Scientific opinions vary on whether the soil should be worked at depth or whether, conversely, it should be left ‘untouched’ with a constant cover of vegetation or green waste. A group of researchers at the University of Illinois have attempted to answer this question. We report their results here because their advice is unequivocal: from the agronomic point of view, for the best results the soil should not be worked.
In order to reach this conclusion the researchers compared data from 62 scientific reports.
Already in the US ploughs and heavy machines are no longer used on one third of agricultural land. There are a number of factors supporting this method of cultivation: in addition to the well-documented recovery of a higher level of biological fertility, there is the saving in fuel costs and the costs of the machines themselves. In addition, unworked soil produces larger and healthier harvests, thanks above all to the microbial activity present in the soil which, through no longer being disturbed, is healthier and more fertile.
Let’s remind ourselves once again: a gram of soil contains millions of bacteria, hundreds of thousands of fungi, thousands of invertebrate (mites, collembola, nematodes, etc.) to which we must add the work of earthworms and vertebrates. The soil is a complex ‘factory’ in which the various agents that are present work together in harmony to attack vegetable waste and organic matter by shredding, aerating and humidifying it in order to make the nutritional elements it contains available to seeds and plants. Working the soil at depth with heavy machines alters this life cycle.
For many years, and still today, agricultural colleges have taught students that ploughing the soil (at depths of between 20 and 40 cm, and ‘breaking’ it down to 90-100 cm) aerates the soil and therefore improves its structure and productivity. These actions are justified by reference to an increase in bacterial activity and the multiplication of beneficial species caused by aeration and the work of the air, sun and rain. Scientific studies, however, show the opposite: that bacteria harmful to plants multiply, the mycelia of the fungi needed to transform organic matter so it becomes available to the plants become more fragile, and vast areas of soil become compacted and in some cases ‘vitrified’.
Direct sowing, on the other hand, reduces the bacteriological diversity but encourages the species involved in fertility, increases the vitality of the fungi and improves their effectiveness in breaking down organic matter. This has been shown to be true across the world, in different conditions of climate, soil and agronomy (such as harrowing the soil surface, the use or not of pesticides, or the practice of direct sowing under permanent cover).
The University of Illinois study was thus able to look at different pieces of scientific research carried out in different parts of the world, giving it a global overview and leading to a single conclusion: ploughed soil has lower levels of microbial activity and a smaller microbial mass as well as much lower enzyme activity than soil that is not ploughed.
This conclusion accords with the work of European researchers. For example, in France for more than a decade there has been a network for measuring the quality of the soil by identifying the average level of bacterial life present in a gram of soil. The platform is called Genosol and it can identify how rich the soil is, and therefore inform farmers about the real state of fertility of their soil.
Further details can be found in this article: Meta-analysis approach to assess effect of tillage on microbial biomass and enzyme activities