Compost teas are plant-based sprays made by aerating finished compost in water. Aeration activates beneficial micro-organisms and helps them multiply. Compost teas improve soil biology when sprayed on the soil, and as foliar sprays they aid disease suppression and nutrient uptake in crops. Over the last decade the Oregon-based micro-biologist Dr Elaine Ingham has become a leader in the field of aerated compost teas. Ingham has established a network of Soil Food Web (www.soilfoodweb.com) laboratories in Europe, North America, South Africa and Australasia. These labs allow farmers to analyse the microbial life on their crops, in their soils, in their compost and in their compost teas.
Compost teas need the right balance of nutrients and the right range and populations of beneficial micro-organisms. The base for the kind of fungal-dominated compost teas which are favoured in wine growing could be made from from well aged (6-18 months decomposition) woody compost (from carbon-rich woodchips and chipped vine prunings or woody weeds like gorse and broom), Peter Proctor’s cow pat pit preparation 502-507, hay steeped in liquid manure slurry, seaweed tea, vermicasts and vermiliquids, molasses, basalt and lime. (See Biodynamics, compost and soil microbiology for a case study).
Once mature, the fermented compost is mixed in good quality water (ideally very slightly acidic, de-gassed of any chlorine) and then aerated. Aeration can last up to 24 hours and is necessary because most beneficial bacteria, fungi, protozoa, and nematodes are aerobic. Microbes present in compost teas must be alive when they are applied to crops or soil so they can produce exudates that help them stick to the crops, even in heavy rain. Spraying compost teas as a fine mist and at low pressure protects the micro-organisms’ cell structures. Rates are commonly 1kg of compost aerated in 100 litres per hectare for a soil drench, and double this for a foliar application (depending on vine canopy height).
Soil drenches are effective during periods when high levels of soil microbes need to be encouraged: in spring, before flowering, after véraison and after harvest. For a foliar application around 70% of the leaf surface needs to be colonised by the beneficial micro-organisms for them to be effective.
Elain Ingham (2003) says the beneficial micro-organisms in compost teas have several roles. They can induce what is called systemic-induced resistance in crops. Microbes in the compost tea surround the plant surface, but because plants cannot differentiate beneficial micro-organisms from non-beneficial ones an immune response is triggered, making plants more resistant.
Fifty per cent of the carbohydrates produced in crop leaves end up in the root system. It makes sense therefore for any food that plants produce as exudates from their roots or leaves which is intended specifically for soil bacteria and fungi is accessed only by beneficial disease-suppressive organisms rather than disease-causing ones. Once safely in situbeneficial organisms perform what is called niche occupation, meaning they take the space disease-causing organisms would otherwise occupy, and they take their food too. They can even predate disease-causing organisms (protozoa feed on bacterial disease organisms for example) and produce compounds that inhibit their growth. Beneficial bacteria and fungi play the key roles in protecting crops above ground, while below ground whereas bacteria and fungi are key, protozoa and nematodes also play vital roles too.
Wine-growers increasingly say they want fungally dominated vineyard soils, commonly a ratio of between 2.5:1 fungae to bacteria. The vine is a woody plant and woody plants need fungal-dominated “forest-floor” soils dominated by mycorrhiza. Soils too high in bacteria mean vine roots may lack the mycorrhiza allowing them access the soil system or soil food web as Dr Ingham calls it. The greater diversity of that mycorrhizal fungi, the greater diversity of nutrients they can access, meaning plants feed more efficiently (saving on fertilizer). Beneficial microbes also make the soil more airy and freer draining. Weedkillers and synthetic fertilizers inhibit the mycorrhizal fungi and create a more bacterial system.
Compost teas are not mixed with regular vineyard sprays containing copper or sulfur; in fact the aim is for the compost teas to render the use of copper or sulfur redundant. However, compost teas can be used in rotation with sulfur and copper sprays, to maintain microbial diversity in the vine’s leaf canopy. Wine-growers spraying eradicant or broad-spectrum fungicides eliminate rot-causing organisms by sterilizing a horizontal strip along the vine canopy where the grape bunches grow. However, because disease organisms live throughout the leaf canopy, both above and below the sprayed zone, they can quickly recolonise the sprayed area, for example when washed down there by the next rains. The disease organisms then find the grape bunches provide them with a perfectly sterile site upon which to grow and quickly reproduce. However, if compost teas are also being used then no one single fungal organism, especially aggressive disease-bearing ones, can easily become dominant, which reduces disease pressure.
All the nutrients that beneficial bacteria need should already be present if the compost being used for the tea had been well prepared. If not, food sources like molasses, complex sugars, fulvic acid and vegetable juice can be added to the compost tea. Food sources for fungi include kelp/seaweed extracts, rock dust, and fish oil/emulsion, or humic acids. In the United States because of fears of E. coli compost teas (even aerated ones in which E. coli cannot survive) made from or using raw manures can only be applied to vineyards no later than 90 days before the grapes are harvested.
Compost teas should also be used in conjunction with regular teas when the latter are being used as foliar feeds. Crops take in more nutrients (from foliar feeds) when their leaves are covered in bacteria and fungi (from compost teas) because these organisms respire carbon dioxide. Carbon dioxide around the leaf surface causes the leaf stomates to open quicker and to stay open for longer than usual. This then helps plants assimilate more easily any nutrient material contained in the foliar feed, potentially reducing the frequency, strength and cost of foliar feed applications.
Dr Elaine Ingham., ‘Compost tea, promises & practicalities’, Acres USA 33/12 (Dec 2003).