The function of nitrogen is to build up the green leaves and stems of the plant. When nitrogen is withheld, the leaves tend to be light green in colour and smaller in size. When nitrogen is applied to growing crops in a quickly available form, the speed at which the plant is growing will increase.
An overdose of nitrogen will cause coarse, strong, luxuriant, and sappy growth. This makes the plant liable to attacks of fungus diseases and delays ripening of the crop.
Nitrogen, then, is useful for crops where one desires large, succulent green leaves, as in the case of cabbages, but must not be withheld from all crops.
Phosphorus (or phosphates, as components of phosphorus are commonly called) seem to possess the power of causing fruitfulness, and is valuable in increasing root production. Plants starved of phosphorus make little root growth, and, in consequence, they suffer tremendously during periods of drought. If you want to keep your plant growing steadily, firmly, and continuously, then, phosphates will help you. Another point in favour of this group is that it tends to hasten the ripening of plants.
The main value of potash is in the way it "harden up" a plant. Soils that have had sufficient potash applied to them grow plants with firmer leaves, more resistant to disease, and generally hardier altogether. Potash, for instance, assists in forming strong fibre, and with fruit it certainly plays a part in producing fruits of a better colour which tend to keep longer. It is useful in the case of lighter soils, as these are normally deficient in this food. It is even claimed that potash appears to increase the scent in flowering plants.