Nitrogen, phosphorus and potash are the "big three" of the many nutrient elements needed in the soil by plants for proper growth. These three are listed by numbers in that order on every package or bag of fertilizer (thus a 5-10-5 fertilizer product contains 5 per cent nitrogen, 10 per cent phosphorus and 5 per cent potash or potassium).
These "plant foods" do not occur as pure elements but as compounds with other chemicals. These compounds may be simple or quite complex but they share one quality-they can be attacked by bacteria, fungi and other organisms and broken down into quite simple products which plants can absorb. A fundamental quality of these simple products is that they must be soluble in water.
THE WAY PLANTS FEED
If we are to understand how plants feed, an old misconception must be discarded at once. Plants cannot "eat" the way animals do. Plants have no alimentary canal, no means of using undigested organic compounds such as protein, bone, straw and other "foods" applied to soil to supply them with nutrients. Even after such materials have gone through thorough decomposition in a compost pile, they may need to be broken down or rotted still more before their complex protein can be reduced to "available food" for plants.
Whether we call this process "decay," "digestion" or "organic breakdown," it involves exposing plant and animal wastes and byproducts to soil organisms. These use part of the foods for their own life processes, but leave behind less complicated materials as end
products. These simpler materials are water soluble and can be taken up by roots of plants.
Another widely believed misconception is that plants reach out for fertilizer in the soil, drawn by some force which tugs at the roots. Vivid proof that this is not so was shown to me in the studio-greenhouse of John Nash Ott who photographs plant growth in lapsed time. Plants were growing in a soil-filled box with a glass front which allowed a clear view of the roots. Fertilizer was placed here and there in the soil. But the roots grew in various directions, apparently unaffected by the fertilizer. In fact, it could be seen that some roots had passed within a fraction of an inch of one concentration of fertilizer.
Perhaps the most complicated substance plants can use directly is ammonia, a relatively simple molecule of nitrogen and hydrogen. Rhododendrons and other acid-soil plants can use ammonia directly, but many others require it to be broken down still further into nitrate nitrogen.
Contrast this simple chemical with the protein molecule, so complex that it offers all-but-insurmountable obstacles to chemists attempting to synthesize it.