get closer to being equal and the soil is said to be at field capacity after two to three days.
As plants use the water, it decreases finally to a point where the plant can no longer draw water in — this level is called permanent wilting point. Only the water between field capacity and permanent wilting point is available for the crop. These values are different in different soil types, as seen in Figure 1.
Sandy soils will only hold about 1 inch of plant-available water per foot of soil, where a silt loam can hold over 2 inches. A typical corn or soybean crop root system will explore 4 to 6 feet deep into the soil, making 8 to 12 inches of plant-available water in storage in a loam soil if the soil is at field capacity.
On irrigated fields, farmers usually only plan to use 50% of the plant available water in the top 3 feet before applying irrigation water to assure the crop will not suffer any water stress. The amount of water the crop can use from a 3-foot depth of loam soil without any water stress is equal to about 3 inches on loam soils and about 1.5 inches on sandy soils.
Why Do Plants Need Water?
Water is one of the main requirements plants need to grow, thrive and produce a top yield. The plants use water in several ways, including:
- Transpiration to keep them cool and allow CO2 into the leaves.
- Keep the soft-walled cells turgid (pumped full of water). This is why most plants wilt when they are water-stressed.
- Water moves nutrients into the plant from the soil and where they are needed in the plant.
- Water transports the carbohydrates from the leaves to growing parts of the plant and storage (grain, fruits, etc.).
- Water (H2O) is broken down into hydrogen and oxygen in the plant for nutrients.
Most of the water is used to keep the plants cool. The sun provides the energy the plants use for photosynthesis. However, the process only uses a small amount of the sun’s energy, the rest heats up the plants. To prevent overheating, the plant uses evaporation of water out through the leaves for cooling. The process is like the cooling effects humans get from sweating.
Corn requires 2,500 to 3,000 gallons of water to produce 1 bushel of grain. Soybeans require as much as 10,000 gallons of water to produce 1 bushel of grain. So, a fully watered corn crop needs about 24 ac-in of water from rain and irrigation or 5,428,632 lbs. of water per acre. Most of this water will go into the crop roots and out through the crop leaves to keep the plant cool.
How Do Plants Get Water From the Soil?
Plants have an ingenious process called transpiration that not only keeps them cool by evaporating water, but also provides the energy needed to pull the water from the soil into the roots, up through the plant, and into the stems, leaves and flowers. In addition, transpiration does not require any energy from the plants. The system, from a simplistic view, is like getting a drink through a drinking straw. The roots, xylem, etc. are the straw and the evaporation process is like the suction from your mouth to pull the drink up the straw.
The method plants use to regulate water use from transpiration is by opening or closing tiny holes on the leaf surface called stomata. The purpose of stomata is to allow water vapor out to drive transpiration and to allow CO2 into the plant to be used in photosynthesis. Sunlight causes the stomata to open in the morning to allow CO2 in when photosynthesis starts, and at sunset, the lack of light closes them. The plants can also close down the stomata when under water stress to reduce water loss, which is good, but it also limits the supply of CO2 entering the plant, limiting photosynthesis and thus crop yield.
The flow created by transpiration works well if the available water in the soil is adequate. However, as the soil becomes drier, more and more energy is required to pull the water out of the soil, causing stress to the plants and lowering its final yield. In contrast, too much water in the soil can also cause plant stress and lower yields by creating an unhealthy environment for the plant roots and soil microbes plus leaching.
Evaporation — The process of any substance, including water, being converted from a liquid to a vapor and being carried off. The process requires a lot of energy, which results in a cooling effect.
Transpiration — The process of water moving from the soil into plant roots, up through the plant, and evaporating from the leaves, stems and flowers.
Evapotranspiration (ET), also called Crop Water Use — Refers to the total amount of water evaporated from the soil surface, including dew or rain for plant surfaces, plus the water that moves into the roots and leaves the plant as water vapor or is used for plant growth.
So, the goal of farmers is to keep the soil in the plant-available water range by draining excess water from too much precipitation with drainage tile and ditches or adding irrigation water when it does not rain. This will keep soil water levels optimal for crops to perform well. It is important to keep in mind that over-irrigation can lower yields.
How Do Weather Conditions Impact Crop Water Use?
The weather conditions around the crops determine how much water the crop will use and how much is lost from the soil surface on any given day, called evapotranspiration or crop water use. The weather factors that influence crop water use include air temperature, humidity, wind speed and sunlight intensity.
Increasing temperature, wind speed, and sunlight intensity will increase water use, whereas increasing humidity will lower water use. Hot, windy, cloud free days with low humidity will be the highest crop water use days. These are also the days when plants may go into water stress if the water supply in the soil is getting low, often resulting in yield loss.
How Much Water is Enough for Top Plant Performance?
The simple answer is, just enough to prevent the plant from being stressed. Plants have a wide range of soil water levels where they can function at top performance. A little like a truck that can run just as well with a full tank of fuel vs. 1/8-tank. But unlike a truck, plants can start shutting down water use when the water supply starts to get short by closing their stomata. However, the consequence of the reduced water use may reduce plant productivity, lowering the final yield.
What Happens If I Don’t Have Enough Water?
For a variety of reasons, farmers may not be able to keep soil at optimal plant-available water levels. The growth period the crop is in during the water shortage has a lot to do with how much yield loss will result. For grain crops, water stress has the smallest impact during the ripening and vegetative growth period. While water stress during the flowering growth period (for corn this is the tassel, silk and pollination period) has large impacts on yield. Forage crops need optimal soil water during the vegetative stage, because we want a large plant.
Figure 2 shows the relative relationship between yield decreases and ET deficits for individual growth stages for corn. The chart can be understood by knowing that the upper righthand corner represents a fully watered crop with full yield, thus no reduction of ET or yield, as indicated by the zeros. The 0.5 on the top axis represents a 50% decrease in ET because of water stress, and the 0.5 on the right axis represents a 50% reduction in yield.
For example, a 20% ET deficit (on the top axis) would result in an 8% yield decrease (shown on the left axis) during the vegetative stage, whereas the same ET deficit would result in a 30% yield decrease during the flowering (silking) stage. The 8% yield decrease would be for a 20% reduction in ET from stress for each day from the time the corn emerges up through tassel. This level is highly unlikely. More common is seeing a few hot afternoons when vegetative corn will have some leaf rolling, which Nebraska research has shown that the plant height can be shortened, but no yield loss. In addition, this mild water stress can save some water.
Conclusion
Crops can optimally function with a wide range of soil water levels. However, too much or not enough can cause poor yields, particularly in critical growth stages. Farmers use both drainage to remove excess precipitation, and irrigation, if available, to add water when rain is insufficient. Extreme caution should be used if scheduling irrigation during the month of June to not over-apply water and leach the nitrogen fertilizer.
It is also important for farmers to understand the effect of water deficit for their crop based on the crop growth stage. For grain crops, some water stress during the vegetative and ripening stages will usually not affect grain yield, whereas water deficit during flowering can dramatically reduce grain yield. However, if a crop is grown for forage, water stress during the vegetative stage will reduce the overall plant stature and thus lower the forage yield.
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