Conservation Agriculture

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For centuries, farmers have used tillage for agricultural production. There were several reasons for adoption of tillage including the oxidation of the organic matter to release needed nutrients for crop production. Farmers also used tillage to make it easier for them to plant seed into the ground, to manage crop residues and organic amendments, and to control weeds, pests and diseases. With the introduction of mechanical power and tractors tillage became even more widespread and manufacturers developed various implements such as mould board and disc ploughs that inverted the soil. However, it soon became apparent that intensive tillage resulted in various negative effects on the environment. The soil was more exposed to climatic events leading to erosion and loss of topsoil. A good example of this was the dust bowl in the USA in the 1930s. Farmers used mould board ploughs to bury the native grasses and prepare the soil for crop production. This exposed the soil surface to rain and wind. The result was the dust bowl with large quantities of topsoil removed by wind and washed away by water. Conservation tillage was introduced to remedy this problem. In conservation tillage only minimal ploughing is done to make it easier to plant seed with the available seed drills. Conservation tillage also leaves previous crop residues on the soil surface to protect it from wind and rain. A minimum of 30% soil cover is required to be called conservation tillage, a major management practice in US farming today.

Conservation tillage still results in soil disturbance in the surface layers. This disturbance affects both the physical and biological properties of the soil. CA goes one step further and reduces the surface tillage to a minimum. 'Conservation agriculture' is a term coined by the Food and Agriculture Organization (FAO) in the last two decades and has three important pillars (FAO, 2009):

1. Reduction in tillage: the objective is to achieve zero tillage, but the system may involve controlled tillage seeding systems that normally do not disturb more than 20-25% of the soil surface; aims are to reduce soil disturbance, energy use and production costs and to increase profitability.

2. Retention of adequate levels of crop residues and surface cover on the soil surface: the objective is the retention of sufficient residue on the soil to protect the soil from water and wind erosion; aims are to reduce water runoff and evaporation, to improve water productivity and to enhance soil physical, chemical and biological properties associated with long-term sustainable productivity. The amount of residues necessary to achieve these ends will vary depending on the biophysical conditions and cropping system.

3. Use of crop rotations: the objective is to employ diversified crop rotations to help moderate/mitigate possible weed, disease and pest problems; aims are to utilize the beneficial effects of some crops on soil conditions and on the productivity of subsequent crops, and to provide farmers with economically viable cropping options that minimize risk.

There are many reports describing the benefits of CA (Hobbs et al., 2008). Wear and tear on farm equipment is decreased as a result of less use. Diesel use for land preparation is significantly less. The benefits also include better and more stable yields through timelier planting or buffering of moisture stress, reduced production costs, improved soil physical and biological properties, improved water infiltration, less soil and wind erosion and a potential for biological control and less disease and pest incidence.

The CA principles are applicable to a wide range of crop production systems from low-yielding, dry, rainfed conditions to high-yielding, irrigated conditions. However, the techniques used to apply the principles of CA will be very different in different situations, and will vary with biophysical and system management conditions and farmer circumstances. Therefore, there are various forms of CA. Specific and compatible management components (pest and weed control tactics, nutrient management strategies, rotation crops, appropriately scaled implements, etc.) will need to be identified through adaptive research with active farmer involvement. Applying CA essentially means altering literally generations of traditional farming practices and implement use. As such, the movement towards CA-based technologies normally comprises a sequence of step-wise changes in cropping system management to improve productivity and sustainability.

In South Asia the term 'resource conserving technologies' has been coined to describe some of these intermediate steps towards the complete implementation of all the CA principles. Resource-conserving technologies can be applied on both flat and raised-bed planting systems. For example under gravity-fed irrigated conditions, a raised-bed system with furrow irrigation may be more suitable than planting on the flat since the furrow system will allow irrigation water to be managed more efficiently. Therefore, a first step will be to implement a conventionally tilled raised-bed system as a resource conserving technology in preparation for the next step, permanent raised beds. The permanent raised-bed system uses the same principles as CA but first forms a bed and furrow system which is then kept permanent, with only minimal soil disturbance and reforming of the beds, if needed, after each crop. The crops are planted on top of the beds and a layer of crop residue is left as in CA planted on the flat. There are several advantages to permanent beds including improved water productivity as well as 'controlled traffic' since the compaction associated with tractor wheels is restricted to the furrows between beds where plants are not drilled (Sayre, 2005).

As soil tillage is primarily used for weed control, this has been a major concern in adopting zero tillage and CA systems. Tillage systems often induce changes in composition of weed species and densities. Weeds are often initially greater when farmers shift to zero-tillage systems and this is one of the few negative aspects of CA; essentially, farmers substitute herbicides for tillage. Zero tillage often favours perennial (broadleaf and grass) species compared to conventional tillage (Carter et al., 2002) as tillage destroys and prevents these plants from setting seed, but zero tillage has been reported to successfully control annual broadleaf weeds over time when the right weed control practices are implemented and the seed bank gets depleted by not tilling (Arshad et al., 1998). In South Asia, where zero-tillage wheat is planted after rice, the grassy weed Phalaris minor germinates less because of less soil disturbance (Hobbs and Gupta, 2003). Also, the mulch residue cover can control weeds by excluding light (Ross and Lembi, 1985). The introduction of herbicide-tolerant crops such as soybeans, maize, cotton and canola has helped to reduce the problems of weeds associated with zero tillage in many countries where CA has significant acreage. In this case, glyphosate, a broad-spectrum herbicide, is used to control weeds in combination with herbicide-tolerant crops (Roundup Ready™ crops). Additionally, crop rotation, one of the other pillars of CA, leads to diversification of cropping practices and therefore changes weed populations and species composition, leaving less opportunity for an individual weed to become dominant.

