Scientific Foundations of Crop Rotations
Scientific foundations of crop rotations — a complex of reasons based on experience and knowledge accumulated by science that determine the need for crop rotation.
For the first time, the reasons for crop rotation were summarized by D. N. Pryanishnikov, based on works devoted to:
- plant nutrition — A. Thaer's humus theory, the theory of mineral nutrition of J. von Liebig;
- formation and accumulation of harmful substances in the soil under continuous cropping, by R. K. Maker, A. de Candolle, and others;
- the symbiotic ability of legumes and various effects of plants on soil properties, primarily on structure, by P. A. Kostychev and V. R. Williams.
Modern harsh market conditions dictate the requirement for the possibility of flexibly changing the structure of sown areas, both by changing crop rotation and by using repeated and continuous cropping. Therefore, deep knowledge of the theory and practice of crop rotation in this area allows agronomists and enterprise managers to increase the sustainability of agribusiness.
Classification of the Foundations of Crop Rotation
The reasons for rotation are divided into:
- physical;
- chemical;
- biological;
- economic.
Many years have passed since D. N. Pryanishnikov summarized them, knowledge in this area has expanded substantially, but the four groups of reasons he identified remain relevant.
Crop Response to Continuous and Repeated Cropping
Based on long-term experiments, crops were found to respond differently to continuous cropping. The greatest sensitivity to continuous cropping is shown by cereal crops, especially winter crops, and also in the absence of fertilizers. Yield increases from introducing crop rotation are greater than from fertilizers and range from 50 to 73%.
Table. Effect of fertilizers and crop rotation on crop yields, t/ha.
| Crop | Without fertilizers | With fertilizers | ||||
|---|---|---|---|---|---|---|
| Continuous cropping | Crop rotation | Increase from crop rotation, % | Continuous cropping | Crop rotation | Increase from crop rotation, % | |
| Winter wheat | 2.03 | 3.38 | 66.5 | 2.88 | 4.42 | 53.5 |
| Spring wheat | 1.26 | 1.89 | 50.0 | 1.87 | 2.51 | 34.2 |
| Winter rye | 1.11 | 1.92 | 73.0 | 2.23 | 3.07 | 37.7 |
| Barley | 1.31 | 1.98 | 51.1 | 2.26 | 2.97 | 31.4 |
| Oats | 0.92 | 1.42 | 54.3 | 1.43 | 1.86 | 30.0 |
| Potato | 10.94 | 14.05 | 28.4 | 18.94 | 23.05 | 21.7 |
| Maize for silage | 16.47 | 19.95 | 21.1 | 29.21 | 31.37 | 7.4 |
| Sugar beet | 6.99 | 16.99 | 143.0 | 18.18 | 30.18 | 66.0 |
Repeated cropping, as well as prolonged continuous cultivation in the same place, leads to the spread of weeds such as common windgrass (Apera spica-venti), scentless mayweed (Tripleurospermum inodorum), rye brome (Bromus secalinus); in spring crops — white goosefoot (Chenopodium album), hemp-nettles (Galeopsis spp.), spurreys (Spergula spp.), cleavers (Galium aparine), knotgrass (Polygonum spp.), etc. For example, if winter crops were sown after winter crops, infestation with scentless mayweed (Tripleurospermum inodorum) was 650 plants/m2, after a vetch-oat mixture — 127, after barley — 40, after clover — 25, and after bare fallow — 5 plants/m2 (table).
Table. Weed infestation of crops and crop yields depending on their cultivation conditions.
| Crop | Weeds, plants/m2 | Weed mass, g/m2 | Yield, t/ha | |
|---|---|---|---|---|
| total | including perennials | |||
| Winter wheat | ||||
| - continuous cropping | 605 | 22 | 560.3 | 2.8 |
| - convertible crop rotation | 182 | 15 | 186.1 | 3.9 |
| - grain-row crop rotation | 153 | 10 | 143.7 | 3.7 |
| Barley | ||||
| - continuous cropping | 459 | 87 | 487.3 | 2.5 |
| - convertible crop rotation | 108 | 30 | 297.4 | 2.9 |
| - grain-row crop rotation | 115 | 23 | 206.9 | 3.2 |
| - specialized grain crop rotation | 279 | 45 | 306.5 | 2.6 |
| Potato | ||||
| - continuous cropping | 66 | 8 | 897.4 | 11.5 |
| - convertible crop rotation | 159 | 16 | 537.5 | 18.6 |
| - grain-row crop rotation | 135 | 12 | 497.6 | 17.9 |
| - specialized grain crop rotation | 187 | 15 | 520.4 | 19.3 |
The effect of continuous cropping may vary depending on soil-climatic, agronomic, and other conditions. A number of agricultural practices, such as irrigation and fertilization, can reduce the effect of continuous cropping.
