The amount of water needed to leach out salts properly varies greatly, but roughly
speaking between 5% and 35% extra water is needed in addition to the normal irrigation
water used, as shown on the table below:
TDS
(ppm)Extra water
needed
640
900
1280
1800+5%
15%
25%
35%
Fertilisers and Leaching
When the soil is leached beneficial nutrients are also lost, together with the salts, so it is
preferable not to apply fertiliser soon before leaching is in operation. If fertilisers are
used during leaching periods, some fertilisers are available which are less soluble than
others and so these should be used if possible.
When to Leach?
Leaching can be done at any time of the year but it is normally done in the cooler, wetter
seasons—for good reasons. At that time plants need less water, rainfall is higher, and
there is enough time to leave the irrigation equipment in one place to ensure thorough
leaching. An additional reason is that water is often scarce and more expensive in the
hotter summer months.
B) Problem’s with Saline Irrigation Water
Even low levels of sodium or chlorine ions in irrigation water can accumulate in the soil
and become harmful to some plants. Unfortunately the techniques used to remove salts
from water are very expensive and use large amounts of energy/electricity. The
techniques involve either reverse osmosis or electrolysis, and are generally only
practical for irrigation schemes where the supply of energy is cheap.
The best way, indeed the only really effective way, to ensure that salts do not
accumulate in the soil is to monitor the actual levels of salts on a regular basis. Due to
the fact that the salt level rises and falls—sometimes dramatically, in rivers for
example—the irrigation can be scheduled so that water is applied when the salt content
is low. Alas, water is often most critically in demand at the very time that water sources
are at their lowest, ie most salty, point.
Water that is used to irrigate plants can often become up to 3 or 4 times more
concentrated when it enters the soil, especially when it is around the root zone. This fact
is important to grasp, as crops which are tolerant to about 5000 ppm TDS should not be
irrigated with water which contains more than about 2000 ppm TDS on sandy loam soils
or 1000 ppm TDS on clay soils.