The Farmer’s Dilemma: nitrogen and water quality
Managing a crops fertilizer needs in a corn production systems is, and probably always will be a challenge. Typically, farmers only recover about 50% of the fertilizer they apply in the crop, a statistic that people in other fields find appalling. But farmers understand that having sufficient nitrogen in the soil is crucial for the corn crop to produce high yields, and that the delivery system for that nutrient is indirect, leaky, and there are just too many loss mechanisms and variables beyond the farmer’s control.
Managing nitrogen in corn consists of adding a nitrogen source to the soil, and that’s when our management and control opportunities usually end. We then hope it doesn’t rain too much or too little, and that it’s not too hot nor too cold, and that most of the applied nitrogen will be held in the soil and converted to plant-available nitrate in the soil solution about when the crop needs it. The nitrate form is dissolved in the water held between soil particles, which means that as the crop takes up water, it also takes in a good portion of the nitrogen it needs through passive uptake. The problem is (or can be) that nitrate is also the form that is most vulnerable to loss; it can be lost through leaching when excess water moves and carries the nitrate with it, or it can be lost through denitrification under saturated soil conditions. The farmer’s nitrogen dilemma is to make sure there is enough nitrogen in the root zone when the crop needs it, but avoid the risk of having too much nitrate in the system that can be lost with excessive rain.
The crucial period for corn nitrogen uptake occurs from about V8 (8 leaf collars are exposed) until the tassel or flowering stage. High yielding corn hybrids grow so fast during this period that some experts refer to it as the “grand growth period” of corn development. Corn plants add a new leaf about every three days during this period, and can use up to 2 inches of water per week through evapotranspiration just prior to flowering. A high-yielding corn crop can take up from 7-9 lbs. N per acre per day during the last three weeks of vegetative growth. That is a huge nitrogen appetite. According the WinField® United nutrient calculator, a corn crop that produces 240 bushel per acre will contain about 216 lbs. N in the grain at the end of the season with an additional 108 lbs./acre in the stover. Two-thirds of the total N should be in the plant by the time if flowers for optimum grain yield, with the rest taken up during reproductive growth stages.
To summarize: Corn needs lots of nitrogen, in the nitrate form, in the soil during late vegetative growing stages. This is also the time with risk of large rainfall events that overwhelm the system and cause significant loss of nitrogen. New crop growth/nitrogen process modelling tools that simulate crop N status and forecast potential deficiencies in time to correct them show promise in managing these risks. High clearance applicators are giving farmers an option to put more N on just when the crop needs it, and we are getting more farmers to use nitrogen stabilizers to help regain some of that control of nitrogen supply after application. We want to work with farmers to address the challenge of nitrogen management and we know that farmers will find a way.