Understanding Tropical Soils: Part 4

This article is continued from Understanding Tropical Soils: Part 3.

by Dexter B. Dombro

Biochar added to soil retains nutrients.

If you have ever seen the rapid drainage of tropical soils, you will understand why changing the CEC so that soil retention improves and nutrients don’t just filter or wash away is such an exciting prospect. However, having a pile of biomass converted to charcoal means we are not quite there yet. There is still the process of charging the charcoal so that it becomes biochar and can be added to the soil. What do we mean by charging? Simply put, charcoal is negative, and if we added it to the soil directly it would suck any nutrients in the soil away from the plants or trees it is supposed to benefit. This means we first have to convert the charcoal into something that is neutral or even positive, so that it retains the nutrients in the soil and makes them available to the plants or trees, without the nutrients being washed or filtered away.

Liquid fertilizer made with cow urine.

There are a lot of different ways in which biochar can be charged. At Amazonia Reforestation and CO2 Tropical Trees we use an ancient recipe known as Jivamritham, which is basically a soup or liquid fertilizer rich in microfauna. It is made out of 10 kg of cow manure, 10 liters of cow urine, 2 kg of legume flour (beans or lentils), 2 kg of molasses or panela or jaggary (first run-off of sugarcane), a handful of local soil, which is how local soil microfauna is added, and finally topped off with 200 liters of water. 250 liter tanks are used to make Jivamritham in plantations, but one can use smaller containers by simply reducing the ingredient ratios. The soup is stirred 3 times a day and is ready for use by the third day. The charcoal is simply immersed in the soup, allowing soil bacteria and other microfauna to saturate it. Once saturated (a few hours) mycorrhizzal fungi, compost or other goodies can be added, but basically we now have charged the charcoal and created biochar.

Making 200 tons of compost.

Not everyone has cows, so another popular charging agent is compost tea, made with compost, worm castings, sugars and water. There are a number of compost tea recipes available on the internet for home gardeners wishing to charge charcoal and create biochar. The home garden mix is usually 10% biochar to 90% compost. There are also recipes using sea weed (kelp), and even commercial fertilizers, though on principal any use of agrochemicals kind of defeats the purpose of using organic materials like biochar to improve the soil. Obviously, plantations need large scale, affordable solutions that are less labor intensive than a home gardener might employ. One of the best books available on composting is the Humanure Handbook by Joseph C. Jenkins, who describes in clear terms how best to make compost using human waste and other organic materials.

Fertilizer factory at La Pedregoza.

It is important to mention that biochar is best when reduced in size. Some biochar retorts are designed to work with wood chips. This means that crushing the charcoal afterwards is not necessary, but most retorts are based on turning larger chunks of wood, bamboo, corn or cane stalks and so on into charcoal, so the charcoal needs to be crushed before being charged. One simple method is to put the charcoal into canvas or vinyl bags and to then drive a truck, tractor or car over the bag, crushing the charcoal. Caution should be exercised when crushing charcoal, as one does not want workers to inhale the dust. The crushed charcoal is then charged, which means that one has a lot more surface area in the biochar to with soil bacteria, mycorrhizzal fungi and other microfauna can adhere.

Branch line is root extension of trees.

At Amazonia Reforestation and CO2 Tropical Trees the charged biochar is either tilled into the soil or mixed into slurry with clay and compost and then poured onto the soil. In existing tree cultivations a worker uses a spade to dig a quick hole beside each tree, into which one or two shovels of biochar mixed with compost are dumped and then covered up. Those holes should be right below the branch line of the trees, as that generally corresponds to where the tips of the trees’ roots will be located. As a rule of thumb, planters and foresters look to add at least 1 kg of biochar to every square meter, so in a one hectare woodlot (2.47 acres) that translates into 10,000 square meters, meaning that 10 tons of biochar would be required to enhance and improve the CEC of the soil.

Adding lime to soil is costly.

Now that we have found a solution to the problem of soil nutrient retention (the CEC), there remains the vexing problem of soil acidity in the tropics. While many native trees and plants are adapted to high soil acidity, many commercial tree species and agricultural plants are not. The traditional method to deal with this problem has been to add large amounts of dolomitic lime (Ca) to the soil, in order to balance the pH or make the pH more alkaline. This process can be fraught with problems as lime can kill soil microfauna, and severely burn plant roots. This means that foresters need to take a number of precautions when applying lime to the soil.

Cow urine has microelements.

The best method is to add the lime to the soil at least 4 weeks before one begins to plant. This allows the lime to filter into the soil, to be better distributed (after tilling or cultivating), to change the pH of the soil and to hopefully allow some recovery of the soil microfauna. It should be mentioned that there are some commercial lime preparations that are a little less harsh on the soil, such as Calfos, a mixture of dolomitic lime and phosphorus that acts as a kind of fertilizer at the same time. One benefit of lime in the soil is the fact that calcium (Ca) is essential for cell formation, so if one wants good plant growth, a certain amount of lime needs to be in the soil to stimulate and assist in cell formation as the tree is growing. 

The organic planter intent on implementing natural silviculture processes in a tropical tree plantation has another option to combat soil acidity. A by-product of making biochar is wood ashes. Wood ashes are a wonderful fertilizer, rich in all sorts of micro elements required for tree growth, and also rich in calcium. In fact, one can overdo wood ashes so care must be taken when they are added to the soil, but an immediate benefit is the fact that most of the micro elements in the ashes are more biologically available then those in commercial agrochemical mixes, and the alkalinity factor of wood ashes (pH of 10.4) is higher than that of lime (pH of 9.6). This means that using biochar retorts creates 2 products using local resources and eliminating transportation costs. Even better, as soil nutrient retention and microfauna increases and organic material starts to exist in the soil, this also reduces soil acidity. In Part 5, I will discuss and compare the difference between natural silviculture and agrochemicals and their impact on the soil, and explain why the soil has a soul that needs to be protected, as it is a vital component of biodiversity and a healthy planet.

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