1. Petar Yankov, Miglena Drumeva
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CROP ROTATIONS – SUBJECT MATTER, CLASSIFICATION, ORGANIZING PRINCIPLES
Petar Yankov, Miglena Drumeva

2. INDEX Introduction Monocrop Reasons for crop rotation
3.1. Biological reasons for crop rotation
3.2. Chemical reasons for crop rotation
3.3. Physiological reasons for crop rotation
3.4. Economic reasons for crop rotation
4. Previous crops
4.1. Previous crops for bread cereals – wheat, rye
4.2. Previous crops for cereal forage crops – barley, oats
4.3. Previous crops for rice
4.4. Previous crops for late root crops
4.5. Previous crops for vegetable crops
4.6. Previous crops for annual legumes – bean, peas, vetch, soybean, lentil, chick pea, faba bean
4.7. Previous crops for perennial legumes – alfalfa, clover, sainfoin

3. INDEX Classification and basic principles of crop rotations
5.1. Field crop rotations
Field crop rotations on non-irrigation areas
Field crop rotations in irrigated areas
5.2. Fodder crop rotations
Meadow-pasture crop rotations
Pre farm crop rotations
5.3. Specialized crop rotations
Crop rotations on sloping terrains
Vegetable crop rotations
Tobacco crop rotations
Rice crop rotations
Cotton crop rotations

4. 1
INTRODUCTION
Crop rotation is the consecutive annual or periodical alternation of the agricultural crops in a certain number of fields for a given interval of time.
This alternation should take into account the ecological peculiarities of the area and its landscape with a view of increasing soil fertility and the yields from the grown crops. The rotation of crops closely relates to the agronomy practices, and especially to soil tillage, fertilization, plant protection, control for erosion, etc.

5. This is an example of intensive crop rotation (Fig. 1):
1. INTRODUCTION
This is an example of intensive crop rotation (Fig. 1):
2nd field
3rd field
1st field
4th field
5th field
Figure 1: An example of crop rotation (Image from UkrLandFarming on line at

6. 2
Monocrop
The growing of the cultural plants for several years on the same area without any alternation is called monocrop. Depending on the response of the plants to growing without rotation, the crops are divided into the following three groups:
Crops, which do not tolerate continuous cropping or even repeated planting on the same place – sunflower, clover, alfalfa, peas, sugar beet, pepper, etc.
Crops, which to some extent tolerate second planting on the same place. Such crops are wheat, barley, early tomatoes, etc.
Crops, which tolerate longer continuous cropping – maize, tobacco, cotton, potatoes, rice, rye, oats, etc.

7. 3 3.1. Biological reasons for crop rotation Reasons for crop rotation
Under continuous cropping, the species composition of weeds decreases as a rule, at the expense of the density of the adapted species. It is especially harmful and unacceptable to plant repeatedly crops, which are attacked by parasite weeds.
Repeated planting, continuous cropping or alternation of crops attacked by the same diseases and pests are favorable prerequisites for mass proliferation of certain diseases and pests. Thus for example, the repeated planting of cereals after cereals (especially wheat after itself) preconditions the mass occurrence of cereal ground beetle, Sunn pest, basal (root) rot, brown rust, etc.

8. 3.2. Chemical reasons for crop rotation
The nutrition regime of soil is significantly enriched through the root and post harvest residues, which remain in the soil after harvesting of the crops. The agricultural crops leave different amounts of organic matter of various chemical compositions.
The differences between the cultural plants, as determined by their effect on the nutrition regime of soil, are dependent of the position of their root system at different depths and the utilization of nutrients from different layers down the soil profile. Another factor determining the nutrition regime is the ability of the crop types for nutrients uptake. For example, lupine and buckwheat are capable of taking up soil phosphorus from compounds that are not easily available.

9. 3.2. Chemical reasons for crop rotation
The crops have different effects on the nutrition regime of soil depending on the growth stage, at which they are harvested – as a green mass or at maturity. The post harvest residues from non-leguminous crops harvested as green mass are rich in nitrogen. The variations in the content of nitrate nitrogen in soil are conditioned by the way of growing the plants. The more intensive loosening of the soil in the inter-row spaces of root crops improves the aeration, which favors the activity of the nitrogen-fixing and nitrifying bacteria. Therefore, more nitrates remain in soil after root crops in comparison to cereals sown on a whole field surface.

10. 3.3. Physiological reasons for crop rotation
Growing of perennial grass mixtures (grasses and leguminous species) improves soil structure. The significance of the physical properties of soil becomes greater in regions with expressed water and wind erosion.
There are great differences between the separate cultural species with regard to their demands for water. The perennial leguminous grasses need much more water than the cereals. The plants with weak and shallow roots draw water mainly form the surface soil layers, while the plants with well developed root systems that burrow deep in the ground take water from the deeper soil horizons. There are also great differences between the plants according to the duration of their vegetative growth cycles and to the time when they need water most. The winter cereals draw highest amounts of water and deplete the soil moisture mainly in spring, while the late root crops – in summer.

