Tillage is an essential management technique that provides a suitable seed bed for plant growth and in some areas of the world, the only non-chemical method to control weeds, tte tillage tools pulled upwards by a tractor are designed to apply an upward force to cut and loosen the compacted soil, sometimes to invert it and mix it. tte other main negative effect of driving a tractor across a field is compaction of the soil. Compaction may result in an increasing shear strength through an increase in bulk density, followed by low infiltration and increased runoff and erosion. tte compaction generally extends to the depth of the previous tillage, up to 300 mm for deep ploughing, 180 mm for normal ploughing and 60 mm with zero tillage (Pidgeon and Soane 1978). Spoor et al. (2003) completed a study summarizing the risk of soils to compaction in relation to texture and wetness.
Conventional Tillage is the standard system of tillage practice involving ploughing, secondary cultivation, with disc harrowing, suitable for planting for a wide range of soils, tte mouldboard plough inverts the soil in the plough furrow and moves all the soil in the plough layer to a depth of 100-200 mm. Secondary disc cultivation helps form seed beds and remove weeds by breaking up the cloddy surface produced by ploughing, tte increased surface roughness due to tillage can be a successful deterrent to water erosion (Cogo et al. 1984).
Contour Tillage for planting and cultivation can reduce soil loss from sloping land compared with standard cultivation up-and-down the slope. It is inadequate as the only conservation practice to reduce soil loss for lengths greater than 180 m at 1° steepness (Troeh et al. 1980). ttis technique maybe effective against soil loss and water erosion only for low rainfall intensity. Protection against more extreme storms is enhanced by supplementing contour farming with strip cropping, discussed later in this paper. Contour ridging and connecting the ridges with cross-ties over the intervening furrows, thereby forming a series of rectangular depressions that fill with water during rain can be very effective in controlling soil erosion along slopes, ttis practice known as tied ridging, should be used only in well drained soils to minimize water logging and damage to crops. Tied ridging on sandy soils in Zimbabwe with no till over a period of three years gave soil losses of less that 0.5 t ha4 compared with up to 9.5 t ha4 for conventional ploughing with mouldboard under maize cultivation (Vogel 1994).
Conservation Tillage can be defined as any practice that leaves at least 30% cover on the soil surface after planting. Numerous studies have examined the effects of different types of conservation tillage as outlined in table 1 (Natural Resources Conservation Service, USDA 1999). tte success of various systems is highly soil specific, landscape, climatic pattern and also dependent how well weeds, pests and disease are controlled, tte main barriers to adoption are the expense of specialized equipment for managing cultivation in crop residues, problems of weed control and increases in pest, particularly rodents.
Conventional Standard practice of ploughing with disc or mouldboard plough, one or more disc harrowing, a spike-tooth harrowing and surface planting.
No Tillage Soil undisturbed prior to planting, which takes place in a nar row,
25-27 mm-wide seed bed. Crop residue covers of 50-100% retained on surface. Weed control by herbicides.
Strip Tillage Soil undisturbed prior to planting, which is done in narrow strips using rotary tiller or in-row chisel, plough-plant, wheel track planting. Intervening areas of soil unfilled. Weed control by herbicides and cultivation.
Mulch Tillage Soil surface disturbed by tillage prior to planting using chisels, field cultivators, discs, or sweeps. At least 30% residue cover left on surface. Weed control by herbicides and cultivation.
In general the better drained, course- and medium textured soils with low organic content respond best and the systems that are not successful occur on poorly drained soils with high organic content or heavy soils, where the use of the mould-
board plough is essential, tte effectiveness of the tillage practice is very dependent on the types of crops planted and the use of crop rotations.
No tillage system usually restricts tillage only to that necessary for plantings and carried out by drilling directly into the stubble of the previous crop. Weed control by herbicide application is an essential part of this system, ttis conservation technique has been found to increase the water-stable aggregates in the soil compared to disc cultivation and ploughing (Suwardji and Eberbach 1998; Mrabet et al. 2001). It is not suitable in soils that easily compact and seal because it can lead to lower crop yields and greater runoff.
In the Corn Belt of the USA, where maize, soybean and wheat are cultivated in rotation, no tillage is one of leading technologies to control erosion in areas. Moldenhauer (1985) reviewed the various tillage systems in this area and showed that annual soil loss under no till on a range of erodible soils was 5-15% of that from conventional tillage. In the southern regions of the USA, no till is recommended on ultisol soils which have become severely eroded after 150 years of continuous cropping. In this area, Langdale et al. (1992) studied three no tillage systems under soybean production as an alternative to the conventional system of growing soybean with a bare fallow over winter: Soybean with winter cover using in-row chisel; soybean with barley as winder cover and using fluted coulters; and soybean with rye as a green manure and using fluted coulters, tte respective mean annual soil losses for the four systems (starting with conventional tillage) were 26.2, 0.1, 0.1 and 3.41 ha4, respectively. One other environmentalbenefit to no tillage operations is that there is no exposure of the sub-layer soils to oxidation and release of carbon dioxide to the atmosphere.
In strip tillage, the soil is prepared for planning along narrow strips, with the intervening areas left undisturbed. In a single plough-planting operation, typically up to one-third of the soil is tilled, tte plough-plant systems caused the least soil compaction, conserved most soil moisture and reduced losses of organic matter, nitrogen, phosphorus and potassium (Quansah and Bonnie 1981). In studies conducted in Ghana, the technique reduced soil loss in maize plots from multiple rain storms totaling 452 mm to 0.2 t ha4 compared with 1.41 ha4 for traditional tillage using hoe and cutlass (Baffoe-Bonnie and Quansah 1985).
Mulch Tillage system in general has been successful to reduce water and wind erosion and to promote the conservation of soil moisture in drier wheat-growing areas (Fenster and McCalla 1970). tte soil is prepared in such a way that at least 30% of the surface is covered with plant residues, or other mulching materials, are specifically left on or near the surface. Mulch tillage is a broad term and includes practices such as no-till, disk plant systems, chisel plant systems, and strip tillage systems. Sometimes a cover crop, usually a legume, is specifically grown within the cropping cycle to produce mulch material. Another variant of planted fallowing, practiced in North America, is referred to as summer fallow or ecofallow. tte latter is a system of fallowing in which weed growth is restricted by shallow cultivation or by using herbicides to conserve soil moisture. Crops are grown every other year or once in 3 years, ttis type of "cropless" fallow is mostly used in arid climates to conserve soil moisture, without having to resort to irrigation.
Minimum Tillage or reduced tillage is a practice using chiseling or disking to prepare the soil while retaining a 15-25 % residue cover. Minimum tillage is not an easy option; it demands commitment, time and patience, ttis option is selected based on the soil and cropping needs of a particular region. Drier and more stable structured soils are best suited to minimum tillage. Chisci and Zanchi (1981) found that minimum till was suitable for silt clay soils in northern Italy under continuous wheat production. Soil moisture was maintained and in turn reduced the cracking while promoting high infiltration rates. Despite higher runoff, annual soil loss was almost one third of that under conventional tillage. In the U.S. Corn Belt, farmers use chiseling in the autumn to produce a rough surface but retain residue cover, followed by disc cultivation in the spring to smooth the seed bed and cover the residue, ttis can reduce soil loss by an order of magnitude compared to conventional tillage (Siemens and Oschwald 1978).
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