lundi 12 octobre 2015

55- The organic matter (supplement)


Following my article No. 54 about the organic matter , Luc Opdecamp, one of my Belgian loyal readers, agronomist philosopher, as he defines himself (you can access his blog, in french, by clicking on the link "L'agronome philosophe" in the left column of this blog), made an interesting comment. I felt important to make an article supplement and translate it, because it complements what I wrote while mitigating my opinion on the subject.
As I like questionnings and subjects of reflection, I present it to you as is, with some explanations and comments for people who are not specialists:

"If no-tillage technique reduces CO2 emissions into the atmosphere, it increases it in the soil. Yet, the CO2 produced by the microbial and root respiration dissolves in water to form carbonic acid H2CO3. The latter is active in soil weathering and mineralogical evolution by acidolysis. Mineral components such as calcium, magnesium, potassium, sodium, aluminum, etc. pass into solution, and are partly absorbed by plants and partly leached to groundwater. Aluminium meanwhile gradually replaces the bases of cations on the adsorbent complex where it can reach toxic levels for many species. This is a very slow but continuous phenomenon that characterizes the acid pedogenesis of well-drained soils under humid (and hot) climates. In the long run, it therefore leads to an accentuated chemical erosion as observed in tropical or temperate forests in already ancient reliefs. "
The pedogenesis is the soil formation process.
This change is not significant in agricultural soils, as Luc explains in a second comment from the same article, because agriculture, by its practices, can compensate and slow down this phenomenon. But it is important to note that the non-released in atmosphere CO2 does not remain inert in the soil, but it undergoes a totally natural chemical evolution, whose effects can be problematic.

"About the incineration of the organic matter, it causes a net loss of nitrogen and carbon which are discharged with the flue gases. By against the elements such as calcium, magnesium or potassium remain in the ash as oxides and can then neutralize the acidity of the soil surface for a few years. This is what justifies and explains the shifting cultivation in the humid tropics and the practice of slash-and-burn farming for natural grasslands."

 Image extracted from

The slash-and-burn farming is a method of preparing a grassy field of pulling up, let dry and then burn the grass, to clear new lands for agriculture, and fertilize them with the ash.
Another mitigation of what I wrote in the article. Note that the beneficial effects of burning technique are interesting, mainly in areas where soil acidification is sensitive.
In short, I remain convinced that the preservation of organic matter in agricultural soils and their enrichment if necessary, is generally a good thing and brings great potential benefits to agriculture and environment.
But it is a technique that must be used with caution, especially in the humid tropical regions.
In temperate climates at least, it is generally considered that a soil is well supplied with organic matter, if it contains at least 2%. This level is indicative, and soils can be very active with a significantly lower rate, others can be almost dead with significantly higher levels.
Having said this, it is more important to worry about the life of the soil itself, than its pure organic content. In the case of orchards, in addition to the restitution of prunings, vegetation cover will play a key role in soil aeration and its ability to promote the revitalization of microbial life.
In short, the farmer has to worry the soil because it is the foundation of its agriculture. Chemical fertilization is not a problem in itself if the dosage, methods of contributions and all the actions that accompany it take into account the impact on soil life.

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