Conventional tillage is the traditional method of farming in which soil is prepared for planting by completely inverting it with a tractor-pulled plough, followed by subsequent additional tillage to smooth the soil surface for crop cultivation. In contrast, conservation tillage is a tillage system that conserves soil, water and energy resources through the reduction of tillage intensity and retention of crop residue. Conservation tillage involves the planting, growing and harvesting of crops with limited disturbance to the soil surface.
Tillage of the soil stimulates microbial decomposition of soil organic matter, which results in emissions of CO2 to the atmosphere. Therefore, minimising the amount of tillage promotes sequestration of carbon in the soil. In the last decades advancements in weed control methods and farm machinery now allow many crops to be grown with minimum tillage (Smith et al., 2008).
Conservation tillage is any method of soil cultivation that leaves the previous year’s crop residue (such as corn stalks or wheat stubble) on fields before and after planting the next crop to reduce soil erosion and runoff, as well as other benefits such as carbon sequestration (MDA, 2011). With this technique, at least 30% of the soil surface is covered with crop residue/organic residue following planting (Dinnes, 2004). It also features noninversion of the soil. This type of soil tillage is characterised by tillage depth and the percentage of surface area disturbed. For example, to plant the crop in figure 1, the planter was adjusted to place the seed 50mm deep and provide a layer of fine tilth 18mm deep across the planted row areas in order to incorporate Treflan, which was sprayed in front of the machine. This was all completed at 20km/h. Conservation tillage methods include zero-till, strip-till, ridge-till and mulch-till. Zero-tillage is the extreme form of conservation tillage resulting in minimal disturbance to the soil surface.
Zero-till involves planting crops directly into residue that hasn’t been tilled at all (MDA, 2011). Zero tillage technology is generally used in large-scale agricultural crop cultivation systems because large machines are required for planting (figure 2). For smaller-scale farms, no adequate machines are available for sowing, although very small scale farmers may do so by hand (figure 3). In zero-tillage, crops are planted with minimum disturbance to the soil by planting the seeds in an un-ploughed field with no other land preparation. A typical zero-tillage machine is a heavy implement that can sow seed in slits 2-3cm wide and 4-7cm deep and also apply fertiliser in one operation (CIMMYT, 2010). The machine contains an inverted T-type furrow opener to open the slits (figure 2). The seed and fertiliser are placed in corresponding boxes and dropped into the slits automatically. The depth of the slits may be controlled by a hydraulic mechanism from the tractor.
Features of zero-tillage include:
- Crop residues are distributed evenly and left on the soil surface
- No implements are used (a) to turn the soil over, (b) to cultivate the crops or (c) to incorporate the crop residues into the soil
- Weeds and cover crops are controlled by a pre-planting application of non-pollutant desiccant herbicides
- A specialised planter is used to cut crop residues on the soil surface and insert the seeds and fertilisers into the soil with minimum disturbance. Generally seed sowing is done when soil moisture content is adequate for seed germination but not so high that the large tractor and planter would compact the soil
- Weed control is also accomplished with pre- and post-emergence herbicides
- Crop rotation is fundamental to zero-tillage because it helps to minimise weed, insect, and disease populations that increase when the same crop is grown year after year on the same ground
- Most experiments with zero-tillage have had increased yields, but in the wetter areas, it took many years to see the crop yields stabilise or increase. However, in drier areas where moisture is the major limiting factor, the effects on yield were seen even in the first year (Kimble et al., 2007)
- Zero-tillage causes stratification of soil organic carbon content with relatively higher concentration in the surface and lower in the subsoil compared to plow-based methods of seedbed preparation. The ratio of soil organic carbon content for zero-tillage to plow-till system remains
Strip-tillage involves tilling the soil only in narrow strips with the rest of the field left untilled (strip-till) (MDA, 2011).
Ridge-till involves planting seeds in the valleys between carefully molded ridges of soil (figure 4). The previous crop’s residue is cleared off ridge-tops into adjacent furrows to make way for the new crop being planted on ridges. Maintaining the ridges is essential and requires modified or specialised equipment (MDA, 2011).
Mulch-till (figure 5) is another reduced tillage system in which residue is partially incorporated using chisels, sweeps, field cultivators, or similar farming implements that leaves at least one third of the soil surface covered with crop residue (MDA, 2011).
