

Size of a pumpkin leaf: 42 cm, that’s 16.5″. Not bad for a sandy soil!

One of the problems a lot of people have is how to improve the fertility of sandy soil. One solution is to add more organic matter (compost, manure, wood chips), but unfortunately if you live in a hot and humid climate the stuff you put in the soil is going to decompose quickly, since microbial activity is so fast. That creates a serious problem, because your poor sandy soil is not holding nutrients. You can add fertilizers, but they are going to leech out of your soil very fast. Because of that your fruit trees, shrubs, and vines will be yielding poorly, and they will be susceptible to diseases and pest damage. What’s worse, the fruits (and vegetables) of your labor will taste plain and they will not have a lot of vitamins and minerals in them.

How can sandy soil be improved?

What you need to do is increase the capacity of your soil to hold nutrients. Then adding fertilizers (either organic or not) will be much more effective, because the stuff you put in your soil will actually stay there. As I mentioned before, compost or mulch are sometimes not the best option because they’re often decomposed very quickly. But there is a way to permanently improve the organic matter content of your soil. It’s called biochar.

What is biochar?

Biochar is a fancy name for charcoal if it’s used as a soil amendment (to improve soil properties).

The benefits of using biochar

Its main benefits are:

Significantly and permanently increasing soil Cation Exchange Capacity (CEC) — i.e. the soil’s ability to hold nutrients

Because of its high porosity it creates lot of habitats for beneficial microbes

Increased water retention

How is biochar made?

Usually biochar is made of agricultural wastes, such as stalks, straw, and wood of no commercial value. Sometimes it is made of manure or animal bones. If it is made of manure or bones, its immediate fertilizing value is higher, but it will not be as permanent. Biochar made of wood or woody organic matter should not be considered as a source of nutrients for the soil since its purpose is not to fertilize your plants or soil, but to create the opportunities for it to be fertile.

It’s mainly used by farmers or gardeners who follow sustainable agriculture practices.

Why should you use biochar in your garden or on your farm?

There are reasons to believe that biochar is responsible for the existence of terra preta. It’s a type of soil that was probably created by Native Americans in the Amazon Basin. It was created by mixing charcoal with waste (manure, bones, food waste, human feces, broken clay pots, etc.).

How fertile is terra preta?

Researchers have measured the CEC of “fresh” biochar made from pine sawdust pellets and pine timber ranging from 22meq to 138meq. (Characterization and Comparison of Biochar, Herbert et al, CalPoly2012). It is also known that as biochar ages its exchange capacity can increase, up to an order of magnitude (10x). In 2006 researchers compared several ancient char-amended soils (terra preta androsols) in the central Amazon with adjacent soils to which char had not been added. The most impressive result was an androsol with an Effective CEC of 213meq compared to adjacent soil with an ECEC of 23meq. This same androsol, estimated to be 600 to 1000 years old, tested as containing 9064ppm Phosphorus and 17 545ppm Calcium, vs the adjacent soil with only 273ppm P and 115ppm Ca. (Black Carbon Increases CEC in Soils, B Liang et al, Soil Sci. Soc. Am. J. 70:1719–1730, 2006) — Ideal Soil 2nd edition, by Michael Astera

By comparison, poor, sandy soil with little to no organic matter would have a Cation Exchange Capacity (the ability to hold positively charged soil nutrients) in the range 1-4.



Corn grown on soil with and without biochar. Terra Preta on your right.

Because of its unusual ability to hold nutrients, it was and still is very fertile….

It’s a remarkably fertile soil, which is rich in organic matter. What’s so unusual about it? There are other places in the world that have soils with high organic matter content. That’s true, but most of those places are in much cooler climates. Because of high microbial activity (both heat and moisture) all “normal” organic matter is quickly decomposed, so it’s almost impossible to bring the soil up to 2% organic matter content.

The term “organic matter” means matter containing carbon (C). What’s unique to biochar is the fact that it’s a type of organic matter that no living things (except for humans — i.e. if we have food poisoning) want to eat. So if biochar is added to the soil it tends to stay there, especially if no till agriculture is being practiced. It’s a good thing to do if you have a lot of brush or decaying wood to clear – you might get rid of weeds and improve your soil in the same time, changing a problem into a solution….

What do you do if you don’t have any brush or weeds to clear?

What do you do if you can’t make charcoal?

What do you do if you don’t want to make charcoal?

