Container or pot plants, should always be grown in an artificial, soilless medium. One of the consequences of this is that they are dependent on a consistent and reliable feeding regime to a far greater extent than the garden plants growing in regular soil.
The best potting "soils", or media, available today are those based on perlite, a natural glass that has been excavated and processed to form the white, ultra light product sold in the garden centers. It is used by landscape professionals mainly because of its light weight, an important consideration in roof and balcony gardening, and because it supplies an almost perfect balance between air and moisture. Another property is that it is chemically inert. While this is generally considered an advantage, it does create a total dependency on readily soluble nutrients being available to the plants. The gardener must take this fact into account when choosing which feeding regime to adopt for the container plants.
The use of compost and other organic materials, while essential for the health, aeration, and mineral balance of garden soils, is virtually irrelevant when growing plants in perlite. The name of the game, unpalatable perhaps to organic gardeners, is simply to ensure a ready supply of mineral nutrient, in other words, chemical fertilizer, at all times. How is this best done?
Clearly, hand fertilizing with the highly soluble products such as Ammonium Sulphate or 20-20-20, is highly laborious and time consuming. For while one can fertilize about once a month with the peat-based potting mixtures, plants growing in perlite, by virtue of it being chemically inert, require spoon feeding at least once every two weeks, if not once a week. The majority of home gardeners will tire of such a regime sooner rather than later.
An elegant alternative is to install a fertilizer pump that adds soluble fertilizer at very low concentrations, to the water that is emitted by the drip irrigation system. This method, known as fertigation, is undoubtedly the most convenient, efficient, and successful way of supplying nutrient to pot plants growing in a soilless, inert mixture such as perlite. There are however a few drawbacks that should be noted.
*Many states require the installation of an expensive backflow, prevention device that has to be approved yearly by a qualified person, rather like a road vehicle license. Failure to do so can lead to criminal procedures in some countries.
*In mild winter climates, plants continue to grow and therefore require some feeding during the winter, albeit at a reduced level. This often results in the need to open the irrigation system (the fertilizer is applied via the irrigation system) even when there is no need to do so – an unacceptable waste of water, especially in dry climate gardens.
An alternative to both fertilizer pumps and readily soluble fertilizers is slow, or controlled release fertilizers. These release the essential nutrients over a set period, ranging from two months to one year. They require some labor, as the pellets have to be inserted into the soil, but this only has to be done a couple of times a year.
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