Brackish water may be contaminated with acids, bases,
salts or organic matter whereas saline water contains only dissolved
salts. Wells that are brackish either all or part of the time also
supply irrigation water. In other areas, farmers use water from lays,
streams and rivers that flow into bodies of salt water and are subject
to tidal fluctuations. The amount of foreign matter in these waters may
vary considerably, so it is important that the water be analyzed
frequently for salt content during the irrigation season. Storage
reservoirs are often constructed to accumulate irrigation water.
How are salts accumulated and removed from the soils?
Salts may accumulate in soils of humid regions if
brackish water is used for irrigation. In humid areas most irrigation
water is applied by sprinkling. Owing to the limited quantity of water
available for irrigation in many areas and the small amount of
sprinkling equipment in use, the average single application consists of
1 to 11/2 inches. As the moisture content of the soil at the time of
irrigation is low, this depth of irrigation water will not penetrate the
soil beyond 6 to 9 inches. Hence, if such light applications are used,
salts in the irrigation water are likely to be confined to the surface
foot of soil. If no rain falls between irrigation, a second irrigation
will double the salt content of the surface foot.
Rainfall must more than saturate the soil to cause
any appreciable leaching of salt. The amount and intensity of rainfall
will determine the movement of salt in the soil profile. Light showers
have little effect. In areas of relatively high rainfall, winter rains
will usually leach salts out of the root zone if drainage is good. Good
drainage is essential for removal of salts. Without it, the danger of
salt accumulation in the surface soil is increased.
HOW DOES SALT ACCUMULATION AFFECT THE SOIL?
The degree to which salt will adversely affect humid
area soils is still subject to further research. If large quantities of
salt should accumulate in the soil, however, they may be harmful to
plant growth. As the salt is leached out of the soil by rainfall, small
quantities of sodium (one of the major constituents of salt present in
seawater) may remain behind in an adsorbed, or tightly held, form.
Should the quantity of sodium in this form accumulate in the soil in any
appreciable amount, a poor physical structure will result. Soils
containing relatively large quantities of adsorbed sodium are much less
permeable to air and water and tend to form a hard crust when dry. Under
average conditions for irrigation in humid areas, it is doubtful that
adsorbed sodium will be a problem.
HOW DOES SALT AFFECT THE PLANT?
High levels of salt accumulation in soils affect
plant growth in two ways. First, as the salt concentration of the soil
increases, water becomes less and less available to plants. Loss of
water from the soil by evaporation and plant use following brackish
water irrigation causes the salt concentration of the remaining water in
the soil to become greater, and thus moisture is less available. For
this reason, plants grown on salt-affected soils require irrigation more
frequently than those grown on a soil low in salt content. Plants
growing under conditions of relatively high salinity are usually stunted
and tend to have a bluish-green colour. A reduction in growth may also
occur at moderate levels of salinity in the soil, depending on the salt
tolerance of the crop grown.
Second, plants may be affected by a direct toxicity
of one or more of the constituents of the salt added by irrigation
water. Toxic constituents frequently affect fruit trees, but their
effect on most field and truck crops is negligible.
HOW MUCH SALT WILL CROPS TOLERATE?
Crop growth generally decreases with increasing
salinity in the soil. Some plants are more tolerant to salinity than
others. Crop plants may be divided into three groups: good salt
tolerance, moderate salt tolerant and poor salt tolerance. In the list
that follows, the plants are ranked in order of decreasing salt
tolerance in each crop group.
GOOD SALT TOLERANCE
Field crops: Barley, beets, cotton.
Forage crops: Salt grass, bermuda grass, barley,
Vegetable crops: Garden beets, kale, asparagus,
MODERATE SALT TOLERANCE
Field crops: Rye, wheat, oats, sorghum, corn.
Forage crops: Sweet clover, alfalfa, orchard
Vegetable crops: Tomato, cabbage, potato,
lettuce, sweet corn, pepper, squash, carrot, onion, peas, cucumber.
Fruit crops: Fig, grape, cantaloup.
POOR SALT TOLERANCE
Field crops: Field beans
Forage crops: White clover, ladino clover.
Vegetable crops: Radish, celery, green beans.
Fruit crops: Pear, apple, orange, grapefruit,
plum, apricot, peach, lemon.
HOW MUCH BRACKISH WATER CAN A GROWER USE?
The amount of brackish water that can be used depends
on the salt concentration of the water, the number of irrigation between
leaching rains, the salt tolerance of the crop, and the salt content of
the soil before irrigation. This guide is based on two assumptions: i)
That there is no intervening rainfall of sufficient intensity to cause
leaching and ii) that there is no salt accumulation.