The white powdery substance is from salt accumulations on the soil surface. Since Colorado is in a semi-arid region, there isn't enough rainfall to leach salts from the soil in most cases.
After a rain when the soil begins to dry out, the salts begin to rise to the soil surface. As moisture evaporates from the soil surface, the salts are left behind and begin to accumulate as the white powdery substance you refer to.
Salts can be naturally occurring, such as from deposits of gypsum in the soil, or from sodium salts. They can also be introduced by applying composts, manures, or other amendments. When used in moderation, the salts from composts and manures are generally not a problem. If the amendments are over applied, then salts can accumulate to levels that can prevent plant growth.
The term alkali is sometimes used to indicate salty conditions since the major components of salts are calcium, magnesium, sodium and potassium. These are chemically referred to as alkali metals (sodium and potassium) or alkaline earths (calcium and magnesium). Alkali more often refers to the sodium concentration in the soil, which can reach very high levels, causing poor water infiltration and poor plant growth. It can also cause a condition called black alkali, where the soil just beneath the surface of a sodium-affected site becomes black.
The best way to evaluate salts is to have the soil tested for pH, electrical conductivity, and possibly sodium absorption ratio (the ratio of sodium to calcium and magnesium concentrations). The soil test will determine what type of approach should be taken to correct a salt problem.
In some cases where naturally occurring salts are not too high and the soil is sandy, additional water can be applied to leach salts from the root zone. Where the soil is clay-like, it may be necessary to install a drainage system composed of perforated PVC pipe or other materials to allow the salts to leach through the soil and to drain away from the site to a dry well or French well. If the soil is high in sodium, then there may be a need to add gypsum or sulfur to the site to help flush sodium from the soil with water and remove it from the root zone. When gypsum is added along with water, the gypsum will dissolve slowly and the soluble calcium will exchange with the sodium on the soil particles allowing sodium to be flushed from the root zone with water.
In soils high in calcium carbonate, sulfur can be applied which will react in the soil to form highly diluted concentrations of sulfuric acid. The acidity created from the sulfuric acid will then dissolve the existing lime in the soil making calcium more soluble, which can then exchange with the sodium on the soil particles and allow sodium to be flushed from the root zone with water.
If salts were imported to the site with soil amendments, it may be possible to remove the affected soil and import new topsoil to the site. Another way to reduce the effect of salts of soils impacted with salty amendments is to dilute the effect of the imported amendments with a material low in salt such as peat moss or low salt topsoil.
There is not a product available that can neutralize salts and turn them into a form that is not detrimental to plant growth.^^^
