The main object of manuring is to restore to the soil, in a more or less available form, the foods that have been taken out of it by the growth of crops. It is evident that if everything taken from the soil were again replaced, there would be no loss at all. But if all the crops grown were put back again, there would be far more material returned to the soil than ever came out of it. There is still a popular impression that the entire weight of a crop comes from the soil, and from the soil only. The air and water get but very little credit for the important part they play in providing, after all, by far the greater weight of every plant. Water itself may be looked upon not only as the means by which foods from the soil are drafted to all parts of the living tissues, but also as a distinct food or manure, as it supplies both oxygen and hydrogen.

Before we consider the application of manures to the soil it is necessary to refer again to van Helmont's experiment with the Willow, already described at p. 108. That experiment clearly proved that the great bulk of a plant's weight came, not from the soil, but from the air and from water. Carbonic acid gas (of which there are only 4 volumes out of 10,000 in the atmosphere) supplies all the carbon that is necessary, so long as the cultivator is sensible enough to allow his plants sufficient space and light, and does not overcrowd them. This carbon, which makes up the great bulk of the dry weight of every plant, is obtained absolutely free of charge from the air, and there is not the slightest danger of the supply becoming exhausted. The carbon from the air and the water from the soil make up from 95 to 99 per cent of the weight of all plants, thus leaving from 1 to 5 per cent of material to be provided by the soil alone. Perhaps this fact will be made more clear by the following analysis of the wheat plant, taken from Sowerby's Thorough Cultivation: -

Composition Of Wheat Plant

From this table it becomes evident that the art of manuring the soil is narrowed down in a very remarkable degree. As with wheat, so with other crops. The cultivator has not to concern himself with providing oxygen, carbon, or hydrogen so long as he allows his plants plenty of fresh air and a proper supply of moisture. He is actually relieved of the burden of finding over 93 1/2 per cent of the material which makes up his crops. The other 3.386 per cent, consisting of soda, magnesia, sulphuric acid, chlorine, oxide of iron, silica, and manganese, he also very rarely has to trouble himself about, as they are generally present in great quantities in the soil. But he must remember that those inorganic foods can only be liberated and brought into an available form by the constant use of the spade, the fork, the plough, the hoe, &c; in fact, by cultivation or tillage operations. If the soil is not cultivated, these foods remain dormant, inactive, and insoluble, and therefore worthless to any crop.

Having 97 per cent of the bulk of his crop practically provided free of charge, except for labour, the cultivator has to devote his energies to supplying the other 3 per cent, made up of nitrogen, phosphoric acid, potash, and lime. Now, the soil is by no means deficient in these foods. In the Broadbalk Field, Rothamsted, it has been found that a soil which had been cropped, but had not been manured for fifty years, still contained 2500 lb. of nitrogen, 2750 lb. of phosphoric acid, 6750 lb. of potash, and 62,250 lb. of lime to the acre at a depth of only 9 in.

These figures are remarkable, and cultivators would do well to remember them. If a soil that has been cropped, but has been unmanured for fifty years, still contains such large quantities of the most important plant foods, it ought to follow as a matter of course that a soil which has been cropped and has also been manured for the same period should show far larger quantities of these particular foods. Such, however, is not the case, as the experiments carried out at Rothamsted prove. The addition of certain manures often has the effect of liberating too freely some of the plant foods, and as they cannot be absorbed by the crop, they are lost in some way, or at least cannot be accounted for (see p. 128). '

To appreciate all the factors in the case it is necessary to remember what has been already emphasized, that only very small quantities of nitrogen, phosphoric acid, potash, and lime are taken from the soil. It has been estimated that fruit trees and ordinary farm crops take from the soil from 50 to 100 lb. of nitrogen, 20 to 50 lb. of phosphates, 30 to 150 lb. of potash, and 150 to 200 lb. of lime. Comparing the figures with the supplies still remaining in the Broadbalk Field at Rothamsted after fifty years, and with the supplies that are said to be in a fertile soil, it is evident that only small quantities are liberated as food for each crop. The usual deduction made is, that as these supplies of nitrogen, phosphates, potash, and lime are to be found in a soil after fifty years, therefore they are regarded as unavailable and probably useless. This view seems to be quite erroneous. Why should these vast supplies become immediately soluble? Would it not be a dire calamity if they were to become so, and if they vanished in one season? The result would be complete and absolute sterility, and succeeding crops would have to starve. Apart from this, it would be a physical impossibility for any crop to take up or absorb 62,250 lb. of lime, 6750 lb. of potash, 2750 lb. of phosphates, and 2500 lb. of nitrogen - altogether, 74,250 lb. (over 33 tons) per acre. In a Turnip crop weighing 33 tons, only about 2 or 3 per cent of the dry weight, say from 1400 to 2000 lb. would come from the soil, the remainder coming mostly from the air and water.

In the Bulletin (No. 103) of the Cornell University, U.S.A., for October, 1895, the following interesting figures appear in connection with an experiment on some Wagner Apple trees - thirty-five to the acre.