With the drought our perpetual salt problems are exacerbated due to less water and often more saline water.  The question keeps coming up if gypsum (calcium sulfate) can help correct the problem.  And the answer is maybe, but along the coast, probably not.  The problem there is confusion about what is a saline soil and what is a sodic soil.  A saline soil is one that is dominated by salts, but has a pH below 8.5 and can have a white crust that will actually taste salty.  A sodic soil is one dominated by sodium, has a pH above 8.5 and can be saline, as well.  Often though, there is a brownish cast to the surface salt crust.  This is caused by dispersion (dissolved) of soil organic matter caused by the high pH.  It's like cooking with vinegar when you make ceviche out of fish.  Saline soils often have a high calcium content and may have sodium, but at a very low ratio compared to calcium.  Most of the sodic soils in California are found in the Central and Imperial Valleys.  Along the coast, the soils, if they have a problem, are largely saline.

The way gypsum works, is that the added calcium displaces soil sodium, pushing it lower in the soil column.  The process also requires a lot of water to move the sodium through the soil column.

So the answer is, along the coast, gypsum is unlikely to improve soil conditions.  However, there are other instances where it might help. In the San Luis Obispo area there are lots of serpentine derived soils that have a high magnesium content relative to calcium.  And they commonly aren't saline, just an imbalance between the two cations. This can lead to infiltration problems and calcium deficiency in plants.  Apples are especially sensitive to this high Mg:Ca ratio and develop a condition called “bitter pit”, a surface, brown pitting in the skin. There are other crops, like celery that are especially sensitive, but even avocado can be mildly affected. In the case of magnesium imbalance, gypsum can help.

I just raked up all the leaves under the avocado and it looks so nice. PUT THEM RIGHT BACK. The avocado is shallow-rooted and really depends on the natural leaf mulch to protect its roots. In fact the roots will actually colonize the rotted leaves as if it were soil. This mulch is also a first line of defense against root rot. The decomposing leaves create a hostile environment to the microorganism that causes the disease. The mulch also helps to reduce evaporative loss of water and therefore reduces water needs. Avocado growers will actually spread mulch in cases where trees are too young to produce adequate leaf drop for mulch or in windy areas where mulch has blown away. The key to remember is that the mulch should be kept at least six inches away from the trunk to avoid collar rot which can be causes by keeping a moist mulch against the trunk.

Mature avocados may be a big tree, but they have very shallow roots. The bulk of them are in the top 8 inches of soil. The tree therefore does not have access to a large volume of stored water. As opposed to a deep rooted walnut, they need frequent, small amounts of water. A young tree in the summer might need multiple applications per week, but because the root system is small, each application may only be 5-20 gallons. An older tree with its wider rooting pattern may go a week to a month between irrigations depending on the weather and rainfall. Proper irrigation is the best way to keep the avocado from getting root rot. Both over and under irrigation can induce the conditions for root rot, although over irrigation is more common. And remember, it is not just the amount applied at an irrigation, but the timing that is important, as well. Because you are managing such a shallow root system, just poking your finger into the root system will tell you if there is adequate moisture there before you irrigate again.

The canopy is thinning. The leaves are small and yellow. There is dieback in the canopy, with leafless tips on the branches. You dig around under the canopy in the wetted area of the sprinkler and you can't find roots within 6 inches of the soil surface or if you do find them they are black. There is little mulch under the tree. There are weeds growing under the tree. All these are signs of root rot disease. But it is also a sign of lack of water, because that is what is happening – there are no roots to take up water. And one of the things a gardener will often do, is start watering the diseased tree more, thinking it is lack of water, which if it is diseased only makes the condition worse. Adding more water to a tree that can not easily take it up only creates an asphyxiation that makes matters worse. Irrigation and mulch are the two most important factors for avoiding the disease.

So what do you do if you have disease? There are fungicides that are available from the nursery, but there are a number of things that you can do before applying something like that. First of all get a handle on the irrigation. Make sure you are irrigating to the tree's needs. Check soil moisture before irrigating. Make sure the tree is not getting supplemental water from another area such as a lawn sprinkler. Make sure there is a good thick, woody mulch under the canopy. Adding gypsum (15-20 pounds per tree) evenly spread under the canopy can also help, but reviewing and modifying the irrigation practice is the most important thing that you can do.