Giving ploughs the push
A man stands with a watering can, holding it as high as he can. While still able to tip it up, he pours water on to two open boxes, one containing bare soil, and the other where soil is covered with straw. Water draining from spouts at the lower edge of the boxes is collected for comparison to demonstrate how a covering of straw can protect soil from water erosion. The result - farmers and extension workers in Santa Catarina state, southern Brazil find that 95% of water erosion is caused by the impact of raindrops on bare soil. The power of the drops detaches soil particles, which are then swept away in run-off water and deposited elsewhere: on lower land, on a road, or in a river. However, if the soil is covered, the detaching of particles is prevented and erosion greatly reduced.
In Santa Catarina loss of productive land has been a major problem, and early attempts at erosion control by farmers focused on physically stopping the run-off by laboriously building terraces. But not only did erosion continue, the farmers found that the excessive soil disturbance and compaction on the terraces actually reduced infiltration and increased the amount of run-off. Then came interest in green manures; but these were generally understood to be crops that would need to be ploughed into the soil before seeding and, as the most erosive rains generally fall at planting time when the manure crop has already been ploughed in, erosion continued to be a problem.
Hence, the gradual adoption and development of minimum tillage, whereby the land is cultivated with a minimum of field operations. With low-till techniques the soil is protected with either living cover crops - sometimes planted, sometimes spontaneous - or dead plant matter (stubble and stover) left from the previous harvest. In the case of cover crops these may be killed, either with herbicides or by cutting, immediately prior to planting. Crops are then planted either directly through the covering material, or in narrow strips within which the organic material has been partially incorporated into the soil. In this way the need for ploughing is reduced or eliminated.
The results of the minimum tillage/low-till approach have been mixed. On the one hand degraded soils have been restored to health, land preparation costs have been reduced and yields have improved. However, farmers have learned that continuing to plant the same cover crop and main crop year after year can lead to problems such as soil compaction, nutrient concentration in the topsoil, and a rise in certain pests, diseases and weeds. The solutions, which are still being researched by close co-operation between farmers and extension staff, centre on the need for crop rotations, both in terms of the manure crop and the main crop.
Such 'Conservation agriculture' methods are now being applied on nearly one third of Santa Catarina's cropped area, and are being widely promoted by the UN Food and Agriculture Organisation (FAO), not least in Asia. In India, Pakistan, Bangladesh and Nepal 'low-till' methods are being adopted, particularly for the planting of wheat after rice. Traditionally, farmers try to 'rebuild' their soils after rice cultivation by repeated ploughing. However, this has the negative effect of encouraging moisture to evaporate and organic matter in the soil to be lost by oxidation. Repeated ploughing is also expensive and time consuming. It can delay the planting of wheat to such an extent that the crop fails to ripen before the hot dry months, and can only be saved from shrivelling by expensive irrigation. Such difficulties hit poor farmers hardest, as they have less access to irrigation, and can least afford the loss from shrivelled grain.
Not surprisingly the low-till method, whereby wheat is sown directly into the post-harvest rice stalks, is becoming especially popular with poorer farmers, particularly as land preparation is less costly; instead of struggling to access expensive ploughing equipment, farmers in many areas have been able to hire 'seed drills' or 'planters', at relatively low cost. In addition, yields have been shown to be higher. It has also been found that the undisturbed rice stubbles provide channels in the soil for the wheat roots to grow in, and they decompose to form a natural fertilizer. Less soil disturbance also means that less weeds germinate, and the earlier planting of wheat, usually by three or four weeks, means that the wheat crop shades out weeds more effectively, lessening the need for herbicides.
In the world of agricultural development there is much talk of the need for a 'doubly-green' revolution; the two sides of this are to increase productivity while maintaining soil, water and environmental resources. Clearly low-tillage cultivation could be a key part of this, and for countries with rising populations and increasingly exhausted cropland, such 'doubly-green' strategies offer a significant contribution towards sustainable food production.