Conservation Agriculture (CA) is an agricultural management approach that evolved to sustainably address issues of soil erosion as described in Chapter 10 and soil management as described in Chapter 9 for crop production. The Food and Agriculture Organization (FAO) of the United Nations (UN) defines CA as an approach to farming (crop production) that sustains and improves production for both food security and profits while also preserving and enhancing environmental resources (United Nations, 2015). CA is based on the following three principles:
These principles are widely applicable to many different crops, climates, and soils; for example, they have been successfully applied in a range of climates from the Arctic Circle to the tropics.
Cultivation of crops requires planting of seeds. There are many approaches to planting seeds, from direct seeding where seed is dropped into a slot that is then pressed closed with minimal soil disturbance to intensive tillage that prepares a “clean” seedbed with primary tillage or inversion of the top layer of soil with a moldboard plow followed by secondary chisels, disks, and harrow tillage resulting in a cycle of erosion that has severely degraded soils. CA advocates minimum soil disturbance, where crop seeds are inserted directly into the soil through the residue of the last crop, omitting the steps of seedbed preparation with intensive tillage. Direct seeding/sowing, direct drilling, no-till farming, zero tillage, and no-tillage are all terms used to describe this approach. Large seeds such as maize and beans can be placed at a desired depth with mechanized equipment called no-till planters or drills that cut through the residue, open a slot and place the seed into that slot, ideally closing the slot so that the seed is in good contact with the soil and the soil is covered by the residue. Very small seeds may be broadcast and will germinate without incorporation just as occurs in native ecosystems.
If tractor-drawn planters are not available, no-till or minimum tillage can also be carried out manually, typical of small scale farming, using several methods including planting basins, planting spots, rippers, planting sticks or jab-planters, and animal-drawn planters. A planting basin is a hole dug 6 in. (15 cm) wide by 12 in. (30 cm) long by 6–8 in. (15–20 cm) deep and spaced in rows about 30 in. (75 cm) apart, using a hoe as the digging implement. Basins are prepared during the dry season and after being dug, inputs such as manure, fertilizer, and lime can be placed in the basin, which is then partially filled with soil to the planting depth. At seeding time after the first rain, multiple seeds are placed in the basin and then covered with soil and/or mulch. Shortly after germination the plants are often thinned to 2–3 plants per basin. After basins are dug the first year, they can be reused in subsequent years thus saving labor. Basins direct rainfall to the seed area and provide a framework for more precise placement of fertilizer.
Planting spots are more shallow planting holes dug to the seed planting depth with a hand hoe, sowed with seed, and then covered with soil, and can be planted with less time than basins in the dry season or just after the rains begin. About 2 weeks after the crop emerges, a stick can be used to make a hole about 2.5 in. (10 cm) away from the seedling to place fertilizer for plant growth.
A wooden stick with a pointed end, called a planting or dibble stick can be used to create a hole to a certain depth. The seed is dropped in the hole, which is then stepped on to close the hole and ensures seed contact with soil. Jab planters are light-weight, hand-carried, and -operated devices that have a seed hopper connected to a shaft that delivers 1–3 seeds at a time to a steel furrow opener at the end of the shaft. The action of pulling the two handles apart and simultaneously jabbing the tip of the shaft into the soil, sets the seed into the tip. A second action of pulling the handles together delivers the seeds and/or fertilizer into the hole created by the tip. Jab-planters can come with one or two hoppers for seed and fertilizer. Jab planters can also be used to fill gaps that are noticed after germination.
Rippers are chisel-pointed tools that open a narrow slot (furrow) about 2–4 in. (5–10 cm) deep for sowing seeds by hand, a machete or planting stick. A mechanical planter attached to the ripper itself can also be used to insert the seeds into the slot. Ripped lines are usually spaced about 30 in. (75 cm) apart.
Several varieties of animal-drawn, hand-operated, automatic, and semi-automatic planter devices are available for purchase and use that can reduce the time for seeding.
