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Monday, February 9, 2015

White Crust on Soil Surface Is Alkali or Salt

Q. I dug up some of our native soil and amended it with 50% planting mix. The next day after it dried, this white substance appeared on the surface. Is this salt or alkali? I know my soil will effervesce when you pour vinegar over it.
Salt deposit left on soil surface of the readers soil.

A. Salt and alkali are pretty much the same thing. Alkali should not be confused with alkaline. Alkaline refers to pH. Alkali refers to salts. Soils that create bubbles or effervesce when vinegar is applied to them have quite a bit of calcium carbonate, or lime, present.
Salt deposit left on drip emitters

The old-timers who grew things in desert soils would refer to "white alkali" and "black alkali". I think the word “alkali” has remained in our vocabulary, but not the difference between white and black types, and it is just as confusing now as it was then.

White alkali refers to the white salt accumulation on the top of soils. These white salts were usually sodium sulfate, sodium chloride, and magnesium sulfate.

Black alkali on the other hand left blackish spots on the soil surface usually composed of sodium carbonate and organic matter. It was understood that black alkali was more damaging to plants in soils than white alkali.
Salt deposit left on soil surface after an irrigation
In fact, when ranking these salts and their injurious effects on crops, sodium carbonate was considered the worst while sodium chloride (table salt) was somewhere in the middle and sodium sulfate the least injurious. When you had black alkali salts, many farmers would just give up and walk away from those soils.

The salts in your picture look like white alkali. I would have to guess it is a mixture of salts containing calcium, sodium and magnesium and carbonates, sulfates and chlorides. You wouldn't know unless you submitted the soil for a soil test.

When salts accumulate on the soil surface it is best to take a flat-nosed shovel and scrape off the top inch or so and dispose of it. Then begin activities that reduce the salt content.
Salt deposits left on block wall after water evaporates.
Most salts are removed with water and flushing. Level the soil surface as much as possible. Sprinkle the soil with water to push the remaining salts deeper into the soil. The idea is to push the salts deeper than the roots of your crops.

If the soil does not drain easily, there may be a high sodium problem. If this is the case, apply gypsum to the top of the soil and rake or rototill it in as deep as you can. Then begin your irrigations.

Gypsum is used to remove sodium from the soil and replace it with calcium. Sodium is a bad player in soils and prevents drainage. Substituting calcium for sodium improves drainage. Also, mixing compost in the soil will help in the removal of salts and make them less injurious to plants.

Learn more about salts, soil, water and irrigation of the desert

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