Nitrogen fertilisation: when to count on soil organic matter

Last week, soil scientist Neil Miles discussed the role animals play in the nitrogen fertilisation of pasture. This week, he tells farmers how to get the most out of soil organic matter.


Pastures obtain nitrogen from three sources:
fertilisers, animal excreta and the soil organic matter. For proper nitrogen fertilisation scheduling, farmers must allow for nitrogen to be released from soil organic matter.

Matching nitrogen applications with grass growth
Figure 1:  shows typical daily growth data for perennial ryegrass in the high-lying, colder areas of the country, showing that it varies from approximately 20kg/ha/day of dry matter in mid-winter to over 70kg/ha/day in early summer. Because of this range in growth rates, intervals between grazings tend to be long (typically 60 days) in the very cold period and short (18 to 21 days) during the early summer growth flush. Fertilisation schedules must match these wide variations.

Figure 1: Typical monthly growth rates of perennial ryegrass in higher-lying areas.

Experience indicates that farmers tend to apply little or no nitrogen during the mid-winter period, and not enough for the pasture to realise its full growth potential during the spring/early summer flush. The tendency to apply little or no nitrogen in June-July is understandable, when growth is slow and a long time between grazings is experienced. However, there’s essentially no release of nitrogen from the soil organic matter during mid-winter, and the pasture is critically dependent on nitrogen fertiliser for growth and survival during that time.

The amount required in winter is low, but intervals between applications shouldn’t exceed 30 days. In severely nitrogen-deficient pastures growth stops completely, and yields in spring are depressed since pastures are slow to grow, as conditions become favourable. Signs of nitrogen deficiency in ryegrass include a reddish-brown leaf colour (often mistaken for potassium deficiency) and a stark contrast in colour and growth between urine patches and the rest of the pasture.
In spring/early summer, pasture growth rate is at a maximum, intervals between grazings are short, and soil organic matter releases minimal nitrogen because of low soil temperatures. This creates a large demand for nitrogen fertiliser.

Figure 2: shows a typical nitrogen fertilisation schedule incorporating the above information.

Tapping nitrogen from soil organic matter
Soil organic matter levels build up under permanent pastures. Nitrogen is present in the soil organic matter at a concentration of about 5% of the dry weight. In fertile, productive soils, the ratio of carbon to nitrogen (C:N) in the organic matter is remarkably constant, and usually falls in the range 9:1 to 13:1. This implies that as organic matter builds up under the pasture, so do considerable amounts of nitrogen. In fact, pasture topsoils may hold between 5 000kg/ha and 15 000kg/ha of nitrogen – over 95% of which is in the organic matter – which is available for plant growth.

Figure 3: Predicted total and monthly nitrogen (N) release from the soil organic matter in two soils differing in organic carbon (C) contents.

Figure 4: Possible nitrogen fertilisation application rates and schedules for perennial ryegrass pastures with high and low soil organic matter contents.

Figure 5: Variation in kikuyu’s response to nitrogen fertiliser in summer months.

But researchers have found that, annually, between 3% and 6% of this nitrogen is released for uptake by plant roots. Since this release is facilitated by soil microorganisms, it’s maximised when soil temperatures favour microbial activity. Thus the nitrogen supply from the soil organic matter peaks in the second half of summer and is minimal in winter and early summer when soil temperatures are low (Figure 3).

The relatively low fertiliser nitrogen rates recommended for January to March (Figure 2) reflect these contributions from the organic matter. On long-term, well-fertilised pastures in which soil organic matter levels have increased, farmers can save considerably by withholding or reducing nitrogen applications when the pasture can rely on releases from the organic matter (Figure 4). In kikuyu pastures, responses to nitrogen fertiliser are marked in the first half of summer, while less is required between January and March (Figure 5).

Contact Neil Miles on 084 577 7087