Mineral Fertilisation of Pastures

Environmental Issues Land subjected to constant grazing, repeated fire or other unsustainable forage exploitation slowly becomes degraded because it loses its fertility if restitution of soil nutrients, either in the form of faeces, organic fertiliser or inorganic fertilisers, or a combination of all three, is insufficient. | Environmental Issues Objectives Techniques Costs Targeted Livestock System Impact Context of Application Monitoring: EIA, Indicators References

Objectives

Mineral fertilisation has two objectives:

- to correct as far as possible problems of soil fertility. Deficiency of certain elements or low availability for plants conditions the level of fertility and therefore the potential of plant production.
- feeding the plants, which for a pasture means stimulating growth of forage plants and thus increasing their productivity and quality.

Wide expanses of natural pastures in hot countries are on soils which are moderately fertile and often deficient in phosphorus. Numerous experiments carried out in Australia, tropical Africa, and India have shown the considerable benefit obtained merely by adding phosphorus to the soil.

The use of nitrogen also increases grass production noticeably.

In semi-arid areas, with a rainfall of between 300 and 400 mm/p.a., the effect of fertilisation is minimal, since productivity is limited mainly by water availability. When plants are given enough water, nitrogen often appears to be a factor limiting biomass production. To produce high biomass of good nutritive quality with annual forage grains (maize, sorghum), heavy compound fertilisation (N, P, D, Ca, etc.) is recommended.

The addition of minerals (calcium, for example, or gypsum in certain instances) is sometimes used to compensate serious deficiency (acid soils or those leached of calcium) or to attenuate an excess (saline soils, those with too much magnesium, etc.).

Fertilisation is also a method of rehabilitating degraded rangeland.

Techniques

On rangelands, spreading fertiliser is an investment which can be justified only if one is looking for results over many years. This is then fertilisation in depth, to correct the major deficiency of a nutrient in the soil. Fertilisation with phosphorus or the addition of calcium fall within this category.

On natural pastures or sown meadows, in addition to in-depth fertilisation whose effects take many years, the addition of fertiliser elements is a very important means of managing meadow sustainability. Fertilisation is especially recommended when establishing improved pastures in order to place the sown plants in the best possible conditions.

On meadows cut for the production of forage stocks, compound fertilisation is indispensable to compensate for loss of fertilising elements by forage removal. It is preferable to reserve the use of compound or nitrogen fertilisers for deep soils, without surface run-off, and on forage grains with high productivity, due to their cost.

On intensive meadows and forage crops, fertilisation stimulates forage production and enables high production to be obtained. Excess fertilisation can lead to an excessive amount in the soil and water leaching out the nutrients.

Spreading of fertilisers is done mechanically behind a tractor, sometimes by air.

Costs

Costs of fertilisation vary greatly from one country to another, depending on price of fertilisers, cost of machines and labour costs. Calculation of costs of fertilisation includes inputs (fertilisers, energy), cost of spreading, depreciation of equipment and labour costs.

The amount of fertiliser should be calculated according to number of years planned to benefit from this input, which can then be compared to the benefit expected.

Targeted Livestock Systems

This refers primarily to mixed systems, and also pastoral systems, especially ranching and farming.

Grazing System

LGA | LGS1 | LGS2 | LGS3 | LGH1 | LGH2 | LGH3 | LGT1 | LGT2 | LGT3
Y | Y | Y | Y | Y | Y | Y | Y | Y | Y

Mixed System

MCG | MCR | MCC | MFF | MEF
Y | Y | Y | Y | N

Industrial System

IFP | IPL | IPG | IRM | IDU | ISL | ITN | IMP
N | N | N | N | N | N | N | N

Impact

- Positive environmental impact

- Improvement of soil fertility.
- Improvement of flora diversity.
- Improvement of plant cover.

- Negative environmental impact

- Risk of pollution of groundwater.
- Risk of toxicity from overdoses.
- Development of certain weeds.

- Impact of productivity on Livestock (milk, reproduction, meat, health, etc.)
Due to the greater quantity of forage obtained and improved chemical composition of this forage, the impact is improvement of animal production resulting from their grazing.

Context of Aplication

- Favourable factors:

- Land tenure competition encouraging intensification,
- Drop in rangeland productivity,
- Poor nutritive value of many plant species,

- Unfavourable factors:

- Communal status of certain pastures.
- Costs of inputs and their implementation not sufficiently compensated for by improvement of production, or lack of financial resources to make this investment.
- Insufficient availability of inputs or equipment.
- Poor retention of organic matter by some soils (poor use of fertilisers).

Monitoring: EIA, Indicators

- Quantity of fertilisers applied, for a given region or per unit area of forage.
- Retention of minerals by forage plants, mainly nitrogen and phosphorus.
- Changes in pasture flora.
- Quantity of biomass produced.

References

Breman H., Sissoko K., 1998. L’intensification agricole au Sahel. Paris, Karthala, pp. 996.

Robert M., 1996. Le sol : interface dans l’environnement, ressource pour le développement. Paris, Masson, pp. 244.

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