Leaching Requirement (LR)

The avocado is the most salt sensitive fruit tree. In order to maintain acceptable yields it is necessary to leach the soil of salts deposited mainly by the irrigation water. If the electrical conductivity (EC) of the soil solution (ECe) is greater than 0.4 decisiemens/meter (dS/m), there is a need for additional water for leaching in order to maintain maximum yield potential. The salinity threshold for avocado is 0.4 dS/m (Dr. Jim Oster, Emeritus Specialist, Univ. of CA, Riverside). The threshold is the maximum ECe value that Hass avocados on Mexican seedling rootstocks can tolerate without a potential yield decline.  ECw is the electrical conductivity of the water source provided by the water district or measured with an EC meter.  ECe is the average salinity of the root zone and could be measured by preparing an extract from a soil paste saturated by distilled water and measured with an EC meter. 

The Rhoades equation to calculate the leaching requirement is:

For example, if ECw is 0.6 dS/m, then the LR equals 0.6/((5 x 0.4)-0.6) which is equal to 0.43.

The water requirement (WR) needed to achieve this LR depends on crop evapotranspiration (ET) according to:

Assuming the annual ET of avocados is 40 inches and the salinity of the applied water is 0.6 dS/m, then WR equals 40/(1 - 0.43), or 70 inches. 

Since rainwater contributes to the overall soil EC, any measurable rain needs to be taken into account when assessing the leaching requirement. If rain provides 20 % of the water used by the avocado trees, it will lower soil salinity by an equal percentage. Thus a grower can potentially irrigate with 1.0 dS/m water without yield decline providing adequate leaching is judiciously applied.

To know more about irrigation water and salinity, Electrical Conductivity (EC), Leaching Requirement (LR) and Leaching Fraction (LF) visit the following sites:

Salinity and water management in avocado - Joseph Shalhevet - Avocado Brainstorming '99 - Pages 84 - 91

Overview of Salinity effect on crop production -  (Abstract, Full Document  Adobe Acrobat File) - UC ANR publication 8066 by Stephen R. Grattan, UC Davis. 

Water quality for agriculture - R.S. Ayers and D.W. Westcot- FAO Irrigation and Drainage Paper 29 Rev.1

Irrigation Water Quality - Salinity and Soil Structure Stability - Queensland Department of Natural Resources

Handbook No.60 Saline and Alkali Soils - Diagnosis and Improvement (Abstract, Full Handbook)

http://archive.globe.gov/sda-bin/wt/ghp/tg+L(en)+P(hydrology/Conductivity) - Electrical Conductivity Protocol - from Globe teaching Guide

 http://wow.nrri.umn.edu/wow/under/parameters/conductivity.html - Electrical Conductivity - from Water onthe Web

http://soils.usda.gov/sqi/files/chpt5.pdf - Electrical Conductivity Test - from USDA

http://www.ag.ndsu.nodak.edu/aginfo/procrop/sls/saltst07.htm - Soil Testing for Salinity.- North Dakota State University