Weeds compete with crops for moisture, nutrients, space, and sunlight. Weeds can also harbor disease and pests that can attack crops. Traditional tillage agriculture used the plow as the major weed control technique incorporating weeds and residue into the soil, providing a clean seed bed in which to plant and also providing nutrients for the subsequent crop from the incorporated plant residues. In CA, weeds are killed and left in place using herbicides, by hand weeding or using equipment to cut or crush weeds, avoiding soil disturbance as much as possible and leaving plant residue on top of the soil.
Herbicides do not disturb the soil and can be quick and easy to apply. Herbicides require special application equipment such as sprayers or wipers and they require knowledge or training to determine which herbicides are appropriate and how to prepare, handle, and apply them correctly and safely. Herbicides may not be accessible or affordable by smallholder farmers.
Weeding with hand tools or hoes can be used to manually dislodge weeds. Though hand weeding is harder work, it generally disturbs the soil less than using a hoe. Knife rollers kill weeds and cover crops by bending and crushing them, and can be used before seeding the crop.
In CA, weeds are also controlled by planting crops closer together to shade out weeds. Ideal crop spacing depends on soil moisture, temperature, and fertility, and has the objective of avoiding competition between the crop plants for water and nutrients. Weeds are also controlled using cover crops, mulch, and crop rotations as described in the following sections.
Making sure the soil is continuously covered with the main crop, mulch, crop residues, and/or cover crops protects the soil from the eroding force of raindrops and excessive heating by the sun. Preventing the eroding force of raindrops on exposed soil reduces soil crusting and surface sealing enabling greater rainfall infiltration and reduced surface runoff and flooding. Constant soil cover also reduces soil moisture evaporation losses thereby increasing soil moisture content available to growing plants during dry periods.
There are two main types of soil cover, including: (1) living plant material such as crops and cover crops; and (2) mulch, compost, and/or dead plant material, which includes crop residues and cuttings or leaves from grass, shrubs, and trees. A combination of mulch and cover crops may be used to keep the soil covered.
By reducing the force, speed, and splash effects of raindrops, residues, mulch, and cover crops allow higher infiltration of water into the soil and reduce runoff, which also decreases soil erosion. The residues also form a rough physical barrier that reduces the speed of water and wind over the surface. Reducing wind speed decreases evaporation of soil moisture.
Soil cover provides the following benefits:
One of the biggest general benefits of CA may be reduced soil erosion. Erosion of soil into waterways leads to filling water reservoirs, lakes, and streams with sediment, reducing water storage capacity and the useful life of reservoirs and dams. Sediment in surface water increases wear and tear in hydroelectric installations and pumping devices, which result in higher maintenance costs and necessitates earlier replacement.
Because more water infiltrates into the soil with CA rather than running off the soil surface, streams are fed more by subsurface flow through soil than by surface runoff. Thus, surface waters are cleaner and more closely resemble groundwater in CA than in areas where intensive tillage and accompanying erosion is more prevalent. Greater infiltration will reduce flooding, by causing more water storage in soil and slower release to streams. Infiltration also recharges groundwater, which can increase water supplies of wells and springs. Sediments in surface waters have to be removed from drinking water supplies, so a reduction in eroded sediment in streams can lead to lower costs for water treatment.
A cover crop can be planted during the cropping season in between the crop rows, known as “intercropping.” For example, a low growing plant such as beans can make a good intercrop with a tall plant such as corn. Cover crops may be planted following the growing season to cover the whole field, though in arid or temperate climates with cold winters, it may be hard to grow or maintain a cover crop during the non-growing/dry season. A cover crop can also be planted in the main crop residue towards the end of the growing season, which gives the cover crop a head start to grow at the beginning of a cold or dry non-growing season.
Cover crops have multiple uses, including as edible seeds or vegetables, as animal fodder, firewood or fencing material, medicine, or to increase soil fertility as with nitrogen-fixing legumes. A cover crop that is used to add nutrients such as nitrogen to the soil is known as a green manure cover crop (GMCC). A cover crop is selected based on how it addresses the needs of the farmer or cropping system.
Some cover crops may be harvested for food or as cash crops such as winter wheat (Triticum aestivum). Cover crops may also be selected based on their ability to produce high amounts of residue for fodder or for strong root development that can pierce hard pans and reduce soil compaction, such as radish (Raphanus spp.) or pigeonpea (Cajanus cajan). Some cover crops, such as marigolds (Asteraceae), have insect pest repelling capability, or weed suppression capability with allelopathic compounds, such as ryegrass (Lolium perenne L.), which can inhibit the germination and growth of some weed species.
