Plowing, also known as conventional tillage, is the mechanical process of turning over and mixing the top layer of soil. This practice has been employed for centuries to prepare a seedbed for planting and to control weeds. While it achieves immediate goals, the mechanical disturbance has profound negative effects on the physical, chemical, and biological health of the soil. This regular disruption impacts the environment and the sustainability of agriculture.
Physical Destruction of Soil Structure
The most immediate consequence of plowing is the breakdown of soil structure. Healthy soil is built on aggregates—small clumps of soil particles held together by organic matter and microbial networks. Plowing shears through these stable aggregates, pulverizing them into fine, loose dust that lacks structural integrity.
This physical breakup destabilizes the soil, collapsing the network of pores and channels that allow for the movement of air and water. Once the fine particles settle, the soil becomes denser and less porous. The repeated pressure from the plow and heavy machinery also creates a dense, compacted layer just below the depth of tillage, often called a “plow pan” or hardpan.
A hardpan restricts water infiltration, forcing runoff rather than soak-in, and physically impedes plant root growth. Roots cannot penetrate this dense layer to access deeper water and nutrients, leaving plants vulnerable to drought and reducing crop health. The destruction of structure also leads to surface crusting, which prevents new seedlings from emerging and limits water entry into the soil.
Accelerated Loss of Organic Matter
Plowing rapidly depletes soil organic matter (SOM), the reservoir of carbon derived from decomposed materials. SOM is crucial for soil fertility and water-holding capacity, but mechanical tillage exposes it to oxygen. When the soil is turned over, the sudden increase in aeration accelerates the decomposition process carried out by soil microbes.
Microbes break down the SOM at an unnaturally fast rate, releasing stored carbon into the atmosphere primarily as carbon dioxide (\(\text{CO}_2\)) through carbon oxidation. This chemical loss depletes the long-term fertility of the soil, increasing reliance on external fertilizers to maintain crop yields. The continual loss of organic carbon also reduces the soil’s ability to act as a carbon sink, contributing to atmospheric \(\text{CO}_2\) levels.
A decrease in SOM translates to fewer water-stable aggregates, compounding structural failure. Soils with lower organic matter are less resistant to compression, making them more prone to compaction and degradation. This creates a cycle where tillage causes SOM loss, which makes the soil more susceptible to the physical damage of subsequent tillage passes.
Disruption of the Soil Food Web
The mechanical action of plowing physically harms the complex biological community that sustains healthy soil. The soil food web includes organisms from microscopic bacteria and fungi to macro-organisms like earthworms, all essential for soil health. Tillage displaces, injures, or kills these beneficial organisms and destroys their intricate networks.
Earthworms, natural engineers that create burrows for aeration and water infiltration, suffer a systematic decline in ploughed soil. Studies show that earthworm populations can be over 100% higher in undisturbed soil compared to conventionally tilled land. The larger species that create permanent, deep burrows are particularly susceptible to the mechanical action of the plow.
Microscopic fungal networks, especially those formed by arbuscular mycorrhizal fungi (AMF), are also severely damaged. AMF create extensive hyphal networks that act as extensions of plant roots, transferring nutrients and water. Tillage equipment severs these fragile filaments, impairing the fungi’s ability to colonize new roots and reducing their positive effects on soil aggregation and plant health.
Consequences: Erosion and Reduced Water Retention
The combined physical, chemical, and biological damage from plowing results in two major environmental outcomes: severe erosion and poor water management. When soil structure is destroyed and pulverized into fine particles, it becomes vulnerable to being carried away by wind and water. This loose, exposed soil is easily transported, a process known as tillage erosion, which moves topsoil downslope on inclined fields.
The loss of stabilizing organic matter and the collapse of pore spaces mean the soil cannot absorb rainfall effectively. Instead of infiltrating, water runs off the compacted surface, carrying away nutrient-rich topsoil and increasing the risk of downstream flooding. This accelerated erosion removes the most fertile layer of soil, leading to a decline in agricultural productivity over time.
Furthermore, reduced organic matter and lack of pore space diminish the soil’s capacity to hold water, making the land susceptible to drought. Degraded soils cannot retain enough moisture to sustain crops during dry periods. This loss of water retention capacity, coupled with restricted root growth caused by the hardpan, makes the agricultural system more fragile and less resilient to changing weather patterns.