THE CROPS IN A ROTATION 511 



little danger of the exhaustion of this element, but Nature's 

 supply of potassium and phosphorus must eventually be 

 supplemented. 



Live stock farming aids in conserving these elements, for 

 live stock products remove much less of them than grains, 

 hay, and cotton. If the manure is properly handled and 

 returned to the land, the exhaustion of the soil will be very 

 slow, but it will be constantly taking place. The products 

 which are sold will remove some of the potassium and phos- 

 phorus, while there will also be a considerable loss by 

 leaching from the soil and from the manure. Some phos- 

 phorus and potassium should occasionally be added from 

 outside sources in the form of purchased feeds or of fertilizers 

 in order to maintain or to increase the fertility of the soil. 



WHAT A ROTATION SHOULD CONTAIN 



669. Classes of Crops in a Rotation. So far as their 

 arrangement in a rotation is concerned, field crops may be 

 divided into grass, grain, -and intertilled, or " fallow," crops. 

 Grass crops include all the plants which are grown in meadows 

 and pastures, such as the perennial forage grasses, clovers, 

 and alfalfa. These remain on the land for two or more 

 years and increase the supply of vegetable matter by the 

 mass of stubble and roots which they produce. All annual 

 crops not intertilled will be designated in this discussion as 

 grain crops. They are sown too thickly to allow intertillage, 

 and occupy the land but a few months. They exhaust the 

 supply of humus and plant food elements, and are also 

 exhaustive of soil moisture. This class of crops includes 

 wheat, oats, barley, rye, flax, buckwheat, millet, and all 

 annual forage crops similarly produced. Intertilled crops 

 are planted in rows wide enough apart to be tilled during a 

 large part of the growing season. They are also exhaustive 



