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Soil Management in Vegetable Production

Mortimer Livan

The major vegetable producing areas in Guyana are the Corentyne Coast, Black Bush Polder, Lower East Coast Demerara, West Coast Demerara and the Parika Backdam. The soils in these areas range from sandy loam (sand reefs) to heavy clays and organic (pegasse soils). These soils have an inherent low fertility status, which is influenced by high acidity (soil pH is below 55.5), low water and nutrient retention characteristics (sand reefs), high water holding capacity with a tendency for flooding (clay soils). A fertile soil could be described as one that provides all the plant nutrients in the right proportion with adequate aeration and soil moisture. Soil management plays a very important role in vegetable production for it maintains/improves soil fertility and provides a good medium for plant growth. The soil not only acts as an anchor for plants and provides them with nutrients and water, but also as a haven for numerous pests. Soil management is the sum total of all the tillage operations, cropping practices, lime, fertilizer, and other treatments applied to the soil for the production of a crop. A large amount of pests of vegetables either live in the soil, spent part of their life cycle in the soil or remain dormant in the soil waiting for a suitable host plant for its rejuvenation. Hence, a number of soil management factors will impinge on pest management for the production of vegetables. Some of these factors are as follows:

Tillage

This is the operation, practice and science of tilling the land. The idea is to make the soil suitable for the production of a crop i.e. to change the soil’s physical properties so that it will have the best conditions for optimum crop growth. The tilled soil should be left to weather for at least two weeks. This will allow for the release of nutrients from the exposed soil minerals.

Tillage is necessary to:

	Create conditions favorable for germination and root growth.
	Manage crop residues.
	Control insects and diseases.
	Control weeds.

Mulching

Mulch is a layer of material such as dried grass, paper, plastic, or crop residue, which is placed over the surface of the soil. The cost of mulching material as well as the cost of application must be considered. There are several advantages and disadvantages of using mulch.

Advantages

	It covers the surface and thus reduces evaporation. Therefore, it aids in soil moisture conservation.
	It reduces the impact to raindrops on the soil surface and, therefore, reduces soil wash and erosion.
	It shades the soil surface from direct sunrays, therefore, it prevents excessive heating of the soil during the day.
	Entrapment of insects in the soil - where plastic mulch is used the insects are trapped beneath the mulch.
	The amount of weed growth is reduced since the mulch smothers weeds.
	There is the addition of organic matter to the soil that improves soil structure and enhances microbial activities.
	Nutrients are added to the soil due to decomposition of the mulch, e.g. coffee husks add K (potassium).

Disadvantages

	It can increase the pest population if the mulch harbours insects.
	It can increase the incidence of pests especially if the mulch is a crop that is an alternate host for the organism or if either infected or infested crop materials are used as mulch.
	The cost of labour and production increases.
	If not done properly, it can smother the crop.
	Waterlogging may occur if it is done in the rainy season.

Fallowing

This is the practice of leaving the land to rest for a lengthy period of time to restore fertility. There are two types:

Flood – Here the land is submerged with 6 –12 inches of water for a period not less than three months.

It has the following advantages:

	It kills the pest living in the soil.
	It increases soil nitrogen content.
	It improves soil structure hence, better root growth and development.

Its disadvantage is the availability of water to keep the land submerged.

Dry (bush) fallow – cover crops are grown or the land is allowed to revert to its natural vegetation.

Dry fallow has the following advantages:

	It breaks the life cycle of the pests, thereby reducing the pest population.
	It provides a cover on the land that reduces the effects of soil erosion.
	It increases the organic matter content of the soil when the leaves and other plant parts are returned to the soil or, if the plant is ploughed back into the soil as in green manuring.

Crop Rotation

Crop rotation is the growing of crops belonging to different families in succession e.g. cabbage followed by tomato followed by bora followed by ochro.