All crops, according to their response to continuous cropping, can be divided into groups:
1. The first group consists of crops that cannot tolerate repeated, let alone continuous, cropping. These include sunflower, sugar beet, flax, pea, beans, vetch, clover; vegetable crops: tomato, eggplant, pepper, cabbage, cucumber. The return interval to the previous field for most crops in this group is, as a rule, at least 3–4 years.
2. The second group consists of crops resistant to repeated cropping without a noticeable decrease in yield. These include barley, oats, winter and spring wheat, winter rye, millet, potato, buckwheat, carrot, and leafy vegetables. The resistance of crops in this group depends strongly on cultivation conditions.
3. The third group consists of crops that respond weakly to crop rotation and are resistant to continuous cropping. These include rice, maize, hemp, tobacco, cotton. Despite the self-compatibility of crops in this group, it is recommended that they be periodically rotated with grain legumes, cereals, and other crops.
This division of crops is rather arbitrary, but it can be used in developing crop rotations taking into account the principle of compatibility and incompatibility of crops and the principle of the periodicity of returning crops to the same field.
Separate practices that help increase crop resistance to repeated cropping include the use of varieties resistant to characteristic diseases and pests. Chemical and biological preparations may have a similar effect. For example, the use of Fusarium-resistant fiber flax varieties, clubroot-resistant cabbage varieties, and broomrape-resistant sunflower varieties makes it possible to shorten the return period to the previous field or to cultivate them repeatedly. The use of nematicides — preparations for controlling sugar beet nematode — allows repeated cropping of sugar beet.
Chemical Reasons for Crop Rotation
Chemical reasons for crop rotation are related to differences in crop requirements for nutrients and in the ability to assimilate their poorly available forms.
No crop is capable of increasing the accumulation of ash elements in the soil, except nitrogen, but crop rotation makes it possible to use their soil reserves more rationally and to increase fertilizer efficiency. Thus, a crop with a deep root system uses moisture and nutrient reserves from the sub-arable layers, and its root residues enrich these layers with organic matter and mineral elements. Deep-penetrating root systems (up to 3 meters or more) are found in alfalfa, clover, lupine, and cucurbit crops. Shallow root systems are characteristic of flax, buckwheat, millet, annual grasses, rapeseed, cucumber, and onion.
Many crops are able to assimilate nutrients from poorly available compounds: legumes, through nitrogen fixation, can meet 50–97% of their own nitrogen requirements; lupines, buckwheat, and mustard are able to assimilate phosphorus from poorly available soil and fertilizer phosphates. After mineralization of the root and stubble residues of these crops, the nutrients contained in them become available to subsequent crops that do not have such biological characteristics.
Chemical reasons for rotation are related to the nutrient balance that develops within an enterprise. Part of the nutrients is removed with the harvest, while another part, in the form of plant residues and feed, is returned with straw and manure.
Organic Matter
Agricultural crops affect the balance of soil organic matter to varying degrees. The leaving of plant and root residues after harvest replenishes soil reserves. According to the amount of plant residues left in the soil, plants are arranged in the following descending sequences:
- Non-Chernozem zone: perennial grasses — maize for silage — winter cereals — spring cereals — grain legumes — potato;
- Forest-steppe zone — perennial grasses — winter wheat — maize for grain and silage — spring cereals — sunflower — grain legumes — sugar beet.
By changing the structure of sown areas, it is possible to model the organic matter balance. For example, an increase in the share of perennial grasses in the area structure leads to accumulation of organic matter with a slowdown in its decomposition processes, while at the same time reducing the content of available nutrients in the soil. Conversely, an increase in the share of row crops and bare fallow in the structure of sown areas under insufficient application of organic fertilizers leads to a decrease in soil organic matter reserves.
The introduction of catch and green manure crops into crop rotation makes it possible to further increase the input of plant residues. In southern regions under irrigation, they leave up to 10 t/ha of residues per crop rotation, while in the southwestern and central parts of the Non-Chernozem zone — 3–5 t/ha.
In addition to accumulating organic matter, plant residues leave 22–52% of nitrogen, 2–48% of potassium, and 19–52% of phosphorus of their total amount in the harvest.