11. 3.4. Economic reasons for crop rotation
The proper rotation of crops preconditions the quality tillage of soil, the rational usage of fertilizers and irrigation water, etc., with a view of obtaining higher yields with better quality and low cost.

12. 4 4.1. Previous crops for bread cereals – wheat, rye Previous crops
Very good previous crops for winter wheat are the early harvested crops – vetch mixtures, vetch for hey, peas mixtures, early potatoes, etc. (Photo 1).
Good predecessors for wheat are the fertilized and well cultivated late root crops – maize, sunflower, beet, oilseed rape, cotton, soybean, pea nuts, and potatoes.
Photo 1: Wheat in Dobrudzha region, Bulgaria

13. 4.1. Previous crops for bread cereals – wheat, rye
In comparison to wheat, rye is less demanding to the previous crop and tolerates planting after itself; this is explained with its more developed root system, its ability to draw nutrients from less soluble compounds in soil, its fast growth rate in spring and its better resistance to weeds (Photo 2).
Photo 2: Rye (picture on line at

14. 4.2. Previous crops for cereal forage crops – barley, oats
The best previous crops for barley are the legumes, followed by the root crops – sunflower, sugar beet, maize (Photo 3).
Oats is considered less demanding to the previous crop and the soil because it has a powerful root system and ability for uptake of less available nutrients. These advantages and the smaller area on which it is sown allow placing it the last field of the crop rotation after late root crops, the areas of which are not occupied by wheat or barley.
Photo 3: Barley (picture on line at

15. 4.3. Previous crops for rice
It has been found out that fallow land is the best predecessor for rice, but leaving the land to lay fallow is not cost-effective in contemporary agriculture.
Second best are the perennial grasses – clover, alfalfa, which develop well on rice soils. The grass mixtures are also suitable predecessors.
Most often, however, rice is grown after annual previous crops (a root crop or a cereal crop). Practice has shown that rice is self-tolerant for 3-4 years without yield decrease if weeds are kept under efficient control and proper fertilization is applied.

16. 4.4. Previous crops for late root crops
Maize tolerates repeated planting and short-term continuous cropping, especially under organic and mineral fertilization (Photo 4). The perennial grasses are good previous crops for maize. The cereals sown on the whole field area are widely used previous crops for wheat. After cereals, a second forage maize crop is usually sown under irrigation conditions or in humid years.
Photo 4: Maize in Dobrudzha region, Bulgaria

17. Photo 5: Sunflower in Dobrudzha region, Bulgaria
4.4. Previous crops for late root crops
Sunflower is a crop, the yields from which are highly dependent on the proper choice of a previous crop (Photo 5). It does not tolerate planting after itself due to the danger of occurrence of downy mildew, phoma, sclerotinia, Orobanche, etc. Cereals sown on the whole field area are used as a predecessor for sunflower. Sunflower should be planted on the same field after 5 years.
Photo 5: Sunflower in Dobrudzha region, Bulgaria

18. Photo 6: Cotton in Chirpan region, Bulgaria
4.4. Previous crops for late root crops
Cotton is not very demanding to the previous crop. It tolerates sowing after itself for several years. In Bulgaria, cotton is commonly grown after cereals planted on the whole field area (Photo 6). Alfalfa is a very good previous crop for cotton.
Sugar beet requires rich, well-cultivated soils. It does not tolerate repeated planting because it suffers heady attacks by beet root weevil and nematodes. Planting in the same field is recommended not earlier than 4-5 years. The winter cereals wheat and barley are good predecessors for sugar beet.
Photo 6: Cotton in Chirpan region, Bulgaria

19. 4.4. Previous crops for late root crops
Potatoes tolerate repeated planting after themselves, especially when grown in a more humid and cooler climate (Photo 7). Clover and alfalfa are very good previous crops for them. The annual legumes such as peas, vetch, beans, chick pea, etc., are also very good predecessors, as well as some vegetable crops – cucumbers, early cabbage, bulb onion. Potatoes should not be alternated with tomatoes, pepper, and egg plant due to the same pests and diseases that attack them.
Photo 7: Potatoes (picture on line at

20. 4.4. Previous crops for late root crops
The areas with tobacco are limited, and the tobacco plants are sown in field late in spring (Photo 8). This allows using pre-cropping of the tobacco fields with winter vetch and pea, rye for fodder, etc. Growing of intermediate crops is a soil-protective practice and a source of forage. Sunflower, potatoes, tomatoes and pepper are considered unsuitable predecessors for tobacco because they are attacked by the same diseases and pests. A good previous crop for tobacco are the cereals planted on the whole field area.
Photo 8: Tobacco (picture on line at