Crop rotation is a key principle of CA because it helps to control weeds, diseases, and pests and it helps to improve soil fertility and structure. Crop rotation involves the diversification of crop species and the sequence they are grown, or the variety of crops grown together (in association) for perennial species. Rotating crops increases the biodiversity of the soil environment and reduces the carry over and growth of crop-specific pests and diseases that result from planting the same crop in succession (monocropping). Crop rotation increases biological activity in soils by providing different food sources and diverse rooting structures. Some crops have strong deep roots that allow nutrients and moisture to be extracted from deeper layers in the soil, while other plants have shallow fine roots that create structure and channels at higher layers. These different types of roots enable nutrients to be extracted and recycled over a larger area. Different plants extract and store different nutrients, so varying the crops planted improves the distribution and balance of the major plant nutrients (nitrogen, phosphorous, and potassium) and minor plant nutrients available in the soil, contributing to diversity of the biota above and below the surface.
Crop rotations are planned according to objectives of food, fodder, and residue production; pest and weed control; and nutrient uptake and production. Some crop rotations may have a cycle of seven or more different crop species in a sequence. Species diversity can be achieved by planting a mixture of cover crop species in one season. Crop selection is also based on soil and climate conditions. Crop rotations, multiple crops grown in a year, and/or intercropping can reduce risk, since a single crop can fail due to a drought or attack by pests.
Crop rotation helps to replace plowing of the soil by aerating the soil with different types of roots, adding the organic matter from roots at various depths in the soil, recycling nutrients, and controlling weeds, pests, and diseases that can live in the residue and soil.
Though these are well known principles, in CA they are combined to work in concert, for example, maintaining soil cover and reducing soil disturbance by not tilling reduces the erosion that results in loss of soil, reduced soil fertility, and soil compaction. All three principles work together to increase biodiversity and soil organic matter, which increases soil fertility. These principles capitalize on natural biological, chemical, and physical processes above and below ground, especially the physical, chemical, and biological properties of organic matter that holds water and nutrients like a sponge, thereby supporting both plant and animal life and natural ecosystem services.
The constant addition of crop residues from not tilling and keeping continuous soil cover with residues, mulch, and cover crops lead to an increase in the organic matter content of the soil. In the beginning this is limited to the top layer of the soil, but with time this will extend to deeper soil layers. Organic matter plays an important role in the soil; fertilizer use efficiency, water holding capacity, soil aggregation, rooting environment, and nutrient retention all depend on organic matter.
In CA soil plants' roots and macrofauna such as earth worms perform “biological tillage” also called soil bioturbation, in place of the mechanical tillage of plows. Charles Darwin estimated in his book entitled: The Formation of Vegetable Mould Through the Action of Worms With Observations of Their Habits (1881) that earthworms could turn over 16 tons of soil per acre (40 tons per ha) per year, and Darwin points out with great foresight that:
The plough is one of the most ancient and valuable of man's inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earthworms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organized creatures.
Though all three principles are generally required to realize optimal results, often CA is adopted in a step-wise fashion with one or two principles. Though it may appear easier, for example, to implement no-till to start, there can be a large cost such as the proliferation of weeds and pests when only no-till is applied.
The events leading up to the development of CA began with the Dust Bowl in the Great Plains of the USA. Following the U.S. Federal Homestead Acts in the late 1800s, which settled much of the Midwestern plain states, and the increase in wheat (Triticum aestivum L.) prices during World War I, farmers plowed millions of acres with new gasoline tractors in the southern great plains states of the US through the 1920s, referred to by many as the “great plow-up.” When an 8-year drought began in 1931, agricultural land dried up and 100 million acres of plowed, unprotected cropland lost most or all of its topsoil from wind erosion. The U.S. Soil Conservation Service, established in 1935, addressed soil erosion with concepts to protect the soil including windbreaks, contour strips, terraces, grassed waterways, and contour plowing, the first change in conventional tillage.
