Ploughing/tilling actually, somewhat counterintuitively, prevents water from draining, interrupts the nutrient cycle in soil, and increases penetration pressure. In a natural ecosystem, a grassland, for example (as a natural analog for standard farming practices), the soil is extremely porous, even in areas where the subsoil is dense clay. Over millenia, the constant and intensive growth of plants adds soil organic carbon, which acts as a sponge for nutrients and water, allowing plants to access the nutrients and water that they need without drowning. Plant growth also adds air channels to the soil as roots grow, force apart the inorganic substrate, then die, leaving a channel of air and organic matter where the root once was. This effect compounds over time with the activity of the other soil biota, which colonize the soil and bring nutrients to the plants in exchange for root exudates and organic matter. Some symbiotes, fungi especially, can âmineâ the soil, extracting phosphorus and other minerals from the inorganic compounds in the soil which they then feed to the plants in exchange for root exudates. Mycorrhizae can also connect different plants and shuffle nutrients around, helping to maintain the health of the whole community.
However, when the soil is tilled, the air channels, pockets of organic matter, fungi, etc. are all torn up, and the soil is exposed to sunlight, killing much of the biology. The increased sunlight, heat, and surface area exposed to sunlight after tillage also volatilizes much of the soil organic carbon and nutrients, releasing them to the atmosphere (contributing to climate change with emissions of carbon dioxide, nitrous oxide, etc.) and depleting the soil. In addition, when the plants that are on the soil are taken away (i.e. when a field is left fallow) their half of the bargain with the soil biology goes away. They stop being able to supply the soil with the exudates and organic matter that it needs to sustain itself, so the soil starves. All of these effects compound to deplete the nutrient availability in the soil and destroy the soil structure and the biology that maintains soil structure (which harms drainage and root penetration). Frequent tillage also creates a hardpan just below the tillage depth, which further compounds the soil penetration and drainage problems.
So how can we solve these problems? Well, obviously, the best way to solve problems caused by tilling is to stop tilling, but that doesnât solve the underlying reasons why farmers till. Farmers primarily till for three reasons: to break up the soil for root and water penetration (as you said), to control weeds, and to more quickly decompose crop residue (with the intention of clearing the previous crop to make planting easier). These problems can be solved with reduced or no-till. First, mechanically breaking up the soil, as written above, will not increase root penetration and water infiltration. It does initially, which is why people think it works, but thatâs only because the soil is aerated when tilled. Once the soil has a chance to settle, it compacts more than it had before ploughing. The best solution to break up the soil is to increase organic matter content, increase soil biology, and keep a wider diversity of plants in the soil year-round (through cover cropping, if necessary). These methods do take a long time to work, so an initial planting of cover crops like daikon with a deep taproot can break the hardpan enough that a crop can be grown, then over time the soil improves through the aforementioned methods. Second, weeds can be controlled (without herbicides) through a variety of methods. The most effective on a large scale would probably be mulching. Instead of removing the previous crop, it can be rolled over to flatten it against the ground, which shades out any weeds. Flattening the previous crop has the added benefit of increasing soil organic matter as it decomposes, in addition to the many benefits of mulches in general. If the weeds are severe or especially rhizomatous, tillage may be necessary, but a shallow plough can achieve the same effects on weeds without damaging the soil as much. Third, ploughing the previous crop under and returning the field to fallow does provide a starting point to easily plant, but the effect is minimal when considering the effort taken to prepare the field before planting. A flattened crop can, with perhaps a bit more effort, be planted into as well as a ploughed-under crop. There are special seed drills that farmers can buy that cut through crop residue, but they arenât absolutely necessary as long as the crop is rolled in the same direction as the seeds are sown.
Some sources and further reading: