When salts build up in soil, or in the growing medium, their level of concentration might become imbalanced.
Salts are added to soil with the irrigation water and with applied fertilizers. Applying more water than needed by the crop leaches the salt below the root zone, deeper into the soil or out of the growing medium when growing container plants. Knowing how much to leach, and when, is important.
The leaching requirement
The leaching requirement can be estimated from the equation:
LR = ECw / (5 x ECe - Ecw)
Where LR is the minimum leaching requirement for the crop, ECw is the electrical conductivity of the irrigation water in ds/m (irrigation water salinity) and ECe is the soil EC tolerated by the crop, measured in a soil-saturated extract.
The total irrigation water amount that has to be applied to meet both crop demand and leaching requirement can be estimated from the equation:
AW = ET / (1 - LR)
Where AW is the amount of irrigation water that has to be applied, ET is the crop water demand and LR is the calculated leaching requirement.
Here’s an example:
ET = 30m3/ha/day
ECe = 2.5 ds/m
ECw = 1.2 ds/m
LR = 1.2 /(5 x 2.5 - 1.2) = 0.1
AW = 30 /(1 - 0.1) = 33.33 m3/ha/day
Simply knowing the total amount of irrigation water to apply is not enough for managing salinity—irrigation intervals must also be considered. These intervals depend not only on the water demand of the crop, but also on factors such as the salinity threshold of the crop and the soil’s capacity to hold water.
Applying the same amount of water to two soils with different characteristics will result in different wetting patterns and depths. Irrigation depth in a heavy soil is lower than in a sandy soil since heavy soils hold more water than sandy soils; therefore, heavy soils require higher application amounts of irrigation water, at larger intervals, in order to prevent the accumulation of salts that exceed the salinity threshold of the crop.
It must be noted, though, that salinity in the root zone increases between irrigations as a result of crop water uptake and water evaporation from soil. The timing of leaching is not critical, provided the salinity threshold of the crop is not exceeded.