We use these organisms as tools because they are the source of nitrogen (as well as carbon and other nutrients) for plants. Slide #2 shows the carbon to nitrogen ratio (C/N) that is found in various organisms. All nutrients that plants absorb have to pass a region of intense interactions between roots, microorganisms and animals, called the rhizosphere. So, we want the bacteria, fungi, protozoa and beneficial nematodes to be present in the right proportions when the compost is ready to be applied to the soil. As can be seen on slide #2, bacteria and fungi contain more nitrogen than protozoa and nematodes: they can be called bags of fertilizer. Protozoa and nematodes regulate bacterial numbers by grazing on these organisms. A simple calculation shows that one protozoan or nematode has to ingest 6 bacteria to get the right amount of carbon. However, by feeding on 6 bacteria it ingests too much nitrogen (it requires one molecule of nitrogen but ingests 6). This nitrogen is in excess and is released into the rhizosphere. This nitrogen is now available to plant roots. A simplistic calculation indicates that per unit of hypothetical soil around a root, there are so many protozoa and nematodes that they release 35 times the amount of nitrogen needed by a hypothetical root.
When a compost pile is deemed to be ready for applying to the soil at Frog Hollow, it has been closely monitored in terms of temperature cycles and humidity and the biomass of the various organisms is calculated. We especially make sure that compost is dominated by beneficial fungi (fungal to bacterial biomass ratio 2:1 or more) because fruit trees prefer a fungally dominated soil
This past week application of compost teas has been continued at Frog Hollow Farm because some bacterial and fungal pathogens can still harm the grapes, pearls and apples. We will start discussing what is the evidence that vermicompost teas can suppress certain plant pathogens and how this is applied at Frog Hollow.