Scheduling Final Irrigation Events for Corn and Soybeans

Insights

• Soil water holding capacity plays a role in estimating plant water availability to help schedule final irrigation events.

• The goal of the final irrigation event should be to provide adequate water to support reproductive development, yet allowing for proper soil profile depletion once plants reach physiological maturity.

Attention on irrigation is often focused on critical periods of pollination and kernel development. However, yield reductions of corn or soybeans can occur at any point prior to physiological maturity if the crop water demand is not satisfied. Conversely, excessive watering near maturity potentially reduces return on investment (ROI) from greater energy costs and can create logistical harvest issues if fields remain excessively wet.

Understanding Available Soil Water

Soil serves as the primary water reservoir for plant uptake. Approximately 50% of the soil profile is comprised of pore space (the area between soil particles), which serves as the area of soil water retention. As pore space fills with water, the soil reaches its saturation point and forces oxygen needed for root growth and nutrient uptake out of the profile. Any extra water will percolate down through the profile by gravitational force. After 24 to 48 hours, gravitational water movement will begin to cease as soil water returns to field capacity.

The remaining water is held in place by adhesive bonds with soil particles and electrostatic cohesive bonds between other water molecules, preventing further drainage of the soil profile. Soil water holding capacity will vary by soil particle size. Smaller sized silt and clay soil particles have more binding sites available for water molecules to adhere to, increasing water holding capacity compared to larger sand particles. However, not all water in soil is accessible to plants due to the strong adhesive relationship between water molecules and soil particles. When plant roots cannot overcome the remaining moisture bond with soil, it is referred to as reaching the permanent wilting point.

The amount of water held by the soil between the permanent wilting point and field capacity is referred to as plant available water. Soil texture again dictates the capacity of this pool, which is a function of its electrostatic properties and pore sizes (Figure 1). Table 1 shows representative available water contents for common soil textures found in the Corn Belt.

Since not all water in the soil profile is available for plant uptake, it is recommended to maintain varying minimum amounts of profile water to avoid stress prior to the next irrigation event. The maximum allowable depletion (MAD) point indicates how much of the total available water in the profile can be utilized before the crop begins to undergo water stress. A conservative rule of thumb is to maintain MAD levels of 50%.

However, they can be as high as 70% at early vegetative crop growth stages when plant water use rate is lower (Table 2). For example, a silty clay loam soil at field capacity with corn in the V8 growth stage can extract more water per foot of soil at 70% MAD levels before needing irrigation than could have been extracted if using 50% MAD levels (1.8-inches H2O/ft soil AWC × 70% MAD = 1.3-inches; versus 1.8-inches H2O/ft soil AWC × 50% MAD = 0.9-inch). In comparison, only 1.0 inch per foot of soil could be extracted from a field with a sandy loam soil (1.4-inches H2O/ft soil × 70% = 1.0-inch).

Crop Water Requirements

Crop water use is a function of two micrometeorological principles: 1) water evaporation from the soil surface, and 2) plant transpiration, which is the exchange of water vapor for carbon dioxide by openings in plant leaves (stomata). Approximately 70 to 80% of crop water use is attributed to transpiration, with the remaining related to evaporation. Crop growth stage, weather conditions (e.g., temperature, humidity, wind speed) and management practices (e.g., tillage, crop residue, plant density) all affect daily crop water use. However, the greatest proportion of the total water requirement occurs during reproductive development (70 and 85% for corn and soybean, respectively).

Daily water demand during reproductive development does decrease as corn and soybeans approach maturity. However, it does not reach zero until full maturity is reached. For example, according to Table 3, one inch of water is still required by soybeans from R7 (denoted by the observation of one mature pod on the main stem) and R8 (95% pod maturation). Although slight, yield reductions can still potentially occur if this water demand is not satisfied at these crop stages.

Planning the Final Irrigation Event

The goal of the final irrigation event should be to provide adequate water to support reproductive development, yet allowing for proper soil profile depletion once plants reach physiological maturity. A theoretical and economical goal is for available soil water to be depleted to 40% of available water

capacity by physiological maturity. To effectively estimate the amount of water applied by the last irrigation, the following information is needed: predicted crop maturity date, estimated remaining crop water demand, and current available soil water in the profile. Although rainfall can periodically influence this final irrigation event, it is often best to omit it from the final water balance unless it can be estimated with high confidence. Once crop growth stage is determined, the remaining total water requirement for either corn or soybeans can be estimated before any credits are applied using the workflow in Table 3. Accurate estimation of remaining soil water occurs through use of soil water sensors. Measurements should be taken at 48-inches for corn and 36-inches for soybeans, as most water uptake by roots occurs with those depths. When multiple soil types are present in the field, the soil with the least water holding capacity should be used to ensure that the soil’s contribution to the water balance is not overestimated. Once these input variables are collected, the amount of water required for the final irrigation event, if any, can be determined through the workflow in Table 4.

An effective final irrigation event meets final crop water demand while minimizing excessive water application. Crop growth stage, current soil water balance, soil physical properties, and future weather events all impact the final amount of irrigation water required, meaning field-by-field calculation is necessary. Contact your Golden Harvest Seed Advisor or Agronomist for further help fine tuning final irrigation events.