The importance of timely nitrogen application

When will a shortage of N hurt a crop and how early is too early to apply it? Some answers to these and other questions that may affect your yield

by KEITH REID

For optimum crop growth, it is important that each nutrient be available to the plant in adequate amounts when the plant needs it and placed where the plant can reach it. This is a special challenge for nitrogen, since it keeps changing into different forms. There has been lots of talk about picking the right rate of nitrogen to apply, but I would like to switch things around and talk instead about making sure the nitrogen supply is there at the right time.

The general rule, because nitrogen is so mobile, is to apply nitrogen as close to when the crop needs it as possible. The realities of crop production mean that this isn’t always feasible, so we need to understand how close is close enough. An additional complication is the supply of nitrogen from the breakdown of organic materials, so we are not just trying to match crop uptake, but rather the difference between the crop needs and the supply from the soil.

When will a shortage of N hurt the crop?
Like most things in biology, the response of plants to a nutrient shortage is not as simple or straightforward as we might like. At some points in a plant’s growth, a shortage of nitrogen can delay growth, but it will compensate by growing faster when the nitrogen supply is restored.

This fact is even used in some transplanted crops, where a nutrient deficiency is induced in transplants to “harden them off,” and then a starter solution is applied to the roots at planting as a ready supply of nutrients. The rapid growth induced by this helps the plant to overcome transplant shock and get established quickly.

If the shortage of nitrogen lasts too long, however, the reduction in growth and yield potential is permanent. Thus, if winter wheat doesn’t receive topdress nitrogen until it is coming in to flag leaf, or a crop of corn runs short of nitrogen before sidedress nitrogen is applied, the yield will be reduced significantly.

Complicating the picture even more are the situations where the crop appears to respond to a greater quantity of added nitrogen than it actually takes up at that point in growth. Corn will take up roughly 10 pounds per acre of nitrogen prior to sidedress, and absorbs most of the balance of its nitrogen requirements during the rapid growth from knee-high to tasselling.

Despite this low demand for nitrogen by the seedling, there has been a consistent response to 30 or 40 pounds per acre of nitrogen at planting, particularly in no-till fields. We don’t know whether this additional nitrogen is to make up for immobilization of the fertilizer in cold soils, or if the crop responds to some signal that says, “There’s lots of nitrogen -- make more yield potential!” It may even be a combination of these factors, but we do know the response is real.

How early is too early?
The chief concern with early application of nitrogen is the amount that can be lost, either through leaching or denitrification, during the time when the crop is not able to take up the nitrogen from the soil. A secondary concern is that early nitrogen might feed the weeds, resulting in greater competition with the crop along with reduced nitrogen supply.

It is well documented that corn doesn’t need as much nitrogen fertilizer if some of the nitrogen is applied sidedress rather than pre-plant. The reduction in nitrogen requirement varies from 20 per cent on clay soils down to zero on sandy soils. This represents the amount of nitrogen that is lost, under average conditions, during the six to eight weeks between planting and the period of rapid uptake by the crop. The same loss will not apply to all crops, since there is a wide range in how rapid early growth is.

Spring cereals are unlikely to respond to delayed nitrogen application because of their rapid life cycle. Full season crops, like tomatoes or pumpkins, are much more likely to respond to applying a significant part of the N requirement just before they begin rapid growth.

How much do I lose with early application?
In Ontario, unlike parts of the western cornbelt or the Prairie provinces, we have never recommended fall application of nitrogen. This means that we don’t have a lot of data about the losses from early application, but we can look to other areas for some ideas about what we might expect in Ontario.

There was an interesting study in Illinois that applied ammonium sulphate every month from September to June, and then compared the corn yields to different rates of nitrogen applied pre-plant. In rough figures, it appeared that there was about 10 per cent nitrogen loss for every additional month in the soil, so someone applying nitrogen in November was losing about 50 per cent of what was applied before the crop could use it.

Doubling the nitrogen rate could make up for the yield loss, but it is an expensive solution and it does raise concerns about environmental impacts. The authors blamed most of the loss on denitrification. Losses during the winter may be slightly less in Ontario, because our soils will tend to be colder than Illinois, but we can still expect significant losses.

Does the same thing apply to manure?
The complication with manure is that part of the nitrogen is in the ammonium form, and part is organic, and the proportions of each vary widely between manure types. If liquid hog manure is applied in the fall, we can expect the amount of loss to be very similar to fall-applied fertilizer, because most of the nitrogen is in the ammonium form.

If solid cattle manure is applied, the picture is much different because most of the nitrogen is in the organic form. Fall-applied solid manure may have equal or higher available nitrogen compared to spring manure because more of the organic N has a chance to mineralize and become available to the crop. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs based in Stratford. keith.reid@ontario.ca