Permanent ground cover is a critical aspect of CA. Results from rainfed and irrigated long-term trials (> 10 years) in Mexico, show that not zero tillage as such, but the combination of zero tillage with the retention of sufficient soil-surface crop residue resulted in increased physical, chemical and biological soil quality. Moreover, the data show that zero tillage without residue retention resulted in soil degradation beyond the conventional tillage practice (Govaerts et al., 2005, 2006a, b, 2007a, b; Limon-Ortega et al. , 2006). Ground cover can be provided in various ways; probably the easiest way is to leave the anchored residues from the previous crop. It has been found that this anchored residue does not create a problem for planting the subsequent crop. The height at which the crop residue is cut will determine the quantity of straw left on the field. This can be an issue in dryland agriculture where crop yields are low and the residue left after harvest is not sufficient to provide ground cover. There are also issues where the crop residues have other uses. For example in some countries the crop residues are removed and used as feed for animals. In this case farmers may not leave enough residues in the field to obtain successful CA. In these areas solutions have to be found to increase the overall biomass productivity of the system in order to meet all farmer and soil needs. Improved fodder sources should also be part of the improved management package (Govaerts et al., 2005; Verhulst et al., 2010). Another way to provide permanent soil cover is to grow a cover crop. This is a crop that is grown for its biomass rather than any grain yield. After the crop has reached sufficient size it is knocked down or killed but is not incorporated into the soil. The cover crop can be leguminous and help fix N or can be another crop species that provides good biomass. Introduction of cover crops can however be very challenging in some environments, depending on the climate conditions and the difficulty in convincing a farmer to grow a crop that will not give any immediate economic return.

Permanent soil cover is important for several reasons (Verhulst et al., 2010). Results from two long-term trials established in the early 1990s in different agro-ecological systems in Mexico clearly show the importance of crop residue retention on soil aggregation (Fig. 10.3). The two systems were: (i) a low-input, semi-arid, rainfed system in the rainfed central highlands (2240 m above sea level) with zero tillage on the flat; and (ii) a high-input, arid, irrigated system in the north-western part of the country with zero-tilled permanent raised beds. Since organic matter is a key factor in soil aggregation, the management of previous crop residues is a key to soil structural development and stability. It has been known for many years that the addition of organic substrates to soil improves its structure (Ladd et al., 1977). The presence of crop residues over the soil surface prevents aggregate breakdown by direct raindrop impact as well as by rapid wetting and drying of soils (LeBissonnais, 1996). Moreover, aggregates are more stable under zero tillage with residue retention compared to conventional tillage and zero tillage with residue removal (Carter, 1992; Chan et al., 2002; Filho et al., 2002; Hernanz et al., 2002; Pinheiro et al., 2004; Li et al., 2007 - all as cited in Verhulst et al., 2010 and Govaerts et al., 2009b). Soil macro-aggregate breakdown has been identified as the major factor leading to surface pore clogging by primary particles and micro-aggregates and thus to formation of surface seals or crusts (LeBissonnais, 1996; Lal and Shukla, 2004). Under permanent soil cover wind erosion and rapid wetting (i.e. slaking) result in less aggregate breakdown, preventing surface crust formation (LeBissonnais, 1996; Scopel and Findeling, 2001; Lal and Shukla, 2004). As a result infiltration of water is generally higher in zero tillage with residue retention compared with zero tillage with residue removal (Fig. 10.4). In addition, the residues left on the topsoil act as a barrier, reducing the runoff velocity and giving the water more time to infiltrate; the residue intercepts rainfall, absorbs its energy and releases it more slowly for infiltration into the soil. The 'barrier' effect is continuous, while the prevention of crust formation probably increases with time (Scopel and Findeling, 2001). The increased aggregate stability and reduced runoff result in lower soil erosion in CA (Carter, 1992; Chan et al., 2002; Filho et al., 2002; Hernanz et al., 2002; Pinheiro et al., 2004; Li et al., 2007; Govaerts et al., 2007c - all as cited in Verhulst et al., 2010 and Govaerts et al., 2009b). The biomass is also a source of food for microbes including various bacteria, fungi, nematodes, earthworms and arthropods. The residue retained on the soil surface provides residue-borne pathogens and beneficial soil microflora with substrates for growth. This can induce major changes in disease pressure in CA systems. However, functional and species diversity are increased, creating more possibilities for integrated pest control. The effect of CA on soil mesofauna is variable, but in

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Fig. 10.3. The effect of residue management on soil aggregate distribution and stability expressed as the mean weight diameter (mm) obtained by dry sieving (grey bars) and wet sieving (white bars) in the zero-till treatments of (a) the long-term rainfed sustainability trial in the highlands of Central Mexico (described in Govaerts et al., 2005), and (b) the long-term irrigated sustainability trial in Ciudad Obregon, North Mexico (adapted from Limon-Ortega et al., 2006). Differences between values of the presented parameters were tested for significance using least square difference grouping and treatments with different letters within the same typography differ significantly at P < 0.05. Bars indicate standard error.


Burning Removal

Partial Full retention retention

Residue management

Fig. 10.4. The effect of residue management on time-to-pond (s) in the zero-till treatments of the long-term irrigated sustainability trial in Ciudad Obregón, North Mexico during the wheat phase of the rotation (adapted from Verhulst et al., 2009). Differences between values were tested for significance using least square difference grouping and treatments with different letters differ significantly at P < 0.05. Bars indicate standard error.

general macrofauna abundance is stimulated (Verhulst et al., 2010). This biological activity is also critical for improving nutrient cycling and improving surface soil physical properties. The biological activity combined with the previous crop's root channels results in interconnected soil pores that lead to improved water infiltration (Kay and VandenBygaart, 2002). This is important for reducing water erosion and increased storage of soil moisture in the soil profile.

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