When we talk about fertilizers and plant foods, we’re really talking about the same thing. Plant foods provide the same nutrients to the soil as do manure and other fertilizers, but in concentrated form.
Like any other living thing your house plants must absorb foods in order to live and grow. The foods they take in come in two forms and from two sources, 1) from the air in the form of gasses which are “breathed” in by the foliage, the most important gas to plants being carbon dioxide. 2) from the earth in the form of soluble minerals which are absorbed with water by the roots. There is not much we can do about the constituents of the air, but we can control to some extent, the chemical and mineral make-up of the soil.
No matter how good the soil is you use in your pots, it is inevitable that sooner or later it is going to need some supplements to do a proper job of feeding your plants. Soil out in the open is replenished year after year with decaying vegetable matter, shifting topsoil, and natural fertilizers; but your potting soil, isolated as it is indoors, has no chance for natural replenishment. This is where fertilizers and plant foods come into the picture. The function of the fertilizer and the plant food is to put back into the soil those mineral nutrients which have been depleted by constant use. Although all soils are made up of hundreds of ingredients, the ones most necessary for growth are nitrogen, phosphoric acid, and potash. These ingredients are to be found in varying degrees in all organic animal manure, and, of course, to a much more important degree in commercial plant foods.