This post might best be retitled ‘reasons not to plough’. The effect of a gardener’s soil management on the environment is small compared to agriculture. My recent posts about soil have examined how minimum cultivation helps build-up of soil organic matter and in ‘Why Gardeners dig:5’, how in traditional horticulture, nutrient release from the breakdown of organic matter as a result of soil cultivation, is harnessed to benefit plant growth. It is the detrimental consequences of organic matter breakdown that I want to discuss in this post.
When organic matter degrades it
- adds carbon dioxide into the atmosphere
- releases soluble nutrients, in particular nitrogen, which can leach into ground water.
There is also a problem of phosphate pollution when leachate from organic matter left standing on hard surfaces such as tarmac, goes directly into the drains. Soil absorbs phosphate very tightly and normally there is very little leaching from soil. When wind and water erosion physically takes soil into water, then phosphate pollution will happen. It is significant that erosion occurs when soil has been loosened by cultivation. In the UK phosphate pollution of water courses is principally caused by industry and detergents.
Soil organic matter sequesters carbon
The soil in an established pasture might contain in excess of 4% organic matter. More, if you include the bio-mass of living grass. After 25 years ‘under the plough’ this level might be halved. When considered on a worldwide scale, this represents millions of tons of carbon dioxide released into the atmosphere.
Inevitably when any wild vegetation is brought into cultivation, carbon dioxide will be released. On a global scale the amounts are huge.
Modern farmers can now make their contribution to reducing carbon dioxide release and indeed with enlightened methods, can return carbon to the soil. Although we do not see much movement to ‘no till’ in the UK, on a world scale there are many examples of otherwise vulnerable soils which retain fertility when minimum cultivation methods are used.
Farm field overlooking Yorkshire Wolds. No local fields have been ploughed this year. It has been too wet.
Pollution of water by nitrates and phosphates
These nutrients promote luxuriant growth in aquatic algae and other water plants. When this growth dies it’s decay depletes water of dissolved oxygen, to the detriment of animal life in water courses, lakes and eventually the sea. Not all nitrate leachate entering waterways is from degradation of soil organic matter. Poor fertilizer practices are a significant source of pollution. Organic growers are not absolved of blame - their manures and organic materials also release large amounts of nitrate when they decay. Organic methods, and particularly minimum cultivation, do however tend to minimize this leaching.
|Blue lupin as green manure|
Agricultural practices that help sequester carbon
The modern farmer can do much to maintain a fertile soil and restore the soil after the detrimental effects of poor past practice.
- Converting to minimum cultivation methods. Farmers can use shallower soil cultivations. It has been shown that deep cultivation, other than to break up a known cultivation-pan, brings no agronomic benefit, uses far more energy, and greatly increases loss of soil organic matter.
- For many crops the use of herbicides ends the need to cultivate for weed control.
- Farmers can follow one crop immediately with another. Far better than overwintered, ploughed fields leaching nitrate.
- Use of green manures.
- It is traditional practice to ‘repair’ damaged agricultural soil by grassing down for a few years. Recent research suggests that inoculating grass seed with mycorrhiza, which add glomalin to the soil, will accelerate this process.
- Carbon can be directly sequestered in the soil as biochar. I will soon be blogging about the enormous benefits of adding charcoal to the soil!