How do we optimize the release of N from the soil?

Thursday, December 5, 2013

Making better use of this free nitrogen can reduce your fertilizer costs per bushel of yield

by KEITH REID

For many of us, nitrogen (N) management for crops begins and ends with the application of fertilizer or manure. This ignores the N reserves in the soil, which constitute a significant part of the total N requirements for every crop we grow (see Table 1). Making a better use of this "free" nitrogen could reduce our fertilizer costs per bushel of yield. Most of the nitrogen in the soil is in the organic form. Typical soils in Ontario contain anywhere from three to six tonnes of organic N in each hectare of topsoil (about 10 per cent of the amount of organic carbon). This represents a huge reservoir of N, but not all of it could (or should) be available to crops.

Plants cannot use organic N directly. It must be broken down into mineral forms (ammonium or nitrate) before the plants can absorb it through the roots. The release of available N results in decreasing levels of organic matter in the soil. For example, when virgin land is cultivated for the first time, the oxygen mixes with soil and encourages organic matter breakdown, which then releases N.

If our only goal was to extract nitrogen from the soil, we could keep farming to maximize organic matter breakdown, but there are two big drawbacks to that approach. The first is that the organic matter that remains in the soil will get progressively harder to break down, which will result in declining N "yield." The second and more important drawback is that such an approach would negatively affect the soil's structural stability, water-holding capacity, drainage, nutrient-holding capacity and, ultimately, the productivity of the soil.

How do we optimize the release of N from the soil organic matter while maintaining adequate levels of humus (organic matter) in the soil?

The key is to treat organic matter both as a stable matter, which stays in the soil for decades or centuries, and an active matter that turns over quickly. To use a financial analogy, the stable fraction is like a long-term Guaranteed Investment Certificate (GIC), while the active fraction is like a chequing account used for everyday deposits and withdrawals.

Active organic matter is made up of the crop residues and decaying roots, plus added organic materials like manure, along with any dead bacteria and fungi. As the fungi and bacteria in the soil use the carbon compounds in the organic matter for breathing, nitrogen is released back into the soil in mineral forms, which can be absorbed by the plants. This continuous process supplies crops with N directly from the soil, without adding fertilizer. This also prevents the accumulation of a large pool of mineral N in the soil that could be lost to the environment.

Of course, this process is not perfect. Some losses through leaching or volatilization are inevitable, and each year a small amount of the active organic matter is converted into stable organic matter. To keep the N cycling working well, we need to be adding more organic N each year than we anticipate losing. This means including crops or cover crops in the rotation that produce an excess of organic matter (like forages). It also means adding manure and other organic materials, and avoiding losses through erosion by wind or water. This becomes even more critical as we consider removing crop residues for biomaterials.

The decline in N cycling through the active organic matter will not happen quickly, so the impacts may not show up for many years. It is easier, however, to work to prevent this decline than to restore the cycles once they have been damaged. BF

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