pecially in soils
of a very compact texture, or "water-logged" soils. In such cases, the
deficiency of these gaseous food elements may become a limiting factor in
plant growth.

Water is often a limiting factor in plant growth. Experiments which have
been repeated many times and under widely varying conditions show that when
water is supplied to a plant in varying amounts, by increasing the
percentage of water in the soil in which the plant is growing by regular
increments up to the saturation point, the growth of the plant, or yield of
the crop, increases up to a certain point and then falls off because the
excess of water reduces the supply of air which is available to the plant
roots. Hence, abundance of water is, in general, a most essential factor in
plant growth.

Under normal conditions of air and moisture supply, however, the plant food
elements which may be considered to be the limiting factors in the
nutrition and growth of plants are the chemical elements mentioned in the
list above.

AVAILABLE AND UNAVAILABLE FORMS

The plant food materials which are taken from the soil by a growing plant
must enter it by osmosis through the semi-permeable membranes which
constitute the epidermis of the root-hairs, and circulate through the plant
either carried in solution in the sap or by osmosis from cell to cell.
Hence, they must be in water-soluble form before they can be utilized by
plants. Obviously, therefore, only those compounds of these elements in the
soil which are soluble in the soil water are _available_ as plant food. The
greater proportion of the soil elements are present there in the form of
compounds which are so slightly soluble in water as to be _unavailable_ to
plants. The processes by which these practically insoluble compounds become
gradually changed into soluble forms are chiefly the "weathering" action of
air and water (particularly if the latter contains carbonic acid) and the
action of the organic acids resulting from decaying animal or vegetable
matter or secreted by living plants.

THE VALUE OF THE SOIL ELEMENTS AS PLANT FOOD

Analyses of the tissues of plants show that they contain all of the
elements that are to be found in the soil on which they grew. Any of these
elements which are present in the soil in soluble form are carried into the
plants with the soil water in which they are dissolved, whether they are
needed by the plant for its nutritio