We tend to focus on the roots of plants as water conductors, but the leaves and stems are equally important in the transferal of moisture, gasses and micro-nutrients. Stomata, tiny apertures on the surface of leaves, regulate transpiration by opening and closing in response to environmental changes. In a remarkably efficient way to reduce water loss through evaporation, the stomata close in midday heat. The plant wilts, thereby allowing less leaf surface to be exposed directly to the sun’s rays. For this very reason, one should not water plants at the height of day since this would undermine the plant’s natural protective response to retain moisture.
The fact that a leaf’s cell walls are permeable allows us to supply nutrients directly to the plant in the form of foliar sprays. Indeed, foliar feeding is an efficient means to fertilize plants as nutrients are more quickly absorbed through the leaves than from the soil. Compost teas have been used for generations as an economical liquid fertilizer, applied as foliar sprays or soil drenches. By simply steeping a handful of organic matter in a bucket of water, a potent brew is ready when it becomes the color of ice tea.
This kind of tea is called a leachate, and its nutritional makeup varies depending on the organic matter used. Emulsified fish parts, for instance, provide nitrogen and phosphorus while kelp is rich in micro-nutrients. Some use a raw manure leachate, but I discourage this for the obvious reason that the concoction could be a lethal stew of pathogens. In England, chopped comfrey leaves steeped in barrels become a smelly brew called Black Jack that is said to be especially beneficial to tomatoes. Even the composition of the compost starter affects the nutritional outcome of the brew. Compost made primarily from decayed tree byproducts, such as leaves, wood shavings, and sawdust, would be high in beneficial fungi and therefore better suited for shrubs and trees. Meanwhile, compost derived from vegetation and manure would result in a solution high in beneficial bacteria that would be best for vegetables.
Recently, there has been increased interest in actively aerated compost tea which differs from compost leachate in that a steady supply of oxygen during the brewing process stimulates the proliferation of beneficial, aerobic bacteria and fungi. Simply place four cups of aged compost in a pantyhose and suspend this in a five-gallon bucket of non-chlorinated water. A small aquarium air pump supplies oxygen through plastic tubes secured to the bottom of the container. The resulting tea is ready in 24 to 36 hours and is a potent fertilizer that also protects the plant from many fungal and mildew diseases. As with good watering practices in general, foliar sprays should be applied in the early morning so that the leaves do not remain wet for long. Make sure to cover both the top and underside of the leaves to achieve maximum benefits.
I intend to spray my plants regularly this season with actively aerated compost tea as a precautionary tactic to thwart bacterial spot and various fungal diseases that damaged my leafy greens, tomatoes and cucumbers last year.