One of the first to perceive and publicly question the damage caused by tillage using the moldboard plow was Edward Faulkner in his book Plowman's Folly (1943), paving the way for others to seriously consider and explore no-till. Simultaneously Masanobu Fukuoka began experimenting in Japan with no-till concepts; however, his work One Straw Revolution (1978) did not get global attention immediately.
After the invention of the herbicides 2,4-D, atrazine, and paraquat in the 1940s and 1950s, no-tillage research gained momentum in the 1960s in the USA and the UK, and the first mechanized demonstration farm trials showed effective use of no-till in 1961. With the development of equipment, several US farms and universities demonstrated successful applications of no-till with a variety of crops, and US universities set up Extension Programs to promote no-till. Equipment manufacturers began selling no-till planters in the mid-1970s, which made no-till practical to adopt by large and medium scale farmers.
No-till also gained a foothold in Brazil, due to a government policy encouraging a shift from livestock farming to cropping systems in the high rainfall hilly areas in southern Brazil, to take advantage of rapid growth in global demand for soybeans in the 1960s. Farmer use of the plow had produced severe soil erosion, which dramatically reduced yields. A Brazilian farmer contacted the University of Kentucky in the early 1970s, which provided access to early no-till equipment, launching a collaboration in Brazil between farmers, researchers, and equipment manufacturers. During the 1980s farmers and researchers supported by the Brazilian government and industry adapted equipment for clay soils; and from their experiences added the practices of rotation and cover crops to no-till to form the basis for the three CA principles. This was first adopted by larger Brazilian farmers in the 1980s and then smaller farmers in the 1990s.
From the 1970s to the 1990s, farmers, researchers, and equipment manufacturers in Brazil and the USA developed and advanced no-till farm equipment and management practices to improve performance of field operations and crops, reaching a base of adoption that enabled demand and corresponding supply by industry of no-tillage farm equipment.
With a lot of the earliest research in to-till taking place in the USA, the largest no-till area in the world as estimated in 2009 was in the USA with 88 million acres (35.5 million ha), though the percentage of cropland under no-till in the USA is 35%, and only 10% of US cropland is under continuous no-till, with the other 25% using some form of tillage, like strip tillage. Table 11.1 shows the countries with the greatest percentage of cropland under CA as reported to the FAO between 2008 and 2014 (FAOSTAT, 2014). As of 2007, about 76% of global land under no-till was in the Americas, 12% in Australia and New Zealand, 5% was in Asia, 4% in Russia and Kazakhstan, with only 1% in Europe, and 1% in Africa.
Table 11.1 Area of land under no-till in the top countries as reported to FAO from 2009 to 2014
| Country | Million acres | Million hectares | Area of CA as % of total arable land |
| USA. | 88 | 36 | 22 |
| Argentina | 67 | 27 | 71 |
| Brazil | 79 | 32 | 44 |
| Australia | 44 | 18 | 36 |
| Canada | 45 | 18 | 39 |
| Paraguay | 7 | 3 | 62 |
| Uruguay | 3 | 1 | 36 |
Though initial research also took place in the UK and other countries such as Nigeria and Kenya, adoption has been slow in Europe and Africa due in large part to traditional farmer practices.
The UN FAO began formally promoting the adoption of CA around the world since 2002 and provides an extensive source of information, educational materials and adoption status on its website (at: www.fao.org/ag/ca).
Making a major change to a farming operation can have high startup costs including planting equipment and the time required to learn and effectively apply a new system. Growing crops and managing a farm is complex and despite the effectiveness and relative simplicity of CA, applying the three principles depends on the conditions and must be tailored to the specific climate, crop, and soil. In addition to the crop and environmental setting, agriculture depends heavily on management decisions such as the planting populations and the timing of planting, fertilizer applications, and weed control. Effective management decisions are crucial for achieving optimal yields with CA.
