How and Why Conservation Agriculture Works
To better understand how and why the system works to enhance and sustain agricultural production while conserving natural resources, we should consider each of the essential features of conservation agriculture one by one. These include:
- Maintaining permanent or semi-permanent soil cover. Plants are left growing or killed and their residues left to decompose in situ. The primary function of this is to protect organic matter-enriched topsoil against chemical and physical weathering. Plant residues intercept energy from falling raindrops, provide a barrier from strong winds, and moderate temperatures, improving water infiltration and decreasing surface evaporation from sunlight. Surface cover also favors enhanced levels of biological activity by providing food for soil microbes, especially in tropical and sub-tropical areas.
- Minimum soil disturbance. No-till (NT) does not involve any loosening of the soil except for a very small area immediately surrounding where the seed is planted. This lack of soil disturbance serves to maintain overall soil structure, including aggregate stability and porosity, both of which promote the exchange of water and gases and provide habitat to an abundant and diverse population of soil biota.
- Regular crop rotations. Well-balanced crop rotations can neutralize many of the pest and disease problems associated with not tilling the soil, including the proliferation of insect pests and other harmful bacteria, viruses and fungi, by increasing the diversity and abundance of beneficial soil biota that can help keep pest and disease problems in check. Rotating crops also interrupts the life cycle of many weeds, thereby leading to a reduction in overall weed growth. These benefits translate to a typical yield increase of about 10 percent of crops grown in rotation, as compared to those grown in monoculture.
- Utilization of green manures/cover crops (GMCC's). Cover crops are grown specifically to help maintain soil fertility and productivity. GMCC's increase soil organic matter (SOM) levels in at least one of two ways - by decreasing erosion and/or by adding fresh plant residues to the soil. Leguminous cover crops offer the added advantage of being able to fix nitrogen from the atmosphere and add it to the soil, thereby increasing overall nitrogen availability for other crops. Cover crops are usually mowed, sprayed with chemical herbicides or otherwise killed before or during soil preparation for the next economic crop. It is generally recommended that you leave a week or two between the killing of the cover crop and the planting of a primary crop in order to allow for some decomposition to occur as well as to lessen the effects of nitrogen immobilization and allelopathic effects.
- No burning of crop residues. Since crop residues are the principal element of permanent soil cover, they must never be burned or otherwise removed from the soil surface. Rather, plant residues are left on the soil surface in order to protect organic matter enriched topsoil from erosion while also adding fresh organic matter upon decomposition. Burning not only creates significant air pollution but also dramatically increases mineralization rates, leading to the rapid depletion of soil organic matter and nutrients from the soil. However, in some situations farmers need to think of the tradeoff between removing residues to feed their animals and leaving them to feed the soil. A win-win situation would do both and as yields and biomass increase over time, both become more feasible.
- Integrated disease and pest management. Conservation agriculture depends heavily on enhanced biological activity to help control insect pests and other disease causing soil organisms. Integrated pest management (IPM) entails the judicious use of crop rotations and other beneficial plant associations as well as chemical pesticides, herbicides and fungicides to control insect pest and disease problems. Over time, the enhanced biological activity and abundance brought on by no-till and other CA technologies results in decreased applications of agrochemicals.
- Reduction in fossil fuel use and greenhouse gas (GHG) emissions.
Compared to conventional tillage, which often requires 4-8 tractor passes in a typical growing season, no-till significantly reduces the use of tractors and other heavy farm machinery and thus diesel. Furthermore, the increased levels of soil organic matter (SOM) and plant-available nitrogen typically found in CA soils greatly reduces the need for applying large amounts of chemical fertilizers, many of which require significant fossil fuel energy to process. Thus, overall fossil fuel use and greenhouse gas emissions are greatly reduced.
- Controlled / limited human and mechanical traffic over agricultural soils. As mentioned above, the number of tractor passes over a given field is significantly reduced under CA, as compared to conventional tillage systems. However, increased bulk densities have been reported under CA, though this can be corrected by limiting the use of heavy farm machinery when soils are wet and most prone to compaction and/or by converting to a permanent raised bed system.