Soil Management on Organic Farms
Management Practices to Improve Soil Health
Organic farmers use a wide variety of practices to maintain or improve soil health in organic production systems. These practices generally are part of long-term, site-specific management programs that aim at developing fertile and biologically active soils that readily capture and store water and nutrients, have good tilth, and suppress plant disease. Deliberate and routine carbon inputs are essential to achieving this goal in organic production environments. Special care is needed to select organic carbon sources that will ensure short-term productivity while building long-term soil quality.
Tillage prepares land for seeding or transplanting. Cultivation practices—implemented either before or after crops are in the ground—manage weeds and improve soil aeration and water infiltration. Traditional clean tillage disrupts soil organisms, reducing their numbers and, often, their diversity. For instance, clean tillage can also disrupt the hyphal network of mycorrhizal fungi, which can lead to their decline over time. Clean tillage also breaks down soil aggregates and produces a bare soil that is vulnerable to erosion and more subject to temperature changes than soils that have a mulch or vegetation cover. Under direct exposure to sun and high temperatures, soil organisms break down soil organic matter rapidly. The rate of organic matter decomposition further increases when tillage breaks soil aggregates into smaller pieces, increasing their surface area and exposure to oxygen.
Conservation tillage systems reduce the rate of organic matter oxidation compared with the conventional tillage systems, balancing microbial activity and slowing down decomposition of root biomass and below ground organic matter (See Figure 4.2). Residue left on the soil surface slows decay, thereby maximizing residue carbon input. Conservation systems use tillage practices that are defined by the percent of residue cover left on the soil surface, such as ridge-tillage, strip-tillage, and no-tillage.
Cover crops are widely seen as an important part of soil quality management in organic production systems. Cover crops are planted to provide ground cover or green manure between cash crops. Cover crops contribute numerous benefits to soil health. They keep the soil covered during the winter and other periods of time when crops are not growing, reducing the risk of erosion. The biomass produced by cover crops is usually returned to the soil, enhancing organic matter levels. Cover crops with taproots can create macropores and alleviate compaction.
Crop rotations involve a systematic farm plan, where the crop planted in one field on the farm changes every year or every season (See Figure 4.3). While soil quality derives from inherent parent material, climate, and topography, crop rotation also affects soil quality and health. More diverse crop rotations that include more crops in the rotation tend to have better soil quality. Longer crop rotations have been shown to improve the soil’s physical properties, decrease erosion, reduce nitrogen leaching potential, improve soil organic matter, and provide competitive crop yields.
Incorporating intercropping principles into an agricultural operation increases diversity and interaction between plants, arthropods, mammals, birds and microorganisms resulting in a more stable crop-ecosystem and a more efficient use of space, water, sunlight and nutrients (See Figure 4.3). Furthermore, soil health is benefited by increasing ground coverage with living vegetation, which reduces erosion, and by increasing the quantity and diversity of root exudates, which enhance soil fauna. This collaborative type of crop management mimics nature and is subject to fewer pest outbreaks, improved nutrient cycling and crop nutrient uptake, and increased water infiltration and moisture retention. Soil quality, water quality and wildlife habitat all benefit. For a more in depth discussion on manure go to Chapter 8, Intercropping on Organic Farms.
While composts and manures are frequently considered to be mainstays of fertility management programs in organic systems, these amendments often vary widely in nutritive value and thus are increasingly being applied as a basic carbon source to enhance overall and long-term soil health. The carbon content of these materials is also quite variable, though it generally ranges from 20 to 40 percent on a dry-weight basis. Annual applications of composts and manures at rates of three to five tons per acre are common in organic farming systems. Such organic amendments add significant amounts of carbon to the soil and are generally associated with improved tilth, lower bulk density, and increased water infiltration.
The use of livestock manure to build up soil health, specifically soil organic matter, is a practice that has been embraced by organic farmers. Increasing soil organic matter improves soil structure or tilth, increases the water-holding capacity of coarse-textured sandy soils, improves drainage in fine-textured clay soils, provides a source of slow release nutrients, reduces wind and water erosion, and promotes growth of earthworms and other beneficial soil organisms. Most vegetable crops return small amounts of crop residue to the soil, so manure, compost, and other organic amendments help maintain soil organic matter levels. Raw manure is high in nutrients, especially readily available nitrogen. Fresh, non-composted manure will generally have a higher nitrogen content than composted manure. However, the use of composted manure will contribute more to the organic matter content of the soil.
The most important benefit of using compost is the increase in soil organic matter in addition to adding nutrients to the soil. Generally, compost contains relatively low concentrations of nitrogen, phosphorus, and potassium compared to manure. Just like manure, the nitrogen content of compost will vary according to the source material and how it is composted. In general, nitrogen in the form of ammonium (NH4+) or nitrate (NO3¯) is low in compost compared to manure.
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