Because tillage mineralizes incorporated organic matter through microbial decomposition, it provides a flush of nutrients similar to a fertilizer application. This has a short-term benefit for the immediate crop, but over the long-term tillage reduces this nutrient stock through the loss of soil and organic matter content from erosion and decomposition, while CA builds the organic matter stocks and reduces erosion. Because of this short-term effect, tillage has been associated with increased fertility and has had the appearance of being a valuable tool, until most of the organic matter is lost and the soil is degraded. Without adding nutrients back into the soil, such as with compost, mulch, manure, and/or fertilizer, crops will not have sufficient nutrients. This soil degradation resulting from tillage may be one reason for the historical use of fallow periods and shifting agriculture to enable natural ecosystems to restore nutrients through re-vegetation.
Adding nutrients such as fertilizer, can often make up for the loss of organic matter and nutrients in degraded soils, however, this is often costly. Fertilizer also does not make up for the many positive qualities of organic matter, such as water holding capacity. Even with fertilizer additions, over time farmers find that their crop yields decline with degrading soils.
Farmers used plowing and burning of plant residues to increase fertility and to control weeds, disease and pests. So, one of the greatest barriers to adoption of CA is changing farmers' intuitive understanding about tillage. Disease and pests can increase with the change from tillage agriculture to no-till. Managing pests and weeds without plowing or burning is not simple or obvious. Integrated pest management is critical for success in CA, and it requires knowledge and training. Crop rotation and diversity halts the growth of specific pests and plant diversity takes advantage of the chemical and physical interactions of different species. While synthetic pesticides, especially herbicides are indispensable especially in the beginning, after beneficial organisms become established and organic matter is increased, it is possible to reduce the use of chemical pesticides, herbicides, and fertilizers.
Because the change from conventional to CA requires significant startup costs, is a change from deeply ingrained traditions, has a steep learning curve, and most often produces greater weeds and lower yields in the first few years, there is a significant barrier to adoption. Weeds are noted as one of the biggest problems and can take several years to get under control. Also during the transition to CA some pests and disease can create problems until a more diverse biological community takes hold. The main benefits of CA may take from 3 to 7 years to be realized (Pope, 1989).
Erosion and severely degraded soils in South America and in the southern USA helped to spur adoption of no-till in those areas, because decreasing yields on eroded and degraded soils represented a greater cost. By offering subsidies, the government of Brazil was able to promote adoption by farmers and the development of a no-till equipment industry. Without educational programs and government support, it may be difficult for the average farmer to overcome the barriers to adoption.
Also, there is no exact recipe for applying CA to a specific location (soil and climate). Achieving a new farm ecosystem balance requires farmer observation and testing, that is, adaptive research on the farmer scale and the sharing of knowledge gained between local farmers. Realizing many of the benefits of CA, can take time, even decades. Apprehension about converting from tillage agriculture to CA can be overcome with the formation of farmer communities that provide a forum for the exchange of ideas and knowledge among farmers practicing CA, building a foundation of local knowledge.
Though continuous no-till provides minimum soil disturbance, some farmers that have adopted CA will occasionally till for various reasons such as incorporating lime in dry climates, or combating herbicide resistant weeds, increasing nutrient mineralization, or to control certain pests.
In 2012 the USDA estimated no-till practices at 92 million acres in the US. Advancements made in genetically modified crops, such as soybeans resistant to the herbicide glyphosate, and the development of new fertilizers, insecticides, and herbicides have contributed to adoption by large and medium scale farmers.
Equipment and fuel costs are the greatest cost consideration for large scale farmers considering adopting CA. In the early 1990s lighter, precision seeding models were released that helped to spur adoption. In Brazil there are now 300 different models of commercial no-till seeders (Calegari, et al., 2013). While the tractors used for no-till planting do not require as much power or fuel, reducing those costs, the switch to CA does require an initial investment in new planting equipment. Analyses show that labor, fuel, and equipment costs are smaller over time, but additional costs such as herbicide and pest management can offset these savings. Comparison of CA systems with conventional tillage systems has not shown consistently higher financial returns except where erosion was an issue degrading soil fertility and where shorter field preparation time with CA enabled more crops or double cropping during a growing period.
Over the long term, CA has been shown to increase profitability over conventional farms in some areas, as a 10-year comparison of 18 large and medium-sized farms in Paraguay showed a 300% increase in net income while the net income on conventional farms fell during that same period (Sorrenson, 1997). In the US, greater yields were found with adoption of no-till in the western and southern regions where the water conserving aspects of CA helped address water shortages, however in northern regions, especially on poorly drained soils, yields could be lower where no-till postponed planting dates and shortened the total growing season with soils that stayed cooler longer in the spring.
