Most soils consist of mineral and organic materials. Soil develops from parent rock material and organic matter influenced by external factors, such as climate, organisms, topography, time and parent material.
The size of the soil particles determines the soil texture. Sand particles are the largest of the particles, and are more than 0.05 mm in diameter. Silt particles have a diameter between 0.002 mm and 0.05 mm, and clay particles are the smallest with less than 0.002 mm (see Figure 2). The percentage content of the different particles determines the soil type.
Figure 2: Proportional size of soil particles
Figure 3: Soil classification
Sandy soil contains 80-100 % sand, 0 -10 % silt and 0 - 10 % clay. Clay loam is composed of 27 - 40 % clay and 20 - 45 % sand.
Soil classification displays a simplified textural triangle which allows for determination of soil classes.
It should be noted that there are different classification systems, e.g. the International classification and the USDA classification (see Table 1). The USDA classification is used in Figure 3. The German classification system (DIN 18196) is not considered here.
Table 1: Grain Size Chart (Source: http://en.wikipedia.org/wiki/File:Wentworth-Grain-Size-Chart.pdf)
Soil texture is an important property related to soil water. Sand soils are porous, have large spaces between the particles, absorb and retain little water, and dry out quickly: they are also called light soils. Loam soils are loose and more porous, and hold water better than sand soils. Clay soils, also called heavy soils, have little air between particles and hold water tightly; thus, minimal water is available to plants.
Organic matter originates from plant residues and manure. It is converted and incorporated into the soil under natural conditions by animals and micro-organisms, and under agricultural systems by tillage.
Clay particles and organic matter form durable and stable aggregates, called a clay humus complex, which is the result of activities of soil organisms, such as worms.
(See also organic matter management).
Soil structure is the way particles of sand, silt, and clay are assembled (FAO, 1985). The structure and the content of organic matter determine the infiltration rate of water into the soil and the water retention capacity of the soil. Both have effects on irrigation efficiency. Soil provides fixation for the plants, and supplies water and nutrients to the roots.
Soil water characteristics are determined by soil particles and their sizes, the soil type, as well as by the pores between the particles. The narrower the pores, the higher the capillary rise of the soil water (see Figure 4).
The forces influencing the soil water are:
- adhesion, which attracts the soil water to soil particles;
- cohesion, which attracts water molecule among themselves; and
- apillarity, which lets water move against the gravity force. Capillarity is an important factor in soil salinization processes.
Soil water is differentiated as follows:
- Gravitational water is soil water moving through the soil as influenced by gravitation. It is not plant available.
- Capillary water (also called soil solution) is water in the micropores of the soil and is held against the force of gravity. Capillary water is held by cohesion and adhesion and most of it is plant available.
- Hygroscopic water is held very tightly on the surfaces of soil particles. Clay holds more hygroscopic water than sand. This part of the soil water is not plant available.
A soil is saturated when all soil pores are filled with water, which normally occurs after sufficient precipitation or irrigation. Field capacity refers to the soil moisture content after all gravitational water has drained. The wilting point is the soil moisture percentage where plants are unable to take up sufficient water for growing.
For more detailed information on soil and water, see: Weave, J. Root development of field crops. 1926. 3
Figure 4: Microscopic view of soil
Figure 5: Capillary tubes
- ↑ Pidwirny, M. (2006). "Soil Pedogenesis". Fundamentals of Physical Geography, 2nd Edition. http://www.physicalgeography.net/fundamentals/10u.html [03-01-2013].
BMZ/GIZ/Huppert et al. (1989): Management of Irrigation Systems: Guiding Principles.
E. Mückenhausen: Die Bodenkunde und ihre geologischen, mineralogischen, geomorphologischen und petrologischen Grundlagen, 1985, ISBN-13: 978-3769005110
Scheffer/Schachtschabel: Lehrbuch der Bodenkunde, 2002, ISBN 978-3-8274-1444-1
W. E. H. Blum: Bodenkunde in Stichworten, 2007, ISBN: 344303117X
M. Ashman, G. Puri: Essential Soil Science: A Clear and Concise Introduction to Soil Science, 2008, ISBN: 978-0-632-04885-4
K. Müller-Sämann and J. Kotschi (1994): Sustaining Growth. Soil fertility management in tropical smallholdings.
WEAVER, J. E., 1926: Root development of field crops, London URL: http://www.soilandhealth.org/01aglibrary/010139fieldcroproots/010139toc.html
FAO 2005: The importance of soil organic matter, Rome