Green Manures

The practice of green manuring is not new, in fact the practice dates back to the ancient Greek and Romans around 300 B.C. and even earlier in China, where texts noted the importance of using grasses and weeds to enrich the soil. So the practice of applying green manures is as old as time.   

Green manures are synonymous to cover crops, catch crops and shade crops and are intentionally turned into the soil before the crops mature, providing nutrients to the subsequent crop. Again, we can think of green manures as cover crops. They also provide the same benefits to the soil as when used as cover crops to

• reduce runoff

• Manage soil nutrients

• Improve soil structure

• Bioavailable soil nutrients 

• Suppress weeds

• Reduce pests

• Conserve soil moisture

• Provide a suitable habitat for soil microorganisms (living soil) 

• Improves water filtration

It is important to be clear that green manures are not the same as compost or synthetic fertilizers. Compost is typically the managed decomposition of organic matter, specifically farm waste such as trimmings, non-seed weeds, prunings and other non-diseased plant matter. The organic matter is removed from the field and processed to later be added back to the field for humus and available nutrients. 

Synthetic fertilizers are man-made chemicals that are designed to deliver macro and micro nutrients to the plant without much regard to soil health. Sometimes the synthetic fertilizers are derived from petroleum, ammonia, natural gas, and inorganic waste materials. 

Common green manures suitable for Hawai’i are:

Legumes (Used to add nitrogen)

1. Cowpea - Fact Sheet

2. White Sweet Clover - Fact Sheet

3. Sunn Hemp (Tropic) - Fact Sheet

4. Pigeonpea - Fact Sheet

5. LabLab - Fact Sheet

6. Woolypod Vetch - Fact Sheet

Non-Legumes

1. Annual Ryegrass - Fact Sheet

2. Barley - Fact Sheet

3. Buckwheat - Fact Sheet - Can make phosphorus available to subsequent cash crop

4. Black Oats 

5. Azolla for wetland Kalo - Fact Sheet

6. Cereal Rye - Fact Sheet

7. Sorgum-Sudan grass - Fact Sheet

If you are considering green manures as a soil nutrient management practice, a producer must consider the proper green manure for their operation. You must first consider the primary objective or reason for planting them as part of your farm management system. Many cover crops meet most soil health objectives, it is good practice to decide which cocktail of green manures (legume/non-legume; legume mix) to use while considering the best time and place to incorporate your green manure into the crop rotation. Because Hawai’i has a year round growing season, we must consider a 12 month crop rotation that incorporates green manures.

Things to consider before applying green manure into your soil management plan.

1. Soil type 

2. Average soil temperature

3. Soil pH

4. Compaction/porosity

5. Soil moisture

6. Organic residues