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Wednesday, January 17, 2018

Conservation agriculture

There is a 'new' sort of agriculture practiced by a handful of farmers in diverse locations around the world.  Conservation Agriculture is not a descriptor although the words describe to some extent what it is.  It is, rather, a name given to a suit of farming methods which taken together are called Conservation Agriculture.

This so called Conservation Agriculture involves a) not ploughing the soil, (and hence, direct drilling) b) rotating crops in a random fashion, with longer periods between growing the same crop, c) leaving all the unused parts (stems, leaves and, of course roots) of the past crop on the land as a mulch and d) the planting of cover crops between commercial crops.  It is not absolutely against using chemical fertilizers but results in great reduction or even  elimination of the use of such chemicals.  In addition it may involve grazing down the standing crop residue and/or cover crops, and thus converting them into dung and urine. If grazing is used, it is very intense, very infrequently.  It may also involve,  the incorporation of char into the soil.  To find more detail on the methods go to this site or to get a historical perspective on the fate of societies that didn't preserve their soils, to this site.  What I would like to explore in this blog is the logic behind the methods.

Let's take a corn plant as an example.  The seed grows into a plant and the plant uses water, carbon dioxide and various minerals to build it's roots, stems, leaves, and seeds (the corn we eat).  The energy to transform these simple, low energy substances into complex, high energy compounds comes from the sun and this captured energy is now in the form of chemical energy.  The resulting chemicals (largely cellulose along with many other  compounds in lesser amounts) can be burnt as a fuel but can also be 'burnt' by soil organisms just as we 'burn' the corn in our bodies for energy.  The soil organisms  incorporate some of this stover into the substance of their bodies, especially proteins and vitamins just as we do with the corn seeds.

Saprophytes (funguses) are specialist in using dead plant material for their sustenance.  Think of the fungus growing on dead wood in moist conditions.  But these are the fruiting bodies of the fungus.  Most of the fungus consists of thin filaments (mycelia) that extend through the media and collect nutrients.  Of particular importance in-so-far as we are talking about soil health for crop production is that the funguses not only use dead organic material for energy but can also mobilise minerals in the soil that are in insoluble, mineral form and make them available to plants.  Many of the funguses grow their mycellia inside or around root hairs and exchange the nutrients they have mobilized  for energy rich compounds that the plant provides. Anything to encourage the growth of these funguses and to avoid disrupting the mycellia that extend throughout the soil is good for the crop.  Therefore we put lots of organic material on the soil where the fungus can access it and we do not plough

 The obvious question is why don't we mix this material into the soil.  Firstly, this would involve ploughing and hence the disruption of the mycellia of the funguses but there is another reason.  If there is a large amount of reduced carbon (cellulose and other compounds) in the soil, the micro-organisms that produce cellulase* and hence can access this source of carbon and energy, will scavenge all the available soluable nutrients from the soil to build their bodies.  The funguses are not the only organisms that can utilise cellulose.  Many single cell soil organisms have the same ability.  If a considerable amount of cellulose is incorporated into the soil, there will be nothing available for the growth of the crop you have planted.  Put the organic material on the surface and it is gradually incorporated into the soil and nutrients are still available for the crop.  But the surface layer of mulch has other benefits.

* The enzyme that can break down cellulose.

The surface mulch shades the soil and keeps it from heating up so much.  The soil looses less water by evaporation, leaving more for the crop.  The mulch softens the blow of the rain and slows the flow across the ground and hence avoids sealing the surface of the soil and increases infiltration.  Again more water for the crop.

A word here about trophic levels.  As a first approximation, only ten percent of the material consumed is fixed into the next trophic level.  10 tons of algae will make one ton of krill and one ton of krill will make a tenth of a ton of whale.  Sounds good since the 90% excreted is mineralized. Some of it is in a form that can be taken up by plants, but here is the rub.  If there is lots of cellulose around, the micro-organisms which can break down cellulose will use the cellulose as energy and scavenge all the mineralised material, which has been released by other organisms, leaving none for the plants.  Of course as the quantity of remaining cellulose decreases, more and more of the mineralised nutrients will be available for the plants.

So, the next thing is why do we plant a cover crop when the main crop has been harvested.  First we capture more sun energy in the form of the chemical energy of the cover crop and hence produce more organic carbon for the soil organisms.  Secondly we scavenge any left over soluble nutrients from the soil and turn them into a slow release fertilizer (the bodies of the plants).  As this organic material breaks down it releases its nutrients into the soil over time.

If we include a deep and a shallow rooter, we scavenge nutrients throughout the depth of the soil as well as spreading roots through the soil which will not only disintegrate over time but will provide passages for water and air to penetrate the soil.

If we include a legume that is either inoculated with the appropriate rhyzobium bacteria or finds the correct bacteria in the ground, atmospheric nitrogen will be taken from the air and turned into a nitrogen compound that can be used by the next cash crop.  Since most of the nitrogen compounds produced will be incorporated into the leaves, stems and seeds of the legume, it is important that this material be left in the field to enrich the soil.

If we include a root crop such as a radish or turnip, as they later disintegrate, in addition to releasing their nutrients, they create tunnels for water to infiltrate.  They also often are deep rooted which will help to scavenge nutrients from lower levels.

It is important to cut down the cover crop or trample or roll it into the surface of the soil before it sets ripe seeds.  You don't want the plants of the cover crop to themselves become weeds.

If you decide to graze the cover crop, it is grazed very heavily for only a day or two.  This tramples some of the crop into the surface of the soil, ensures that all plants are utilized and not only the favorites, including weeds that you have not planted, and turns the cover crop into urine and manure.  This short sharp grazing leaves lots of time for the soil organisms to  sort out any surface damage and to incorporate the animal excretement into organic material.

When the cash crop is then planted by direct drilling, it has all the best of the soil structure and soil organisms to support it.

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