Soil Organic Matter Solutions for Peri-urban Market Farms

Soil organic matter is key to soil health. Organic matter supports fungi and macro-invertebrates in the root zone, increases aggregate stability, absorbs water and nutrients, and helps to reduce crusting and compaction. Maintaining sufficient soil organic matter levels in vegetable fields is challenging, as most vegetables produce little crop residue, and growers rely on tillage to create seedbeds and control weeds, insects, and diseases. One solution is to rotate vegetable crops with high-biomass cover crops. This often is not economically practical for peri-urban market farmers because of high land costs and the need to maximize the length of the growing season. At the University of Rhode Island we have been investigating ways to build soil organic matter without removing land from vegetable production. Zone tillage combined with a killed rye covercrop increased soil organic matter by 8.7% after three years, but yields were very low as the covercrop interfered with weed control and competed with the crop for nitrogen. Over the same time period soil organic matter in the conventionally tilled treatment decreased by 15% despite winter cover crops of cereal rye. Perennial living mulch of ryegrass and white clover in the wheel tracks maintained soil organic matter at a constant level. Yields were comparable to or better than the conventionally-tilled treatment in the first year. In years 2 and 3 yields were similar to the conventionally tilled treatment for melons, cucumbers, and carrots but significantly lower for tomatoes, lettuce, and brassica. The living mulch competed with the vegetables, and complicated weed control at the interface between mulch and planting row. It also complicated use of plastic mulch. Intercropping with crimson clover after vegetable establishment did not decrease yields, but it also failed to prevent loss of organic matter. Another approach to maintaining soil organic matter is to amend soil with waste organic matter from the surrounding urban area. After one growing season there were no clear differences in soil organic matter levels but also no significant differences in yields of sweet corn, potatoes, or butternut squash when organic waste materials applied at a rate of 9500 kg organic matter/ha were compared to control plots receiving only synthetic fertilizer. Six materials are being tested: dehydrated food waste; composted yard waste; class A biosolids + yard waste co-compost; recycled paper fiber waste; gelatin manufacturing waste; and mixed feedstock compost made from yard waste, stable sweepings, fish processing waste, and food waste.