Organic Crop Production: Soil Conservation Practices
Soil is the production base of all agricultural systems and soil conservation is one of the pillars of sustainability. Soil quality has deteriorated due to wind and water erosion, as well as farming practices that reduce soil organic matter content. Soil conservation promotes practices that stop the decline in soil quality and, over time, improve soil quality.
Conservation practices are generally those that reduce wind speed, reduce rate and amount of water movement, and/or increase soil organic matter levels. No one conservation management system is suited to all situations due to differences in soil type, topography, type of farming operation, and climate. Furthermore, it may be necessary and desirable to employ a number of conservation practices. For example, rather than plant a large number of shelterbelts on a particular field, it may be better to plant a lesser number and reduce tillage operations or include green manure more often. The challenge to producers is to find and employ conservation practices best adapted to their farm.
The challenge is greater with organic crop production, however, because those conservation practices that use herbicides are not an option. Also, some common organic crop production practices such as post-emergent harrowing for weed control are destructive to the soil. Accordingly, the organic crop producer may need to employ additional or more aggressive conservation measures if practices such as post-emergent harrowing are used.
The amount of residue produced and the rate of decay vary among crops. The combination of these two factors determines the quality of residue in relation to its value for soil conservation. Field pea and flax generally produce about half the residue of cereals and canola. Residue from canola, field pea, lentil, and sunflower usually decays very rapidly (particularly when tilled), leaving the soil surface unprotected. Conventional tillage summerfallow after oilseed or pulse crops is not recommended, unless an extensive system of wind barriers will prevent wind erosion and water erosion is not a concern. Even where recropping is practiced, excessive tillage after oilseed and pulse crops can promote serious erosion.
Cereals usually produce an acceptable level of crop residue that decays at a moderate rate. The surface cereal residue levels required for erosion control are listed in Table 1. When designing a crop rotation, factors of this nature should be considered.
Table 1. Approximate surface cereal residue levels required for erosion control
Tillage equipment type significantly influences residue conservation (Table 2). Tillage implements such as a wide blade cultivator or rod weeder conserve significantly more residue than a cultivator which, in turn, is better than a discer. The addition of harrows to a field or heavy duty cultivator doubles the amount of residue buried. The addition of a rodweeder to a cultivator does not significantly affect residue reduction.
Table 2. Approximate residue conservation with various tillage implements
The need for each tillage operation should be carefully considered. For example, in the brown and dark brown soil zones fall tillage may be of little benefit unless a heavy infestation of moisture-consuming weeds is present. In many cases fall tillage reduces moisture conservation by disturbing standing stubble, which diminishes its effectiveness as a snow trap and exposes the soil to erosion. In the brown soil zone standing stubble will conserve one-half to one inch more moisture over winter than bare soil. In contrast, fall tillage may offer some advantage in the more humid black and gray soil zones where moisture is rarely limiting and where crop residues may pose a problem during preparation of the seedbed. Even in these areas, however, fall tillage may not result in increased yields.
In all situations, avoid tillage under wet soil conditions as this can degrade soil structure and significantly reduce surface residue levels.
Extended Crop Rotations
The benefits of an extended crop rotation are numerous and include improved fertility, tilth, aggregate stability, moisture storage, and resistance to soil erosion and degradation, as well as reductions in insect, weed and disease problems. All of these factors contribute to increased productivity and most have a significant positive effect on soil sustainability. Decisions regarding cropping strategies should consider not only short-term benefits, but also their long term effects on soil and environmental quality. A diverse rotation should include cereals, oilseeds, pulses, fall-seeded crops and forages. The level of crop diversity determines the significance and degree of the rotational benefits. Selection and management of legume species (pulses and forage legumes) within a rotation is a vital aspect of achieving diversity and supplying nitrogen through symbiotic nitrogen fixation.
Complementary Rotational Crops
Growing legumes in the rotation provides both nitrogen and non-nitrogen benefits to subsequent crops. Properly inoculated/nodulated legumes fix 50 to 90 per cent of their nitrogen requirement from the air. The remainder is obtained from the soil. However, nitrogen is also exuded from legume roots during the growing season and the legume residue decomposes and recycles the nutrients faster than non-legume residues, thus more nitrogen is usually available to the subsequent crop than if a non-legume had been grown. Furthermore, research has shown that the non-nitrogen benefits (such as disease suppression, tilth improvement, etc.) of growing legumes in the rotation may result in increased yields.
