PRESS BULLETIN 128
Florida Agricullural Experimecn Slallon
HOW FERTILIZERS INJURE CITRUS TREES
By B. F. Floyd
It sometimes happens that ill effects follow the use of fertilizers in
citrus groves. This may result from two causes-the solution of the fer-
tilizer in the soil may be too strong for the tender rootlets, or it may
act as a true poison.
Absorbing Portion of the Rootlets
All materials obtained from the soil by the orange trees are taken in
through the rootlets, dissolved in water. Insoluble fertilizers, like cotton-
seed meal, are first changed by partial decay into soluble salts, such as
nitrate of lime. In most cases only a small portion of the rootlet can take
water and dissolved material from the soil. This part begins just back
of the tip, and is usually only a fraction of an inch long. Usually this
absorbing part is covered with minute root-hairs, which increase the surface,
and allow more material to be taken from the soil. The remainder of the
rootlet is often covered with brown cork, and is more or less waterproof.
This part serves only as a conductor for the raw material (water and salts)
taken in, and for the food which passes back for the use of the growing
parts near the root tips.
Structure of the Rootlets
Under the microscope the rootlet is seen to be a mass of cells, each of
which usually consists of a cell-wall, enclosing the living substance (proto-
plasm) and the cell sap. Let us suppose a mass of shoe-boxes placed end
to end, in close contact, in such a way that the whole would appear in out-
line as a rootlet many times enlarged. Suppose each of these boxes to be
lined half an inch thick throughout with a substance similar to the white of
an egg, and completely filled with water; also the walls of the boxes to be
If this mass of boxes was then reduced in size until it was as small as
an ordinary rootlet, the boxes could only be seen by the aid of the micro-
scope, and each box would correspond to a cell. The wall of the box would
be the cell-wall; the white of egg, the protoplasm; the water, the cell-sap;
and the water-soaked condition of the walls of the boxes would be similar
to that of the cell-walls of the root.
October 23, 1909
The cell-wall is dead matter and serves mainly for support and pro-
tection. The cell-sap is a watery solution of sugar, salts, and other ma-
terials. The living protoplasm, which surrounds the cell-sap, exercises a
selective power, by allowing only certain salts to enter and only certain
ones to escape.
Absorption of Water
The ability of the rootlets to take up water and distribute it through
the plant depends mainly upon the strength or concentration of the cell-
sap. The rootlets absorb water when the concentration of the cell-sap is
greater than that of the soil solution. When the soil solution is the more
concentrated, water is drawn from the cell-sap, and the tree wilts. A soil
solution stronger than one-half per cent. is usually considered to be too
strong for the health of plants. A heavy application of certain readily solu-
ble fertilizers or other salts (such as nitrate of soda, or common salt), may
bring on this condition. When the action is severe, the roots are some-
times said to have been burnt.
Water is also absorbed and carried in the cell-walls. But the amount
thus obtained is not nearly sufficient to maintain the tree.
Absorption of Fertilizers
Fertilizers and other salts in solution in the soil water enter the rootlets
by diffusion. Large quantities of water may be taken into the tree and only
a small quantity of the fertilizers or other salts that were in the solution
be carried along. If the quantity per volume of any particular salt in the
cells of the root is equal to or greater than that in the soil solution, the
rootlets are probably unable to take up any of it from the soil. Thus, an
orange grove may suffer from the lack of a particular fertilizer, though
there may be a small quantity of it present in the soil.
Effects of Poisonous Salts
Some salts may act as poisons, and, in certain quantities, kill the proto-
plasm with which they come in contact. On the other hand, very small
quantities of these salts may act as a stimulus when absorbed. An example
of this is copper sulphate or bluestone, which is used so generally in the
treatment of dieback. This salt is placed on the soil or beneath the bark.
When placed beneath the bark, its poisonous effect is evident from the
large wounds that are produced. The course of the salt, as it diffuses up
and down the stem, can be traced by the extension of these wounds. An
excessive quantity placed upon the soil will kill the tree. This excessive
quantity is very small in comparison to the amount of a fertilizer that would
be required to injure the tree.
State papers please copy.