Soil is cultivated to produce favourable conditions for the establishment of crops. These conditions include favourable soil moisture for germination and early plant development. This is achieved by maximising the amount of water that infiltrates the soil and by reducing weeds and volunteer plants growing during the water-harvesting season. Tillage is also effective in eliminating compaction and managing excess stubble. Traditionally, weeds were controlled using mechanical cultivation such as ploughing with a mouldboard plough (conventional tillage) or applying shallow cultivation to kill weeds while retaining stubble on the surface (conservation tillage). Another planting method, namely minimum-till (also called no-till) in which the seed is directly sown in untilled soil, has become available due to cost-effective means of killing weeds with broad-spectrum herbicides (chemical cultivation), limiting the impact on soil micro-organisms. Whichever system the producer chooses, good crop establishment and economic factors remain the main issues that need to be considered.
Conventional tillage.
Conventional tillage is recommended for a cropping system in which the risk of root disease is high, and the risk of wind and water erosion is minimal. The use of a mouldboard plough causes the topsoil layer to be inverted and leaves virtually no stubble on the soil surface. It effectively kills germinated weeds but has the disadvantage of bringing weed seeds from deeper layers to the soil surface where it germinates. Mouldboard ploughing should always be followed with secondary cultivation to get rid of clods and new weed infestations.
Mouldboard plough.
Conservation tillage
Conservation tillage is highly recommended in all areas where the risk of wind and/ or water erosion is high, because of the low clay content of these soils. These areas are usually less prone to root disease, as the rainfall is lower, and soils are well drained. Conservation tillage can also give good results in high rainfall areas if used in a crop rotation system. Under dryland conditions, farmers in the North-Western Free State producing on deep sandy soils on a shallow water table have successfully implemented reduced tillage for many years. A dry climate and high yield potential are ideally suited for reduced tillage systems.
No-till (Direct seeding)
The increasing use of crop rotation systems and the development of new technology have created new opportunities to implement direct seeding systems successfully. The current high cost of diesel and the reduction in the price of glyphosate-based herbicides makes reduced tillage methods even more attractive to producers. No-till has been established successfully in many areas in South Africa, including some parts of the Winter Rainfall Region and some irrigation schemes, especially in KwaZulu-Natal. In the Eastern Free State, these systems are more problematic due to high disease pressure, but with good management, these problems can be overcome. One of the main aims of direct seeding is to minimise disturbance of the soil surface to prevent surfacing and germination of new weed seeds, to maximise coverage of the surface by residue and to prevent the impact on microbial communities in soil. This further suppresses the germination of weeds and enhances the uptake of water by the soil. A properly functioning no-till planter is then used to open a narrow slot by pushing away crop residues from the plant row. A tine is used for proper fertiliser placement and breaking of the surface and sub-soil compaction.
No-till and Conventional till planter
A tine used for fertiliser placement in no-till
Benefits of No-Till Farming
Soil is tilled to loosen it for oxygen and water to reach the area where roots will grow. Using a mouldboard plough doesn’t necessarily produce such soil. Ploughing and disking a field result in soil with a broken structure lying atop a heavily compressed plough pan (the undisturbed layer that the plough doesn’t reach). This broken-up soil is very prone to be compacted by rainfall. In addition, many passes must be made over the field with very heavy equipment, the wheels of which further compress the soil. The untilled ground starts off being less compacted than a heavily machine-worked field, and it stays that way. What’s more, the earth that has become compressed by tillage or machinery will return to a less compacted state after a few years of no-till planting.
Tilling put fertilisers and crop residues into the soil, making nutrients readily available to the roots of the plants. In no-till farming, crop residues are left on the surface, where the nutrients that result from their decay can leach into the soil. This leaching process is far more thorough than you might imagine. Fertilisers, including anhydrous ammonia, phosphorus, and potassium, are also as effective on the no-till fields as on the ploughed plots. And with the right equipment, these ingredients can be placed directly into the planting trench (where they’re most needed) during seeding. As for the potential problem of the planter fouling with residues, specially designed no-till planters have a device that cuts a slot through surface dry matter. Besides, studies have shown that the accumulation of this surface material levels off after a few seasons of no-till practice.
Conventional ploughing does prepare a seedbed. No-till farming also does this, but just in a much more restrained way. In normal tillage, the entire field is turned into a seedbed that may be mounded for planting. With no-till one simply prepares a narrow trench of the appropriate depth. Studies have shown that plant roots develop at least as well in a no-till field as in a ploughed one and that the lack of mounding exposes less of the soil to air and evaporation. Conventional ploughing is also done across the contours of the land to prevent soil erosion. But once again, this rationale doesn’t hold up when you look at the results possible with no-till. The crop residues on a no-till field prevent runoff to an amazing extent. On slopes that are too steep to plant, no-till fields have consistently shown limited topsoil loss after downpours of several millimetres of rain per hour. That same vegetative cover also makes the no-till field less susceptible to the effects of wind erosion. This type of agriculture truly offers a solution to the problem of topsoil loss.