A significant amount of nitrogen applied to fields is never taken up by plants.
Understanding how much nitrogen (N) crops actually need can help decrease needless expenses and potential environmental issues.
Why it matters: Nitrogen loss to the environment can damage local water sources causing human health concerns and harm to aquatic life. Inefficient fertilizer applications are also a wasteful, unnecessary expense.
Dr. Kate Congreves with the University of Saskatchewan explained at the recent Ontario Fruit and Vegetable Convention that about 47 units of nitrogen — of the total 94 units applied to the field — is lost to the environment.
“You have 100 units of nitrogen produced at the manufacturing facility, by the time you get to the field you may have lost some, but not much — once you apply that nitrogen to the field the amount that actually gets in the crop drops from 94 units [applied] all the way to 47 units being taken up by the plant,” says Congreves.
There are many opportunities from the time of application to harvest for N to be lost and growers can work to lower this.
The application of fertilizer helps to increase yields until the crop reaches a plateau. Then no matter how much fertilizer the farmer applies, it has no effect on yield. Applying fertilizer on fields below the plateau helps to ensure economic return.
As well, the more nitrogen applied above the amount required by the crops, the more risk that it will be lost to the environment.
Farmers who better understand their soil types and their nutrient requirements, will achieve a higher agronomic, economic and environmental return for the fertilizer investment.
Congreves and her team at the University of Saskatchewan completed a replicated trial of broccoli, carrots and sweet corn over a three-year period. With five different rates of N fertilizer applied — from zero N to a high rate of nitrogen — they found no difference in yield response, regardless of the rate of nitrogen fertilizer.
In situations such as this, with no nitrogen yield response, attention should be paid to soil tests because they tell the story about whether any nitrogen fertilizer is necessary in the first place, says Congreves.
Nitrogen use efficiency measures how efficient the crop is at using all the nutrients that are present.
Congreves found that in her broccoli, carrots and sweet corn trials, there was a decrease in the crop-use efficiency as more nitrogen was applied, in all three vegetable crops.
“If you erred on the side of caution and had not applied a lot of nitrogen fertilizer, that’s the case where you have the crops with the most efficient use of nitrogen,” says Congreves.
Her study showed that as farmers apply more nitrogen fertilizer, they risk reaching a point where it is excessive and that decreases the crop’s nitrogen-use efficiency.
There is also a greater chance for the nitrogen to be lost to the environment.
On the other end of the spectrum, if a farmer applies less nitrogen than what is required, there is a risk of decreasing yield compared to fields in which more nitrogen was applied, but a higher nitrogen-use efficiency.
It comes down to farmers properly managing their nutrients, which is often through the 4R nutrient stewardship program — right rate, right time, right place, right source — and which involves accurate soil sampling.