The continuous growing of crops naturally results in the exhaustion of the soil; day by day each plant is assimilating food from the soil, and at the end of the year the garden is deficient in active plant food. The keen gardener endeavours to prevent this weakening of the feeding power of the soil, and by means of natural and artificial manures, he carefully maintains and improves the condition of the soil. On the other hand the feeding of the soil may be neglected; then partial starvation of the plants is inevitable and certain, and the value of the ground is lessened. Money invested on manures is money well spent ; false economy in this direction is the path to failure.
We have already shown that the soil is simply a food storage for the plants, but before the nutrient matter is available it must be soluble, since plants feed by imbibition alone. There may be a large supply of insoluble food matter, but this is of no value except as a reserve supply which, under the combined action of soil working, water and air, is transformed into an active supply.
The three most important constituents of this food supply are nitrogen, phosphoric acid and potash, and manures containing these three important foods are readily obtainable.