Conserving Soil in Organic Crops
Soil is the production base of all agricultural systems. 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 organic matter content. Soil conservation promotes practices that stop the decline and, over time, improve soil quality. In general maintaining soil cover, living plant roots and limiting soil disturbance are the best ways to limit erosion.
Conservation practices generally reduce wind erosion, reduce rate and amount of lateral water movement in fields, and increase soil organic matter levels. There is not one way of conserving soil that is well suited to all situations, due to differences in soil type, topography, type of farming operation and climate.
In organic crop production, the challenge increases, because conservation practices that involve pesticides are not an option. In every case, it is generally more effective to use a number of conservation strategies. 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 to keep soil surface covered. The challenge is determining which conservation practices are best adapted to each farm.
Crop residues include roots, chaff, stems and leaves are left after a crop is harvested. Crop residues are the prime source of organic matter replenishment. These residues improve several soil properties, such as water infiltration, water storage and particle aggregation. Crop residues also contain nutrients, including nitrogen, phosphorus, potassium, sulphur and micronutrients. Stubble left standing overwinter will trap snow and slow evaporation of soil moisture in spring, as well as prevent erosion. Moisture conservation is important to soil conservation because the additional moisture will improve crop growth the following year.
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
|Wind erosion control|
|Soil texture||Cereal residue required (lb./ac.)|
|Water erosion control|
|Field slope||Cereal residue required (lb./ac.)
|Steep (16-30%)||continuous grass|
|Very steep (>30%)||native vegetation|
Crop residue conservation during tillage is affected by equipment type, speed, depth and frequency of tillage, as well as soil and climatic factors. Limiting tillage depth, speed and the number of operations all conserve crop residue and soil moisture. Shallow tillage allows crop residue to accumulate near the soil surface, where it will be most effective in reducing wind and water erosion, improving infiltration and reducing evaporation. Reducing tillage speed usually lessens crop residue burial.
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 rod weeder to a cultivator does not significantly affect residue reduction.
Table 2. Approximate residue conservation with various tillage implements
|Tillage implement||Residue reduction per operation (%)||Residue left after four operations (%)|
|Wide blade cultivator (35 inch sweeps)||10||50-60|
|Rod weeder||5-10||no data|
|Heavy duty cultivator (16-18 inch sweeps)||20||30-40|
|Heavy duty cultivator with rodweeder||20||30-40|
|Heavy duty cultivator with harrows||40||15-20|
|Field cultivator (9-12 inch sweeps)||20||30-40|
|Field cultivator with harrows||40||15-20|
|Tanderm disc - offset disc||30-70||5-15|
|Moldboard plow||90||no data|
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.
Trapping more overwinter snow with "tall" or "sculptured" stubble enhances moisture conservation. Tall stubble refers to stubble that is cut 12 or more inches high, usually when straight combining. Sculptured stubble refers to when a swather-mounted clipper or deflector cuts strips of taller stubble with each swath, or when alternate swaths are cut at normal height and taller.
Zero-till, direct seeding is not usually associated with organic crop production because herbicides cannot be used. Some Saskatchewan organic producers, however, do practice direct seeding. Thus, producers may wish to consider this conservation practice when fall and spring weed pressure is low, and previous crop straw and chaff have been adequately spread.
Extended Crop Rotations
Summerfallowing is destructive to the soil because no new organic matter is returned to the soil during the fallow year. Tillage also speeds the breakdown of soil organic matter and predisposes the soil to erosion. Extending crop rotations is a conservation practice because it reduces the incidence of summerfallow.
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.
Forages contribute significant amounts of organic material to the soil. They also offer an alternative commodity in the form of hay, silage or seed. Semi-permanent forage production should be considered on soils, which are inherently low in productivity and/or vulnerable to erosion. Forage production for two to four years should also be considered as part of a normal crop rotation.
Selection of forage species and management practices can be tailored to specific problems such as drought, excessive soil moisture, salinity, poor soil structure, low pH and other problems.
Complementary Rotational Crops
An ideal rotation should be as diverse as possible including cereals, oilseeds, pulses, fall-seeded crops and forages. A diverse crop rotation, in addition to many other benefits, can help soil nutrient availability, because different crops have a different demand for and ability to remove particular nutrients.
