AUTHORS NOTE: The following article discusses salinity management considerations for SANDY SOILS (e.g. sands, loamy sands). A follow up post will be made for finer texture soils (loams, silts, and clays). For the most part, this article describes issues with the EAST SIDE of the San Joaquin Valley. NOT ALL SOILS ARE THE SAME. Please note that the following guidelines may need site-specific adjustments.
During low rainfall years, salt burn often shows up in sandy soils. This salt burn is a combination of accumulation of sodium within the soil, the use of well water, and the low exchange capacity of the soil.
Starting with the soil, we often use the term cation exchange capacity (CEC) which is the amount of cations (positive charged ions like sodium, magnesium, calcium, potassium, etc.) that can bind to the soil particle surface. In the sandier areas on the east side of the San Joaquin Valley, CEC values can be quite low, with values less than 10 meq/100 g of soil. To put that in perspective, sandy loams are in the teens through 20s, silts and clays are in the 30s to 40s. This value is important as it indicates the amount of cations the soil particles can hold. The higher the CEC, the more cations that stick to the soil, preventing them from entering the soil water (soil water is the amount of water that is held between soil particles – it is what the tree drinks), reducing exposure to the roots of the tree. Regardless of the CEC, once the soil is saturated with cations, the excess will stay within the soil water. As the soil salinity increases, the tree’s roots have a greater exposure impacting them by affecting the osmotic movement of water (e.g. essentially making the tree work harder for water) and eventual toxicity.
In most sandy areas, interestingly enough, salinity levels within the water are often low. ECs commonly range from 0.5 – 1.0 dS/m, a value in which many would think sodium uptake issues would not occur. This would be true if accumulated salts are flushed out of the rootzone. Salts accumulate in the rootzone due to the ability for the plant’s roots to exclude them. For the most part, roots exclude between 95-99% of the sodium applied. As water is applied, the tree sucks up the water leaving the sodium behind. As subsequent irrigations occur, the sodium begins to accumulate. After multiple irrigation cycles (e.g. once a week through the summer is about 20 irrigation cycles), the soil will gradually become saltier than the water applied. At some point, the concentration of sodium exceeds the tree’s ability to exclude them. The sodium is then picked up by the tree, moves to the leaves, and burn occurs.
The combination of the accumulation of sodium with the low cation exchange capacity is why sodium toxicity occurs rapidly on sandy soils, especially when there are no winter rains or irrigation to help leach the sodium. The good news is that within sandy soils, sodium can leach almost as fast as it has accumulated. This can be done by emulating rain through winter irrigations. If possible, an application of at least 4-6 inches of water before the end of November. This will fill the soil profile (check to be sure by probing to 5 feet). If over a half inch of rain occurs before then, it can be subtracted from the targeted total. This water should be applied in 18-24 hour shots. After an application, wait 4-5 days, and apply another irrigation until the targeted total is reached.
After the soil profile is filled, another 4-6 inches of water will be needed. This can fall as rain or irrigation. After the soil profile is filled, any subsequent rains can count as leaching water. The leaching process should be completed by mid-January. This means that if Jan 1st comes around and no rain has fallen, winter irrigations following the episodic patterns as described above should begin. Do not wait until mid-Feb. Root expansion occurs in mid-January, and irrigating later than this could create anaerobic conditions, which kill the newly developed fine feeder roots.
Applications of calcium can help with the leaching process. Calcium can help displace the sodium from the soil’s charge sites. Although helpful, they are not required. I would rather see the money spent on pump time than on gyp as water is more critical than calcium at this point. If applying gypsum, make sure to apply potassium AFTER the gyp applications. Potassium is a single charged like sodium and will also flush out of the system.
On sandy soils, I have seen orchards recover within two-three years as it takes time for the trees to “flush” out the accumulated salts. Usually, the trees will express symptoms in the following year, but they are reduced. In the subsequent years, the damage will be less prevalent and the tree will respond with increasing yields.