Crop Rotation

For 2,000 years, since the Romans spread their farming practices throughout the Roman Empire, European farmers followed a Roman cropping system called "food, feed, and fallow." Farmers divided their land into three sections, and each year planted a food grain such as wheat on one section, barley or oats as feed for livestock on another, and let the third plot lie fallow. On this schedule, each section lay fallow and recovered some of its nutrients and organic matter every third year before it was again sown with wheat. Farmers following the "food, feed, fallow" system typically only harvested six to ten times as much seed as they had sown, and saved a sixth to a tenth of their harvest to sow the following year. Low yields left little grain for storage; crops failed and people often starved during years of flood, drought, or pest infestation.

The size of agricultural allotments in Europe gradually increased beginning in the fifteenth century, allowing farmers more space to experiment with different crop rotation schedules. By 1800, many European farmers had adopted a four-year rotation cycle developed in Holland and introduced in Great Britain by Viscount Charles "Turnip" Townshend in the mid-1700s. The four-field system rotated wheat, barley, a root crop like turnips, and a nitrogen-fixing crop like clover. Livestock grazed directly on the clover, and consumed the root crop in the field. In the new system, fields were always planted with either food or feed, increasing both grain yields and livestock productivity. Furthermore, adding a nitrogen-fixing crop and allowing manure to accumulate directly on the fields improved soil fertility; eliminating a fallow period insured that the land was protected from soil erosion by stabilizing vegetation throughout the cycle.

Subsistence farmers in tropical South America and Africa followed a less orderly crop rotation system called "slash and burn" agriculture. Slash and burn rotation Adjacent fields of rice and wheat, Sacramento Valley, California. JLM Visuals. Reproduced by permission. involves cutting and burning nutrient-rich tropical vegetation in place to enhance a plot of nutrient-poor tropical soil, then planting crops on the plot for several years, and moving on to a new plot. Slash and burn agriculture is a successful strategy as long as the agricultural plots remain small in relation to the surrounding rainforest, and the plot has many years to recover before being cultivated again. Large-scale slash and burn agriculture results in permanent destruction of rainforest ecosystems, and in complete loss of agricultural productivity on the deforested land.

Crop rotation fell out of favor in developed nations in the 1950s, when farmers found they could maintain high-yield monoculture crops by applying newly developed chemical fertilizers, pesticides, and weed killers to their fields. Large-scale commercial agriculture that requires prescribed chemical treatments has become the norm in most developed nations, including the United States. However, substantial concerns about the effect of agricultural chemicals on human health, and damage to soil structure and fertility by monoculture crops, have led many farmers to return to more "natural" practices like crop rotation in recent decades. So-called conventional farmers use crop rotation in concert with chemical treatments. Organic farmers, who cannot by definition use chemical treatments, rely entirely upon methods like crop rotation to maintain soil health and profitable crop yields.