1.2 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 percentages of the three particle classes usually are drawn in a textural triangle where a soil type can be determined (Fig. 1.158).

Figure 1.158. Soil classification according to sand, silt and clay content (textural triangle).

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 pro­duce 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 objec­tives 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.