THE ORGANIC MATTER IN THE SOIL
In early September, for business purposes, I had to prepare a technical paper to be included in a dossier submitted to the Andalusian administration. It was a simple explanation of the rationale for the grinding of wood in the orchards of all types, so that the assigned official who is not necessarily an agronomist, may easily understand technical and environmental motivations. It seemed to me an excellent basis for this article of the blog.
The plants feed from the soil in the form of nutrients released into the soil solution, that is to say in water available to plants. These factors, constituting the plant, are nitrogen, potassium, phosphorus, calcium, sulfur, magnesium, and many microelements, generally metals. Only a few plants can extract nitrogen from the air, legumes, and other like orchids extract almost all of their food from the air (that's why you must never cut the aerial roots of orchids, otherwise they die).
Naturally available nutrients (that is to say, not counting the contributions of chemical and organic fertilizers) for the plant in the soil, are coming, in the vast majority, from the decomposition of organic matter.
Organic matter comes from the life cycle of plants that live there. In a natural forest, dead plants or plant parts fall on the ground, and then begins a long decomposition process, leading to the same elements, again made available to the plants of the place. A natural forest without human cleaning or maintenance operation, does not need fertilizer, although it produces a huge amount of vegetal material in form of shoots, wood, leaves and fruits of all present types.
What is produced comes from what plants absorb.
What plants absorb comes from the decomposition of organisms, plants and animals which live and die on place.
What dies on place, except for some animals and microorganisms that can come from the outside, comes from indigenous plants and animals.
This is the cycle of life. Nothing is created nor disappears, almost, at least in this specific place.
The wood is formed by a specific and hard structure, which gives it its rigidity and resistance, lignin. This is a problem in the decomposition process because, apart from animals such as rodents or insects, only fungi can attack it. They settle on dead wood, and there make their cycle by feeding of lignin. These fungi, specific for this function, need a lot of moisture, air and light to live. They remain in the ground surface, in the first few centimeters (0 to 10 cm depth approximately). That is why the sunken ships, or the remains of wood buried under several meters of soil, can be discovered in very good condition after several centuries. Fungi, unable to live in these lack of air and/or light conditions, don't develop and don't attack the wood. For the same reason, a city like Venice, whose buildings are based on wooden posts, is still standing although on a large lagoon, constantly bathed by water.
Once broken down lignin, the organic material can begin the process of humification (humus transformation), with the active participation of a large number of living organisms in the soil, especially bacteria and earthworms. This is an aerobic process, which means it only can be conducted in the presence of a sufficient amount of air, so only in the surface layers of soil (from 0 to about 40 centimeters).
In agriculture, we extract the crops for the production of food, straw, wood, textile fibers or materials for biomass or for industry. The result is that gradually, the soil is impoverished if extractions are not fully recovered.
To maintain soil in the same situation of fertility, it is necessary to compensate for these extractions by fertilizer inputs. They only compensate for the loss of nutrients, not the loss of organic matter or fertility.
Soil fertility must be long-term maintained by organic matter input, so as to compensate extractions which tend to deplete the soil not only in terms of fertility, but also in terms of soil life.
On woody crops (trees and shrubs), the pruning consists of a selection of productive shoots to improve the production quality. The removed wood can be recycled (biomass, compost, cellulose, firewood, etc.), burned or returned to the soil.
Wood sent for recycling may have of economic interest, since in general it is sold, or pulls a benefit of a different kind.
A tractor equipped with a mill, it is the most common way to take advantage of pruning in summer and winter.
Burning should be reserved for very unusual cases of severe disease problems, since it is a direct waste, with polluting effects, accompanied by the destruction by fire and heat, of surface soil life. Many farmers are very reluctant to let the crushed wood, for fear of spreading disease. Yet it must indicate that there is an alternative to burning, composting, since, in its natural process, the temperature can reach 70 degrees for many hours, which is an excellent sterilizing agent that eliminates bacteria, fungi, insects, and even weed seeds.
Grinding is the most effective way to combine the needs of agriculture (to walk and move in the orchard), with the natural behavior of wood cycle in a forest. It is a process, using a very fashionable word, totally sustainable.
If the case of extracting or burning the pruning wood, it is necessary to compensate for these extractions by fertilizer inputs. Yet fertilizers without organic matter (chemical or biological) only compensate the extraction of elements, but are not able to maintain the activity of soil life. Gradually, the soil loses its structure and fertility. The crop conditions are always more difficult, and productive results get worse. It is necessary to bring organic matter in one way or another, for example as compost or composted manure.
Extractions of nutrients that represents one crop are generally quite limited, and easily offset by moderate fertilization. If we add the extraction of wood, they are much higher amounts. It is here that appears the real risk of destroying the soil structure, of loss of fertility, and of environmental pollution, because of the amount of fertilizer to be brought.
