Soil Conservation

There is general agreement that there are three basic principles that are used in conservation agriculture:

• Minimal soil disturbance.
• Maintain a protective cover on the soil.
• Practice crop rotations.

Following and understanding these principles are the basis for developing successful conservation agriculture systems, for both large-scale commercial farming operations and subsistence level systems common in many parts of Africa.

Minimum Soil Disturbance

Rather than disturbing all the soil in a field – typically down to a depth of 15cm (6 inches) as little soil as possible is disturbed. This is typically achieved by directly placing a seed into a small slot formed in the soil with a no-till planter for mechanized operations, or jab planter or even a stick for small-holder farmers.

In modern-day agriculture tillage operations (ploughing, disking, harrowing etc.) require a considerable amount of energy, different pieces of equipment and can have considerable negative effects on the soil structure.

Maintain Soil Cover

Soil erosion is the result of several natural and manmade factors. For the most part, things like rainfall (amount and intensity), the erodibility of the soil and the slop cannot be modified by farmers. Bare soil will erode much more than soil with crops or other plants growing on it. After crops have been harvested it is the non-harvested residue can be laid on the surface of the soil to protect it during the time when there are no crops in the field.

The one thing that farmers can do to reduce erosion is to protect it from the impact of falling raindrops. This can be done by maintaining an adequate soil cover, either with vegetation or residues from the previous crop.

Click here to view a video that explains soil erosion types and causes.

Practices that remove residue after harvest (such as burning), should be avoided or kept to a minimum.

Cost Saving Structures

The following cost-saving structures are all used as small structures in catchment areas of less than 5 km2. The structures are established at less productive times when labour employed for extensive farming can be utilized better. The structure should not be built on very steep slopes. However, the soil type will determine what constitutes a safe slope. The structures should be regarded as part of a pasture rehabilitation program aimed at long-term benefits. The purpose is to conserve moisture and to establish a seedbed.

The structures should be established in a strategic spot. This is usually where a few small gullies converge, where the slope is slighter than in the rest of the donga, or where a reef has been exposed clearly in the right position with regard to the slope of the donga.

If the flow of the water is diverted by the small structure, the water must be left on a safe, hard stratum, or in the original bed where there is no chance of erosion.

Gutter Structures

The gutter structure is used to move water from a more elevated part of the donga bed to a lower-lying terrace. The structure can be built from concrete, bricks or stone masonry.

An upstream cut-off wall of about half a meter below the floor, and a downstream cut-off wall extending all the way to the reef, is very important. The floor against a slope must be cast on a firm base. The water must be returned to the existing donga. The structures can be erected without great expense by using materials occurring in the vicinity, for example, sand and building/concrete stone.

Tyre Structures

Structures made from old car tyres must be attached very firmly to iron poles driven into the ground in each stack of tyres that are piled up. The tyres must be tied together, and they must be filled with stones. A layer of shade cloth must also be attached to the upstream surface of the tyres. This must extend from ground level to the top of the tyres. The tyres must be inserted into the sides of the dongas where they are used, in such a way as to prevent water from flowing between the tyres and the walls. Where excavations are made in the sides of the walls, the soil must be thoroughly compacted against the tyres after the piles had been stacked. The iron poles around which the piles are stacked must be anchored to one central pole upstream from the stacks. The upstream iron pole must be driven into the ground as deep as possible.

These structures are not recommended unless great care is taken to stack them properly. Fast flowing water can cause any number of problems.

Small Concrete Structures

Small concrete structures, brickwork or stone masonry can be used in small gullies where the reefs are clearly exposed. The spill-over part of these small structures is not built higher than half a meter. The structures can be built in any shape. The shape must be simple and practical. If structures are built higher than half a meter they have to be designed, and correctly spaced buttresses must be added on the downstream side.

These structures are built to conserve moisture and to establish a seedbed on a bare reef. The reef must be cleaned very thoroughly to enable the concrete to bind. All loose material must be removed. If the reef is brittle, it cannot be used. The structures must be well-recessed into the sides of the donga. If the recesses are dug neatly, the brick or stone-work, or the concrete can be put in place without shuttering. Shuttering can be used where necessary. It is very important that the structures should have a floor on the downstream side to prevent erosion of the stone and damage to the structure. The object of this work is not to form dams.

Once the work has been overgrown or silted over the idea is that it should blend in with the environment. To keep transport costs down as far as possible, the structures should preferably be built with materials found in the vicinity. No brittle stones should be used for building since this would weaken the structure. Use stones of 150-200 mm in diameter for stone-work.

Bare Spots (Brackish Spots)

Branches can be used to cover bare spots to prevent them from drying out so soon after rain. It is advisable to break up the spots with an implement to form holes in which water can collect.

