What is the N-supplying Power of Your Soil?

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Q: Why are some soils more fertile than others?

Dr. Khan:

Most soils of the Midwestern USA are very fertile because they are moderate to high in organic matter, developed on deep loess deposits and have good nutrient and water supplying potential. These soils need less nutrient inputs from fertilizers, as opposed to those soils that are lower in organic matter with a limited rooting depth, and thus lower in overall nutrient supplying capacity.

Q: How much nitrogen is required by a crop over the growing season?

It takes about 1.2 pounds of N to grow a bushel of corn grain, so 240 pounds of N would be required in producing 200 bushels per acre. In a typical growing season, the period of peak demand runs from mid-June through at least mid-July. Uptake decreases after tasseling, as the crop remobilizes N from the lower canopy to the developing grain.

Q: How do plants fulfill their nitrogen needs?

Dr. Khan:

Plants take up the bulk of their nitrogen from the soil’s own storehouse of organic nitrogen – not from N-fertilizer! Soil organic nitrogen becomes plant-available through a microbial process called mineralization.

Numerous field trials have shown that Midwestern soils can supply up to 70% of the nitrogen needed by corn, and that some can supply even more. Nitrogen fertilizers are needed only to supplement what the soil can supply, when nitrogen demand is high at certain times in the life cycle of the plant.

It is important to understand that soils differ in N-supplying power, because this is crucial to optimizing fertilizer N rates. Good soils have greater N-supplying power, and they need less fertilizer N to grow high yields. Poor soils have lower N-supplying power, and they need more fertilizer N to fulfil plant demands.

Q: How much of the N-fertilizer applied ends up in the plant, and what happens to the rest?

Dr. Khan:

We know from numerous isotope studies that as soil fertility increases, the plant takes up less fertilizer nitrogen and “nitrogen use efficiency” decreases. Most Midwestern soils are very fertile, and nitrogen use efficiency typically ranges between 15 and 50%. [Note: This range in NUE is for whole crop N uptake. For grain only, NUE would typically range from 10 to 40%.] In other words, the majority of the nitrogen applied does not end up in the crop. Some of the unused N is taken up by soil microbes, but a substantial proportion is subject to loss, either by nitrate leaching that causes water pollution or by denitrification that releases gaseous forms of nitrogen to the air.

Knowing your soil’s N-supplying power before applying nitrogen is a key step toward making better use of your fertilizer investment to boost profits, while reducing the environmental consequences of excessive fertilization.

Q: How is FertiSaver-N used to measure the N-supplying power of the soil?

Dr. Khan:

The FertiSaver-N system measures the soil’s content of mineralizable organic nitrogen, an abundant fraction in most Midwestern soils that reduces the amount of fertilizer N needed for corn production. By taking soil organic nitrogen into account, variable-rate N management becomes a reality for reducing N fertilizer rates without sacrificing yield.