Nitrogen
Continuous cropping of certain crops that consume large amounts of soil nitrogen, such as sugar beet, maize for silage, cabbage, and cotton, can lead to nitrogen depletion of the soil. Other crops that promote nitrogen fixation by nodule bacteria, primarily legumes, by contrast, are able to accumulate nitrogen annually. For example, annual legumes under favorable conditions accumulate up to 50–70 kg of nitrogen per 1 ha, perennial leguminous grasses — up to 120–150 kg of nitrogen per 1 ha, which is equivalent to applying 400 kg of ammonium nitrate. However, repeated and continuous cropping of legumes leads to leaching of accumulated and unused nitrogen from the soil into groundwater, as well as to accumulation of phytotoxic substances, as a result of which yield sharply decreases. Therefore, rotating legumes with nitrogen-absorbing crops eliminates the negative consequences of continuous cultivation, ensuring rational use of the accumulated nitrogen.
Phosphorus
Potato, legumes, and winter cereals are distinguished by high phosphorus removal with the harvest.
Crops show different abilities to assimilate poorly available forms of soil and fertilizer phosphorus. The greatest ability to assimilate poorly available forms is found in lupine, oats, buckwheat, potato, mustard, sugar beet, and sainfoin, which, through root exudates, convert sparingly soluble soil phosphates and phosphorite meal into soluble forms. Rotation of these crops makes it possible to use soil phosphorus reserves more efficiently.
Potassium
The greatest potassium requirement is noted in sugar beet — up to 200 kg K2O at a yield of 30 t/ha, potato — up to 300 kg K2O at a yield of 30 t/ha, cereals — up to 50–60 kg K2O at a yield of 3 t/ha, as well as fodder root crops, cotton, and vegetable crops.
Physical Reasons for Crop Rotation
Physical reasons for crop rotation are determined by the influence of crops on the structure, density, aggregation, water regime of soils, and resistance to erosion processes. The reasons for this influence are the biological and morphological properties of crops: the mass and distribution of roots, decomposition of plant residues, and features of cultivation technology. The dense aboveground part, especially of solid-seeded crops, helps protect soils from erosion and improves the water and thermal regimes of soils, while organic matter formed from the decomposition of plant residues affects agrophysical indicators of fertility.
According to their ability to form soil structure, crops can be arranged in descending order: perennial grasses — annual legume-cereal mixtures — winter cereals — maize — spring cereals — fiber flax — potato — root crops.
Crops consume soil moisture to varying degrees. The transpiration coefficient serves as an indicator of consumption. Industrial crops and perennial grasses are distinguished by the highest water consumption, strongly drying out the soil at great depth, which can affect the yield of subsequent crops. Winter crops require more moisture than spring crops.
Moisture reserves in a crop rotation are affected by the period between harvesting the predecessor and sowing the subsequent crop. During this period, moisture accumulates due to atmospheric precipitation and snowmelt water, which is especially important in arid regions. Rotation of crops with different water consumption and root penetration depth makes it possible to regulate soil water reserves. The greatest moisture accumulation effect is achieved by using bare fallow, which guarantees high yields of winter and spring wheat in arid regions of the steppe zone.
Biological Reasons for Crop Rotation
Biological reasons for crop rotation are determined by different responses to diseases, pests, and weeds. This is because each crop has its own characteristic pests, diseases, and weeds. Continuous or repeated cropping promotes the exponential accumulation of characteristic pathogenic organisms capable of causing complete death of the crops. D. N. Pryanishnikov gave numerous examples of attempts at continuous cultivation of a number of crops that ended unsuccessfully primarily because of the accumulation of pests and diseases. The specificity of pests toward certain crops makes even modern methods of controlling them ineffective. This brings crop rotation to the fore. On the contrary, the specificity of pests makes them relatively harmless to other crops, owing to which crop rotation makes it possible to control the prevalence and harmfulness of most of them.
Accumulation of disease agents (fungi, bacteria, viruses) occurs in the soil and on plant residues. For example, flax dies under continuous cultivation because of the accumulation of the fungus causing Fusarium wilt. When sunflower is returned to its previous field earlier than after 7–8 years, it is affected by powdery mildew, so short-rotation crop rotations are unsuitable for its cultivation. Continuous cultivation of maize promotes accumulation of pathogens of Fusarium and blister smut, winter wheat — pathogens of leaf rust and loose smut, cotton — pathogens of wilt. The causative agents of root rots of barley and wheat are harmless to oat crops, which is why oats are recognized as a “sanitary” crop in crop rotations. Winter rye can be heavily affected by ergot, but this disease does not harm other cereal crops. Clubroot of cabbage affects only plants of the Brassicaceae family, but does not affect Solanaceae, Apiaceae, and other families.
