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"The farther we get away from the land, the greater our insecurity:" HENRY FORD.
Plants need four main chemical elements to enable them to grow: nitrogen, phosphorus, potassium and calcium. These elements are in the form of salts, and the plants dissolve them from the soil fragments by means of their own juices. There are about twenty additional chemicals that are also needed, among which are magnesium, sulphur, iron, manganese, boron, iodine, zinc, copper, and chlorine. These are required in such small quantities that they are termed trace elements. Usually they are present in sufficient amounts in the topsoil, humus or compost that makes up your potting soil; sheep-manure water brings some of them, as do the soluble chemicals mentioned in Chapter 6. If you think that your potting soil is lacking in one or another of them, compounds containing them-Esminel being one-may be added.
Nitrogen, phosphorus and potassium are the three elements that are in short supply in some soils, and we add them by using "fertilizers" from time to time. State and Federal laws require that all materials sold as fertilizers have their analysis displayed on the container, and the fertilizers themselves are often known in terms of this analysis: the first figure showing the number of pounds of nitrogen; the middle figure, the phosphorus; and the third figure, the potassium. Thus, 5-10-5 stands for 5 pounds of nitrogen salts, 10 pounds of phosphorus, and 5 pounds of potassium-20 pounds of actual plant food in 100 pounds of material.
Granted the fact that plants dissolve from the soil these chemical elements, it is natural to assume that all we need do is to supply them direct without soil, and our troubles will be over.
Several good formulas have been worked out to give a good culture solution, and gardensupply stores carry them. Be sure to use these only if it is stated clearly on the package that the contents will make a culture solution for chemical gardening.
One way to use these solutions, which will be very weak if you mix and dissolve them correctly, is to suspend or float the plants with their roots in a tank. Make rings of cork or light wood to serve as small rafts through which roots may be threaded. Another way is to place a cover of small-mesh chicken wire on the tank level with the surface of the solution. Work the roots through the spaces into the water.
One good arrangement is to have the plants growing in coarse sand, which has been sterilized by boiling, in a series of glazed containers and arrange to pipe the solution so that it will drip into each container. Have a siphon pipe with a spigot to remove the solution from the bottom.
If you use the floating or suspending system, the solution should be agitated every hour or so with a paddle; or a simple air pump will serve. Some fresh solution should be added every day, and the entire solution should be replaced at weekly intervals.
Some years ago hydroponics became popular; it was hoped that indoor gardening would now be a more exact process, with constant moisture and controlled feeding. It was thought that there would be no trouble with soil-borne diseases and insects. Although hydroponics is employed to a small degree in commercial horticulture, amateur chemical gardeners still have their problems with light and temperature, and much of the enthusiasm has been dissipated. The process is interesting; if you wish to study it, books and Government bulletins may be consulted.