Clover (Trifolium species) is a genus of flowering plants recognized for its influence on soil health in agriculture and gardening. This legume, which includes hundreds of species, has been used historically as a rotational crop to revitalize fields depleted by continuous planting. Clover is a significant asset to the soil, functioning as a biological mechanism that enriches the earth beneath. By engaging in a unique chemical process and developing a tenacious root system, clover actively enhances the physical structure and nutrient profile of the soil where it grows.
The Process of Nitrogen Fixation
Clover’s primary benefit stems from its ability to convert atmospheric nitrogen into a form usable by plants, a process known as nitrogen fixation. This conversion is performed by specialized soil bacteria from the genus Rhizobium. The clover and the bacteria engage in a symbiotic relationship where both organisms benefit.
The bacteria colonize the clover roots, stimulating the formation of small structures called nodules. Inside these nodules, Rhizobium takes nitrogen gas, which is inaccessible to plants, and transforms it into ammonium. This ammonium provides the clover with the nitrogen needed for growth.
The clover supplies the bacteria within the nodules with carbohydrates, serving as an energy source for the conversion process. When the clover plant dies or is tilled into the soil, the nitrogen stored in its biomass is released through decomposition. This slow release of nitrogen then becomes available to surrounding or subsequent non-legume crops.
Improving Physical Soil Structure
Clover physically alters the soil profile through its extensive root architecture. Varieties like White Dutch produce a dense, shallow root mass, while others, such as Red Clover, develop deep taproots that penetrate several feet into the ground. This root growth breaks up compacted soil layers, a process known as natural aeration.
The deep roots create vertical channels in the soil, improving the infiltration rate of water and air. When these roots die and decompose, they leave behind macropores, enhancing drainage and reducing surface runoff. This improved structure supports a healthier environment for other plant roots and beneficial soil organisms.
The decomposition of the root mass also contributes organic matter deeper into the soil profile. This incorporation of carbon-rich material helps stabilize the soil structure, leading to better soil tilth and resilience against erosion and compaction.
Clover as a Ground Cover and Living Mulch
The above-ground growth of clover offers protective and competitive benefits for the soil surface. Its dense canopy forms a natural protective layer that shields the soil from wind and rain. This coverage significantly reduces soil erosion and prevents the washing away of valuable topsoil.
Clover is often employed as a “living mulch,” grown simultaneously with cash crops or maintained in a lawn setting. The canopy shades the soil surface, which lowers the soil temperature and reduces water evaporation. This conservation of moisture is beneficial during dry periods, helping to maintain soil hydration for adjacent plants.
The rapid and dense growth of many clover varieties also makes them effective at suppressing weeds. By blocking sunlight, clover out-competes weed seeds and seedlings for space and light. This biological control method reduces the need for herbicides, contributing to a healthier soil ecosystem.
Practical Application and Integration
The beneficial properties of clover can be integrated into various systems through the strategic choice of species and management practices. Common varieties include:
- White Dutch Clover (Trifolium repens) is low-growing and perennial, suitable for lawns and permanent living mulches.
- Red Clover (Trifolium pratense) is taller and typically biennial, often used for short-term cover cropping and hay production.
- Crimson Clover (Trifolium incarnatum) is a quick-growing annual variety that provides rapid soil coverage and substantial nitrogen contributions.
To maximize nitrogen transfer, clover is often planted as a cover crop and then terminated before the next crop is planted. Tilling the clover into the soil converts it into “green manure,” quickly releasing the stored nitrogen as the biomass breaks down. This practice ensures nitrogen is available for the subsequent crop, often providing the equivalent of 60 to 170 pounds of nitrogen per acre.
When planting, use pre-inoculated seeds or apply a Rhizobium inoculant. This ensures the specific bacteria needed for effective nitrogen fixation are present. For pastures or lawns, clover is often frost-seeded in late winter, allowing freeze-thaw cycles to help the seed settle for spring germination.