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HOME GARDEN TECHNOLOGY LEAFLET 5

SOIL IMPROVEMENT

Soil must have all the necessary nutrients and adequate moisture to support good plant growth and development. The structure of the soil must allow air and water to penetrate, so root systems can develop. Without good root systems, plants cannot effectively take plant nutrients from the soil. The soil structure must also allow excess water to drain away.

THE LIVING SOIL SYSTEM

Most plant nutrients are naturally recycled, going from the soil into the roots and then back into the soil through fallen leaves, dead plants and crop residue. Worms, insects and other tiny organisms feed on the fallen leaves, dead plants and crop residue and change them into humus, which makes the topsoil dark and gives it a good structure.

FIGURE 1
Topsoil is the best part

Different types of soil

Some soils, such as river plain and volcanic soils, are naturally fertile. Others are naturally infertile. Some soils lose their plant nutrients (humus) through clearing and the exposure of the soil to rain or wind, continuous cropping without restoring plant food or fertilizer, and the regular burning of fields. Subsoil, which contains little humus, is not fertile. Some common features of soils are shown in Table 1.

TABLE 1
Common soil types and treatments

Soil type	Features	Methods of improvement
Sand	Poor structure Poor fertility Cannot hold water	Add organic matter and fertilizer regularly Use green manure crops Add anthill material Practise minimum tillage
Silt	Poor structure	Add coarse organic matter
Clay	Dries hard Holds too much water	Add organic matter, compost and gypsum^*
Acid subsoil	Subsoil layer is toxic to some plants	Grow shallow-rooted plants (vegetables) Apply ground limestone (based on soil analysis results) and manure
Loam	Mixture of sand, silt and clay	Maintain soil fertility with periodic application of fertilizer and compost

* Mineral consisting of hydrated calcium sulphate that occurs in sedimentary rocks and clay and is used in the making of plaster, cement, etc.

To grow good crops, a farmer must improve soil fertility and structure. Cultivation activities such as hoeing improve soil structure and reduce water loss by evaporation. Rotating crops with deep and shallow roots, or crops with high and low plant nutrient requirements helps maintain soil fertility and structure.

PLANT NUTRITION

Crops grow well only if the soil contains enough nutrients, water and air to promote growth. Table 2 explains how to recognize when plants lack nutrients and what actions to take.

HOW TO MANAGE SOIL FERTILITY

If properly managed, poor soil can become productive. Manure and compost can improve soil structure, while chemical fertilizer is needed for higher production. Home garden managers should apply these plant nutrients to the soil just before planting, with regular additional applications as the crops grow. The basic method of application is to mix the compost, organic matter, manure or chemical fertilizer into the soil just prior to planting, and then apply smaller amounts alongside the plants about every two weeks until harvest.

TABLE 2
Signs of insufficient plant nutrients, and the actions to take

Nutrient	Function	Signs	Actions to take
Nitrogen (N)	Growth in leaves and stems Provides green colour and pest/disease resistance	Pale green or yellow leaves Poor growth Leaf fall	Pest problem Apply compost, green manure, animal manure, chemical fertilizer
Phosphorus (P)	Bean, seed and fruit (early) maturity Root formation Drought resistance	Stunted growth Poor formation of side shoots and flowers	Disease Apply chicken manure, wood ash, ground animal bones, chemical fertilizer^_
Potassium (K)	Strong roots and stems Fat seeds and fruit Helps move nutrients the plant	Curled and wrinkled or burnt leaves Uneven ripening, particularly of fruits	Poor growth Apply wood ash, manure, compost, ground maize cobs, banana leaves and stems, chemical fertilizer^_

* Local nitrogen-rich fertilizer.

^_ Local phosphorus-rich fertilizer.

^_ Local potassium-rich fertilizer.

Fertilizer

The quickest way to put plant nutrients into the soil is to use chemical or mineral fertilizers containing one or more of the three chemical nutrients needed by plants (see Table 2). Since fertilizers can wash away, they should not be applied too early before the planting. They are expensive, but extremely concentrated, so only one handful for every 4 m² is needed. Never heap fertilizer too close to a plant. It may burn the plant's roots or stem. Instead, spread the fertilizer out and lightly mix it into the soil's surface. Improper use of chemicals can also cause harm to humans. For information on safe application techniques, see Home Garden Technology Leaflet 10, "Safe and effective crop protection".

Compost

Compost is easy to make and inexpensive. All that is needed is time, space in the garden, and access to materials such as animal and kitchen waste, leaves and grasses.

Home garden managers who are unsure about what to apply should consult the local agricultural extension worker. Information on how to make compost, green manure and mulch is provided in Home Garden Technology Leaflet 6, "Special techniques for improving soil and water management".