All crops require 16 essential nutrients, and in Saskatchewan, most soils are deficient in nitrogen and available phosphorus. Some soils are also deficient in sulphur and potassium. Micronutrients are rarely deficient. For example, canola and alfalfa have a high demand for sulphur and should not be grown on deficient soils. Where copper is deficient, as in some grey soils, barley or rye will provide comparatively better yields than the other cereals.
Growing legumes in the rotation provides both nitrogen and non-nitrogen benefits to subsequent crops. Properly inoculated/nodulated legumes fix 50 to 90 percent of their nitrogen requirement from the air. The remainder is obtained from the soil. in addition, nitrogen is also exuded from legume roots during the growing season and can be utilized by the companion crop if intercrops are grown. 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 to be the 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.
Annual Crop Barriers in Crop
Barriers of "taller" annual crops have been used to a limited degree in low residue-producing crops. A divider is placed in the seedbox so that two rows of wheat or flax are seeded every seeder width or two of lentil. At harvest, the lentil is combined and the barrier strip left standing to trap snow and prevent wind erosion during the upcoming winter.
Perennial Grass Barriers
Perennial grass barriers are two rows of grass planted perpendicular to prevailing winds to reduce wind erosion, trap snow and reduce evaporative losses. Barriers should be placed 30 to 60 feet apart, depending on soil type; closest on sands, moderately spaced on clays and furthest apart on loams. Barriers may be placed further apart if other soil conservation practices are also being used.
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.
Shelterbelts can effectively reduce wind velocity for a distance approximately 20 or more times their height. This is usually sufficient to control wind erosion for a distance of approximately 10 or more times their height when planted perpendicular to prevailing winds. Their effectiveness, however, does depend upon shape, porosity and maintenance as well as height. Shelterbelts have the added benefit of increasing crop yields, particularly of less drought-tolerant crops such as canola and alfalfa..
Strip cropping consists of alternating strips of crop and summerfallow at an angle perpendicular to the prevailing winds. The strip width varies depending on soil texture. Sandy soils are the most prone to wind erosion, followed by clays, then loams. Strip cropping works well for loams to clays where eight to 10 strips per quarter section will significantly reduce the potential for wind erosion. With sandy textured soils, however, too many strips are required to be manageable. Keep field equipment sizes in mind when establishing strip widths.
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.
Cocktail Cover Crops
A cover crop mixture is one way to get the benefits of multiple cover crops, and on steep land, a high-residue cover crop to prevent erosion should be included in the cover crop mixture. Cereal crops should be included at a rate of one-third to one-half in the mixture and the others can be legume crops. Some of the cocktail cover crops will either be killed by fall frost remaining on the soil surface until spring planting, providing valuable soil protection or start growing the following spring. These crops may be removed by tillage or used for short term livestock grazing or grown to maturity in the case of winter wheat or fall rye.
Green manuring involves legume crops, or incorporation with any other green, fresh vegetative material into the soil. Green manure crops add nitrogen and organic matter to the soil, improve fertility and soil tilth. Weeds are also regarded as green manure. The extent of soil improvement depends on the type of legumes and quantity of plant material returned to the soil.
The greatest benefits usually occur by using biennial (sweetclover) or perennial legumes (alfalfa or true clovers) as green manure on poor structured soils with low organic matter levels. If sweetclover is used as a green manure crop, it should be incorporated into the soil at the time of approximately 10 per cent bloom. When plowed down at this stage, the legume crop will contribute the maximum amount of nitrogen to the soil and also allow soil moisture recharge for the following year's crop.
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 Indian Head lentil and specific field pea varieties 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.
The spreading of livestock and poultry manure provides not only nutrients required for plant growth but has a major beneficial effect on soil tilth and particle aggregation. The organic materials contained in manure act as binding agents in stabilizing soil structure. This positive change in soil structure caused by the addition of manure is equally, if not more important, than the nutrient contribution provided. Changes in structure of this nature positively affect water infiltration, water holding capacity and aeration, as well as resistance to wind and water erosion.
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 based on nitrogen content. 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.