What is soil?

A pretty basic question for the ground under our feet.  Plus, Dr. Dan Davidson explains Calcium Compartmentalization below.

What is Soil?

By Dr. Dan Davidson, Agronomist & Farmer

When you pick up a hand full of soil did you ever think about what it contains? Soil contains minerals, organic matter, water and air space (pores). When you break this down by volume soil is comprised

  • 45 percent minerals
  • 25 percent water
  • 25 percent air
  • 5 percent organic matter or biological life

Minerals comprise the bulk of the soil. Some of those minerals are soil particles including sand, silt and clay. Other minerals are natural and insoluble like calcite, dolomite, amphibole, apatite (phosphate minerals), marl, iron and aluminum oxides, quartz, feldspars, mica, silicates, etc. Soil minerals play a vital role in soil fertility since mineral surfaces serve as potential sites for nutrient storage.  As minerals weather, they release these stored, vital nutrients. And elements like iron, aluminum and calcium are important structural components in soil particles.

How Calcium is Compartmentalized

Calcium (Ca) is an important mineral in the soil as well and is found in in various components.

  • Structural calcium in the soil lattice (calcium aluminum silicates)
  • Calcium in limestone or Ca2+-CO32-
  • Soluble calcium (Ca2+)in the soil solution
  • Calcium on the exchange complex (Ca2+)
  • Organic Ca found tied up in soil residues or living soil plant flora and fauna.

Calcium is found in three states in the soil; available, unavailable and relatively insoluble. Calcium is not mobile and as such is not considered a leachable nutrient; hence it will not move down through the soil with water like nitrate and sulfate. However it is not as immobile as phosphate that ties up quickly. Many soils contain high levels of insoluble calcium such as calcium carbonate (a.k.a ‘limestone’) and crops grown in these situation can show a calcium deficiency.

Most silt and clay soils have 2500 to 4000 ppm calcium while sandy soils 500 to 1000 ppm calcium. And in lbs per acre, those numbers are double. However some of that calcium is not available and will be tied up in the mineral and structural forms while the rest will be considered exchangeable or available.

It is important when running a soil test to understand what your total calcium is, what is tied up and not readily unavailable and what is available. And if you have high levels of bicarbonate in your soil, that means much of that available calcium will fix to it (becoming unavailable). It is worthwhile understanding if you have good levels of available calcium and hopefully low levels of bicarbonate that won’t tie up available calcium.

Dr. Davidson posts articles on soil health and management related subjects. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919.