Each conservation tillage method requires its own type of specialised or modified equipment and adaptations in management.
- Increases the ability of soil to store or sequester carbon while simultaneously enriching the soil.
- Improves soil water infiltration, thereby reducing erosion and water and nitrate runoff.
- Improves the stabilisation of soil surface to wind erosion and the release of dust and other airborne particulates.
- Reduces leaching of nutrients due to greater amounts of soil organic matter to provide binding sites.
- Decreases evaporation and increases soil moisture retention, which can increase yields in drought years (Suddick et al., 2010).
- Reduces the number of passages of equipment across the field, thereby reducing the cost of fossil fuel and the associated carbon emissions to the atmosphere.
- Reduces the loss of pesticides and other applied chemicals. This is because higher infiltration rates with more surface residue results in less runoff moisture holding capacity due to higher soil organic matter that results in less leaching.
- Adoption of reduced tillage in humid, cool soils would primarily affect the distribution of SOC in the profile, unless carbon inputs were increased (Lal et al., 1998).
- Specialised, expensive equipment is required, or much hand labour in the case of very small scale growers.
- Requires more herbicides and pesticides than standard conventional practices to control weeds and other pests.
- Due to the large size of the original soil carbon pools, the contribution of conservation tillage can appear to be small, and a significant amount of time is required to detect changes.
- Sizable amounts of non-CO2 greenhouse gases (N2O and CH4) can be emitted under conservation tillage compared to the amount of carbon stored, so that the benefits of conservation tillage in storing carbon can be outweighed by disadvantages from other GHG emissions.
According to Brown (2008), zero-till is widely used in five countries in particular: 15 million hectares in the United States, 24 million hectares in Brazil, 18 million hectares in Argentina, and 13 million hectares in Canada. Australia has 9 million hectares under zero-till, making a total of 79 million hectares for these five countries with the most hectareage. Worldwide, the use of zero-till is increasing. In 1999 it was used on 45 million hectares and by 2005 it had more than doubled to reach 95 million hectares. Using the latter figures, all other countries than those in the top five accounted for only 17% of the total.
For conservation tillage in general, in the developing world, it has been most successful in Brazil and Argentina (Abrol et al., 2005). In these countries, 45-60% of all agricultural land is said to be managed by conservation agriculture systems. In the 2001-2002 season, conservation agriculture practices are estimated to have been used on more than 9 million hectares in Argentina and 13 million hectares in Brazil. In Africa, the Africa Conservation Tillage Network (ACT) was established in 1998 to promote conservation agriculture as a sustainable means to alleviate poverty, make more effective use of natural and human resources, and reduce environmental degradation (Abrol et al. 2005).
- Less labour time and cost are required under a reduced tillage system due to fewer tillage trips and cultivation operations for seedbed preparation. The savings range from $2.47/ha to $19.13/ha (Kimble et al., 2007).
- A large number of studies have estimated the potential fuel cost savings as a result of reducing tillage. They range between $3.58/ha and $28.29/ha (Kimble et al., 2007).
- Generally, reduced tillage systems have lower machinery repair and maintenance costs due to less use of tillage implements (Kimble et al., 2007).
- Zero-tillage technology reduces costs of field preparation up to US$70 (Rs. 3200) per hectare (Verma and Singh, 2009), and it also saves time and labour (up to 10-20%). A saving of fuel consumption by 26.5-43.7 litres per hectare (Verma and Singh, 2009) results in reduced fuel cost and reduced carbon emitted to the atmosphere.
- Zero-tillage can save farmers around 1 million litres of water per hectare (100mm) compared with conventional practices due to the mulch on the soil surface which reduces evapotranspiration (Rehman, 2007).
- Zero-tillage increases soil carbon from 0.1 to 0.7 metric tonnes ha-1yr-1 (Paustion et al., 1995) under sub-tropical conditions.
The largest barrier is the weight and cost of the specialised planters required to penetrate the soil covered with the previous crop. The use of these planters is mainly restricted to richer countries where the fields are relatively large. For growers with small farms in poor countries, the large amount of hand labour required is a barrier.
Abrol, I.P., R.K. Gupta and R.K. Malik (Editors) (2005): Conservation Agriculture. Status and Prospects. Centre for Advancement of Sustainable Agriculture, New Delhi pp. 242.
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