21. 4.5. Previous crops for vegetable crops
The best previous crops for pepper are the plowed grass mixtures, alfalfa, garden bean and pea, while late cabbage, wheat, barley, soybean, and maize are good predecessors.
The medium early tomatoes give highest yields after the annual legumes – peas and bean. Potatoes, watermelons and spinach also belong to the group of good previous crops for tomatoes.
Potatoes, bean and peas are good previous crops for late cabbage (Photo 9). Pepper and the medium early tomatoes are satisfactory predecessors for this crop.
Photo 9: Potatoes (picture on line at

22. 4.6. Previous crops for annual legumes – bean, peas, vetch, soybean, lentil, chick pea, faba bean
The annual legumes are not very demanding to the previous crop (Photo 10). Good previous crops for them are the fertilized cereals sown on the whole field area and the root crops. It should be kept in mind, however, that when well fertilized, the root crop areas are used for planting of winter cereals, not annual legumes.
Photo 10: Bean in Dobrudzha region, Bulgaria

23. 4.7. Previous crops for perennial legumes – alfalfa, clover, sainfoin
The perennial legumes are grown as pure crops or in grass mixtures with other legumes or cereal grasses (Photo 11). They should be planted on well-cultivated and weeds-free areas because they grow slowly during the first year and suffer from weed infestation. Such conditions are available after fertilized and properly grown root crops, which are very good predecessors for the grasses.
Photo 11: Clover (picture on line at plantvillage.org/en/topics/clover/diseases_and_pests_description_uses_propagation)

24. 5
Classification and basic principles of crop rotations
5.1. Field crop rotations
Field crop rotations on non-irrigation areas
When designing field crop rotations for non-irrigation regions, the following issues should be taken into account:

25. 5.1. Field crop rotations
Field crop rotations on non-irrigation areas
The rotations should include a minimal number of crops. In most regions, the maximal number of crops in one rotation should be 4-6, and in regions with intensive grain production – 2-3.
If possible, in a crop rotation only one technical crop should be involved (sunflower, sugar beet, cotton, etc.) with a more specific technology of growing and harvesting. Its relative percent should not be greater than % from the total area of the crop rotation.
The crop rotations should be designed to ensure predominant alternation of deep-root crops with crops with shallow root system, which are not attacked by the same weeds, diseases and pests. Annual legumes or manure-fertilized root crops have to be involved to improve soil fertility.
Alfalfa can be included in such crop rotations; it has to be sown only in fields suitable for its growing as an emergency extra crop outside the main rotation.

26. 5.1.2. Field crop rotations in irrigated areas
An important peculiarity of crop rotations in irrigated areas is that they can be supplemented with additional crops. The additional cropping can be done as follows:
Growing a second crop. The second crop follows crops harvested during the first half of summer. The second crop is harvested in autumn and usually spring plants are cultivated after them. A second crop following barley and wheat is most common in Bulgaria. Most often, this second crop is forage maize. Some vegetable plants – cabbage, tomatoes, cucumbers, beans, peas, etc. – are often grown as second crops as well.

27. 5.1. Field crop rotations
Field crop rotations in irrigated areas
Growing winter intermediate crops (pre-cropping). Pre-crops are sown in autumn (September – October) and are harvested before planting of the main crop (usually at the end the end of April – first half of May). Winter rye, winter vetch, and repko (a hybrid of winter rape with Chinese cabbage) are most suitable for pre-crops. This method of additional cropping is particularly applicable on sloping terrains to reduce water erosion. The intermediate crops can also be used for green fertilization (sideration). Legumes (forage pea, lupine, melilot), which develop fast in spring and decompose quickly after plowing, are suitable siderates. The green mass should be plowed at least days before sowing (transplanting) of the main crop.

28. The following is an example of crop rotation:
5.1. Field crop rotations
Field crop rotations in irrigated areas
Growing of spring pre-crops. Spring pre-crops are sown in spring and harvested before planting of the main crop. Peas for hay is most often sown as a pre-crop of tobacco, maize, etc. The early potatoes, spring vetch and others are also suitable.
The following is an example of crop rotation:
1st field – alfalfa (З-year usage);
2nd field – maize;
3rd field – wheat + silage maize as a second crop;
4th field – sugar beet;
5th field – barley + grain maize as a second crop;
6th field – barley + maize as a second crop.

