Jan 30

pH, soil, and nutrient availability

Many of you have asked about the relationship between nutrient availability and soil pH, so I thought I would address this in a post. Alot of this info is pretty technical, but if you take some time to break it down it is really pretty straight forward.

When small quantities of inorganic salts, such as the soluble mineral matter of soil and commercial fertilizers, are added to water they dissociate into electrically charged units called ions.

The positively charged ions (cations) such as hydrogen (H+), potassium (K+), calcium (Ca++) magnesium (Mg++), ammonium (NH4+), iron (Fe++), manganese (Mn++), and zinc (Zn++) are absorbed mostly on the negatively charged surfaces of the soil colloids (microscopic clay and humus particles) and exist only in small quantities in the soil solution. Thus, the humus-clay colloids serve as a storehouse for certain essential ions (cations).

The negatively charged ions (anions), such as nitrates (N03-) phosphates (HPO4–), sulfates (SO4–), and chlorides (Cl-), are found almost exclusively in the soil solution and can therefore be leached away easily with overwatering.

The roots and root hairs are in intimate contact with the soil colloidal surfaces, which are bathed in the soil solution, and therefore nutrient uptake can take place either from the soil solution or directly from the colloidal surfaces (cation exchange). The soil solution is the most important source of nutrients, but since it is very dilute its nutrients are easily depleted and must be replenished from soil particles. The solid phase of the soil, acting as a reservoir of nutrients, slowly releases them into the soil solution by the solubilization of soil minerals and organics, by the solution of soluble salts, and by cation exchange.

A more dramatic increase in the nutrient content of the soil solution takes place with the addition of commercial fertilizers. As plants absorb nutrients (ions) they exchange them for other ions.

For example, for the uptake of one potassium (K+) ion or one ammonium (NH4+) ion, one hydrogen (H+) ion is released into the soil solution or directly into the soil colloids by the process of cation exchange. Similarly, for the uptake of one calcium (Ca++) or one magnesium (Mg++) ion, two hydrogen (H+) ions are released by the root. Thus, as the plant absorbs these essential cations, the soil solution and the colloidal particles contain more and more hydrogen (H+) ions, which explains why the removal of cations (ammonium (NH4+) nitrogen is a good example) by crops tends to make soils acidic, i.e., having a low pH.

Also, as the plant (absorbs essential anions such as nitrates (NO3-) and phosphates (HPO4-), the soil solution is enriched with more and more hydroxyl groups (OH-) and bicarbonates (HCO3-), which explains why the removal of anions (nitrate (NO3-) nitrogen is a good example) by crops tends to make soils alkaline, i.e., having a high pH.

Whew, that was a mouthful, right? It doesn’t take a degree in chemistry to understand nutrient/ pH relationships in soil, but it helps!

1 comment

  1. dcp511

    hanks for sharing your blog with all of us, very imformative.

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