Loosening

Loosening of the soil also breaks the upper crust and brings some of the subsoils to the surface. To this end, the spade plough or ripper can be used.

Stone Stacks (With or Without Wire Baskets)

Stone stacks can be used almost everywhere with great success. The stones must be stacked correctly to enable them to absorb the force of the water. Where steep slopes occur, it is preferable to stack stones in wire baskets. The stones must be stacked together tightly to prevent the water from readily washing away loose stones. It is very important that the direction of the flow of the water should not be altered. The water must keep flowing in the donga and must merely flow over the structure. The slides of the stone stack must be higher than the part over which the water is supposed to flow.

The stone stacks are used to establish plant growth which would increase the carrying capacity of the area. They retard the run-off rate of the water, which results in increased infiltration of the soil. In the long term, this can benefit the underground sources of water. The paddocks, in which the structures occur, must be continuously monitored to ensure that the vegetation is established around the structures is not damaged by grazing stock. If seeds are sown to accelerate rehabilitation, only seeds that belong in the immediate vicinity must be used. Avoid all strange or alien seeds, which could result in problems with alien vegetation.

In areas where more than 500 mm of rain occurs annually, rehabilitation work can be done with grass sods. The sods can be placed in strategic spots where water is available, to cover the ground and prevent further erosion.

Click here to view a video that explains gully rock sheets.

Crop Rotations

Click here to view a video that explains crop rotation.

Farmers have long known the value of crop rotations for keeping their soils productive and crops healthy.

Crop rotations include growing crops from different plant families, and leguminous and non-leguminous plants in successive growing seasons. This helps to reduce the buildup of soil pests and diseases that might accumulate between seasons when growing the same crop.

Here you do not plant the same crop in the same place year after year, but you make sure that a different crop is planted in each space. This is a good idea because:

• It prevents or stops the accumulation of insects and diseases. If the same crop is planted some insects and diseases will become more every year!
• Different crops use different nutrients or plant food stored in the soil. In this way, you do not overuse some of the plant food while not using others.
• The soil can be covered all year round.
• Some crop adds nutrients or nitrogen to the soil. Examples are beans, peas, broad beans, soya beans, peanuts, cowpeas, Lucerne and clover.
• It prevents the soil from building up bad or negative reactions to specific plants. An example here is nematodes on tomatoes and swiss chard. Nematodes are very small worms, that we cannot see with our eyes. They live in the soil and feed on the roots of your plants.
• There is no build-up of specific weeds.

In the first season after applying compost and or manure heavy feeders or nitrogen, consumers are planted. In the second season, the light feeders are planted and in the third season, legumes are planted. This is followed by another application of compost or manure and the cycle is repeated.

In trench beds, where the organic matter is decaying slowly in the soil, you may want to start with legumes, move on to heavy feeders or nitrogen consumers and then move on to light feeders. This is because during the decaying process plant nutrients will take a while to become available for use by plants. The legumes can fix most of their own nitrogen and are thus a better starting point.

Prepare the land or bed well. Put a lot of compost or manure in your bed (4 full spades/square meters). Then, start by planting a fruiting crop. Leaf crops are needless and can follow fruit crops. Then root crops can follow leaf crops without much addition of plant food. Root crops like fertile soil, but do not like fresh manure or compost. It has to be well rotted. Then, nitrogen fixers can follow, with the addition of little or no plant food. Then you need to prepare the land well again. Start once more with fruiting crops.

Preparing the land or bed: This would mean trenching or double digging or the addition of a lot of compost/manure forked into the top 30 cm. You will need at least 4 full spades for every square metre. A general recommendation is to place 30 tons of compost or manure on a hectare of land. This comes to about one half of a wheelbarrow load for every square metre. This is about the same as 4 full spades!

Advantages and Disadvantages of Crop Rotation

Advantages:

• No build-up of pests and diseases.
• Soil nutrients are used effectively.
• Soil moisture is used effectively.
• A healthy living soil can be built up over time.

Disadvantages:

• Without a plan, either drawn or written on paper, it is difficult to remember which crop is to follow.
• It can be tricky to decide which rotation to follow when inter-planting is also used.

Norfolk Four-Course Rotation

This section illustrates the Norfolk four-course rotation system, which allows sustainability that increases productivity from these farmlands.

Year 1 - Fields are sown to wheat, which is harvested and sold.

Year 2 - Root crops are grown on the fields, which are fed to animals. The animals are then sold and the animal manure is used to fertilise the fields.

Year 3 - Barley is sown as green manure.

Year 4 -  Ryegrass is sown, fixing valuable nitrogen into the soil and providing direct grazing for the animals.