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What’s in the ground under your crop?

What is half air by volume, 50 per cent oxygen by weight, and has the capacity to grow a crop?

Is it dirt, earth, mud, sand, clay, silt, muck? Yes, it’s all of these, but what really are Prairie soils made of?

There are four basic types of Prairie soil: dark brown, black, dark grey and brown. Rainfall on these soils, the prime yield-limiting factor, ranges from around 11 to as much as 20 inches annually, with the black soils of the central Prairies having the highest average rainfall and consequently the highest yield potential. Virtually all prairie cropland soils developed under a canopy of perennial grasses and forbs (plants other than grasses). Native grasses with root systems extending up to five feet into the soil made up 85 to 95 per cent of the grassy forage and many types of perennial herbaceous plants and a few shrubs with deeper roots made up the rest of the ground cover.

The word “prairie” translated from the French means grassland. Contrary to the myths of Prairie farm settlement there were few, if any, trees to cut down since grass fires spread every few years.

Soil components

An average cubic foot of dry soil weighs between 75 to 85 pounds. How much of the normal volume is air (or water, if it’s fully saturated). A cubic foot of dry, sandy soil will weigh more than a cubic foot of dry, clay soil. Very sandy soil will hold only one to 1.25 inches of water per foot; a heavy clay soil could hold up to three inches of water per linear foot.

On average, an acre of soil to a depth of six inches should weigh about two million pounds, depending on organic matter, peat soils exempted. So, if you add one pound of powdered actual zinc to this soil you would have a soil concentration in the top six inches of 0.5 parts per million (ppm). Add two pounds of boron to the top six inches of one acre of topsoil and you would have a concentration of 1 ppm of boron.

A three per cent organic matter soil contains 30,000 to 60,000 pounds of organic carbon matter, 4,000 pounds of fixed organic nitrogen, 2,000 pounds of organic phosphorous, 1,000 pounds of microbial mass per acre, as well as bound up sulphur, boron and other micronutrients. These plant essential nutrients are very tightly bound up in this organic “cement” or humus in the soil.

If a field with three per cent organic matter is fallowed for one year it would release on average perhaps 20 to 30 pounds of organic nitrogen, five to 10 pounds of phosphorous, one to two pounds of sulphur and trace minerals, such as boron. The nutrient release from such a three per cent organic soil with no added nutrients, assuming soil available potassium, would be only enough for about a 15- to 20-bushel per acre crop of wheat.

The component parts of soil are normally called clay, silt and sand. When soils are one third of each of these components it is a good loamy soil. Soils high in any component are referred to as clay soils, sandy soils or silt soils:

Sandy soils: coarse rock particles that have poor water holding capacities and are generally low in organic matter and cation exchange capacity.

Silt soils: medium to small, generally round soil particles developed from glacial till with moderate cation exchange capacity.

Clay soils: fine to very fine textured soils with microscopic particles developed from lake sediments with high cation exchange capacities.

Generally, the square foot of soil, excepting the three per cent organic faction, is made up of the following minerals by weight:

  • 50 per cent oxygen (oxides).
  • 25 per cent silicon (silicates).
  • 7 per cent aluminium (aluminates).
  • 5 per cent iron (iron compounds).

This leaves 10 per cent of the remaining soil by weight: calcium, two to four per cent; magnesium, one per cent; potassium, one to two per cent; and smaller percentage amounts of crop-essential nutrients such as sulphur, boron, zinc, copper, manganese, cobalt, nickel, molybdenum. When we get to parts per million and per billion, almost all soils contain detectable levels of arsenic, cadmium, antimony, mercury, lead, silver and gold.

In future articles I will define the roles of macro and micronutrients in the biological life cycle of soils.

About the author

Contributor

Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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