Contrary to what many gardeners think, direct absorption of such a complex molecule is beyond the capacity of any plant root. (One apparent contradiction of this statement is the capture and absorption of insects by certain insectivorous plants, such as the Venus Fly Trap and the Sundews. Insects are lured into special organs where they are caught and "digested" before being used by the plant as food. However, the digestive organ of these plant oddities is a specialized leaf in which protein is fermented into ammonia and nitrogen compounds. These can be absorbed by the leaf and used just as these compounds are used when absorbed by other plants through roots.)
To repeat (and it's worth repeating), the simple nitrogen compounds which garden plants always need, the phosphorus they use at certain stages of growth, the potash so vital to woody plants, the sulfur so often ignored in discussions of plant nutrition, and several other chemicals vital to plant growth must all be in soluble form. Otherwise they are as inaccessible to plants as if they did not exist. This does not mean that the best material is the most soluble. Often
we need controlled solubility-especially with nitrogen-to give a longer feeding period.
Of all food elements needed by plants, none is more important than nitrogen. It is popular to call the "nitrogen cycle" (a process by which nitrogen is used and reused, over and over again) the most important single biological process in the world. While the nitrogen cycle is vital to the continued existence of every living organism, it is, of course, only one of several such basic processes, none of which could be halted without destroying all life.
Nitrogen is so important to plant nutrition that its concentration in a given soil tends to be the #1 factor which controls growth. What we call a worn-out soil is often the result of farming or gardening practices which have exhausted native reserves of this vital element and made no provision for replacement. Nitrogen usually determines whether a soil is rich or poor, whether yields will be high or low.
One reason why nitrogen is so important is that it is essential to all tissues involved in growth and reproduction. Research has proved that the rate of growth in plants is more dependent upon this element than on any other single material.
WHERE DOES IT COME FROM?
We may talk glibly about organic versus inorganic nitrogen, but regardless of whether it occurs as part of animal or plant protein or as any other nitrogen compound, every atom of nitrogen came originally from the atmosphere. Once captured from the skies (whether precipitated by lightning or trapped by a nitrogen-fixing organism), nitrogen must be built into plant protein in order to be available to living organisms.