Water Isn’t Simply a Combination of H2O
As recent rains begin to alleviate the effects of California’s extended drought and leach long-accumulated salts (such as boron, sodium and chloride) from the root zone of our groves, it’s important to remember that salinity will be a perennial issue for California avocado growers because all waters — even rain water — have some dissolved salts in them and low rainfall is the rule, not the exception, in this state.
Most of us think of NaCl when we think of salt, but that’s not the only salt growers need to be concerned with. As Ben Faber notes in a recent blog post, salts are simply a combination of electrically charged ions — and these positively/negatively charged ions separate from one another as they dissolve in water. Thus, water — be it rainwater, well water or domestic water supplies — contains a variety of these ions (bicarbonate, boron, calcium, chloride, magnesium, sodium and sulfate). In addition, natural water contains low concentrations of other elements — such as boron — that can be harmful to California avocado trees. Faber notes that Santa Barbara and Ventura Counties contain “potentially harmful levels of boron for plants.”
When assessing the affects of salinity on California avocado groves you must consider not only the ions in water — whose proportions can change over time — but also the ions in soil and fertilizer. Thus, it is important that when growers evaluate their water, they look not only at its total salinity, but the specific ions in the water. Keep in mind that as trees take up water from the soil, the concentration of salts will increase in the water that remains in the soil. This becomes problematic because the soil salts (ions) compete with the roots for water, thus making it more difficult for the tree to pull water from the soil.
While small concentrations of salts in the soil may not excessively damage trees, a combination of high fertilizer use, poor water quality and poor leaching can significantly reduce tree growth. The first symptom of salt accumulation, or lack of water, is leaf drop; this is followed by tip burn, yellow leaves and poor root growth. If soils salts continue to increase, the tree will drop more leaves exposing the tree and fruit to sunburn and shriveling. These conditions also make trees more vulnerable to disease.
The salinity of water is measured by the electrical conductivity (EC) of the water. When ions (salts) are present in the water, the EC of the solution increases. If no salts are present, the EC is low. One EC (dS/m) is equal to about 640ppm salt. Soil water of about 4 EC, or TDS of 2000, is too salty.
To measure the soil salinity in the avocado grove:
- Collect soil samples at different depths
- Mix the samples with pure, distilled water at a 1:2 ratio
- Add ⅓-ounce soil (10g) to ⅔-ounce (20mls) water; mix and allow the salt to dissolve
- Measure the EC with a salinity pen, and multiply by eight, for total soil EC
When groves have low rainfall amounts, or growers use poor quality water for irrigation, it’s important to leach the soil to remove excess salts from the soil. Keep in mind:
- A sign of poor soil leaching is a tree displaying water stress, despite wet soil
- Effective leaching requires monitoring soil water to determine irrigation volume and duration
- Generally aim to use a 10-20% leaching fraction at each irrigation, to maintain a root-zone salinity of soil water below EC 2
- Leaching fraction is the amount of additional irrigation water needed to maintain the correct salinity; this, however, depends on salt levels in irrigation water
- Appropriate leaching amounts depend on irrigation water salinity and target root-zone salinity