Fertilizers for home gardens can be expensive, heavy, require large storage areas and contribute to nutrient pollution in Hawaii’s streams and groundwater.

Many coastal states are enacting fertilizer laws, such as the 2011 New Jersey Fertilizer Law passed to protect surface and groundwater quality from nitrogen and phosphorous. Overuse of fertilizers containing these important plant nutrients threatens aquatic habitats and the quality of ground and surface water.

All garden soil needs organic matter to maintain life forms such as bacteria, fungi, earthworms and other soil organisms that work together to form healthy, fertile soil.

"Green" manure from different types of grains, grasses and legumes, such as Sunn hemp, buckwheat and common oats, is a great way to produce your own supply of fertilizer and organic matter that works well with other sustainable gardening practices. It has the potential to vastly decrease the amount of nutrients released into Hawaii’s watershed, streams, near-shore areas and groundwater sources.

Green manure seeds of different types are available locally, grow very quickly and within two months can be mulched and incorporated into garden soil. Some of the benefits of using green manure for the home gardener or small grower include providing organic matter and nutrients (such as nitrogen if using legumes), loosening compacted soil, erosion and weed control, attracting beneficial insects, little water and maintenance required, low cost, and year-round growth in Hawaii.

Seeds sprout quickly, and within two months of planting, green manure plants are chopped and the leaves, branches, plant stems and roots are incorporated into the soil.

A section of the garden can be designated for growing green manure, creating an in-house fertilizer supply where green manure can be chopped and carried to other areas of the garden or yard. Green manure crops also can be rotated throughout the garden, providing soil amendments and improving soil structure. Chopped green manure plants can be added to your compost pile.

Planting a green manure crop is a versatile way to build the soil in your garden without the hassle of dealing with large, expensive bags of compost and fertilizer and the need to keep going back to the store for more.

Green manure should be incorporated into the soil before it becomes fibrous and just before flowering and seeding. This ensures the highest composition of nitrogen and prevents loss of nitrogen to the formation of seeds.

A Green Manure Database and a list of sources from which to purchase green manure seeds are available through the University of Hawaii-Manoa College of Tropical Agriculture and Human Resources at www.ctahr.hawaii.edu; search "green manure."

Kimo Franklin is a graduate of the University of Hawaii-Manoa College of Tropical Agriculture and Human Resources and is a certified UH Master Gardener. Email him at bkfisle@hawaii.rr.com. Jayme Grzebik is an urban horticulturist with the UH CTAHR Cooperative Extension Service and statewide coordinator for the Master Gardener Program. Email her at grzebik@hawaii.edu.