Before entering into a practical discussion of the subject of irrigation the student or the reader should fully realize what an important part water plays in the growth of plants. (1) It is a powerful solvent of plant foods. No matter how fertile a soil may be naturally, or how much manure or fertilizer may be added, such foods are valueless without water to dissolve them. Both stable manures and fertilizers often fail to give increased yields because of insufficient soil moisture. (2) Water not only serves as a solvent, but it holds in solution organic acids which are more powerful solvents than water alone. (3) Water is essential to the life of friendly bacteria. (4) Water serves as a vehicle in the distribution of plant foods in the soil.
Its functions in the plant are equally important. (1) Water enters largely into the composition of all garden products. Many vegetables contain over 90 per cent of water. (2) Water is also a medium in the conveyance of food in the plant, so that enormous quantities transpire from the leaves. Several hundred pounds of water are required to produce every pound of dry matter. It is claimed that a well-developed hill of cucumbers will use half a barrel of water in three days. (3) The transpiration of water has an important relation to the fixation of carbon from the atmosphere.