. A civic biology : presented in problems. Biology; Sanitation. 82 SOIL AND ITS RELATION TO ROOTS posed to plant corn, the nitrogen left in the soil thus giving nourish- ment to the young corn plants. In scientifically managed farms, different crops are planted in a given field on different years so that one crop may replace some of the elements taken from the soil by the previous crop. This is known as rotation of The annual yield of the average farm may thus be greatly increased. Five of the elements necessary to the life of the plant which may be taken out of the soil by constant us
. A civic biology : presented in problems. Biology; Sanitation. 82 SOIL AND ITS RELATION TO ROOTS posed to plant corn, the nitrogen left in the soil thus giving nourish- ment to the young corn plants. In scientifically managed farms, different crops are planted in a given field on different years so that one crop may replace some of the elements taken from the soil by the previous crop. This is known as rotation of The annual yield of the average farm may thus be greatly increased. Five of the elements necessary to the life of the plant which may be taken out of the soil by constant use are calcium, nitrogen, phosphorus, potassium, and sulphur. Several methods are used by the farmer to prevent the exhaustion of these and other raw food materials from the soil. One method known as fallowing is to allow the soil to remain idle until bacteria and oxidation have renewed the chemical ma- terials used by the plants. This is an expensive method, if land is dear. The most common method of enriching soil is by means of fertilizing material rich in plant food. Manure is most frequently used, but many artificial fertilizers, most of which contain nitrogen in the. Nitrogen in the soil is necessary for plants. Explain from this diagram how nitro- gen is put into the soil by some plants and taken out by others. form of some nitrate, are used, because they can be more easily transported and sold. Such are ground bone, guano (bird manure), nitrate of soda, and many others. These also contain other important raw food materials for plants, especially potash and phosphoric acid. Both of these substances are made soluble so as to be taken into the roots by the action of the carbon dioxide in the soil. The Indirect Relation of this to the City Dweller. - - All of us living in the city are aware of the importance of fresh vegetables, 1 That crop rotation is riot primarily a process to conserve the fertility of the soil, but is a sanitary measure to prevent infection of the soil,
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