On a garden scale, fertilisers are generally apply to the soil before sowing or planting and again during crop development at intervals of 14-21days.
The types of fertiliser used are usually combinations of nitrogen, phosphorus and potash (NPK) mixtures in various ratios e.g. 1:2:1, 1:2:2 or 2:2:1, depending on the type of soil and the requirements of the plant.
The rates used will also vary, from 50 to 85 g per m3. The fertiliser is mixed thoroughly with the top soil when used as a pre-sowing or planting application, when applied as a regular topdressing. It should be stirred
well into the soil but should not come into contact with the roots. The placement of fertiliser in a band alongside the plants in the row or in a circle around larger crops is often recommended for larger holdings or farms, the fertiliser, also often in a compound or mixed form, is applied to the soil surface before planting It may be left on the surface or hoed into the topsoil.
Harrows are used for this purpose where machinery is available. For most posts seeding operations, the fertiliser is placed in bands or rows along the rows of seeds or seedlings at a distance of 5—7 cm from the crop or seed row and preferably at a depth of 2—4 cm. Fertiliser drills or combine drills (see Chapter l) will perform these operations adequately.
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Subsequent dressings, or side dressings, given to the growing crop are normally placed in bands between the rows of the crop and are not mixed with the soil. Fruit trees are treated by placing bands of fertiliser at a distance of at least 80 cm from the base of the tree, over the actively throwing root zone.
The rate of fertiliser application required will vary with the type of crop, the stage of growth, the soil fertility and the climate and local experience is required to determine the amounts of fertiliser required for any specific crop One recommendation for guinea corn grown in the Zaria area of northern Nigeria, however, is a dressing of 125 kg/ha of sulphate of ammonia but maize requires approximately 310 kg/ha of this fertiliser, in addition to from 2 500 to 12 550 kg/ha of farmyard manure,
Most crops grown in Nigeria show a response to nitrogen, particularly in the savanna areas, and soils which have been exhausted by extensive cropping respond well to nitrogenous fertilisers.
Only moderate responses to potassium, however, are found in the savanna areas, whereas more positive results have been obtained from soils in the forest areas. Pronounced responses to potassium have been obtained from coconuts growing in coastal areas of West Africa.
Crops treated with phosphates have given rather erratic responses, both in the savanna and forest areas and it appears that crop demand for phosphorus is limited to specific soils and is not a general requirement.
Other Organic Manures & Liquid Fertilisers
In tropical regions, the waste products of some industrial operations are rarely available for use on the land, although, in some areas, sawdust, wood ashes,
groundnut and coffee hulls, fish residues, bonemeal, dried blood and town refuse are available.
The nutrient content of all these materials can be extremely variable, depending on their origin and subsequent treatment, and with the exception of dried blood and
bonemeal, they should be well decomposed before use.
They are then probably best applied as surface mulches which will gradually become mixed in with the surface soil.
Dried blood is a rich source of immediately available nitrogen e.g. up to 10% and bonemeal is a good source of phosphate and calcium. It may be applied as a surface dressing or mixed with potting composts, for nursery use. The main advantage of all these organic materials is that they decompose slowly in the soil, improve soil structure and aeration and release any nutrients they contain over relatively long periods.
Inorganic fertilisers
Unlike organic manures, chemical fertilisers are of known composition and can therefore be selected to avoid or treat any soil condition which involves a shortage of any particular element. Fertilisers are much more concentrated than organic manures but most of their constituents are generally more soluble and are therefore of less lasting value. The main exceptions to this are the phosphoric fertilisers, many of which are broken down in the soil fairly slowlyproduced
Liquid fertilisers and slurry
These materials are becoming more widely produced as intensive livestock holdings increase. The disposal of the urine and semi-liquid residues, usually termed slurry, which are produced from cattle and pig units can become a problem, unless pumps, piping and nozzles which can be used to spread the material on the soil surface are available. Several systems for slurry disposal have been devised, and although their use in West Africa is not widespread, they are likely to increase in the future.
On a garden scale, the application of liquid fertiliser to growing crops provides a readily available source of nutrients, the liquid should be well diluted before use, otherwise leaf scorching can result. Liquid manure can be either applied shortly after transplanting or to young seedlings, or as an occasional application during the growing period.