Sunday, March 29, 2015

Organic matter

In my last post, I talked about the importance of a good soil structure for having a high fertility. Another very important factor is the percentage of organic matter present in your soil.
Organic matter is essentially made of carbon, the atom that defines life. Carbon is the basic unit of all living things, from bacteria to humans. As Antoine Lavoisier said: “in Nature, nothing is created, nothing is lost, everything is transformed”.  I’m going to talk about the fate carbon and why it matters in agriculture.

A diverse landscape in Mengsong, Xishuangbanna, Yunnan.

When the leaves fall off the trees, they accumulate on the ground and for litter. After a while, the action of small animals, fungi and bacteria will transform the litter in humus: a dark material made of unrecognizable remnants of leaves, dead microorganisms and animal feces (yes, you’re walking on poo every day!). The difference between humus and litter is the stage of decay at which they are, but the limit is not very clear.

Litter is organic matter which is being eaten by microorganisms, it is transforming, typically the colorful fallen leaves cover that you can see in the forests during Autumn. Litter is the main source of food for the soil animals and microorganisms. Worms feed on litter, their presence in the soil will guarantee a good soil structure because they spend their life digging tunnels. They eat litter and digest it into smaller particles. Nematodes are very tiny round worms, barely noticeable with the naked eye, they will continue the job and eat smaller pieces. Fungi and bacteria eventually degrade organic matter into humus.

Jingmai Ancient Tea Gardens have a thick litter thanks to the big trees.

Humus is organic matter that has reached its final stage of decay, it is almost as stable as stone, it is what’s left after microorganisms have eaten everything. Humus mixes with the soil particles (sand, silt and clay), this is what makes your soil more or less dark.  It is important to have a lot of organic matter in the soil because it increases the water and nutrients capacity of the soil, the fridge is bigger!
Not only the fridge gets bigger as humus is produced, but it is also steadily filled. If organic matter is mostly made of carbon, it also contains nutrients very useful for the plants: nitrogen, phosphorus, potassium, magnesium, calcium… It works as if you were demolishing a house: you couldn’t reuse the concrete, but you could recycle the copper from your electric wires and use it in a new house. Nature works the same, as matter decays, nutrients are made available to the plants.

The soil in Jingmai has a dark color, it is rich in organic matter.

Chemical fertilizers are very handy for the farmers because they are much lighter than organic manure, you need to add up to a hundred times less of for the same amount of nutrients. This is one of the main reasons why they are widely used in the world, from large scale industrial farms to smallholders who don’t have a tractor. They have been the cornerstone of the Green Revolution.

However, their use on the long term creates a major problem: soil degradation. A soil is always degraded because of rain, wind and chemical processes. In order to compensate the losses, you have to continuously add things; this is especially true when it comes to organic matter. After several years without adding organic matter, the soil structure is impacted; it is more vulnerable to weathering and has poorer nutrient and water retention capacity. In other words, you have plenty of food, but your fridge is very small… 

Wednesday, March 25, 2015

For a fertile soil...

For a long time, soil has been seen as a simple support. The philosophy of conventional agriculture is to give each problem its solution: if there isn't enough nutrient, you add fertilizer, if there isn't enough water, you add irrigation and if there's a pest attack, you spray pesticides. In conventional farming, the soil is adapted to the crop by using a variety of inputs. 

Agroceology takes a different approach. The soil is the most important part of the field because many ecological processes occur in it. Changing your agricultural practices will affect those natural processes which will change your soil and create a positive or negative dynamics. Conventional agriculture focuses on the present and neglects those long-term evolutions, this is why soil degradation occurs in many place: because of a poor management of the ecological processes, the soil was kept in a vicious circle for years. The yield was not affected because the loss in soil quality was compensated by extra inputs added. Farming on a degraded soil is possible but it requires an intensive management because everything has to be brought. 

Irrigated and heavily pruned tea trees
A good soil can store nutrients and water, the main things plants need to grow. One of our teacher says "soil is to water what the fridge in your house is to you, and you prefer it big and full of good stuff". Some soils are better than others naturally, because of their texture. Soil has particles of different size, from big chunks of sand to tiny pieces of clay. The smaller the particles, the better the soil can keep things in it, therefore, a clay soil is generally preferred to a sandy soil. The soil texture cannot be changed by agriculture, it would mean bringing tons of new material and it is very unpractical.

However, the structure of the soil is very dependent on what people do on the soil. The structure means how the particles are organised. You want the soil to be bulky enough so that it won't fly away with wind and but not too compacted in order to offer a maximum surface contact between the soil particles and the air. That way, you can store much more nutrients and water : the fridge gets bigger.

The soil is so loose in some parts of Jingmai mountain that heavy rains give that hilly shape to the surface.

In agriculture, the main problem is compaction : it occurs when you use tractors or when you and your cows walk on the field. It is possible to loosen the soil by digging it, but this is hard manual labor. Small dudes can do the job for free : worms. As worms crawl through the soil, they dig galleries that will loosen the soil and therefore improve its structure. This is one of the main reasons why soil life is important.

Ants contribute to the soil structure

With a good structure, plants will grow roots easily and a special kind of fungi called mycorrhiza will fix on the roots and grow a network of filaments that will help the plants catch nutrients. In exchange, the fungi can take carbon from the plant to feed itself. This is one of the great mutualistic relationships known in Nature. It is believed that without mycorrhizae, the nutrient catching capacity of many plants would be severely reduced.

We should think about the soil first, and the crop second. A fertile soil is the guarantee of good yields on the long run. By improving soil life, we can improve the soil structure and therefore increase its fertility. 

Manual weeding as part of an experiment in Jingmai natural tea gardens