A part of the remainder sinks into the soil below the surface. A large portion of this helps to cause the slow rock-decay that forms the soil, and which is known as ground water. It is estimated that within the first hundred feet below the surface of the earth there is a quantity of water that has seeped down; and that would form, if it were collected, a vast reservoir sixteen or seventeen feet in depth spreading over all the 3,000,000 square miles of the area of our country. This is equal to about seven years' rainfall and is a very important part of our water resources. In many places it forms into underground streams or lakes. It feeds all the springs and many of the lakes. Our wells are dug or drilled into this underground water system. It carries away the excess of salts and mineral matter from the soil, the trees strike their roots deep into the earth and draw from it, and last and most important of all, that which sinks immediately below the surface supplies all our plant growth. So that it is this last portion, that which sinks below the ground, and which is sometimes termed the cut-off, amounting to about one-tenth of all our water resource, or about the quantity that flows through the Mississippi River system, that forms the really important part. On this depends all that makes a land habitable, the water for drinking purposes and for plant and animal growth. On it depends the rate of production of every acre of farm and forest land and the life of every animal. Every full-grown man of one hundred and fifty pounds takes into his system not less than a ton of water each year, and every bushel of corn requires for its making fifteen or twenty tons of water. Of the importance of this Professor Chamberlain says: "The key to the problem of soil conservation lies in due control of the water that falls on every acre. This water is an asset of great value. It should be counted by every land owner as a possible value, saved if turned where it will do good, lost if permitted to run away, doubly lost if it also carries away the soil and does destructive work below." The uses of rainfall are given thus: A due portion should go through the soil to its bottom to promote rock decay. Some of it should go into the underdrainage to carry away harmful matter, another portion goes up to the surface carrying solutions needed by the plants. A portion goes into the plants to nourish them, and still another part runs off the surface, carrying away the worn-out parts of the soil. Crops can use to advantage all the rain that falls during the growing season; and in most cases crops are all the better for all the water that can be carried over from the winter. There are many local exceptions, but in general crops are best when the soil can be made to absorb as much of the rainfall and snowfall as possible. This also causes the least possible amount of wash from the land.