Cover cropping is making a comeback. Farmers of yore recognized the benefits of cover crops worked
into the soil as green manure or left on the surface as mulch. Then fertilizers, pesticides, and high-yielding crop varieties reduced the need to take fields out of cash-crop production in order to plant cover crops.
Now, renewed focus on sustainability has revived interest, and USDA’s National Resources Conservation Service promotes the practice. Oregon State University researchers developed a Cover Crop Calculator to help farmers gauge the amount of nitrogen added to the soil so they can reduce applications of fertilizer accordingly. Because many of the cover crops used on the Mainland don’t grow well in tropical climes and soils, local growers want to know what will work here.
Enter Assistant Professor Koon-Hui Wang (Plant and Environmental Protection Sciences), the principal investigator on a three-year $474,043 USDA Conservation Innovation Grant to develop a Cover Crop Calculator for the Tropics. Her project will evaluate 10 annual or perennial leguminous cover crops (pigeonpea, lablab, sunn hemp, vetch, etc.), including those adapted to high or low elevations and acidic or high-pH soils, and look at how factors such as soil pH, tillage practices, and inclusion of grains or other non-legumes affect the calculations.
Nematodes will serve as the barometer of soil health. “To maintain healthy soil, you want a lot of microbial activity to break down organic matter and recycle the nutrients,” Wang explains. While some cover crops are toxic to pest nematodes, healthy soil contains beneficial nematodes that eat bacteria, fungi, or plant pests.
She works with several CTAHR Extension faculty members and USDA’s Waimea agent to conduct field tests on CTAHR plots at Lalamilo, Poamoho, and Waimanalo. Kapa‘a Farms coffee grower Gerry Ross and Big Island farmer Chris Robb are participating, and DuPont Pioneer contributes seed from sunn hemp grown on its Oahu properties. Results will be shared with growers in Hawaii and provided to CTAHR Extension agents in the Northern Mariana Islands.