29. 5.2. Fodder crop rotations 5.2.1. Meadow-pasture crop rotations
The meadow-pasture crop rotations include mainly perennial grasses and annual forage crops (grain fodder crops, root and tuber vegetables and annual grasses), but in some cases certain valuable technical crops also have suitable conditions for development.
This is an example of a meadow-pasture crop rotation:
1st field – perennial grasses;
2nd field – perennial grasses;
3rd field – maize;
4th field – maize;
5th field – annual grasses;
6th field – barley.

30. 5.2.2. Pre farm crop rotations
The pre farm crop rotations are located near animal farms. Thus, on the one hand, transportation expenses are reduced, and on the other hand near-by pastures are ensured for those species and categories of domestic animals, which cannot pass long distances.
The perennial grasses in the pre farm crop rotations are grown for 3-4 years. They are sown as grass mixtures or as pure crops (legumes). After plowing of the grasses, the field is usually planted with silage crops (maize), and in some cases with vegetable crops.
This is an example of pre farm crop rotation:
1st field – alfalfa;
2nd field – alfalfa;
3rd filed – alfalfa;
4th field – silage maize + green rye;
5th field – silage maize + rye-peas mixture as a second crop;
6th field – maize.

31. 5.3. Specialized crop rotations
Crop rotations on sloping terrains
The winter cereals and the winter pre-crops, sown on the whole field area, have good soil protection activity, especially when the crop is with normal density. These crops stay in the field for more than 8-9 months at the time of the main rainfalls, which cause erosion. The spring crops sown on the whole field area also have significant anti-erosion effect, both cereals and grain and fodder legumes, but their comparatively short growth cycle does not allow for long-term protection of soil.
The root crops have lowest anti-erosion value. The area, on which they are grown, is cultivated in autumn, and in winter and spring it is unoccupied and unprotected from the activity of the water drops and the water runoff. The wide inter-row spacing and the vegetation tillage contribute to their lower anti-erosion effect.

32. 4th field – winter cereal crop;
5.3. Specialized crop rotations
Crop rotations on sloping terrains
The soil protection crop rotations are carried out on terrains with gradient of the slope higher than 5°.
On terrains with gradient of the slope up to 10-12°, the erosion processes are not very strong, but in some cases they can be significant, especially on long slopes and light soils. The following crop rotation is suitable in such cases: 1st field – annual legumes sown on the whole field area; 2nd field – winter cereals + intermediate winter crop; 3rd field – root crop; 4th field – winter cereals.
On slopes with gradient higher than 12°, typical soil protection crop rotations are carried out. When the conditions are suitable for growing of alfalfa, the following crop rotation may be introduced:
1st field – alfalfa;
4th field – winter cereal crop;
2nd field – alfalfa;
5th field – annual legumes;
3rd field – winter cereal crop;
6th field – winter cereal crop.

33. 5.3.2. Vegetable crop rotations
The purpose of vegetable crop rotations is regularly and all-the-year-round to meet the demands of the market for variable and fresh vegetables. Therefore, these crop rotations should involve greater variety of vegetable species and cultivars with different earliness – early, medium early and late.
Example:
1st field – lettuce, green onion and garlic + pepper;
2nd field – early potatoes, cucumbers + green beans as a second crop;
3rd field – early cabbage + late cucumbers as a second crop;
4th field – wheat + lettuce as a second crop, green onion.

34. 5.3.3. Tobacco crop rotations
When planning these crop rotations, the condition should be observed to grow tobacco on the most favorable soils. The tobacco with small-sized leaves is a crop, which tolerates planting after itself, while the large-leaved tobacco does not.
This is an example of growing small-leaved tobacco:
This is an example of growing large-leaved tobacco:
1st field – winter cereal crop;
4th field – tobacco;
2nd field – tobacco;
5th field – winter cereal crop;
3rd field – forage peas;
6th field – tobacco.
1st field – winter cereal crop;
4th field – winter cereal crop;
2nd field – tobacco;
5th field – tobacco.
3rd field – maize;

35. Rice crop rotations
Bearing in mind the possibilities for several years of continuous growing of rice, and observing the requirement the main crop to occupy the major part of the area, the rice crop rotations should consist of two elements. The first element serves for enrichment of the soil with organic matter, for building up and maintaining of high soil fertility and nitrogen potential. The second element aims at control of the weed vegetation and conditions the levelling up of the terrain and improving the ameliorative condition of the rice fields.
Example:
1st field – alfalfa;
4th field – rice;
2nd field – alfalfa;
5th field – rice;
3rd field – rice;
6th field – wheat (vegetables).

36. Cotton crop rotations
On the one hand, cotton is demanding to soil, on the other – it is not very demanding to the previous crop and can be planted after itself for several years.
Example:
1st field – legumes;
2nd field – wheat;
3rd field – cotton;
4th field – wheat, barley;
5th field – maize;
6th field – wheat.

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