When good tillage and the addition of organic matter and lime do not render available sufficient plant food, then the supply of available food may be increased by the application of manure and fertilizers. It will be seen that all these classes of properties are necessary to furnish all the conditions for root growth. The proper chemical conditions require the presence of both physical and biological properties and the biological work in the soil requires both chemical and physical conditions. From the farmer's standpoint the physical properties seem to be most important, for the others are dependent on the proper texture, moisture, heat and ventilation which are controlled largely by tillage. Therefore the first effort of the farmer to improve the fertility of his soil should be to improve his methods of working the soil. Every one of these properties of the fertile soil, and consequently every one of the conditions necessary for the growth and development of plant roots, is influenced in some way by every operation performed on the soil, whether it be plowing, harrowing, cultivating, applying manure, growing crops, harvesting, or anything else, and the thoughtful farmer will frequently ask himself the question: "How is this going to effect the fertility of my soil or the conditions necessary for profitable crop production?" MAINTENANCE OF FERTILITY The important factors in maintaining or increasing the fertility of the soil are: The mechanical operations of tillage, especially with reference to the control of soil water. The application of manures and fertilizers, especially with reference to maintaining a supply of humus and plant food. Methods or systems of cropping the soil, with reference to economizing fertility. CHAPTER XVII SOIL WATER The more important tillage tools and tillage operations we studied in Chapters XI and XII. They will be noticed here only in connection with their influence over soil water, for in the regulation of this important factor in soil fertility the other conditions of fertility are also very largely controlled. IMPORTANCE OF SOIL WATER "Of all the factors influencing the growth of plants, water is beyond doubt the most important," and the maintaining of the proper amount of soil water is one of the most important problems of the thinking farmer in controlling the fertility of his soil. NECESSITY OF SOIL WATER The decay of mineral and organic matter in the soil, and the consequent setting free of plant food, can take place only in the presence of moisture. The plant food in barn manures and crops plowed under for green moisture, can be made available only when there is sufficient moisture in the soil to permit breaking down and decomposition. The presence of moisture in the soil is necessary for the process of nitrification to take place.