Advantages

	Controls pests by disturbing their life cycles. This is because the crops keep changing and the pest cannot infect/feed/live on all the crops. This will result in a reduction of the pest population due to a lack of food. Also, the different growth habits of the rotated crops will reduce the weed competition.
	Maintains soil fertility – the addition of a legume will increase the nitrogen level of the soil and improve soil structure.
	Prevents soil erosion – this is due to the different rooting system and growth habit of the crops.
	There is better utilization of land.
	There is better use of soil nutrients by the various crops – this is due to the different rooting system of the crops.
	There may be more ready availability of earnings and food all year round.
	It is satisfying to farmers to grow different crops all year round.

Disadvantages

For the home gardeners and small farmers there is little if any setbacks when using crop rotation. However, a large farmer may invest substantial money into specialized equipment that can be used for only one crop on the rotation. Subsequently, the farmer may be unwilling to switch to another crop for which he does not have adequate equipment.

Drainage

This is necessary to:

	Remove excess water from the root zone.
	Reduce the soil moisture contents to levels that will reduce the build up of pests.
	Provide enough aeration for the plant roots.

Liming

This reduces soil acidity. Crops growing on soils with high acidity tend to have poor root systems, poor growth and low yields. Liming will:

	Improve root growth.
	Improve nutrient availability.

Fertilizer Application

A fertilizer is any substance added to the soil or plant to supply one or more plant nutrients necessary for crop growth. To maintain adequate yields it is necessary to supply the depleted nutrients to the soil in quantities that will depress yields. Plants need about 17 nutrient elements to enable them to carry out their metabolic functions. The efficient use of fertilizers entails the use of the right kind of fertilizer material, using it at the appropriate time and the right amount and placing it in such a way that plants makes maximum use of the material. Failing to observe these criteria can result in the material being lost to the plant due to:

Volatilization and Nitrification – Nitrogen (N) fertilizers. This is the spontaneous breakdown of the fertilizer material and the Nitrogen returns to the air as gas.
Leaching – Nitrogen (N), Potassium (K), Calcium (Ca), Magnesium (Mg) and some micronutrients. Nutrients become dissolved in the soil water and drain down the soil profile.
Fixation - Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), and some micronutrients also and excess of calcium carbonate (CaCo) result in K fixation. This renders the nutrient unavailable to the plant.
Erosion – all materials. The fertilizer material is washed away in the runoff rain.
Antagonism – Between P: Zinc (Zn) &P: Copper (Cu). A nutrient that is in a higher concentration than the other in the soil solution will prevent the uptake of the other. Determination of the type and amount of a nutrient to be used could be best done (by NARI) using soil and foliar analyses. These will give the amounts of nutrients present on the soil and plant tissue. From this the limiting level of each nutrient would be identified and the amount necessary to bring it to an acceptable level, or, the amounts of each nutrient required to increase yields can be determined.

Overuse of fertilizers could result in more harm than good. Excessive use of Nitrogen will predispose the plant of disease (blast in rice) and pest attacks due to the softer cell walls present. In some species the use of nitrogen often induces/prolongs the vegetative phase of the crop and can subject the crop to pest attacks due to the prolonged maturation stave. Excess Nitrogen can also induce Zinc and Copper deficiency. These two nutrients act as catalysts/carriers in many biochemical reactions in the cell, hence their presence in insufficient quantities will result in a reduction of plant productivity. Phosphorus (P) excess will result in a Zinc and Copper deficiency similar to N. Deficiency in P will result in reduced root growth and a stunted plant, sugars and carbohydrates will accumulate in the leaves thereby predisposing the plants to attacks by pests that found the leaves ideal for feeding.

Potassium is an important nutrient on plant nutrition and disease management for if enhances the ability of the plant to withstand attacks by pests.

Calcium is essential for cell wall development, hence a deficiency of calcium will result on plants with cell walls that can be penetrated easily be pests.

Excess use of potassium, Mg Ca can upset the Ca: Mg or Ca:K or K:Mg ratio in the soil this will result on the nutrient in excess being taken up by the plant at the expense of the other. This will result in the plant showing deficiency symptoms for the nutrient that is less. These critical ratios also exist for the other nutrients and creating imbalances in the system will result in deficiency symptoms being exhibited by the crop.

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