The task of tillage is to prepare soils for productive use. Usually tillage is limited to the arable layer of soil, which contains organic matter and where plant life actually can occur. Tillage has to be performed to clear virgin soils of plants and animals for agricultural use. Furthermore, it must be performed to bring the seedlings into the soil and procure for them a good environment for further development.
Another objective of tillage is to control weeds and animals living in the soil, such as mice or slugs. This is, compared to the use of chemical means, an energy and time- consuming way to control pests. Another important point is surface leveling because most operations in mechanized agriculture depend on level surfaces. Irregularities in the soil niveau may be caused by traffic on the soil, harvesting or climatic effects. To- gether with this goes the need to distribute clods and porosity according to plant need (Fig. 1.157).
Figure 1.157. Stratified seedbed (from [1]).
The seeds should be covered by small clods for protection while around the seeds, fine soil should prevail. Under the seeds, porosity must not be too high, while smaller and larger clods should give structure to the soil. Producing this distribution of smaller and larger clods (stratified seedbed) is one of the main objectives of primary tillage.
Producing fine soils for the environment of the seedling and the structure of the seedbed is the main objective of secondary tillage and seedbed preparation. Warming up the soil and bringing air to deeper layers stimulates life in the soil. At the same time, loosening makes it easier for plant roots to penetrate into deeper soil layers. An optimum porosity will also facilitate the infiltration of air and water for the plant roots, and the ascention of water from deeper soil layers during dry periods. Loosening the subsoil may be necessary to break up a hardpan, which can be created by trafficking and smearing the bottom of the tillage zone as it happens with plowing or which may develop naturally as in sodopol soils. Finally, it can be necessary to undertake soil improvements such as bringing down organic matter into the sterile subsoil or bringing up sand/clay subsoil into arable layers containing too much sand/clay in their texture.
Appropriate Tillage According to Soil Conditions
An important characteristic of agricultural soil is its texture. It is usual to divide the smallest mineral particles forming the soil matrix into the three diameter classes of sand (particles between 2 mm and 0.05 mm), silt (particles between 0.05 mm and 0.002 mm) and clay (particles smaller than 0.002 mm) [2]. Gravel and cobbles (over 2 mm) appear in agricultural soils but are usually unwanted because they make tillage hazardous and keep little organic matter. Sand-sized and larger particles can be fractionated by sieving. Silt or clay particles must be estimated by hydrometer or pipette analysis. Soils are classified into several types, according to the distribution of the three particle size classes. The
Figure 1.158. Soil classification according to sand, silt and clay content (textural triangle) (from [2]).
percentages of the three particle classes usually are drawn in a textural triangle where a soil type can be determined (Fig. 1.158).
Soil type has great influence on its workability. Tillage on sands and sandy soils (light soils) is easy at all moisture contents. But water storage capacity is low while infiltration rate and water conductivity are high. This can be an advantage for irrigated soils as there is only little danger of salt residues. Due to high water conductivity and easy warming as a result of a low heat capacity, organic matter is reduced fast and the content of nutrients and humus is comparatively low. Yields on these soils are usually lower than on other soil types. Melioration by incorporating clay or silt into the sand is possible.
Silty and loamy soils (medium soils) are usually the most advantageous agricultural soils. Their ability to hold water is still high enough for plant growth, and they show sufficiently large pores for good aeration. Their nutrition content is comparatively high, and the limits of their workability are much wider than with clay soils. They are usually soils where the highest yields can be achieved.
Because of its influence on soil properties, clay takes in a greater area of the textural triangle than sand. Clay soils (heavy soils) are difficult to cultivate as the soil becomes very hard at low moisture contents, rendering tillage operations almost impossible. At high moisture contents, plastification will make crumbling almost impossible and pro- duce very high draft forces for tillage implements together with severe smearing at the soil-tool interfaces. Thus, these soils can be worked only in a limited range of moisture contents. Water conductivity is low so clay soils can be wet in a humid climate. Porosity is high, but most pores are fine pores that do not allow sufficient aeration. The ability to hold water is high, so drying and swelling can lead to cracks and to a kind of auto tillage
of such soils. These soils can be meliorated by incorporating sand from deep soil layers or by deep loosening.
Soil particles agglutinate to soil aggregates. They form the actual soil matrix with the system of fine, medium and large pores. The medium-sized pores are most important for water storage functions, while large pores are important for conducting air to the plant roots. Fine pores are of less importance to plant development as contained water is inaccessible to plants. Large- and medium-sized pores can be produced by shrinking and swelling in clay soils, by earthworms and other animals in the soil and by tillage operations. Thus, achieving optimum porosity must always be one of the main objec- tives of tillage. An optimum tilth is usually obtained when crumbs of a diameter smaller than 50 mm are obtained and a sufficient amount of stable aggregates remains to create porosity and as a protection against erosion. Especially small aggregates show a small stability against water impact and wind erosion. When particles are destroyed by water, silting-up and encrustation occurs, which can obstruct plant growth. Such crusts must be broken; in order to avoid silting-up and encrustation, minimum tillage, soil cover and mulch should be applied. Generally in humid climates, one should keep soils in a more loose condition in order to avoid silting up and encrustation and to increase aeration.
Under dry climatic conditions, a denser soil condition is advantageous since loosening will reduce the water storage capacity of the soil and make the water supply from deeper soil layers more difficult.
Socio-Economical Aspects of Tillage
Research carried out in the United States showed that, as the amount of tillage de- creases (from conventional tillage to reduced tillage or no-till), the size and the number of machines decreases and costs for machinery and labor decrease, too [3]. Major costs are affected by machinery and herbicides. For corn production, the results are summarized below:
• Ridge-till and no-till are the most profitable systems in different soil types.
• In all cases mulch-till systems (fall chisel, disk or field cultivator) are more profitable than fall plow but not as good as ridge-till and no-till.
• In poorly drained soil, ridge-till is more profitable than no-till. Slopes higher than 4% favour no-till systems.
Time requirements can be reduced by 65% and 43% when replacing conventional tillage with zero tillage and reduced tillage, respectively. At the same time, energy requirements can be reduced from more than 90 kW h/ha for conventional tillage to about 60 and 10 kWãh/ha, respectively, for reduced and zero tillage system.
The adoption of soil conservation tillage systems suffers from many constraints, particularly in developing areas:
• Lack of financial means (difficult procedures to obtain credits and subsidies, high interest rates).
• Climatic conditions impose more risks with the new tillage systems.
• Low technical level of farmers due to lack of research and extension.
In developed countries, energy and labor saving and the protection of soil and water from pollution and degradation are the major aspects connected with tillage. The search for adapted systems assumes that the user’s technical level and the farmer’s financial
funds are taken into account. A simulation based on data of an investigation carried out in Morocco shows that the cost of direct seeding is less than for other tillage systems if the annual planted surface is larger than 60 ha.
There is an urgent need to introduce sustainable agriculture in fragile ecosystems, particularly in the developing countries where low-input farming increases the envi- ronmental degradation and perpetuates low yields and low revenue (Lal [4], Nicou [5], Merzouk [6], Oussible [7], Papendick and Cambell [8]).