After the winter pruning, walk or move can be a problem.
Maintain or enrich the soil with organic matter has significant agronomic benefits:
- - The soil life is much more important and active, which promotes fertility, aeration and its ability to store water from rain or irrigation.
- - Organic matter plays a fundamental role in the clay-humic complex, that is to say on the exchange capacity of nutrients to the plant. The efficacy of the plant nutrition directly depends on it.
- - In orchards, and combined with ground vegetal cover, that is to say, with the presence of grass between the rows of trees, this technique promotes the proliferation of earthworms, promotes the drainage and reduces compaction.
- - In many cases, the most visual benefit is an increase in the uniformity of the crops, which results in an increased productivity without the need to increase the contributions of fertilizer or irrigation water. Natural irregularities in the texture of the soil, combined with the work of the farmer, make that a plantation, initially homogeneous, becomes heterogeneous with time, with powerful and productive trees, and others weaker and unproductive. Win in homogeneity is to ensure that the trees are all powerful and productive.
There is no longer a problem.
The environmental benefits of proper management of organic matter are even more numerous:
- - A significant active soil life promotes biodiversity. For example, increased earthworm populations promote moles and birds populations that feed on them, which promotes the proliferation of snakes and predatory birds, and so on.
- - Increasing the natural fertility of the soil reduces fertilizer consumption, since much of the crop needs are naturally provided by the soil itself.
- - The improvement of soil aeration allows better penetration of rainwater, and a reduction in irrigation requirements.
- - The improvement of soil aeration also allows better drainage, thereby reducing the risk of asphyxiation of crops, as well as rain water losses by surface flow, and improves refill possibilities of groundwater.
- - As in the case of no-tillage technique for annual crops, keeping the soil without plowing, with remaining pruning wood grinded on it, especially if it is combined with green cover, greatly reduces loss agricultural soil erosion due to rain or wind.
- - The improvement of the homogeneity of crop plots allows for better productivity. At the local level, it's a higher income for the farmer. At the global level, it is a guarantee to continue to grow in productivity, that is to say, to produce more, with no need to conquer new lands, to cope with the increase in world population. This is an obvious environmental benefit, although difficult to assess locally.
- - No-tillage significantly reduced the release of CO2 to the atmosphere, which is directly involved in combating global warming.
- - There is a fact little known to farmers themselves, but potentially very important, which is the improvement of the ability of the plant to self-defense because of the richness of soil in beneficial microorganisms. This point requires an entire post on its own.
- - Finally, there is evidence that a more active microbial soil life greatly improves its ability to break down pesticide residues, chemical and biological, reducing the risk of groundwater pollution.
In the following spring, the grass has grown and we no longer see the wood prunings. Yet they are on the ground.
Grinding the pruning wood for its return to the ground is a totally sustainable practice as it actively participates in maintaining soil fertility, in its conservation, in its drainage capacity and its ability to produce, in the long term, important crops in quantity and quality. In the case of degraded soils or very poor in organic matter, external inputs like compost are needed to make them a sufficient workforce. It is a long process that can take several years.
But be careful, to be effective, an organic matter enrichment has to be made in very large quantities. An example :
To increase the percentage of organic matter 1%, in the 30 first centimeters from the ground surface, it has to be taken into account that 30 centimeters of soil are 300 liters per square meter of soil, or 450 kilos, and that a compost generally contains around 40% of organic matter.
So for a garden of 100 m2, it will bring more than 1 ton of good quality compost and a farmer will have to provide more than 100 tons per hectare.
This short calculation is intended to make you understand that the annual return of plant remains is an easy technical, economical and very effective way to keep soil in good conditions. It is interesting, especially for the gardener, to do his own compost with all plant remains he generates, for example, from decorative plants, hedge trimming, damaged fruit and vegetable, scraps or peelings, or cut grass. It's easy, it does not smell bad and it is an excellent nutritional supplement. It only requires to be returned from time to time, and to be kept moist. I want to write a post about compost.
This calculation is also a warning that the usual organic fertilizers are misleading. It is a good source of nutrition, but not a soil enrichment. For that, it would bring in huge quantities.
At the end of a very dry summer, the grass has dried, mixed with the crushed remains of pruning, forming a thick layer of organic material, ready to be decomposed when arrive the first autumn rains. The grass will sprout again, and next spring, we will have new green ground cover, fine and very active.
Without plants, there would be no life on Earth, but without fertile soils, plants could not grow.
The soil is plants life, so that of humans. More simply, the soil is life. We must take care of it, as if we are farmers, gardener, or single owner of a garden.
To care of it, we must first know it, and understand the complex processes permanently taking place under our feet, and in silence. We don't care of it. Yet our life depends on it.