Green plants, straw or leaves, when plowed under or spaded in the soil, are attacked by bacterial agencies which gradually turn these products into humus. The same process occurs when a "compost" is set up. This is made of leaves, manure, soil, straw and other materials thrown into a heap and allowed to decay. Such compost is excellent for placing around plants when setting them out, since it holds moisture, supplies fertility and creates optimum conditions for young root growth. Under practical field conditions, humus may be added to soils by spreading animal manures, followed by plowing them down, or by the growing of heavy green crops such as wheat, rye, cowpeas or vetch and turning the entire mass under with the plow when they are at their height. _Lime and Its Application._--Reference has been made to the fact that calcium is an essential plant food and is frequently deficient in soils. As a matter of fact, the great majority of soils are deficient in calcium and their productiveness is inhibited thereby. Lime supplies calcium and also magnesium as food for plants. Its application accomplishes many other desirable things such as correcting soil acidity. The growth of beneficial bacteria is greatly stimulated in a soil that has had its acidity neutralized by the application of lime. This product, therefore, creates a more congenial condition for the growth of bacteria, which, in turn, make for better crop production. Lime is also beneficial through furnishing the element calcium with which other plant foods combine chemically and thereby become soluble in the soil water. Unless plant foods are in a state of solution, they cannot be absorbed by plant roots. Lime is a potent force in creating chemical reactions in the soil, resulting in the stimulation of growth through increased absorption of essential elements in solution. Lime also benefits soils of a clayey nature through its ability to cement together the fine clay particles and in that way create air spaces so greatly needed in tight clay soils. Lime is beneficial, too, in the case of soils which have a large proportion of sand or large particles, and serves as an agent in creating a better condition of tilth and of moisture retention. It makes little difference in what form lime is applied. It may be purchased and applied in the form of ground limestone, a rock rich in calcium which has been mechanically ground to a very great degree of fineness. It can also be applied in the form of hydrated lime. This is obtained by heating ground limestone and slaking it by adding water. A common example of this is the slaking of lime for whitewashing purposes. Another good source of lime is finely ground shells of oysters or other forms of sea life which collect the calcium from sea water and deposit it in their shells.