The main source of preservation and spread of diseases, primarily fungal, is the plant residues of host plants. Therefore, freeing the soil from pathogens is related to the rate of residue decomposition. The main role in the decomposition of plant residues belongs to saprophytic microorganisms, which compete for life factors with pathogenic fungi. Therefore, all practices aimed at increasing the activity of soil saprophytes help reduce the number of pathogens, that is, improve the phytosanitary condition of the soil. This is achieved by applying manure, using green manures and mineral fertilizers, cultivating legumes, loosening the soil, and other practices. Cultivation of row crops and fallowing, observance of the periodicity of returning crops to the previous field, and avoidance of repeated cropping help reduce pathogenic organisms in the soil.
Soil fatigue is the accumulation in the soil of root exudates that suppress plant growth. Soil fatigue is also associated with the accumulation of characteristic pathogens and their metabolic products.
Weeds
Weeds, due to their ability to adapt quickly to growing conditions, have become specific to certain crops. Therefore, continuous and repeated cropping lead to the accumulation of specific weeds.
For example, wild oat (Avena fatua) is a specific weed of early spring cereal crops, primarily oats. Corn cockle (Agrostemma sp.), larkspur (Delphinium sp.), shepherd's purse (Capsella sp.), field pennycress (Thlaspi arvense), cornflower (Centaurea cyanus) grow predominantly in winter rye and winter wheat crops; amaranth (Amaranthus sp.), common barnyardgrass (Echinochloa crus-galli), yellow foxtail (Setaria pumila), green foxtail (Setaria viridis) — in rice, millet, and maize crops.
In continuous crops of winter crops, the number of winter and wintering weeds increases; in crops of late spring crops — late spring weeds; in fields of perennial grasses — perennial, biennial, wintering, and winter weeds.
Cultivated plants show different competitive abilities in relation to weeds. High competitiveness is shown by crops of summer grasses, winter rye, and winter wheat. Low competitive ability is characteristic of flax, sugar beet, and spring wheat crops. Medium — in barley and maize crops. Some crops may show high competitiveness toward some weeds and low toward others. Sunflower, maize, castor bean, and hemp have the ability to naturally suppress weeds due to broad leaves and tall stands. Winter wheat and winter rye also have this ability due to rapid growth in spring, outgrowing many weeds.
In row crop stands and bare and occupied fallows, the most favorable conditions for weed control develop due to frequent hoeing and soil tillage. Proper rotation with solid-seeded crops makes it possible to keep field weed infestation under control. An increase in the share of solid-seeded crops in the structure of sown areas leads to an increase in field weed infestation, whereas an increase in the share of row crops and fallows reduces it.
Table. Weed infestation of winter wheat crops depending on crop rotation and fertilizers.
| Predecessor | Without fertilizers | With fertilizers | ||
|---|---|---|---|---|
| Number of weeds per 1 m2 | Fresh weight of weeds, g/m2 | Number of weeds per 1 m2 | Fresh weight of weeds, g/m2 | |
| Early potato | 65 | 19.5 | 133 | 39.7 |
| Clover | 79 | 21.1 | 95 | 66.7 |
| Maize for green fodder | 64 | 18.5 | 63 | 43.3 |
| Pea | 65 | 20.8 | 111 | 72.3 |
| Continuous cropping (3–4 years) | 248 | 87.2 | 327 | 278.3 |
According to studies by S. A. Vorobyov, in continuous crops of winter wheat under conditions of the Moscow region, weed infestation was 4–5 times greater than after predecessors.
Crop rotation makes it possible to use competitive advantages as effectively as possible to control weeds specialized to other crops. For example, spring crop stands suppress weeds characteristic of winter crops, primarily winter and biennial plants. Conversely, in stands of perennial grasses and winter crops, early and late spring weeds are sharply suppressed.
Pests
In repeated and continuous crops, favorable conditions are created for the life and spread of pests characteristic of the crop. Crop rotation, along with the use of pesticides, makes it possible to significantly reduce the population of many pests and reduce the damage they cause. In cases where some pests are resistant to control agents or pesticides are of low effectiveness, crop rotation becomes of primary importance, especially against nematodes. The area affected by nematodes in continuous crops of winter rye increases to 66%, and of spring wheat — to 91.3% (Bashkir State Agrarian University). The yield decrease when sugar beet is replanted in fields heavily infested with nematodes is 60–70%. A similar situation was observed in repeated crops or frequent return of potato affected by potato nematode.
