Leaching

Leaching Salts from the Rootzone

Leaching is the transport of salts out of the root zone using irrigation water. The proportion of the irrigation water used specifically for this purpose is the leaching fraction (LF). The leaching fraction (LF) is water beyond that required for tree growth and crop development–that is, for evapotranspiration(ET) . It is also beyond that required to refill the rootzone, so it will percolate below the rootzone.

Required Irrigation = tree growth requirement (ET) + Rootzone refill requirement + requirement for salinity control (LF)

The LF varies, depending on the salinity of the soil and the salinity of the irrigation water. High salinity in either translates into a higher LF to maintain acceptable salinity in the rootzone. The table below provides LF required to sustain a desired rootzone EC_e for given irrigation water salinity EC_w. Source: Pistachio Production Manual, 2008: adapted from Ayers, RS. D.W. Westcot. Water Quality for Agriculture. FAO Irrigation and Drainage paper 29 (reprinted 1989, 1994).

Table 1. Leaching Fraction (%) Requirement

Irrigation Water EC (dS/m)	Desired Avg. Rootzone EC_e
Irrigation Water EC (dS/m)	1	2	3	4	5	6
0.2	2	1
0.5	10	3	2
1.0	33	10	5	3	2	2
1.5		20	10	7	5	3
2.0		33	17	10	7	5
2.5			24	15	10	8
3.0			33	20	14	10
3.5				26	18	13
4.0				33	23	17
4.5					27	20
5.0					33	24
5.5						28
6.0						33

To computer the LF using Table 1: Needed information is the amount of water consumed annually by the orchard. For the example below: assume a value of 42 inches/year

Required Irrigation water = ET*(1 + LF)

When EC_w is 1 dS/m, for an EC_e goal of 3.0: 42*(1 + .05) = 44.1 inches annually
When EC_w is 1 dS/m, for an EC_e goal of 1.0: 42*(1 + .33) = 55.9 inches annually
When EC_w is 2 dS/m, for an EC_egoal of 3.0: 42*(2 + .05) = 86.1 inches annually

Annual or biannual sampling is advised to adjust a salinity management strategy, due to the factors unique to every orchard. It is also advised to establish an actual distribution uniformity (DU) for the orchard irrigation system by field evaluation (that is, by testing the flows and pressure across the system).

Usually, during the season, it is sufficient to irrigate to meet orchard ET needs plus the extra for system non-uniformity suggested below. With this strategy, salinity may rise during the season, but post harvest irrigation, winter rains and spring irrigation is usually sufficient to return salinity to acceptable levels.

for border system with tail return: 15 – 25% over ET
for drip system: 10 – 25% over ET
for microsprinkler system: 6 – 15% over ET