Pros and Cons of cultivating soil

Last week, we learnt that many livestock farmers in Botswana do not practice environmental friendly methods of raising livestock. This week, we will focus on crop farming practices that are also not environmentally friendly. The environment is complex and consists of the atmosphere, climate, vegetation, animals, rocks, soil and water. And it is these last two that are especially important to crop farmers and they must be conserved at all costs.

Our soils are degrading at a rapid rate. But soil is vital for healthy crop growth – it supplies essential plant nutrients, or plant food - such as nitrogen and calcium, and also stores water. Soils also support plants so that they do not fall over.

Now why are our soils in such a poor state? Part of the problem lies in what most farmers learnt when they were attending primary and secondary school. But this should not really come as a surprise! Their teachers probably told them that farmers should cultivate the soil before planting, for example, by using a mould board plough. And, no doubt, their teachers would have given them countless reasons why they should cultivate – it kills insect pests by burying them, loosens the soil so that it can easily absorb rainwater and encourage germination of seeds, weeds are buried, roots can easily penetrate, makes the soil well aerated [ allows air to pass easily through the soil]… But contrary to what many farmers may believe, soil scientists are of the opinion that virgin and uncultivated soils have the highest quality. Therefore, the moment farmers start cultivating their land, the soil will never be of the same quality as before.

The characteristics of a soil include: colour, texture, fertility, acidity or pH, structure, depth and others. But it is soil structure that is most negatively affected by cultivation. Soil structure simply refers to the arrangement of particles in the soil. In other words, it means the way that particles are held together in the soil. There are many types of soil structure.

A virgin, uncultivated soil may have a crumb structure which consists of small rounded crumbs of soil up to about one centimetre in size. If you take a loaf of bread and break it into crumbs then you will get the idea. Such a soil will be well aerated; enable plant roots to grow and push through the soil so that they can draw water from it, and will be neither too dry nor waterlogged. Therefore, plants growing in such soils will be healthy and thrive.

But once we start ploughing the soil, we will destroy this good soil structure. Now ploughing the soil will leave it bare and exposed to the weather. Heavy rain can then easily break up the soil crumbs at the surface into finer particles. Splashing may then throw these particles into the air after which they may now fall back to the ground.

Later, when the soil dries, a hard, dense layer, or crust, will form at the surface so that very little water will now be able to infiltrate the soil [ think of a loaf of bread which is covered with a thin hard crust]. When it rains again, most of the water will flow along the ground as runoff and cause erosion.

Hence no water will penetrate the soil and be absorbed by the plant roots. And the soil will be poorly aerated and seed germination may not take place. Those readers who want to grow vegetables in their gardens may, for this reason, find that the soil surface is as hard as concrete!

The situation is made worse if cultivation is carried out frequently. For example, a farmer may cultivate his land two or three times before planting: firstly, using a disc plough, then a mouldboard plough, and finally, a harrow. And once the crops are growing, the soil may be cultivated many more times to control and bury the weeds.

Ploughing the soil when it is too wet can also destroy soil structure. In this case, lumps of soil, or makwete, may form to produce a soil with a blocky structure. And if the soil is cultivated when it is too dry, the soil crumbs will break down into fine, loose dust which can easily be eroded and removed by the wind. Over much of the central Great Plains of the USA, wheat and maize are grown on a large scale. Due to the intensive cultivation of these crops in the 1930s, much of this area was turned into a “dust bowl” and massive dust storms were a common daily sight. Such erosion removed much of the fertile topsoil and the depth of the soil became shallower. Many farmers became bankrupt and went out of business.

On large commercial farms, the frequent use of large heavy machines, such as tractors, also destroys soil structure. The weight of these machines compacts the soil forming a hard layer of soil, called a hard pan. This feature may form deep down in the soil and may cause waterlogging. This is because the hard pan is impermeable and so prevents the downward movement of excess water deeper into the soil.

At the lands, or masimo, most older farmers practice mixed cropping which is the growing of more than one crop in the same field at the same time. For example, farmers may grow sorghum, maize, pumpkins, beans and marotse together in the same field. But many of today’s farmers have switched to monocropping which is the growing of a single crop in a field at any given time. If maize is planted every year, then the same plant nutrients will be used by the plants year after year.

Maize is a heavy feeder, and so requires fertile soils if it is to grow well and produce high yields. In particular, it needs nitrogen for rapid growth and so this nutrient will become quickly depleted in the soil. This means that the farmer will have to add more fertiliser to his soil, for example, LAN, which contain nitrogen. But as we will see later, the addition of fertilisers can have a negative effect on the soil.

Planting only one crop can also lead to the build- up of pests and diseases. Some of these may also attack nearby indigenous non- crop plants and thus will have a negative impact on the local ecosystem.

To improve the growth of their crops, many farmers add fertilisers to their soil to add those nutrients that are lacking. Organic fertilisers are natural and include kraal manure, compost, and crop residues, or remains.

Advantages of using such fertilisers include, they are easy to obtain, and are cheap or free. And they improve soil structure; this is because manure becomes sticky when wet and so can bind sandy soil particles together to produce soil crumbs. But many farmers look down on such fertilisers because they may contain only small amounts of plant nutrients and so they have to be added in large amounts, and much time may be needed to dig and mix them into the soil.

For these reasons, most commercial farmers add inorganic fertilisers which are made mainly from chemicals. And such fertilisers contain large amounts of plant nutrients, are sold in bags, and much less time and effort are required to add and mix them into the soil.

However, their use may have a negative effect on soil structure.

This is because they may kill soil micro- organisms that help in binding soil particles together.

They may also reduce the activities of other soil microorganisms, such as bacteria and fungi. When plants and animals die, their remains decompose, or decay, through the activities of these organisms.

When fully decomposed, this material forms humus which is dark brown, or black, in colour and increases soil fertility. Since inorganic fertilisers dissolve in water, they may pass downwards through the soil and cause pollution of the groundwater used by human beings and livestock.

Care must also be taken when adding any fertiliser into fields along rivers, especially if the land is sloping.

This is because it may easily be washed into river water and cause the water to become progressively enriched in nutrients, especially nitrogen and phosphorous – a process known as eutrophication. Such nutrients may lead to the rapid growth of aquatic plants in rivers, such as reeds [ lotlhaka]. In time, these plants may cover the sandy river bed and reduce the flow of water in the river - this may easily be seen in the Tati river in Francistown.

The surface of the water may also be covered with a thin layer of greenish algae. This may result in the depletion of oxygen in the water which may cause river organisms to suffocate. The algae also limits sunlight that is available to bottom- dwelling organisms. Some algae are even toxic, or poisonous, to animals and so may kill livestock if they eat it. For these reasons, the presence of algae in rivers can clearly endanger many species including fish.

To control crop pests and diseases, farmers may apply poisonous chemicals, or pesticides, to their crops. But like chemical fertilisers, these chemicals may pollute groundwater and they may also kill useful insects that feed on crop pests.

Finally, farmers must check to see in which direction their land is sloping before preparing the land for planting. This is because problems will arise if they plough down the slope, in other words, from the top of the slope to the bottom. Rainwater will now flow along the furrows, wash away the seeds, and erode the soil which will lead to the formation of gullies.