Microbially enhanced compost extract: does it increase solubilisation of minerals and mineralisation of organic matter and thus improve plant nutrition?

Compost extracts may potentially benefit plant growth through a direct nutritional benefit, through increased mineralisation or solubilisation, through disease protection or by detoxification of the soil environment. Tomato (Lycopersicum esculentum cv. Tiny Tim) and a subsequent sorghum (Sorghum bicolor cv. Sweet Jumbo LPA) grown on a ferrosol and a vertisol was treated with soil drenches of a cattle rumen content compost extract (aerated and non-aerated, sterilised or not). Soil type was the primary factor in determining growth rate of crops. Compost extracts produced by either extraction method had similar effects on plant growth, and there was no difference between sterilised and non-sterilised treatments. Thus the noted growth benefit of compost extracts was not directly biological in nature. The positive impact of compost extract application on plant growth was ascribed to a nutritional effect, attributed to the high doses of compost extract application used (2 Lpot-1, equivalent to 34,000 Lha-1), delivering 3.4g of Npot-1. There was no evidence of increased mineralisation or solubilisation in this exercise.