Guideline Index

Chapter 12: Nitrogen and Nitrogen Fertilisers

12.6 Other issues to be managed when applying N fertiliser

In addition to environmental issues with use of N fertiliser discussed in Section 12.3, there are several other issues that need to be considered, including potential poisoning effects of nitrates/nitrites, soil acidification and maintenance of clover in mixed pastures.

12.6.1 Nitrate/nitrite poisoning in cattle

The potential for nitrate or nitrite poisoning in cattle occurs because the plants can absorb the applied N very quickly once it is in a form (ammonium or nitrate) that they can use. The ammonium and nitrate are then converted to proteins and other nitrogen-containing substances. When might nitrite/nitrate poisoning occur?

Nitrate accumulation in plants is increased in the following situations:

  • High rates of N fertilisers.
  • Grazing using a short rotation and not allowing grasses to grow between 2 new leaves (ryegrass and tall fescue) or 3 new leaves (brome (prairie grass), cocksfoot and kikuyu) to dilute N concentration in leaves prior to grazing (see Section
  • After rainfall following a drought. Soil nitrate accumulates during the drought period due to minimal leaching, reduced uptake by plants and plant decay.
  • Moisture stress.
  • Decreased light (overcast weather, short day-length).
  • Low temperatures.
  • Spraying with hormone-type herbicides, such as 2,4-D.
  • Presence of species known to be ‘high-nitrate’ accumulators (such as capeweed, annual or short-rotation ryegrasses, brassicas, oats etc.).

Nitrate/nitrite poisoning in livestock can occur when they eat plants that have been subject to the above situations.

In addition, high consumption of various weeds (capeweed, variegated thistle, pigweed), crops (maize, sorghum, millet, wheat, oats, barley, immature brassicas in high-fertility situations) and some pasture plants (lucerne, subterranean clover, annual or short-rotation ryegrasses) can also cause nitrate/nitrite poisoning. Symptoms of nitrate poisoning

Ruminants can tolerate high levels of nitrates if the intake is spread over the whole day and if the diet contains high levels of readily available carbohydrates. If these conditions are not achieved, nitrate poisoning may occur, causing irritation in the gut and resulting in scouring and loss of production.

Symptoms may appear as profuse scouring, sudden drop in milk production, rough coat, and occasionally shivering and staggers. If nitrate intake is too high or if conditions in the rumen cause a slow conversion of the nitrites to ammonia, then nitrites build up and may be absorbed into the blood. Nitrite levels can also be high on wet or mouldy hay due to microbial action. The nitrites reduce the ability of the blood to carry oxygen, and death may occur through oxygen starvation.

Symptoms may include animals breathing rapidly and acting in a stressed manner. Their pulse will be weak but rapid. The blood will be dark due to lack of oxygen, and the visible mucous membranes on the gums and inside the nose appear blue. How to avoid nitrate/nitrite poisoning

Plant nitrate levels may be high at an early stage of regrowth (before the 1-leaf stage in most grasses), and then levels decline as the plants grow and synthesise proteins.

Avoid grazing N fertilised pastures until the 2-leaf stage in ryegrass and tall fescue pastures, and the 3-leaf stage in brome (prairie grass), cocksfoot and kikuyu pastures to reduce the risk of nitrate poisoning.

This period also allows for a greater response to the applied N.

In crops such as brassicas which can accumulate nitrates, do not apply N or effluent closer than 6 weeks from the expected grazing date.

12.6.2 Soil acidification

Every nutrient that a plant absorbs has either a positive or a negative charge. When plants take up N (regardless of whether it is from fertiliser or legumes), then they need to exude another nutrient or ion with the same charge in order to balance their overall charge, otherwise they could become too positively or too negatively charged. Ammonia is a positively charged ion (NH4+) and so plants need to exude another positively charged ion to keep their charge balanced, when they take up ammonia. The positive charge they normally exude is a hydrogen ion (H+). The amount of hydrogen ions present in the soil affects the soils pH, with more hydrogen ions resulting in more acidic soil. Therefore, the application of ammonium based fertiliser and the uptake of ammonia by pasture plants acidifies soil.

All N fertilisers that contain ammonium (for example, sulphate of ammonia and DAP) or that produce ammonium (for example, urea) have an acidifying effect on the soil. Nitrogen fixation by legumes also has a minor acidifying effect on the soil. The various N fertilisers have different potentials for acidifying the soil. See Table 12.4.

Table 12.4 Approximate lime requirement to neutralise the potential acidifying effect of N fertiliser.
Table 12.4 Approximate lime requirement to neutralise the potential acidifying effect of N fertiliser.
Source: Roberts and Ledgard (1998) (New Zealand)

Note: In practice, soils have a natural pH buffering capacity, which means that, at recommended N rates (25 to 50 kg N/ha), the acidification effect will be slow and farmers should regularly monitor soil pH and apply lime when pH drops below the recommended pH range (see Chapter 7.6.9 for how to correct soil acidity).

If annual N application rates exceed 250 kg N/ha/year, more regular soil pH testing and liming is recommended to prevent soil acidification (Eckard, 2001).

12.6.3 Maintaining clover in mixed species pastures

Regular use of N fertiliser is associated with lower levels of clover in mixed species pastures. However, legumes such as clover aren’t ‘damaged’ by N fertiliser, as although they can capture their own N from the atmosphere with the help of bacteria in roots (see Section 12.1.1), they will readily use fertiliser N when it’s available. But regular applications of N fertiliser reduce the amount of N fixation by clovers, which removes their competitive advantage over grasses.

Nitrogen fertiliser also stimulates growth of grass more than clover, and this results in increased competition for light, water and other nutrients. Clover plants hate to be shaded, and it’s mainly this effect of N fertiliser in a mixed species pasture that reduces clover content.

Clover shading is the main cause of clover demise in mixed pastures.

Maintaining clover in mixed pasture swards is therefore a combination of grazing appropriately, and applying moderate amounts of N per year (100-150 kg N/ha). Appropriate grazing management includes grazing at the optimal leaf regrowth stage (2-3 leaves for ryegrass and tall fescue, 3-4 leaves for cocksfoot, prairie grass and phalaris, and 4-5 leaves for kikuyu and Rhodes grass) or when significant shading starts to occur (more than 25% of the pasture area is shaded to the point that the base of the pasture can’t be seen through the canopy when standing directly above), and grazing down to an average height of 4-5cm for all species except Rhodes grass which should be grazed to an average of 7-8cm.