Understanding Evapotranspiration (ET) and Its Role in Nutrient Uptake

As the corn plant grows and develops, it takes up water and transpiring it daily through its leaves.  Likewise, soil is losing water through evaporation, especially as air temperatures increase.  Together, called evapotranspiration, a great deal of water can be lost during the growing season (up to 20-22" for high yielding corn). 

Transpiration in corn is involved in the movement of nutrients through the plant, and for plant cooling.  Therefore, sufficient soil moisture is needed at key growth periods to ensure stress is minimized. 

Early in the growing season, between planting and V6 growth stage, soil evaporation is the main component of the combined evapotranspiration, making up to 70%, according to a study published by Iowa State researchers.  However, between V6 and R6 (physiological maturity), the majority of evapotranspiration came as plant transpiration, also up to 70% of the total. 

Figure 1: Plant transpiration and evapotranspiration from planting to maturity with a 110 CRM hybrid using weather from central Iowa run 35 years through the APSIM crop growth model. (Courtesy of Iowa State University)

If there is insufficient water available during key growth periods, minimal ET will occur due to the plant attempting to conserve water, but in turn will also reduce nutrient uptake and plant development. 

In early to mid-vegetative growth stages (V5-V8), water stress will first cause leaf rolling, which then can lead to reduced leaf development and further hinder photosynthetic uptake.  Root development may also be affected, and ear development, in terms of kernel rows and length, can also be affected at this time.  Severe water stress during this time puts the plant in a difficult situation to be able to recover in terms of yield. 

Maximum ET in corn plant development occurs around tassel/silking, or VT/R1 growth stage.  At this stage the plant is moving from vegetative to reproductive development.  Kernel set and fill is about to occur, and new root development also ceases.  Nutrient uptake is key to ensuring nutrients are available for later kernel filling in R3 to R5 growth stages. 

Water stress during R3 to R5 growth stages will usually results in reduced kernel weights, not necessarily kernel counts per ear.  This in turn leads to reduced yields due to the lower seed mass.  Again, because the plant has insufficient water to carry nutrients through it to the developing grain, a reduction in yield occurs. 

Knowing when and how much transpiration can and will occur helps in making decisions when it comes to irrigation.  Ensuring a proper amount of water in the soil and being applied during dry conditions will both replenish the soil moisture and aid in cooling the plants during times of high temperature.  Early root development plays a key part in assisting with water availability.  As noted in a previous article, controlling irrigation during vegetative growth stages allows for roots to grow deeper in search of more moisture.  This assists the plants later as irrigation is applied and more efficient water uptake can take place. 

Summary: 

Water and nutrient uptake go hand in hand.  Water is the transport mechanism for nutrient movement in the plant and provides crucial cooling during periods of hot weather.  Insufficient water leads to water stress in the form of rolling leaves as the plant attempts to protect itself.  In turn, nutrient movement and location is reduced because of this act of self-preservation.  

Evapotranspiration is a natural and necessary process in crop development.  Knowing that peak ET occurs around silking, a crucial point in yield formation, those that have irrigation capabilities can take advantage of this knowledge to ensure proper water, and therefore, nutrient delivery is protected.