If a crust forms on soils already growing a crop, the crust should be broken up, with
a hoe for example, to allow water to percolate down into the soil and to give the roots
access to the air. The crop can be weeded at the same time.
Capping is more of a problem in clay soils, especially those with low organic matter
content, and also occurs in soils with a high proportion of fine sand. It is caused by both
physical and chemical processes, and can be reduced with applications of manure and
other sources of organic matter. It can also sometimes be appropriate to cover the soil,
heavy rains or strong sunshine are likely to occur.
The UNEP has estimated that about 20% of the world's cultivated land and nearly 50%
of all irrigated land is affected by salinity (Flowers and Yeo 1995).
This is not a recent problem; many historians maintain that the ancient Sumerian
civilisation declined partly as a result of irrigation that caused salinisation—the toxic
build-up of salts and other impurities.
Saline soils have a high concentration of ions, both Sodium cations (Na+) and
Chlorine anions (Cl-), and are generally unfavourable to the growth of most plants.
Alkaline soils above about 8.5 pH have only Na+ cations and normally cannot be
Some crops such as barley and cotton are quite tolerant of saline soils and can grow
in soils with more than 5000 ppm TDS (Total Dissolved Salts/Solids). Other more
sensitive crops such as beans and citrus trees suffer from salt stress in soils with only
960 ppm TDS. See list below, Salt Tolerance of Plants pages 16–17.
Salinity of soil is normally measured in terms of its Electrical Conductivity (EC,
measured in deciSiemens/metre—dS/m) or Total Dissolved Salts (TDS—in ppm or
mg/litre). EC is a measure of the conduction of electricity through water, or a water
extract of soil. The EC value represents the amount of soluble salts in an extract,
providing an indication of soil salinity. Saline soils are defined as those with an EC of
greater than 1.5 dS/m for a 1:5 soil water extract and greater than 4 dS/m for a
saturation extract. It can be interpreted in terms of the salinity tolerance of plants.
Conversion Rates: 1 dS/m = 640 mg/litre = approx. 640 ppm TDS.These units are used to indicate the extent of the problem of osmosis, the ability of
plants to take up water through their roots. The unit “ESP” (exchangeable sodium
percentage) indicates the percentage of absorbed sodium ions to other cations that could
be exchanged. Soils are categorised as being sodic with an ESP of 6–14% and strongly
sodic with an ESP of greater than 15%.
Some soils are naturally saline; other soils become more saline, under one or more
of the following conditions:
- when the irrigation water is saline, or when seawater inundates low lying areas;
- when the rate of evaporation is high (high temperature and/or wind speed);
- if the water table is high, as a result of poor drainage.
GROWING FOOD – THE FOOD PRODUCTION HANDBOOK 15
with either a mulch (1He, pages 70–71) or a growing crop, during periods when either
The problem of saline soils is also discussed in 1N. “Irrigation”, pages 93–99.used at all for crop production. “pH” is discussed in 1Cc, pages 20–23.