Clover (genus Trifolium) is a common plant used in lawns, pastures, and cover cropping to improve soil conditions. Homeowners and land managers often ask whether clover needs fertilizer. Clover generally does not require nitrogen fertilizer because of its unique biological relationship with soil bacteria, which supplies its own nitrogen source. However, like all plants, clover still relies on a balanced supply of other nutrients and specific soil conditions to thrive.
The Mechanism of Nitrogen Fixation
Clover is a legume that forms a symbiotic relationship with specialized soil microorganisms called Rhizobium bacteria. This partnership is the reason clover can largely forgo external nitrogen fertilization. The bacteria penetrate the root hairs and stimulate the formation of small, swollen structures on the roots known as nodules.
Inside these root nodules, the Rhizobium bacteria perform biological nitrogen fixation (BNF). They convert inert atmospheric nitrogen gas (\(\text{N}_2\)), which plants cannot use, into a biologically available form such as ammonia (\(\text{NH}_3\)). The clover plant provides the bacteria with carbohydrates and a protected environment, while the bacteria supply the plant with usable nitrogen.
The effectiveness of this process is visible inside the nodule, which should appear pink or red when cut open, indicating the presence of leghemoglobin. This fixed nitrogen is available for the clover’s growth, allowing it to produce between 70 and 150 pounds of nitrogen per acre annually. If the clover seed is not pre-treated, it must be properly inoculated with the correct Rhizobium strain before planting.
Required Nutrients Beyond Nitrogen
While clover is self-sufficient in nitrogen, its growth and ability to fix nitrogen depend on the availability of other nutrients. Phosphorus (P) and Potassium (K) are particularly important elements that clover must obtain from the soil. Phosphorus is necessary for developing a vigorous root system and is directly tied to the energy transfer required for nitrogen fixation.
Potassium plays a role in water regulation, disease resistance, and overall plant health. Deficiencies in this element can cause grasses to outcompete clover in a mixed stand. Clover is less efficient at taking up soil nutrients compared to grasses, making it more susceptible to deficiencies in P and K.
Soil pH strongly influences clover’s health and its nitrogen-fixing capacity. Clover prefers a slightly acidic to neutral soil, with optimal growth occurring when the soil pH is above 6.3. When the soil becomes too acidic, typically below a pH of 5.8, the formation of the Rhizobium root nodules is significantly reduced. Applying agricultural lime can raise the soil pH, which also helps ensure the availability of micronutrients like calcium and molybdenum necessary for nodule function.
Clover’s Function in Soil Health
Clover is often utilized for its external contributions to the soil ecosystem. It is frequently used as a green manure crop, grown specifically to be tilled back into the soil while still green. This incorporation adds organic matter and quickly releases nutrients, providing a fertility boost for subsequent crops.
Even without tilling, clover enriches the soil for companion plants, such as turfgrass. As clover roots and nodules naturally die and decompose, the fixed nitrogen is released into the soil. This nitrogen becomes available for neighboring plants that cannot fix their own nitrogen, reducing the need for synthetic fertilizers in a mixed setting.
The plant’s extensive root system offers structural benefits to the soil. Clover roots help reduce soil compaction and improve structure by creating channels that enhance water infiltration and aeration. The dense foliage acts as a cover crop, protecting the soil surface from erosion and retaining moisture. By shading the soil, clover also helps suppress weed growth, offering a natural method of weed control.