This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
By means of experiment it has been proved that all green-leaved plants at least require certain essential foods to enable them to perform their functions properly. Some of these foods are absorbed from the air under the influence of sunlight, and some are taken from the soil by the roots. These essential foods are -
Iron Soda Chlorine
These thirteen foods are found in all plants in varying proportions. The first four are gaseous and organic, and are driven from the plant by burning. The other nine are found in the ash of plants, and constitute the mineral or inorganic foods. When soluble in water, they are in a condition to be absorbed from the soil under favourable conditions.
Until about three hundred years ago it had been always thought that the soil supplied all the foods of plants and made up the great bulk of the tissues. Jean Baptiste van Helmont (b. 1577, d. 1644), a chemist of Brussels, was the first to disprove the old theory that all plant foods came from the soil alone. He planted a young Willow weighing 5 lb. in a pot containing 200 lb. of soil. He watered the plant daily with rainwater, and grew it for five years. He then weighed the plant and soil again and found that the Willow had increased from 5 lb. to 169 lb., but the 200 lb. of soil had lost only about 2 oz. Van Helmont therefore concluded that the extra 164 lb. weight of the Willow came from the water alone. In this he was wrong. He did not know that the invisible carbon in the air had anything to do with the increased weight of his Willow. Indeed it was not until a Dutch scientist, Jan van Ingenhuisz, published his researches in 1779 that it was discovered that the increase in weight was due to the carbon that had been assimilated from the atmosphere by the leaves during the daytime (see p. 44). It is evident, therefore, that only a very small proportion of plant food is actually taken from the soil itself. That little, however, is absolutely essential; and unless it is in a form easily dissolved in water, so that it may be absorbed by the roots, it is quite useless, and no growth can take place.
The figures on p. 109, compiled chiefly from Dr. A. B. Griffith's works, will give some idea as to the various mineral foods taken out of the soil by different fruit and vegetable crops. These foods must be all soluble in water, and the temperature of the soil must be favourable, otherwise the roots would be unable to absorb them. It will be noticed that the same food is taken up by different crops in different proportions, and there is often a great difference in the composition of the wood and the fruit on the same plant. It should also be stated that the results obtained by different chemists vary greatly, probably owing to the plants tested being taken from different soils and at different times.