The growth patterns of various crops should also be taken into account when planning complementary rotational cropping. Broadleaf crops such as pea, lentil, flax and polish canola generally extract moisture and nutrients from shallower depths than spring-seeded cereals. Fall rye and winter wheat root deeper, earlier in the growing season than spring cereals, using early spring moisture to their advantage. Winter cereals also have an advantage over spring cereals because they usually flower earlier, when soil moisture reserves are more plentiful. Perennial forages can be very deep rooted, using moisture and nutrients from the subsoil. Shallow rooted crops appear best adapted to follow a deep-rooted crop because water recharge is likely to occur only near the soil surface and a shallow-rooted crop will not expend energy rooting deeper in search of moisture that is not there. Medium or deep-rooted crops appear better adapted to follow shallow-rooted crops as they are able to take advantage of any moisture left at depth, not used by the previous shallow-rooted crop.
For more information on legumes in the rotation and nutrients, refer to Soil Improvement with Legumes and Organic Crop Production: Fertility.
Annual Crop Barriers on Summerfallow
A number of commercial strip seeders are available for purchase and mounting on tillage equipment. Following seeding, these barriers are left intact, to protect the soil until planting the following year. This may require adjustments of summerfallow tillage widths or removal of two or more cultivator shanks to compensate for the barriers. The following crop may look poor where the barriers were located the previous year. However, research has found yield losses amount to less than two per cent over the entire field.
Annual Crop Barriers in Crop
Perennial Grass Barriers
Species such as tall wheatgrass work well, as it is usually a weak competitor with most field crops and will not spread beyond the seeded rows. It also grows high enough without lodging to trap snow, helping in soil moisture recharge.
Strip cropping is a more common practice in the drier areas of the prairies where often too little crop residue is present to prevent wind erosion. It can be used in wetter areas, however, provided the strips also run perpendicular to the slope, so that water erosion does not become a problem.
Grain legumes (pulses) can be used effectively as green manure. Due to their annual growth habit these crops will not contribute nitrogen or crop residue to the same degree as a biennial or perennial legumes. They may, however, be more adaptable to an existing crop rotation. Pulses such as Indianhead lentil have been specially developed for this use.
Non-legume crops such as buckwheat have also been used as a green manure crop.
To protect against soil erosion, do not over-incorporate green manure crops.
For more information on green manure and nitrogen fixation by legumes, refer to Soil Improvement with Legumes.
The nutrient value of manure is highly variable depending on numerous factors such as animal type and age, type of feed, amount of straw, and method and time of storage. Typically, barnyard cow manure contains approximately three to five lbs. of crop available nitrogen, four to 11 lbs. of phosphate, nine to 16 lbs. of potassium and about three lbs. of sulphur, per ton of manure. Manure usually has sufficient micronutrients present to prevent plant deficiency symptoms from occurring. Soil testing laboratories test animal manure for nutrient content and make recommendations on manure application rates.
Application rates of manure will vary depending on availability, soil type, location, slope, crop rotation and production practices. To prevent leaching losses and potential environmental contamination, rates of manure application should not exceed what a crop can use in one growing season. Following the application of manure, it should be incorporated as quickly as possible into the soil to prevent nitrogen loss. In addition, changes in soil nutrient levels resulting from manure should be monitored by soil testing on a regular basis.
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The amount of nitrogen fixed varies according to the legume species and variety.
Selection of an appropriate forage species is an important first step in successful forage production.
There are 16 nutrients that all plants must have.
As a component of the AAFC-PFRA Agroforestry Division, the Prairie Shelterbelt Program (PSP) provides technical services and tree and shrub seedlings for establishment of shelterbelts and other agroforestry, conservation and reclamation projects on agricultural and eligible lands in Manitoba, Saskatchewan, Alberta and in the Peace River region of British Columbia.