Green Manure for Greenhouses

Green manure is not the most common form of manure to be used in greenhouses, but if your greenhouse uses planting beds and does not cultivate year-round it can be an effective option. Just like cover crops and green manure benefit outdoor fields, they can be used to protect and enrich planting beds.

• Nitrogen enrichment
• Organic matter is added to soil improving depth and texture
• Improves soil moisture retention
• Inexpensive 
• Reduces or eliminates the need for chemical products
• Preserves soil quality and integrity
• Prevents erosion
• Reduces/eliminates weeds and many pests
• Some green manure crops have value and can be harvested and sold

Cover crops (of any sort of ground cover plant) can be planted to protect fields or planting beds from soil erosion and help the soil retain moisture. Weeds, insects, and pathogens are suppressed when cover crops are planted. They also help to preserve soil structure and microbial populations between the planting of regular crops.

Some cover crops are also good for use as green manures. Green manure is simply a cover crop that has been plowed into the soil prior to planting a regular crop. Most farms using these methods incorporate the use of cover crops and green manuring as part of a crop rotation system. Green manures add nutrients and organic matter to the soil, which improves plant nutrition and soil texture. 

In greenhouse planting beds, the green manure crops can be planted to cover the whole bed then mown with a lawnmower and left for a few days. After that, it can be turned in with the soil and the next crop can be planted. The cover crop adds the value of maintaining the soil texture and preventing erosion. Crops like clover or vetch have been demonstrated to be good green manure crops for greenhouses.

Research on green manuring in greenhouses has not shown any increase in the quality or yield of the primary crop. The main advantage it serves is reducing or eliminating the need to use synthetic fertilizers.

While crops may not do significantly better than when fertilized with chemicals, they do not generally perform worse which is enough to consider if the cost and effects of synthetic fertilizers are really necessary.

One early study found that greenhouse plant bed soils saw a 1.2% decrease in soil organic matter content after three years of growing. By comparison, only 0.6% of soil organic matter was depleted in green manured plant beds. Outdoor fields see about the same levels of organic matter reduction which would require 400 kg of farmyard manure to make up for a 0.6% loss of organic matter.