Conventional tillage (CT) is the traditional and most intensive method of preparing a field for planting, involving mechanical manipulation of the topsoil layer. This practice, which typically disturbs the entire field surface, has been the dominant agricultural method for centuries, beginning with the widespread use of the plow. CT is characterized by the significant inversion and mixing of the soil profile, resulting in a largely residue-free surface before the crop is sown.
The Standard Sequence of Operations
The process of conventional tillage is divided into two phases: primary tillage and secondary tillage. Primary tillage is the initial, deep operation designed to loosen the soil and turn over the soil profile. This phase often uses implements like the moldboard plow, which completely inverts the soil, burying previous crop residue and stubble at depths ranging from 6 to 10 inches.
Secondary tillage refines the soil surface following this deep disruption. These operations are shallower and less aggressive than the primary phase, focusing on breaking down the large soil clods created by plowing. Lighter passes, often performed with disks or harrows, aim to create a fine, level, and uniform seedbed suitable for optimal seed-to-soil contact during planting.
Primary Purposes of Intensive Soil Turning
Intensive soil turning serves three primary purposes. The first is effective weed suppression, as mechanical inversion physically buries weed seeds and existing plants deep beneath the surface, preventing germination. This provides immediate control over vegetation.
A second purpose is the incorporation of crop residues from the previous season into the soil matrix. Mixing this organic material into the topsoil accelerates decomposition by increasing exposure to soil microbes and oxygen. Clearing the surface residue also facilitates the rapid warming of the soil in the spring, allowing for earlier planting and quicker seed germination in cooler climates.
The third purpose is creating a finely textured seedbed for successful crop establishment. Secondary tillage pulverizes soil aggregates to achieve a uniform tilth, ensuring seeds are surrounded by fine soil particles for maximum water and nutrient uptake and consistent seedling emergence.
Impact on Soil Structure and Composition
The aggressive mechanical action of conventional tillage fundamentally alters the soil ecosystem. Continuous turning and mixing severely reduce the stability of soil aggregates—the small clumps of soil particles held together by organic matter. The destruction of these aggregates leaves finer soil particles vulnerable to detachment and transport by wind and water, leading to accelerated soil erosion and loss of topsoil.
Organic Matter Depletion
Intensive aeration resulting from mechanical inversion stimulates a rapid breakdown of soil organic matter (SOM). Increased oxygen exposure boosts microbial respiration, causing organic carbon stored in the SOM to be released as carbon dioxide. This depletion reduces the soil’s inherent fertility, nutrient retention capacity, and water-holding ability.
Compaction and Water Dynamics
The repeated use of heavy machinery for multiple tillage passes often leads to the formation of a dense layer beneath the tilled zone, known as a tillage pan or hardpan. This compacted layer restricts the downward growth of plant roots, limiting their access to water and nutrients in the subsoil. Furthermore, the destruction of natural soil pores reduces the soil’s capacity to absorb heavy rainfall. Water infiltration decreases, increasing surface runoff and the potential for erosion.