In addition to cultivating crops in one place, which promotes accumulation of pests specific to that crop, the influence of preceding crops on subsequent ones should also be taken into account. For example, perennial grass stands promote the spread of wireworms, which severely damage subsequent crops, such as cereals, maize, and potato.
Economic Reasons for Crop Rotation
Economic reasons for crop rotation are determined by the structure of sown areas and by natural and economic factors of agricultural production with the aim of ensuring maximum profitability while maintaining stable farming.
The structure of sown areas is developed for the conditions of a specific enterprise on the basis of production plans, taking into account the productivity and economic efficiency of crops and their effect on soil fertility. Different plants under different soil and climatic conditions can produce unequal amounts of feed units of different quality, which is related to their biological characteristics. For example, sugar beet or maize in one zone may produce a high yield per unit area and, accordingly, be economically profitable, while in another zone, on the contrary, may produce a low yield and be unprofitable.
In economic terms, crop rotation should ensure:
- the most rational and productive use of land with simple or expanded reproduction of fertility;
- the enterprise's needs (if necessary) for feed and seed material;
- uniform use of machinery;
- introduction of modern intensive technologies;
- efficient organization of labor.
As D. N. Pryanishnikov noted, the economic necessity of crop rotation is related to the different amounts and distribution of labor over time required for cultivating different crops on the farm. Thus, cultivating early and late spring crops, because of different sowing and harvesting dates, makes it possible to reduce the load on people and machinery in the same period by 2 times compared with cultivating only early or only late spring crops. Adding winter crops makes it possible to distribute the intensity of field work even more evenly. The use of varieties with different ripening periods gives a similar effect. This reduces the risk of failing to meet optimal deadlines for field work.
A certain ratio of cereals, industrial crops, and fodder crops makes it possible to build a balance between manure-producing (fodder crops) and manure-consuming (cereals, row crops) plants.
Economic reasons are related to the other reasons for crop rotation, since properly structuring crop rotation helps increase the economic return of production. For example, with the help of crop rotation in combination with a number of agricultural practices, it is possible to reduce weed infestation of crops and reduce the population of pathogenic organisms and pests to the economic threshold of harmfulness, thereby reducing costs for agrochemicals and increasing yield, which increases production profitability.
Ecological Reasons for Crop Rotation
Ecological reasons for crop rotation are determined by replacing or reducing the use of agrochemicals through the positive influence of a properly structured crop rotation on field weed infestation and populations of harmful organisms. In addition, crop rotation plays a significant role in the system of soil-protective farming. Through the use of agronomic and biological methods of weed and pest control, oversaturation of land with pesticides is prevented.
Agromeliorative measures aimed at protecting soil and the environment include:
- creation of buffer strips along field boundaries;
- planting of shelterbelt plantations;
- creation of a road network in fields;
- organization of systems for retaining atmospheric precipitation and snowmelt water;
- construction of irrigation systems, etc.
In addition to meliorative measures, soil-protective measures include special agronomic practices of contour soil tillage and shelf and coulisse placement of crops. This provision is regulated by the Federal Law “On Environmental Protection,” aimed at reducing the environmental threat associated with agricultural activity. Thus, crop rotation is an integral part of ecological land use within the boundaries of unified agricultural landscapes.
Prevention of water erosion of agricultural lands that are part of the agricultural landscape (meadows, pastures, forests) is achieved by using special contour methods of soil tillage (slotting, mole drainage, ridging). To prevent wind erosion, for example in steppe regions, shelf placement of crop rotation crops and bare fallows is used across the prevailing winds, combining them with coulisses in a system of no-moldboard flat-cut tillage. Thus, the crop rotation system, like an individual crop rotation, performs a soil-protective function.
The ecological function of crop rotations is manifested in their ability to reduce the content of harmful substances, pesticides, and growth regulators accumulated from agricultural production, reducing the risk of their entry with runoff into rivers, water bodies, and groundwater.
List of Sources
1. Niklyaev V. S. (2000). Fundamentals of Agricultural Production Technology. Farming and Crop Production. Moscow: “Bylina”.
2. Bazdyrev G. I., Loshakov V. G., Puponin A. I., et al. (2000). Farming. Textbook for Universities. Moscow: “Kolos” Publishing House.
3. Evtefeev Yu. V., Kazantsev G. M. (2013). Fundamentals of Agronomy: A Textbook. Moscow: FORUM.
