Crop Response to Soil Salinity

Excess soil salinity causes poor and spotty stands of crops, uneven and stunted growth and poor yields, the extent depending on the degree of salinity. The primary effect of excess salinity is that it renders less water available to plants although some is still present in the root zone. This is because the osmotic pressure of the soil solution increases as the salt concentration increases. Apart from the osmotic effect of salts in the soil solution, excessive concentration and absorption of individual ions may prove toxic to the plants and/or may retard the absorption of other essential plant nutrients.

Some crops are more sensitive to salinity than others. Crops such as pulses, row crops and special crops are particularly sensitive to salinity. The salt tolerance of some crops changes with growth stages. Sensitive crops may exhibit negative effects of salinity at levels <4 dS/m. An EC of 4 is a general salinity rating for traditional annual crops (wheat, canola) which are not significantly affected by soil salinity levels below 4 dS/m. Other rating systems (refer to Manual for Describing Soils in the Field) evaluate salinity with greater detail using the following classes:

1. Nonsaline (0-2 dS/m)
2. Slightly saline (2-4 dS/m)
3. Weakly saline (4-8 dS/m)
4. Moderately saline (8-15 dS/m)
5. Strongly saline (>15 dS/m)

There is no critical point of salinity where plants fail to grow. As the salinity increases growth decreases until plants become chlorotic and die. Plants differ widely in their ability to tolerate salts in the soil. Salt tolerance ratings of plants are based on yield reduction on salt-affected soils when compared with yields on similar non-saline soils. Soil salinity classes generally recognized are given in the following table.

SOIL SALINITY CLASSES AND CROP GROWTH

Soil Salinity Class	Conductivity of the Saturation Extract (dS/m)	Effect on Crop Plants
Non saline	0 - 2	Salinity effects negligible
Slightly saline	2 - 4	Yields of sensitive crops may be restricted
Moderately saline	4 - 8	Yields of many crops are restricted
Strongly saline	8 - 16	Only tolerant crops yield satisfactorily
Very strongly saline	> 16	Only a few very tolerant crops yield satisfactorily