Crop Rotation

Crop rotation changes what grows on a field from one year to the next, and uses fallow, manure and legumes to give back what the harvests take away. It keeps the soil fertile and starves the pests of each crop; in Europe it led from a resting year every other year to rotations with no fallow at all.

What it is

Sown with the same crop year after year, a field tends to yield less unless its nutrients are put back. The plants draw the same nutrients from the same soil, and the pests of that crop find a host waiting for them each spring. Crop rotation changes what grows on each piece of land from year to year, in a planned cycle.

It goes with fallow, a season without a crop; with manure and compost, which return dung and plant waste to the soil; and with legumes such as beans, peas and clover, which add nitrogen from the air. The plate shows a three-field farm and its manure heap. On the right, a bean root is drawn beside a wheat root, above a compost heap cut open.

How it works

Besides water and air, plants need mineral nutrients from the soil, three of them in large amounts. Nitrogen builds proteins and leaves; a cereal short of it turns yellow. Phosphorus serves roots and seeds. Potassium keeps water in the cells and stems upright. Every sack of grain that leaves the farm takes some of each away.

Nodules on a pea root. A working nodule is pink or red inside.

Nitrogen runs short first, although the air is four-fifths nitrogen gas, because plants cannot use the gas. Legumes can, with help from soil bacteria called rhizobia. The bacteria settle in small swellings on the roots, the nodules, and turn nitrogen from the air into ammonia; the plant makes protein from it and feeds the bacteria with sugars in return. When the roots and stems rot, or the whole crop is ploughed in green, much of that nitrogen passes to the next crop. An established stand of red or white clover can fix roughly 85 to 225 kg of nitrogen per hectare in a year, a short or poor crop far less.

Phosphorus and potassium cannot come from the air; they return with manure, compost, urine and ashes.

Changing crops also slows pests and diseases that attack a single plant family: without their host for a few years, most of them decline. Some last far longer. The spores of clubroot, a disease of cabbages and turnips, can stay in the soil for up to 20 years, and onion white rot for at least 15, so infected ground is best kept free of their hosts. Fallow adds no nitrogen, but lets the farmer plough out weeds and gives grazing.

How to build

Tools: a spade or plough, a fork and planks for a bin.

Divide the ground. Mark out four plots of equal size and sort the crops into four groups, keeping related plants together: legumes (peas, beans); cabbages, kale, turnips and radishes; potatoes and tomatoes; onions and roots (leeks, carrots, beetroot, parsnips).

Move each group one plot on every year. A common order is legumes, cabbages, potatoes, then onions and roots. Cabbages follow legumes to use the nitrogen left behind.

Sow green manure on bare ground. After an early harvest, sow a green manure and dig it in while still soft, a few weeks before the next crop. Clover, vetch and field beans are legumes: sow them where cabbages come next, not where peas and beans are due.

A heap that works well steams on a cold morning.

Build the compost heap. On bare soil in half shade, set up a bin about one metre each way. Alternate layers about 15 cm thick of soft green material (grass, vegetable waste, fresh manure from grazing animals) and dry brown material (straw, dead leaves, cardboard), with at least as much brown as green. Add a little soil now and then; keep the heap as damp as a wrung-out sponge.

Turn it and check the heat. Push a long thermometer into the centre. A heap that holds 55 °C or more for several days kills many germs and weed seeds, but only the core gets that hot, so each time it cools, fork the outside into the middle and let it heat again. The compost is ready when dark, crumbly and earthy, after a few months to a year or more.

Use manure with care. Spread manure from cattle, horses, sheep, goats or poultry in autumn and dig it in, never onto growing plants.

Safety. Fresh manure carries germs such as E. coli and Salmonella. Manure that looks well rotted, and garden compost made with it, may still hold them, because a home heap never heats evenly: treat both as raw manure. Spread them at least 120 days before harvesting crops whose edible part touches the soil or can be splashed with soil by rain or watering, such as lettuce, carrots or strawberries, and at least 90 days before other crops. Never put dog, cat, pig or human waste on vegetable beds or in their compost. Wash your hands with soap after handling manure or compost and before touching food, and wash produce in drinking water. Pregnant women, young children, older people and anyone with a weak immune system should not eat raw vegetables from manured ground.

History and evolution

Nitrogen isotopes in charred grain from 13 Neolithic sites, from Greece to Britain, show that farmers manured their crops from about 6000 BCE.

Hesiod’s advice on fallow, around 700 BCE, is one of the earliest written accounts to survive and gives this plate its date. Varro, in 37 BCE, describes lupins or beans ploughed in on thin soil as they begin to pod, in place of dung. Columella, a century later, rated lupins ploughed in as good as the best manure, but doubted that beans grown for seed enriched the land.

In China, the Qimin Yaoshu, a farming manual finished around 544 CE by Jia Sixie, says millet fields must change crop each year and names mung and adzuki beans as the best crops to precede millet.

Between the Loire and the Rhine, many Frankish estates moved in the late eighth and ninth centuries from two courses, half the land resting each year, to three: winter grain, then oats, barley, peas and beans sown in spring, then fallow. Spring sowing needs summer rain, so the system stayed north of the Alps.

Sheep penned on turnips eat the crop where it grows and manure the ground for the barley that follows.

Fodder crops later replaced fallow. The eighteenth-century Norfolk four-course rotation, associated with Charles Townshend, grew wheat, turnips, barley and clover in turn. The economic historian Robert Allen credits nitrogen-fixing plants with about half the rise in English grain yields from the Middle Ages to the Industrial Revolution.

In 1838 Jean-Baptiste Boussingault found that clover and peas gained nitrogen in a soil without organic matter, while wheat and oats did not. In 1886 Hermann Hellriegel and Hermann Wilfarth traced the gain to the root nodules and their microbes, and in 1888 Martinus Beijerinck isolated the bacterium.