There will be an increase reliance on groundwater for 2014. Wells that have been typically relied on to subsidize water allocations are now providing the primary source of water for the drought stricken almond orchards. If using a well, it is important to sample the water. Sampling will determine the characteristics of the water such as dissolved salts, pH, and major cations and anions. Sampling should be performed regularly, and more frequently if well performance or pumping depth has changed.
After receiving the analysis, check the electrical conductivity (ECw). If this value is less than 1.5, then there is a reduced risk of accumulating toxic levels of salt if a leaching fraction of 20% is used. If greater than 3.0, the water may be too hot and should only be considered for use if blended. Data suggests that trees planted on soils with lower exchange capacities (<15 meq/100 g) may not be able to tolerate an ECw of 1.5.
Review the sodium adsorption ratio. If this value is greater than 3, it indicates that sodium makes up the primary cation in the water. Generally, values under 3 are good, values between 3 and 9 are workable with some issues, and over 9 is not recommended for use in almond. If the pH of the water is high or if carbonate levels are high, this will affect the SAR as calcium will precipitate out in the soil. This is taken into account with the adjusted SAR. The SAR can be lowered by adding calcium (gypsum, calcium nitrate, calcium thiosulfate, etc) or magnesium (not as common). The adjusted SAR can be brought closer to the calculated SAR by acidifying the water.
The SAR will take into account sodium, but not chloride and boron. Water containing under 5 meq/L of chloride should have minimal restrictions, while water greater than 5 but less than 15 is usable. Boron can also be toxic. Water with concentrations greater than 3 PPM should not be applied, while values between 0.5 and 3.0 require leaching fractions. Boron can easily accumulate to toxic levels in the soil.
Water applications with good to marginal quality of water should be applied with a leaching fraction. This extra water basically keeps the salt front below the root zone of the tree. There is an exact way to calculate this value. If too lazy for the basic math and soil sampling, the following guidelines or generalizations may be of use:
- If wanting the EC of the soil to match the EC of the water, apply 33% more water;
- If wanting the EC of the soil to be twice the EC of the water, apply 10% more water;
- If wanting the EC of the soil to be three times the EC of the water, apply 5% more water.
Keep in mind that the values listed above for “good” or tolerable ECw, SAR, chloride and boron concentrations all were based on the assumption that a 20% leaching fraction was applied.
Remember also that the lack of winter rainfall means less leaching of salts. Typically, sodium toxicity occurs after one or two seasons when no winter leaching has occurred. In these years it is critical that a leaching fraction be applied to help reduce salt load in the rootzone and prevent tree damage and productivity loss.