Finally, it must be admitted that there are a great many more or less poorly understood or unknown forces at work in all soils which aid in the maintenance of soil-fertility. Chief among these are the low forms of life known as bacteria. Many of these, under favorable conditions, appear to have the power of liberating food from the insoluble soil grains. Others have the power when settled on the roots of leguminous or pod-bearing plants to fix nitrogen from the air and convert it into a form suitable for the need of plants. In recent years it has been found that other forms of bacteria, the best known of which is azotobacter, have the power of gathering nitrogen from the air and combining it for the plant needs without the presence of leguminous plants. These nitrogen-gathering bacteria utilize for their life processes the organic matter in the soil, such as the decaying header stubble, and at the same time enrich the soil by the addition of combined nitrogen. Now, it so happens that these important bacteria require a soil somewhat rich in lime, well aerated and fairly dry and warm. These conditions are all met on the vast majority of our dry-farm soils, under the system of culture outlined in this volume. Hall maintains that to the activity of these bacteria must be ascribed the large quantities of nitrogen found in many virgin soils and probably the final explanation of the steady nitrogen supply for dry farms is to be found in the work of the azatobacter and related forms of low life. The potash and phosphoric acid supply can probably be maintained for ages by proper methods of cultivation, though the phosphoric acid will become exhausted long before the potash. The nitrogen supply, however, must come from without. The nitrogen question will undoubtedly soon be the one before the students of dry-farm fertility. A liberal supply of organic matter In the soil with cultural methods favoring the growth of the nitrogen-gathering bacteria appears at present to be the first solution of the nitrogen question. Meanwhile, the activity of the nitrogen-gathering bacteria, like azotobacter, is one of our best explanations of the large presence of nitrogen in cultivated dry-farm soils. To summarize, the apparent increase in productivity and plant-food content of dry-farm soils can best be explained by a consideration of these factors: (1) the intrinsically high fertility of the arid soils; (2) the deep feeding ground for the deep root systems of dry-farm crops; (3) the concentration of the plant food distributed throughout the soil by the upward movement of the natural precipitation stored in the soil; (4) the cultural methods of dry-farming which enable the weathering agencies to liberate freely and vigorously the plant-food of the soil grains; (5) the small annual crops; (6) the plowing under of the header straw, and (7) the activity of bacteria that gather nitrogen directly from the air. Methods of conserving soil-fertility