The obvious solution is to buy charcoal! But that might be expensive and usually the wood that’s used to make charcoal is acquired by cutting the tropical jungle somewhere in Southeast Asia. That means it is not too good for the environment…. Fortunately there’s a different type of carbon product that microbes can’t consume.

It’s coal.

Yep, that black stuff that’s being mined from the ground.

How do you make biochar to create terra preta?

Because of that we (by we I mean myself and Jacek Kobus) decided to check how good coal mixed with horse manure is in improving properties of the sandy soil. Jacek created an impressive pile of horse dung mixed with culm (brown coal dust). Culm is the cheapest fraction of coal you can buy. We bought 1 ton of culm and then mixed it with 3 parts – by volume – of horse manure.



Biochar made the Polish way – culm (brown coal dust) mixed with horse manure.

Why does biochar research sometimes show a decrease of yields?

I mentioned before that biochar is not a source of nutrients for your garden. The same can be said about coal. Although it contains a lot of micronutrients and trace elements, but they are not available for plants.

What’s more important to remember is the fact that coal or biochar has both a high Cation Exchange Capacity and Anion Exchange Capacity. That means it can hold all sorts of nutrients for plants very well – that’s why you want to use it in the first place! But most of the “place” where the nutrients can be held is initially empty, waiting to be filled up.

Because of that, if you apply it to your garden, field, or pasture straight away, it would suck up and hold nutrients from your soil. If you have infertile, sandy soil, your biochar will be taking and holding nutrients from your soil for months, making the growth of your fruit, vegetables, and cereal less than perfect. That’s why biochar research sometimes shows a decrease of yields after an application of biochar to the soil.

How do you charge your biochar with nutrients before applying it in the garden?

Mixing biochar with manure – “the dry method”

It’s quite simple – just mix your charcoal or coal with some moist animal manure and let it “mature” for at least 2-3 weeks. We used 1 part coal dust to 3 parts horse manure, because that’s what was available. You can also use chicken, pig, or cow manure.

Mixing biochar with urine or another liquid in a barrel or container

Some people also put coal into a barrel, then add urine or any other liquid fertilizer so the coal can “suck up” nitrogen, a bit of potassium, phosphorus, and other trace nutrients that can be found in human urine or a different fertilizer. Again, let it “rest” for at least 2-3 weeks before you apply it to the soil.

One of my readers used a similar method to prepare special biochar for blueberries – he mixed coal dust, sawdust with water, elemental sulfur, and ammonia sulfate to make biochar more acidic.

The results of using biochar made of coal on sandy soil? It’s actually quite impressive….



20 year old cat hunting for rodents in biochar fertilized field of pumpkins

How do you add biology to your biochar to improve the soil food web?

Before you apply “nutrient charged” biochar to the soil you can add some beneficial organism (microbes, mycorrhizal fungi mycelium or mycorrhizal fungi spores) that will improve the biology of your soil even further.

Using biochar as animal bedding

You can also use biochar as the bottom layer of animal bedding. It will soak up excess liquid, tie up nutrients, and limit nitrogen loss. You can also add some rock dust (like granite rock dust or basalt rock dust) to your biochar. Just make sure to add some other bedding material on top of it, like straw, wood chips, or sawdust.

Should you mix biochar with rock dust or not?

If you are planning to add rock dust to your soil, you can add it to the charcoal-manure mixture. It is especially beneficial to mix soft phosphate rock dust with manure, because microbes from manure will help to unlock nutrients from phosphate rock.

Should you dig biochar into the soil?

I advise incorporating biochar into the soil but it will work even if you spread it on top of your soil. Eventually it will get into the deeper soil levels.

If you want to spread it on top of your soil in your garden it will be good if you put some mulch made of “normal” organic matter that will be digested by microbes.

How much biochar should I add to my soil?

Biochar is one of those things that the more you have in your soil, the better it is, but the minimum value I recommend to use in a garden is a 1″ (2.5cm) layer on top of your soil. No matter if you dig it in or not, you will have more than 10% organic matter content in the top 6-8″. If you add 2″, then even after the manure that you mixed with your biochar decomposes, you will still have plenty of organic matter in your soil. That way you can have a Cation Exchange Capacity for your soil from 1-4 to 8-12, which is good enough to grow high quality, nutrient-dense fruits and vegetables. Our pile made of 1 ton of coal dust and 3x as much horse manure was enough to cover 300-400 m² (3200-4200 square feet or 1/12 of an acre) with 5cm (2″) layer of biochar mixed with horse manure.