I have over 40 trees on my property providing lots of leaves to amend my soil. While I know that leaves increase soil organic matter, improve water-holding capacity and soil structure, I have wondered if leaves provide much in the way of nutrients. According to a research study at Rutgers University by Drs. Joseph R. Heckman, Daniel Kluchinsh and Donn A. Derr (http://www.spectrumanalytic.com/support/library/ff/Plant_Nutrients_in_Municipal_Leaves.htm), "leaves are a valuable source of all crop nutrients." They point out that "only a portion of the nutrients are available immediately after application for use by the crop." However, long-term fertility is improved as nutrients are released over time.
The reason for this slow release of nutrients is that the nutrients are part of the plant tissues and need microbial decomposition to be released. In addition, dried leaves are primarily carbon, which supply energy-producing carbohydrates, attracting fungi and bacteria in the soil to start decomposition. These microbes initially use up available soil nitrogen in their development and may cause a temporary nitrogen deficiency. The researchers say that crops grown on soils one year after leaves are added will probably need extra nitrogen fertilizer to compensate for the increased microbe use. As the decay of the leaves continues, the process starts to reverse and the microbes add nitrogen back into the soil. While a one-time application of leaves might deplete soil nitrogen in the following growing season, yearly additions of leaves build soil fertility.
"Of the three major nutrients, (nitrogen, phosphorus and potassium), potassium is the most easily released from leaves and is the most readily available to crops in the first year after leaf waste application," say Heckman and partners. Their research indicated that after three years of applying leaves, the pH (acid or basic) did not change. Over many years, the addition of leaves will eventually start to lower the pH slightly.
Nutrient concentrations in leaves vary widely. The researchers suggest putting down a 6-inch layer annually. Shredding the leaves in a chipper or running over them with a lawnmower will allow them to break down more quickly. Shredded leaves are easier for microbes to decompose. They have to use less energy, which reduces depletion of soil nitrogen.
Remember, healthy soils mean healthier plants. Feed your soil, not your plant.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at firstname.lastname@example.org or 887-2252.