The ancient Romans followed a form of crop rotation that they introduced to the rest of Europe. The system was called “food, feed and fallow”. A farmer divided his land into three plots, and each year planted a food grain such as wheat on one section and barley or oats as feed for livestock on another. The third plot he left alone. In this way, each plot lay fallow and recovered some of its nutrients and organic matter every third year before it was again sown with wheat. Unfortunately, plots were small and low yield meant that there was little grain left over for storage, so people often starved during years of flood, drought or pestilence.
Beginning in the 15th century, the size of agricultural allotments began to increase, allowing farmers more space to experiment with different crop rotation schedules. By 1800, many European farmers had adopted the four-year rotation cycle developed in Holland. This rotated wheat, barley, a root crop such as turnips, and a nitrogen-fixing crop such as clover. Livestock grazed the clover and consumed the root crop in the plot.
In the new system, plots were always planted with either food or feed, increasing yield and livestock productivity. Furthermore, adding a nitrogen-fixing crop and allowing manure to accumulate directly on the surfaces improved soil fertility. Eliminating a fallow period protected the land from soil erosion by stabilising soil and vegetation throughout the cycle. Subsistence farmers in South America and Africa followed a less orderly rotation system called ‘slash and burn’. This involved cutting and burning the nutrient-rich tropical vegetation to enhance a plot of nutrient-poor tropical soil, then planting crops on the plot for several years before moving on.
According to experts, this is a successful strategy as long as the plots remain small in relation to the surrounding forest, and the plot has sufficient time to recover. Large-scale slash and burn agriculture destroys ecosystems, and leads to a complete loss of agricultural productivity on the deforested land.
Crop rotation fell out of favour in developed nations in the 1950s when farmers found they could maintain high-yield monoculture crops by applying newly developed chemical fertilisers, pesticides, and herbicides to their land. Large-scale commercial agriculture requiring chemical treatment became the norm.
In recent decades, concern about the effect of agricultural chemicals on human health and damage to soil structure and fertility by monoculture crops have led many farmers to adopt (in South Africa, at least) a three-year crop rotation.
The following should be kept in mind when planning such a cycle:
Make sure that the soil is suitable for the planned crop. Take into account soil depth, texture and salinity. Study the climate over the various seasons when deciding which crop can be grown successfully at different times of the year.
Investigate the costs of producing various vegetable crops, as well as the income expected at various planting and harvesting times. Remember that prices are higher than normal at certain times of the year.
Diseases and pests
Crops belonging to the same family, such as cabbage, cauliflower and broccoli, or tomato, potato and eggplant, are often attacked by the same group of pests and diseases. For this reason, don’t include related crops in successive plantings or even in the same three-year rotation programme.
Low-growing crops, such as carrots, lettuce or onions, are easily overgrown by weed, so these should follow crops in which weeds were well controlled.
The best use can be made of crop water by rotating deep- and shallow-rooted crops. Crops with shallow roots seem best adapted to follow a deep-rooted crop because water recharge is likely to occur only near the surface, and a shallow-rooted crop will not expend energy in search of moisture that is not there. Medium- or deep-rooted crops appear better adapted to follow shallow-rooted crops, as they take advantage of any moisture left at depth that was not used by the previous shallow-rooted crop.
Crops with high nitrogen requirements such as cabbage should follow a leguminous crop such as green beans and peas, which fix atmospheric nitrogen. Applying too much organic manure can damage certain crops, such as carrots and beetroot. Plant these crops later, after applying organic manure to crops such as tomatoes that respond well to organic fertilisers.
Crops requiring large quantities of nutrients, such as cabbage, should follow crops with lesser needs, such as pumpkin, or less efficient feeders such as potatoes. This will allow them to make use of residual nutrients that remain in the soil after the crop has been harvested.
Sources: ‘Cultivating vegetables – crop rotation’, compiled by Directorate Agricultural Information Services, department of agriculture, in co-operation with the KwaZulu-Natal department of agriculture; ‘Principles and Practices of Crop Rotation’, www.agriculture.gov.sk.ca; ‘Crop rotation’, jrank.org.