Nitrogen is a major nutrient for plants, the main ingredient for fertilizers, and takes a lot of energy to synthesize from natural gas. Legume plants take nitrogen from the air and fix it in a form that plants can use thanks to the special bacterial nodules on the roots. Organic growers like to use legume cover crops to increase nitrogen in the soil without the expense of purchasing fertilizer. More information on cover crops in Florida.
Orchard alleys are typically grass and/or weeds that allow the passage of vehicles through the orchard. There is an opportunity to grow legumes in the alleys to fix nitrogen for the fruit crops in the orchard. Legumes grown in the alley could be mowed in place, mowed and blown on to the tree row, or incorporated into the soil. However, incorporation disturbs the soil, increases weed germination, and may damage surface fruit tree roots. Newer techniques of rolling/crimping to kill off the cover crop and direct seeding equipment can be adapted for orchard use to avoid these problems.
Researchers in Washington State wondered which legume species would be best to provide this free nitrogen, and how much nitrogen was added to the soil by the mow and blow system.
Four perennial legume species, alfalfa, ladino white clover, birdsfoot trefoil, and kura clover were compared with the resident grass cover crop in a mature apple orchard. Legumes were direct seeded into the alley and successfully established.
The amount of nitrogen contributed to the tree row by the cover crops from four mowings in the second season were: alfalfa about 39 lbs nitrogen per acre, ladino and trefoil about 27 lbs nitrogen per acre, and grass and kura about 10 lbs nitrogen per acre. Legume nitrogen appears to be less expensive than other sources of organic nitrogen and may be cost competitive with synthetic fertilizer nitrogen when prices are high. The amount of nitrogen added to the soil depended on the width of the legume planting. Tractor traffic reduced legume performance. To adapt these findings to Florida, we would need to test legume species that will grow here. Some growers are already doing this with cowpeas, sunn hemp, and perennial peanut.
For the full article: Growing Legumes in Orchard Alleys as an Internal Nitrogen Source. 2017. HortScience 52(9):1283-1287. By D. Granatstein, J.R. Davenport, and E. Kirby.