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Getting the most from organic matter

Tuesday, October 2, 2012

Managing organic matter with tillage is an art. As conditions and factors change, you have to change how you practice this art

by PAT LYNCH

If you really understand organic matter and how it affects soil and yields, you will grow better crops. This article is written from the experiences I have gained by watching producers and imagining what is occurring in the soil.

An acre of average soil has about 1,000 tons in the top six inches. This "acre slice" weighs more for heavy soils and less for lighter soils. A heavy clay loam can have four per cent organic matter, which is 80,000 pounds per acre. A high-yielding silt loam may only have 3.25 per cent organic matter. It may only have 68 to 70 tons per acre of organic matter.

Generally, the silt loam soil will out-yield the heavier clay soil, even with less organic matter.
There are three fractions of organic matter. The exact percentages of each type vary between soils. The first type is very stable. It is sometimes referred to as humus. It generally makes up 40 to 60 per cent of soil organic matter and takes years to build. It is the nutrient-holding portion of organic matter and decomposes very slowly, no matter what you do. If you are farming organically and counting on tillage for weed control, it is important to have a soil with lots of humus. Tillage will not destroy this fraction.

The second biggest portion is the active organic fraction. It can represent 10 to 40 per cent of organic matter. It is this fraction that makes the big difference in yield and accounts for why dairy producers can grow such good crops. This is the portion that must be managed for best yields. Too much tillage quickly breaks down this fraction.

This organic matter must be mixed thoroughly into the topsoil. It contains plant tops and roots. The roots of perennial crops are more valuable than those of annual crops, since it takes longer for them to break down. Deep-rooted perennials like alfalfa are especially good, since they break down slowly and go deeper than annual crop roots.

It was the loss of this fraction on many farms in the 1980s, when producers went from livestock to cash cropping, that increased no-till. They noticed a yield increase in no-till initially. These same producers are now finding that using some form of vertical tillage to mix raw organic matter is giving a yield increase. This year, the difference between no-till soys and soys with some tillage may be five bushels per acre.

The final portion of organic matter is the raw plant residues and micro-organisms. These make up one to 10 per cent of organic matter. Manure is part of this fraction. When you look at the yield responses when manure is added, you cannot rationalize the yield increase by the nutrients and small bit of organic matter that is added. The micro-organisms play a major role in increasing yields.

Some soils can have up to 15 tons per acre of soil micro-organisms. This raw plant residue and micro-organism fraction is the most important fraction of organic matter. How you manage this really dictates yield.

A big factor influencing how organic matter works is tillage. If you use a mouldboard plow and layer the organic matter fraction, you need repeated trips with secondary tillage to mix it into the topsoil. If you plow too deeply, you bury this fraction, preventing it from breaking down. Often, we see corn stalks or wheat stubble plowed back up from two or three years previously because plowing was too deep.

If you no-till, this raw fraction stays on the soil surface. This is all right for the short term, but eventually yield will decline until you introduce some tillage to mix these organic matter sources.

You do not have to work the land every year, but some tillage every three or four years helps.
Managing organic matter with tillage is an art. As conditions and factors change, you have to change how you practice this art. BF

Consulting agronomist Pat Lynch, CCA (ON) formerly worked with the Ontario agriculture ministry and with Cargill.

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