Early Summer Crop Water Needs

By Angie Gradiz and Lyndon Kelley

With corn and soybeans in their rapid growth stages, it's time to prepare for irrigation to match the crop water use. While early summer rainfall may have initially provided soil moisture, factors such as cover crops, tillage or insufficient rainfall can affect soil moisture conditions, necessitating the start of irrigation soon.

Irrigation for late season planting after harvested forages such as wheat or rye gets the crops established and developed as they enter the typical drier weather of summer. The first or spring crop will often deplete the soil moisture reserves, resulting in the need for irrigation to germinate the second crop. In some situations, irrigation before planting the second crop can aid in seedbed formation.

Early irrigation also serves as effective management for incorporating fertilizer and herbicides or softening soil crusts to aid crop emergence. However, it's crucial to avoid over-irrigation during vegetative stages. Excess water in the root zone can lead to deep percolation and leaching, especially in sandy soils, which can result in the loss of agrichemicals.

To determine if crops need irrigation, there are simple tips and tools available. Crops in vegetative stages such as corn at V4 to V8 typically use 0.5 to 0.8 inches of water per week, while soybeans at V1 to V2 use 0.4 to 0.6 inches per week under normal conditions. Crops with a full canopy, such as winter wheat and forages before cutting, use 1.5 to 2 inches per week.

While crop water use during vegetative stages is low, it is essential to monitor water deficits. To prevent these deficits, Michigan State University Extension encourages irrigators to compensate for the previous week's water use. However, if rain is forecasted, leave space for it to refill the soil profile to prevent runoff or deep percolation.

Irrigation scheduling is a valuable tool for maximizing irrigation efficiency. One method involves using reference potential evapotranspiration (rPET). In southern Michigan, the amount of water used by well-watered grass in the last week of June is about 1.4 inches, though this can vary based on weather conditions. The Michigan State University (MSU) Enviroweather network provides rPET estimates and related tools for each of its 87 Michigan sites. These data can guide irrigation based on local weather conditions, and the MSU Enviroweather network offers a daily rPET text service to subscribers. 

The rPET needs to be adjusted for the water demand of the specific crop being grown. The ET for annual crops increases until full canopy is reached. Early-season rooting depth limits irrigation application volumes. Applications of 0.75 inches or less are common this time of year to avoid pushing water below the effective root zone. However, avoid making too many small (less than 0.4 inch) applications, as they are less effective and can aggravate disease by frequently wetting the plant. Corn at the V6 stage has an expected effective rooting depth of 20 inches. At the V10 stage, corn has a 23-inch effective rooting depth, and by the VT (tassel) stage, it reaches full effective rooting depth of 36 inches or more, rapidly reaching peak daily water use rates during pollination. Soybeans at the V3 stage have an effective rooting depth of 16 inches, which extends to almost 24 inches by the R1 stage.

For example, the table below shows estimated crop water use for corn and soybeans at different stages according to weather conditions for three locations. In the last week of June, V10 corn will use approximately 88% of the rPET, while V3 soybeans will use about 70%, or almost 1 inch.

Early-season irrigation can be more accurately scheduled by monitoring soil moisture in the root zone rather than using a checkbook irrigation scheduling system for newly emerged crops. Soil moisture monitoring helps determine whether the field is dry or wet, guiding when and how much water is needed. Monitoring multiple depths of soil can determine which soil layers are dry or wet. There are several ways to monitor field soil moisture content. The feel and appearance method is popular and quick. This method involves taking soil samples using a soil probe, auger or shovel, then squeezing the samples firmly in your hand to form a shaped ball and comparing it with photographs shown in the United States Department of Agriculture Natural Resources Conservation Service standard.

A reliable method is using soil moisture sensors. These sensors estimate soil moisture content based on either the force required to pull water out of the soil (tension) or the electrical properties of the soil, both of which change based on soil moisture content and type. Soil moisture data can determine when the soil is getting drier and how much water is stored in the root zone, helping schedule irrigation with more confidence. More information about the soil moisture sensors can be found in MSU Extension Bulletin E3445 “Improving Irrigation Water Use Efficiency: Using Soil Moisture Sensors.”

Many irrigators could save crop inputs by using irrigation scheduling to make informed decisions and ensure optimal yields. For more information on irrigation water use and when to irrigate, see Irrigation Fact Sheet #3, “Irrigation Scheduling Tools." If you are just getting started with irrigation scheduling, download the “Soil Water Balance Sheet." These and other useful resources are available from the MSU Extension Irrigation website.