Agriculture worldwide faces a variety of challenges. In some regions, production must be expanded to provide food for growing populations, in others there are surpluses and agricultural production has to be matched to effective demand. Environmental issues are increasingly constraining agricultural systems.

In Asia, in the year 2030 there will be 0.06 ha available arable land per caput, compared with 0.14 ha in 1990. Yields will have to increase correspondingly. If the requirements of the growing population cannot be met by an increase of soil productivity, the poorest people will be obliged to cultivate marginal land, entailing an extension of land degradation and soil erosion.

It was therefore considered useful to prepare these guidelines on agricultural practices which can contribute to the development of agricultural systems that are both environmentally and economically sustainable. Ecological measures can be successfully implemented only under economically favourable conditions.

Sources of plant nutrients other than mineral fertilizers occupy an important place but are rarely enough to satisfy crops' needs in the region. The guidelines deal with mineral fertilization in the context of these other sources of plant nutrients and of sound agricultural practices. They cover the underlying principles, which have to be developed and adapted to the circumstances of particular countries, regions and farms.

OBJECTIVES OF THE GUIDELINES

1. To explain how mineral fertilizers satisfy plant nutrient requirements by complementing the nutrient availability of soils, restoring and enhancing soil fertility, and how losses throughout the nutrient cycles can be limited.
2. To promote economic crop production through the integration of sustainable agricultural practices and environmental protection.
3. To create public awareness and to provide planners and policy-makers with a sound understanding of the role of fertilizer in sustainable crop production.

CROP NUTRITION AND PLANT NUTRIENT LOSS FROM SOILS

Crops require adequate amounts of plant nutrients to produce optimum yields. Since many soils in the region are not capable of supplying sufficient amounts of all nutrients, the application of mineral fertilizers and organic manures is required to ensure adequate crop production. A balanced supply of nutrients for the crop, through proper rates and timing of fertilizer application, is important in order to minimize losses. Use should be made of all indigenous materials which can increase fertilizer use efficiency.

All crop systems result in some loss of nutrient, whether the site is fertilized or not. The loss may be to the air, to water, or fixed in the soil. Some nutrient losses, resulting from denitrification, volatilization and naturally-occurring leaching, are unavoidable. The quantity of nutrients lost depends not just on the nutrients applied but on the whole agricultural system, in particular the climate, soil type and cropping. Particular care should be taken to minimize soil erosion and runoff by means of appropriate soil conservation measures.

Sustainable production can be achieved only if soil nutrients are maintained at satisfactory levels, supplying additional nutrients, applied at the correct rate and time, if any are lacking.

Organic fertilizers, such as animal manure, crop residues, city wastes etc., have been the traditional means of maintaining soil fertility. They can be composted and enriched to improve their fertilization potential. Residues from agricultural product processing plants should be recycled. There is positive interaction between the integrated use of mineral fertilizer and organic materials. However, the supplies of organic materials are limited and unable to meet the fast growing nutrient demand in the region. The application of the bulky organic fertilizer involves high labour and transport costs. Furthermore, the release of nutrients from organic material is unpredictable and difficult to control.

Mineral fertilizers, on the other hand, can be formulated to provide a balanced supply of the nutrients, readily available to the plants at a time and in quantities best suited to their requirements. They are relatively easy to transport and can be stored without risk to the environment.

On marginal and degraded land, the combination of legume cover crops and mineral fertilizer application provides an adequate technology for rehabilitation and erosion control.

THE QUANTITIES OF PLANT NUTRIENTS TO BE APPLIED

All the plant nutrient sources, and their contribution to the crop requirements, should be identified in order to obtain the best possible nutrient balance. The amount of plant nutrients applied in fertilizers must be assessed for individual fields to ensure that it is matched to the uptake of nutrients by the plant, allows vigorous crop growth and maintains satisfactory levels in the soil.

In order to estimate the crop's requirements, its yield needs to be forecasted. Records of crop performance allow better yield forecasts to be made. The best available local analytical and estimation techniques should be used for assessing the optimum amount of fertilizer to apply.

It is important that the economic optimum for the individual field, based on a realistic assessment of the anticipated yield, should not be exceeded. Some losses occur whether the site is fertilized or not. But, up to the economic rate, the additional nutrient loss, compared with zero fertilizer application, can be negligible. Additional quantities should not be applied as an insurance.

TIMING AND METHOD OF APPLICATION

The timing and method of a fertilizer application will be determined by the farming system, and by the need to avoid nutrient losses to the environment and to maximize crop nutrient uptake.

The amount and timing of nutrient uptake by the crop depends on the variety, its planting date and the crop rotation, as well as on external factors such as soil and weather conditions. Nitrogenous fertilizers should be applied only at times when the crops can use the nitrogen or when there is a very low risk of loss due to leaching. All yield-promoting measures, such as proper tillage practices and crop protection, enhance nutrient uptake, thus improving fertilizer efficiency and reducing the risk of nutrient losses.

When applying fertilizers, unsuitable weather and soil conditions must be avoided as far as possible. Extreme caution should be exercised when applying fertilizers close to natural watercourses.

The fertilizer application equipment must give a uniform spread at the correct rate. It must be carefully chosen, properly adjusted and well maintained.

Under certain circumstances it may be necessary to improve the soil, for example by correcting excessive alkalinity or acidity by means of soil conditioners, before starting to apply fertilizers.

TYPE OF FERTILIZER TO BE APPLIED

The type of fertilizer used should always take account of its environmental impact and its efficiency for economic crop production. Slow-release fertilizers may be appropriate for certain crop and site conditions. Both chemical form and physical characteristics must be matched to local circumstances. Attention should be paid to the physical quality of the fertilizer materials and this quality should be safeguarded, and losses minimized, by good management of transport and storage during the distribution process.

These guidelines are based on deliberations at the FADINAP Regional Seminar on 'Fertilization and the Environment' held in Chiangmai, Thailand, in September 1992. The seminar was organized under the FINNIDA funded project 'Environmentally Friendly Fertilization through Balanced Fertilizer Use'.

Further information :

FADINAP
(Fertilizer Advisory, Development and Information Network for Asia and the Pacific)
ESCAP Rural & Urban Development Division
United Nations Building
Rajdamnern Avenue - Bangkok 10200 - Thailand
Tel: (66) (2) 2803618 - Fax: (66) (2) 2881056
Web: http://www.fadinap.org