thumb|upright=1.5|Effects of crop rotation and [[monoculture at the Swojec Experimental Farm, Wrocław University of Environmental and Life Sciences. In the front field, the "Norfolk" crop rotation sequence (potatoes, oats, peas, rye) is being applied; in the back field, rye has been grown for 58 years in a row.]]

Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. This practice reduces the reliance of crops on one set of nutrients, pest and weed pressure, along with the probability of developing resistant pests and weeds.

Growing the same crop in the same place for many years in a row, known as monocropping, gradually depletes the soil of certain nutrients and promotes the proliferation of specialized pest and weed populations adapted to that crop system. Without balancing nutrient use and diversifying pest and weed communities, the productivity of monocultures is highly dependent on external inputs that may be harmful to the soil's fertility. Conversely, a well-designed crop rotation can reduce the need for synthetic fertilizers and herbicides by better using ecosystem services from a diverse set of crops. Additionally, crop rotations can improve soil structure and organic matter, which reduces erosion and increases farm system resilience.

History

thumb|[[Legumes such as alfalfa, beans, and clover have long been used in crop rotations. They have bacteria in their root nodules which take nitrogen from the air and fix it into the soil as nitrates that crops can use.]]

Farmers have long recognized that suitable rotations such as planting spring crops for livestock in place of grains for human consumption make it possible to restore or to maintain productive soils. Ancient Near Eastern farmers practiced crop rotation in 6000 BC, alternately planting legumes and cereals.

Two-field systems

Under a two-field rotation, half the land was planted in a year, while the other half lay fallow. Then, in the next year, the two fields were reversed. In China both the two- and three-field systems had been used since the Eastern Zhou period.

Three-field systems

From the 9th century to the 11th century, farmers in Europe transitioned from a two-field system to a three-field system. This system persisted until the 20th century. Available land was divided into three sections. The first section grew autumn planted crops such as rye and wheat. The second section grew spring planted crops such as oats; barley; and legumes: peas, lentils, or beans. The third section was left fallow. The three fields were rotated in this manner so that every three years, one of the fields would rest and lie fallow. Under the two-field system, only half the land was planted in any year. Under the new three-field rotation system, two thirds of the land was planted, potentially yielding a larger harvest. But the additional crops had a more significant effect than mere quantitative productivity. Since the spring crops were mostly legumes, which fix nitrogen needed for plants to make proteins, they increased the overall nutrition of the people of Europe.

Four-field rotations

The British agriculturist Charles Townshend (1674–1738) popularised this system in the 18th century. The sequence of four crops (wheat, turnips, barley and clover), included a fodder crop and a grazing crop, allowing livestock to be bred year-round. The four-field crop rotation became a key development in the British Agricultural Revolution.

Modern developments

In the Green Revolution of the mid-20th century, crop rotation gave way in the developed world to the practice of supplementing the chemical inputs to the soil through topdressing with fertilizers, adding (for example) ammonium nitrate or urea and restoring soil pH with lime. Such practices aimed to increase yields, to prepare soil for specialist crops, and to reduce waste and inefficiency by simplifying planting, harvesting, and irrigation.

Crop choice

A preliminary assessment of crop interrelationships can be found in how each crop:

  1. Contributes to soil organic matter (SOM) content.
  2. Provides for pest management.
  3. Manages deficient or excess nutrients.
  4. Contributes to or controls for soil erosion.
  5. Interbreeds with other crops to produce hybrid offspring.
  6. Impacts surrounding food webs and field ecosystems.

Crop choice is often related to the goal the farmer is looking to achieve with the rotation, which could be weed management, increasing available nitrogen in the soil, controlling for erosion, or increasing soil structure and biomass, to name a few. When discussing crop rotations, crops are classified in different ways depending on what quality is being assessed: by family, by nutrient needs/benefits, and/or by profitability (i.e. cash crop versus cover crop). For example, giving adequate attention to plant family is essential to mitigating pests and pathogens. However, many farmers have success managing rotations by planning sequencing and cover crops around desirable cash crops. The following is a simplified classification based on crop quality and purpose.

Row crops

Many crops which are critical for the market, like vegetables, are row crops (that is, grown in tight rows). With much of the soil around the plant exposed to disruption by rainfall and traffic, fields with row crops experience faster break down of organic matter by microbes, leaving fewer nutrients for future plants.

Grasses and cereals

Cereal and grasses are frequent cover crops because of the many advantages they supply to soil quality and structure. The dense and far-reaching root systems give ample structure to surrounding soil and provide significant biomass for soil organic matter.

Grasses and cereals are key in weed management as they compete with undesired plants for soil space and nutrients.

Green manure

Green manure is a crop that is mixed into the soil. Both nitrogen-fixing legumes and nutrient scavengers, like grasses, can be used as green manure. Green manure of legumes is an excellent source of nitrogen, especially for organic systems, however, legume biomass does not contribute to lasting soil organic matter like grasses do. Moreover, a crop rotation must consider in what condition one crop will leave the soil for the succeeding crop and how one crop can be seeded with another crop. and by growing more than one crop at a time in a field. A monoculture is a crop grown by itself in a field. A polyculture involves two or more crops growing in the same place at the same time. Crop rotations can be applied to both monocultures and polycultures, resulting in multiple ways of increasing agricultural biodiversity (table).

! rowspan="3" colspan="3" |

! colspan="3" |Diversity in time

|-

! rowspan="2" |Low

! colspan="2" |Higher

|-

!Cyclic

!Dynamic

|-

!rowspan="2" |Diversity<br/>in space

!Low

!Monoculture,<br/>one species in a field

|Continuous<br/>monoculture,<br/>monocropping

|Rotation of<br/>monocultures

|Sequence of<br/>monocultures

|-

!Higher

!Polyculture,<br/>two or more species<br/>intermingled in a field

|Continuous<br/>polyculture

|Rotation of<br/>polycultures

|Sequence of<br/>polycultures

|}

Incorporation of livestock

Introducing livestock makes the most efficient use of critical sod and cover crops; livestock (through manure) are able to distribute the nutrients in these crops throughout the soil rather than removing nutrients from the farm through the sale of hay.

Polyculture

Polyculture systems, such as intercropping or companion planting, offer more diversity and complexity within the same season or rotation. An example is the Three Sisters, the inter-planting of corn with pole beans and vining squash or pumpkins. In this system, the beans provide nitrogen; the corn provides support for the beans and a "screen" against squash vine borer; the vining squash provides a weed suppressive canopy and a discouragement for corn-hungry raccoons. The “Crop Rotation Practice Standard” for the National Organic Program under the U.S. Code of Federal Regulations, section §205.205, states that