This presentation is by Claude Bourguignon in his book Regenerating the Soil. He makes great points about the difference between tropical and temperature soils. What I see here is the basis for understanding why food forests (
perennial plantings) are the best way to grow food in the tropics. some of his solutions are to me appalling but the understanding of the soil supports for our own solutions.
Chemical composition of the soil. In temperate regions the soils have a high cation exchange capacity but if you go to the tropical countries the cation exchange capacity is very low. It is really surprising that the poorest soil in the world supports the tallest
trees. What the agronomist conclude is that good production of food in tropical countries is impossible because there is nothing in the soil. If the soil is poor and the vegetation is tall and lush it simply suggest that the fertility must be traced not to the soil but to another source.
Exchange capacity: you must have two things, clay and humic acid. Clay comes from the transformation of rocks by plants. Plants attack the rocks and produce clay. Insects and animals attack leaves and the rest of the vegetation and produce humic acid. It is the blending of clay and humic acid that produces cation exchange capacity. In tropical systsems you find very little clay and very little organic matter in the soils. This is because 90% of the organic matter is present in the vegetative
canopy of the forest and only 10% in the soil. In temperature countries on the other hand 60% of the organic matter occurs within the soil and only 40% within the vegetative canopy.
When you put organic matter on tropical and equitorial soils, it invites a good development of soil fauna. In fact, the total porosity of the soil is created by the soil fauna. In equatorial countries this is the result of termite activity to a great extent and worms to a lesser extent.
If there is no organic matter in the soil, the termites disappear, the porosity of the soil vanishes and strong rainfall is unable to permeate the soil. You have erosion.
The lesson to be learned is to follow what happens in the forest and to do what the forest does. Instead of maintaining its nutrients within the soil, the forest channels them into the trees. This is made possible by deep
root systems. Such a well developed system of
roots is essential in order to prevent loss of elements by leaching.
The first thing to do when you want to transform a wild forest of an equatorial country into an agricultural area is to avoid burning organic matter. The correct way to proceed is to just cut the trees, keep the trunks for use, keep all the leaves and all the organic matter on the soil. Then you must directly sow in this organic residue. There is no sense in using chemical fertilizers because they are sure to get leached.
We have two possibilities. We can start from a wild system. I have made measurements in the Amazonian forest in Brazil and found in the forest cover more than 1 ton of nitrogen per ha (2.5 acres), more than 800 kg of potassium, more than 600 kg of phosphorus. If you examine the soil you find almost nothing. So in the nontillage system you cut down the forest, then we make a direct sowing of what we call a pump plant. In Brazil we use plants which are able to grow at the rate of 5 cm per day for roots and 5 meters in 60 days for stems. Afterwards the pump plant is cut down and in this organic matter we grow the paddy. We use rain fed paddy as there is no
irrigation possible. With this system we get two tons of paddy per ha and within three years we reach five tons of paddy per ha without fertilizer.
What is delicate is to get the time factor right for sowing. The mineralization of organic matter starts very fast, so you need to sow within 6 days of the rain commencing. After the paddy crop is harvested you again have a dry season. You must sow a pump plant which is adapted to grow with 300 ml of rain in order to avoid leaving the soil uncovered in the dry season. An exposed soil will kill all the biological activity.
With an eroded soil. It is impossible to start agriculture directly without putting in something. He suggests stable chemicals for these soils. In
permaculture we bring in some mulch from outside the property and start planting “the pump plant” immediately. We could also use small amounts of rock minerals which could not be broken down without the addition of mulch to increase the biological activity.
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