CHAPTER IV

SOIL AS A SOURCE OF PLANT FOOD

Mineral Constituents of the Plant.-There still remains to be considered the mineral matter found in the ash, or that material which remains when the organic part of the plant is destroyed by burning and which corresponds exactly to the ashes left in the stove after burning wood. The substances found in the ash are all derived from the soil. It has not always been thought that they were necessary to plant growth. The earlier writers on agriculture considered only the organic matter of the soil and certain constituents of the atmosphere as of any importance to the plant. These writers thought the presence of mineral matter merely accidental, and due to the fact that the plant took it because it was dissolved in the necessary soil water, and had no way of rejecting, or removing it. Later writers, however, preeminent among whom was Liebig, proved that the ash ingredients are necessary to the plant. A very simple experiment was sufficient to show that at least some of the mineral matter was essential to plant growth. Seeds were planted in quartz-sand in pots, to one of which nitrogen compounds alone were supplied, and to the other, nitrogen and a small amount of plant ash. The plants in the pot which received the ash grew to maturity, while those in the other pot made only a feeble, short lived growth.

Essential and Non-Essential Elements.-The experiment just described proves that there is something in the ash that is required by the plant, but does not show whether only a part or all of the ingredients are

5 6 7 8 9 10 11 12 Experiment to show the essential elements of plant food. Numbers 4, 7, and 11 received all the elements of plant food while one element was withheld in each of the other tests

essential. This question naturally interested a number of investigators, and soon a mass of evidence was at hand. In order to determine which elements are essential, plants were grown, either in specially prepared sand or by the "water-culture method," in such

a way that they were supplied with all the elements. occurring in plants, with the exception of the one element under investigation. If the plant grew to maturity the element which was missing was deemed nonessential. If, on the other hand, the plant failed to develop, that particular element was considered to be essential.

The numerous experiments of this kind which have been carried on show that of the ash constituents potash, lime, phosphoric acid, magnesia, iron and sulphuric acid are absolutely essential to plant growth. Toward soda, chlorine and silica plants seem to be indifferent, as they can grow to maturity in the absence of these substances. For this reason it is generally conceded that only ten of the thirteen elements found in the plant are essential to growth, soda, chlorine and silica being thought non-essential. Accepting this view and referring again to the table on page II it is seen that 1,000 pounds of corn plant contain only 9 pounds of essential mineral matter or about 0.9 per cent. Attention is called to the fact that these experiments extended over only one generation, and that it is possible that an attempt to grow the crop through successive generations in a soil devoid of soda, chlorine or silica might show different results.

One Element can not be Substituted for Another. -The experiments mentioned above have shown, not only that certain chemical elements are necessary to plant growth, but also, that it is not possible to replace these essential elements even by others which are similar in chemical properties. In the chemical laboratory, for example, it is found that soda and potash are very

much alike in their action, and one may be used in place of the other in many operations. It would be a good thing for agriculture if soda could be substituted for potash as a plant food, as compounds of sodium are very inexpensive compared with potash compounds.

Spinach: a full ration soda; b full ration potash

One element of plant food cannot take the place of another in promoting plant growth. The pile on the right shows spinach grown with complete fertilizer. The pile on the left received the same fertilizer with potash replaced by soda

This point has been thoroughly investigated, and it has been demonstrated that soda can not take the place of potash as a fertilizer. As a definite amount of each of these elements is required for a certain yield, and none of the elements can be replaced by another, it seems to follow that the crop produced will be limited by the quantity of the essential element present in least proportion, compared with the requirements of the crop. In other words, if a field of corn can obtain

potash sufficient for only half an average crop, no more than this can be produced no matter how much of the other forms of plant food is present.

How the Mineral Matter Enters the Plant.-It seems evident that the mineral matter must be taken up in some way by the roots. All are familiar with the fact that the soil is not a solid mass but consists of small particles, or "grains," with air spaces between, these spaces in the surface foot amounting to fully half the bulk of the soil. These grains vary in size according to the character of the soil, being very fine in clay, and comparatively coarse in sandy soils. The roots of the plant push down between these soil grains, branching more or less, and spreading throughout the soil. Surrounding the growing tip of the root are great numbers of fine root hairs that work their way in between and around the small soil grains, adher

ing closely to them and Root-hairs on wheat when very young

covering an immense amount of surface. It is on these root hairs that the plant is dependent

and four weeks later. All the water and food from the soil enter the plant through the root-hairs. Note how closely the root-hairs adhere to the soil particles. (After Sachs)

for the absorption of its water and mineral food.