No-till planting equipment has evolved to increase efficiency and yield with more precise seeding. To increase the effectiveness of planters on heavy clay soils, double disc openers were replaced with a ripper tine on some planting equipment. Seeding speeds can result in bouncing and misplaced seeds. Equipment manufacturers develop solutions to these and other problems with devices such as rebounder attachments and other innovations. Precise seed spacing is critical to reduce seed gaps, weed pressure, and competition between plants, and can be accomplished with metering systems. Seed firmers have been developed to increase uniformity in seed depth. Special closing wheels have also been added to cover seeds with soil that is less dense.
Because residue can interfere with planters and seed placement, and can be large for some crops such as corn, double disc openers have been developed to slice through residue to improve seed placement. Some combines used for harvesting also have implements that help to chop and evenly spread the residue. Strip headers have been developed to strip the grain from the stalk and some combines have residue shredders that shred residues as they pass through the combine. Recent equipment innovations have increased speeds of planting, to increase the amount of land that can be planted per day for larger farms.
Some farmers both in tillage agriculture and no-till are adopting precision agriculture with geographic information systems (GIS) and in-field sensors that can provide feedback about the needs of the crop and problems in the field. This data assists with optimal timing of planting, fertilization and harvest to improve yield and profits and lower costs. Equipment manufacturers and fertilizer companies have also come forward with interactive decision support tools for precision agriculture and other services to assist growers.
In many developed countries, especially in Europe, reduced and no-till has not been adopted in large part due to challenges managing weeds without tillage, especially where government policy restricts elevated use of pesticides and herbicides. Some research has explored alternatives to herbicidal control of weeds to address the needs of no-till and organic agriculture and in considering the advent of herbicidal resistant weeds. Research in Brazil has shown that some cover crops such as hairy vetch (Vicia villosa Roth), black oat (Avena strigosa Schreb.), and oil seed radish (Raphanus sativus L.) are effective at reducing weed populations and reducing the amount of herbicide needed. Methods such as increasing cover crop biomass to suppress weeds and allelopathic interactions have been studied with mixed results.
Most progress in adoption by smallholder subsistence farmers has been in South America; adoption has been marginal outside of Brazil, Paraguay, and Uruguay, where government programs have provided support and education through extension services to promote adoption by small farmers. Studies of net farm income of smallholders in South America were greater with CA than conventional practices. Animal driven rippers and seeders were developed in Brazil for small scale farmers, and have been exported to Africa for smallholder farmer mechanization.
Other “homemade” equipment developed by and for small farmers includes the knife roller (roller-crimper) that is designed to bend over and crush cover crops and weeds, and other vegetation flatteners to press the residue down before planting. Wooden and metal subsoilers where the moldboard plow share is replaced with a metal point to reduce soil disturbance have been developed that can be used with draft animals. Also a metal chisel based on the conventional subsoiler design have replaced the metal point with a thinner and longer metal spine developed to pierce through compacted soil layers while disturbing the soil less.
Weed control becomes an issue if there is not enough labor or no access to herbicides. There is also a common view that CA requires increased fertility, which in the past has been accomplished by plowing residues into the soil. Without that flush of nutrients, fertilizer input at least in the initial stages would be required to avoid a reduction in yield with CA. Fertilizer inputs for the smallholder farmer are often expensive or not accessible.
Many smallholder farmers in Africa must consider the use of residues as feed and bedding for livestock where they are integrated into farming. Residues have other purposes such as construction and as a fuel source. Smallholder farmers also face issues in adopting crop rotations, when there are no markets for alternative crops, especially legumes.
Intercropping of legumes may be one solution to increase residue production and nutrient requirements. However, the timing and crop selection are important to avoid competition between crop plants and lower yields.
With respect to equipment in South Asia, two-wheeled tractor-mounted planters are replacing animal drawn plows and planters.
By increasing biodiversity, increasing water infiltration, and reducing soil erosion, CA has multiple benefits for agriculture and ecosystems. In summary, CA: