How much N do you need to sidedress your corn crops?

Friday, June 6, 2014

Even with the best predictions of N availability from the soil, we will still be at the mercy of the weather. But delaying application of most of the N until sidedress time reduces the risk of losses between planting time and the time when the crop needs it

by KEITH REID



Nitrogen fertilizer is tied with the cost of seed as the biggest single expense in corn production, but it also contributes significantly to crop yield. Applying the right rate will have a big impact on your profitability, and also on the environment.  

Nitrogen (N) taken up by the crop comes from a combination of organic N that has mineralized from the soil and fertilizer that is applied. The plant cannot distinguish between these sources, since they both end up being absorbed as ammonium or nitrate. The conversion of organic N to mineral N, however, can vary from field to field and year to year. Waiting to apply most of the N for a corn crop until sidedress time should allow us to account for these variations, and balance the supply from the soil with added fertilizer.  

There are a number of approaches for this accounting, ranging from the simple to the complex. In the Corn N Calculator, there is a credit for sidedress N on fine, and medium-textured soils, indicating lower optimum N rates when N application is delayed. This probably reflects the risk of N losses through denitrification if there is heavy rain in late spring, and sidedress N avoids this risk. The calculator does not, however, account for nitrogen mineralization in individual years.

There are complex models that attempt to follow the soil processes as they are affected by temperature and rainfall. These require a long list of inputs, high-powered computers, and highly skilled technicians to set up and run the programs. They are useful to help understand the processes in the soil, but they are far too cumbersome to be used as a predictive tool for farmers.  

Work is on-going on a number of fronts to find a compromise solution – a model that is simple enough to be used in the field but accurate enough to be an improvement over general N recommendations. This seems simple enough, since we know that biological activity will be greatest under warm moist conditions.

Mineralization, since it is a biological process, should be maximized when these conditions exist, making more N available to the crop. Complications arise, however, when you realize that denitrification rates will also be increased with warm temperatures, so the risk of loss is greater as well.  

Dr. Nicolas Tremblay at the Agriculture and Agri-Food Canada (AAFC) research station in Saint-Jean-sur-Richelieu, Que., has been analyzing data from N response plots across a wide area and over a number of years, and he has found that combining temperature with an index of "Adequate and Well Distributed Rainfall" can give good predictions of the amount of N mineralization. This helps to account for the losses that occur if the rain occurs in heavy downpours, or the lack of mineralization if the soil is too dry.

While this approach shows promise, it is not quite ready for the field. At both AAFC and universities across Canada, work is ongoing to develop ways to account for the variation in the size of the organic N reserves in the soil, and whether the nature of organic matter in different soils can affect how much N is released. There are also questions about how to account for measurements of the N status of a growing crop with tools like chlorophyll meters. Integrating all of these components into a Decision Support System is the subject of our current AAFC project.

Even with the best predictions of N availability from the soil, we will still be at the mercy of the weather after the N fertilizer is applied. We cannot predict if there are going to be thunderstorms a week after the N is applied that will wash it away. We also cannot predict if we are going to have that rare year when growing conditions are perfect for the corn crop, and it could have used more N than normal. These factors are not included in a general N recommendation, either.

Delaying application of most of the N until sidedress time not only reduces the risk of losses between planting time and the time when the crop needs a large amount of N, but it also allows us to account for the weather impacts up to that point. This should get us closer to matching fertilizer applications to what the crop requires, ensuring you get the best returns from your fertilizer investment. BF

Keith Reid is Soil Scientist Agriculture and Agri-Food Canada, Guelph.