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How Old Is Our Dirt?

What’s older than dirt? Lots of things, if you’re talking about Manitoba soils.

In our province, the story began 10,000 to 12,000 years ago, after massive glaciers bulldozed the uppermost layers away, then melted to create the ancient Lake Agassiz.

In terms of world history, that’s not a long time. Egyptian civilization goes back almost 5,000 years.

But if you figure in the fact that the soil-building process grinds to a halt for about half the year, when sub-zero temperatures put everything in a state of suspended animation, Manitoba’s soil is practically brand new in the whole scheme of things.

“Although it’s been about 12,000 years since the glaciers, there’s actually been only about 5,000 years of soil development because we still get six months of glaciated-type conditions where the soil is basically inert,” said John Heard, a provincial soil fertility specialist, in a short “history lesson” he gave here at a recent soil fertility workshop.

“So we can talk about our soils being very young in geological terms compared to other areas.”

HIGHER PH

The other main unique characteristic of our soil, from the heavy, dark clay-based Red River Valley type, to the lighter Newdale, which is highly suitable for crop production, and the sandy-based Almassippi soil, is that the three dominant types tend to have a higher pH value than in other areas.

Unlike the corn belt, farmers here seldom need to add lime to compensate for increased acidity. That’s because the glaciers distributed a generous helping of dolomitic limestone from deposits in the Interlake that were pulverized and added to the soil mix.

“That has led to some poor marketing opportunities for some of you folks,” Heard joked. “If you had a dealership in Iowa, Indiana or even parts of Ontario, every four years you’d be selling five to 10 tons of limestone to your growers to lime up their acid soils.

“But you’re stuck here where we have predominantly high pH soils because of the dominant effect of the limestone.”

Repeated applications of nitrogen fertilizer tend to make soil more acidic over time, but studies have shown the effect is negligible, even over two decades. At any rate, it can be fixed by amending the soil with three tons of lime per acre, he said.

Soil “parent” material makes up half its volume. Water and organic matter make up a further quarter, with the rest is air, or “pore” spaces, between mineral particles. Too-wet conditions, ironically, don’t drown plants, but rather suffocate the roots by driving out the air.

ESSENTIALS

There are 17 essential nutrients for sustaining plant growth, including the three most commonly bought from fertilizer dealers: nitrogen, phosphorus and potassium (N, P and K).

Secondary nutrients are sulphur, calcium and magnesium, generally supplied by the breakdown of soil minerals.

Sulphur is becoming more commonly applied in recent decades, as coal-burning power plants have adopted scrubbers to reduce acid rain, and as higher yields deplete soil reserves.

Interest in micronutrients such as boron, copper, iron, chloride, zinc and others, which plants require in tiny amounts, has been growing, but studies have shown the effect of trace-element “cocktails” on crops has been mixed, with small yield responses only appearing in a third of all plots.

When it’s -40C and the wind is blowing, it’s sometimes hard to see the bright side. The good news is that the bitterly cold winter weather in Manitoba helps prevent soil compaction, though massive machines are driven over the fields year in, year out.

Manitoba’s montmorillonite clay, unlike the kaolinite or illite-based types found in older types of soil in the southern U. S., tends to be broken up each year by the repeated seasonal effects of freezing, thawing, wetting and drying.

“Our type of clay and the environmental conditions help the soil to remediate itself from the compaction of previous years,” said Heard, adding that compressed subsoil sometimes occurs during spring field operations and affects the growing season, but generally it is ironed out by the “ace up our sleeves” – natural, climatic forces.

Experts say ruts with large lips at the edges, where the soggy muck has slurped out, seldom result in compaction, and can be filled in or smoothed out with tillage equipment.

Cation exchange capacity (CEC), which relates to fertility and the ability of soil to retain nutrients, is higher in clay soils, at 15 to 150 per 100 grams of soil, and lower in sand, which ranges from two to six per 100 g. With a negative charge, the clay particles attract positively charged particles, also known as cations, which include ammonium, potassium and magnesium.

Anions, or negatively charged ions, such as sulphate, nitrate and phosphate, are repelled, which in the soil solution makes them more vulnerable to leaching. Red River Valley soil typically has a CEC of 47 compared to 27 in Newdale. The higher the organic matter content, however, the better the CEC rating.

ALKALINE VERSUS ALKALI

Soil scientists have done a good job of confusing farmers with regard to soil pH, Heard said, especially with regard to the difference between “alkaline” soils and “alkali.”

“Alkaline soils are basic, or calcareous, with a high pH. Alkali – we’re trying to remove this term – are saline soils,” he said.

“Farmers are confused, because when we start talking about basic soils, which means a pH of greater than seven being alkaline, they automatically start to think they have salt problems.”

Acidity, or a reading of less than 5.8, is of concern to farmers, he added, because a lower reading on the pH scale means lower phosphorus and micro-nutrient availability, and less soil microbial activity, which can lead to herbicide residue car ryover and product ion problems.

The carbon in organic matter has a buffering effect on pH, which is why the exposed subsoil on eroded knolls tends to show reduced yields.

Manitoba soils are generally free of acidity problems, but areas with leached or heavily irrigated sandy soils may show isolated pockets, he added. [email protected]

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