Keep it in the root zone

Forms that get lost

Most of the nitrogen in the soil is bound up in organic matter, and in this form is held in the soil.

Nitrogen from organic matter slowly becomes plant available as the soil microbes break it down. The nitrogen changes from organic matter to ammonium NH4 (mineralisation), then to nitrite NO3 (nitrification) and then to nitrates NO2 (nitrification again) the form most readily taken up by plants. Nitrates can be converted by bacteria into gaseous forms including N2 and N2O, which can be lost to the atmosphere.

Figure 3. The Nitrogen Cycle (Image source: Pinterest topsoil-screener.com)

Nitrate is highly soluble, and moves quickly with water. So as a wetting front moves down through the soil profile as a result of rain or irrigation, it can easily take nitrate with it. Once the nitrate moves below the crop’s root zone, it cannot be accessed and is prone to leaching and moving into water bodies.

Urea fertilisers can also be lost through the process of volatilisation. This is the direct “evaporation” of urea to the atmosphere and losses can be greater in hot conditions.

Avoiding volatilisation

Nitrogen losses to atmosphere can be minimised by using a fertiliser that is less likely to volatilise such as calcium nitrate or a blended product. If using urea, volatilisation can be minimised by getting the urea into the soil rather than leaving it exposed on the surface. We can bury the fertiliser by incorporation at planting (drilling beside the seed) or cultivating it in, which avoids much of the potential loss. An alternative is washing urea into the soil with irrigation or by applying it just before light rain. 

Figure 4. Incorporating urea fertiliser into the soil at planting minimises risk of volatilisation losses

Avoiding leaching

Avoid leaching by minimising the amount of nitrate in the soil and avoiding excessive rain or irrigation. Ensure application equipment is correctly calibrated so the right amount gets applied.

While rain cannot be controlled, timing applications to avoid expected heavy rainfall events reduces fertiliser losses and helps protect the environment.

Applying nitrogen in a series of smaller applications as the crop develops also helps reduce the nitrate at risk in the soil. A small amount at planting, followed by side-dressing or top-dressing as roots extend and the plants develop provides the necessary nutrients at the time the plant is able to take them up.

Figure 5. Topdressing nitrogen fertilisers at canopy closure avoid high levels of nitrate in soil while plants are too small to take it up

For crops such as sweetcorn, the bulk of nitrogen can be applied at side-dressing before canopy closure. By this stage, the roots have grown down into the soil, and the plant is beginning to grow rapidly so will absorb the available nitrogen quickly.

Soaking up an excess

Keeping deep rooted crops in the crop rotation is an important method of retaining nitrogen in the soil where it can be used by crops. Deep, living roots capture nitrate that has drained down through the soil profile.

If shallow rooting, high nitrogen demanding plants are grown, there is a high chance some nitrogen remains in the soil after harvest. It will be sitting below the crop’s root zone but can be captured if a deeper rooting plant or “catch crop” is grown after the main crop. An example is growing maize or cereals after lettuce or baby spinach crops.

Figure 6. Shallow rooted crops cannot take up nitrate once it has passed below the root zone

If growing a catch crop, manage any fertiliser applications accordingly. Ensure the crop can use all the nitrogen available in the soil before considering any further additions. It is a catch crop! It is meant to recapture lost nutrients.

When the catch crop is harvested (grain or silage) the recaptured nitrogen is exported from the field. Winter planted cabbages with a marketable yield of 68 t/ha of heads, can remove 169 kg N/ha, so provide a way to effectively use any leftover nutrient from a preceding short rooted, high nitrogen demanding crop (Nutrient Management for Vegetable Crops in NZ, p 60).

If the crop is mulched or incorporated, the nitrogen is bound in the organic matter and safe until microbes release it over time. The rate of release depends on the level of biological activity, which in turn depends on how many microbes are present, the soil temperature and soil moisture.

A sweetcorn crop yielding 16 t/ha of ears removes about 174 kg N/ha, 112 kg/ha in the stems and leaves and 62 kg/ha in the ears (Nutrient Management for Vegetable Crops in NZ, p 118). Much of that could be provided by the leftovers following a crop. The 112 kg N/ha in the stalks will become available over time for following crops.

These examples show the importance of soil testing to know firstly, how much immediately available nitrate is present in the root zone and secondly, for measuring potentially available nitrogen to see what extra is expected to become available from soil organic matter as the crop grows.