Wednesday, January 9, 2013

Terra Preta

Certain areas of the Amazon River basin were discovered to be extremely fertile in spite of being in a tropical rain forest area. Tropical rain forests typically have very poor soil because the nutrients are continually leached out by the heavy rains. Why the big difference in these specific areas? The secret is a soil type called "Terra Preta". Terra Preta is artificially amended soil; amended primarily with charcoal. Today we call this amendment "biochar".

What is Terra Preta and Biochar? Biochar is any organic, carbon-based material such as wood, paper, reeds, etc. that is heated in a low oxygen environment until it is carbonized. Charcoal has been made for centuries in outdoor pits filled with wood, lit on fire, and then covered with clay. Today it is cleaner and easier to make it in closed containers (with a small opening to release the flammable gases). Do a Google or Yahoo search on making charcoal if you are interested.

If you have a wood stove, fireplace, or fireplace insert you likely end up with lots of ashes and what do you do with them?  Ashes are good for your garden soil in certain circumstances.  Wood ash can be useful in home gardens, in your compost pile or as a pest repellent.  Wood ash can be a valuable source of lime, potassium and trace elements. Since wood ash is derived from plant material, it contains most of the 13 essential nutrients the soil must supply for plant growth. When wood burns, nitrogen and sulfur are lost as gases, and calcium, potassium, magnesium and trace element compounds remain. The carbonates and oxides remaining after burning are valuable liming agents, raising pH, thereby helping to neutralize acid soils. Ash from a cord of oak meets the potassium needs of a garden 60 by 70 feet, he said. A cord of Douglas-fir ash supplies enough potassium for a garden 30 by 30 feet. Both types of ash contain enough calcium and magnesium to reduce soil acidity (increase soil pH) slightly. In compost piles, wood ash can be used to help maintain a neutral condition, the best environment to help microorganisms break down organic materials. Sprinkle ash on each layer of compost as the pile is built up. Ash also adds nutrients to compost. If used judiciously, wood ash can be used to repel insects, slugs and snails, because it draws water from invertebrates' bodies. Sprinkle ash around the base of your plants to discourage surface feeding pests. But once ash gets wet, it loses its deterring properties. Continual use of ash in this way may increase the soil pH too much, or accumulate high salt levels harmful to plants.

You probably find some charcoal in your ashes too. Charcoal has some interesting properties for improving your soil. Charcoal, finely ground, when added to your soil will hold large amounts of nutrients and moisture.  Charcoal is a fine-grained, porous black carbon, generated from plant materials heated or "burned" in a low oxygen environment. It is non-toxic to plants and people if made from clean, untreated wood. There are many tiny pores in charcoal and the pores will allow air to diffuse into the soil. Plant roots need the air to breathe. The tiny pores will hold water and nutrients and later supply them to plants. More important, unlike other organic fertilizers, charcoal is very stable and it will not decompose to carbon dioxide. So once applied, it will stay in soil for hundreds to thousands of years. The high stability and porosity make charcoal a better fertilizer than most other organic materials.

How much do you want to incorporate into your soil? The general consensus is about five percent by volume. That is far more than you think. So adding small amounts accidentally produced in your fireplace is not going to improve your soil very much, but over time the soil will get better and better. You'll probably need to intentionally produce larger amounts if permanently improving your soil so that you can produce more food is your goal.

I installed a fireplace insert last weekend so that I can partially heat my house with the abundant wood on my property. At the end of the evening I generally close the damper on the insert and that chokes off air to the fire box; this produces charcoal. I will be adding this to my garden all winter long and see how much biochar I can get incorporated into the soil. If it is not as much as I want I will attempt to produce more charcoal using a burn barrel.

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