128 HORTICULTURE FOR SCHOOLS 



necessary to keep down weeds and to maintain a dust mulch, 

 which acts as a blanket in conserving moisture. 



212. Cover-crops. The incorporation of organic matter 

 in the soil to better its physical condition and to increase 

 plant-food is desirable on nearly all land. Where it can be 

 secured, there is nothing better than stable manure. When 

 it cannot be obtained, the growing of cover-crops may take its 

 place to a certain extent, especially if legumes are grown to 

 add nitrogen to the soil. To be effective, the cover-crop 

 should be plowed under to add organic matter. There must 

 be an abundance of moisture in the soil in order that the 

 cover-crop may not rob the trees of their moisture supply. A 

 cover-crop must be of such nature that it will grow well 

 during the time when water is plentiful, either in the rainy 

 season or when irrigation water can be obtained. In many 

 hot semi-arid regions, where water is too scarce to grow cover- 

 crops, cultivation to conserve the moisture results in burning 

 out of the humus in the top soil. For this there seems as yet 

 to be no satisfactory solution. 



213. Inter-cropping consists in growing a money crop 

 between the trees before the latter come into profitable 

 bearing. Whether or not the grower can afford to crop his 

 orchard in this manner depends on the fertility of the land 

 and the crops he can grow. The inter-crop should not come 

 closer to the trees than six or eight feet in any instance. 

 Some common inter-crops are peas, beans, and other legumes, 

 potatoes, pumpkins, squashes, and strawberries. In irrigated 

 sections, the inter-crops are sometimes over-irrigated at the 

 expense of the trees. 



214. Fertilizers. Before taking up the subject of fertil- 

 izers for orchards, it will be necessary to consider fertilizers 

 in general. It has been found by experiments that the plant- 

 foods most likely to be lacking in the soil are nitrogen, phos- 

 phorus, and potassium. A commercial fertilizer contains some 

 one or more of these elements in the form of a compound 



