Soil and water

Physical soil properties
Physical properties of the soil influence to a great extend: Classification of soils
Soil classification is based on sand, clay and sand content.
Soils can be divided according to their "texture", i.e. the distribution of the size of their mineral particles.


The relative proportion of clay, silt and sand in a soil determines its textural class.
Classification triangle of soils

The physical soil characteristics determine to a great extend the effect of soil tillage operations:
ploughing, seed-bed preparation, ridging, harvest, etc.
Soil tillage operations are relatively easy in the soils found at the lower left part of the soil classification triangle.
The more clay a soil contains the more difficult it is for soil tillage. Since these soils are poorly drained, soil is compacted and clods are formed when the tillage is done in still wet soils.
Too much sand is not favourable either since soils with a high percentage of sand have a low stability and have a limited water holding capacity.

Soil moisture retention curve
The soil moisture retention curve (pF curve) gives the relation between soil moisture suction and soil moisture content.
Soil moisture retention curve

A soil is at F.C. (field capacity) or has a pF-value of 2, some 2 to 3 days the soil has been saturated by rainfall or irrigation.
When the soil becomes dry and plants cannot take up water anymore the soil is at W.P (wilting point) or has a pF=4.2.

The amount of water held by a soil in the root zone between F.C. and W.P. and which can be used by plants is described as available water. (F.C.- W.P.= available water)
For sand, loam and clay the values are 6, 20 and 17 volume percent respectively. (Note that 1 vol.% per 10 cm depth corresponds to 1 mm of rainfall).

Readily available water in root zone
In a root zone of 50 cm in sand, loam and clay has respectively 30, 100 and 85 mm of available water. It is an assumption that 50% of the available water (readily available water) can be used by the plants, before growth reduction starts. This means that well irrigated crops with a water use of 5 mm a day, have to be irrigated every 6, 20 or 17 days for respectively sand, loam and clay. Some examples of available water are given.


Available water, drainage and aeration
Available water is low in sand, higher in clay soils and maximal in loamy and silty soils.
In clay soils drainage is often poor.
Soils containing less than 10-15 volume % of air (e.g.clay) at F.C., have insufficient aeration for plant growth.
The soils which are good for potato production are found in the lower left part of the soil classification triangle: Loam, sandy loam, sandy clay loam and loamy sand.