TlMKLY HlMTb KOK FaKMKKS. 67 



nish a supply of green peas. The best varieties for this purpose 

 were found to be Yorkshire Hero and Champion of England. 



The White sweet-clover already mentioned makes a luxuriant 

 growth during the summer, but grows little during the winter 

 when the trees are dormant. For summer growth it seems to be 

 no better than alfalfa. If permitted to grow two seasons, it dies 

 at the end of the second. It may be plowed under green, or sim- 

 ply be allowed to die and decay upon the surface. 



The benefits from the winter-grown green-manuring crops 

 are four-fold at least. In the first place, the soil is thus covered 

 during a portion of the year and the exhaustion of decayed vege- 

 tation by the heat of the sun retarded. In the second place, plant 

 food that would otherwise be washed away by rains and irrigation 

 is appropriated by growing plants, and thus saved for the tree. 

 One of the most important benefits is the improved physical con- 

 dition of the soil, due to the decaying of the green matter turned 

 under. This causes the soil to bake less after irrigation, and to 

 hold its moisture longer. 



The chief benefit to the trees comes from the addition to the 

 soil of nitrogen, one of the most important plant foods. The ni- 

 trogen added is derived from the air mixed with the soil. Most 

 plants are powerless to. use nitrogen from the air with which they 

 are surrounded, notwithstanding the fact that the latter contains 

 about 80 per cent of this element. The members of the pea fam- 

 ily (peas, beans, clovers, alfalfa, etc.) are an exception. They 

 harbor upon their roots colonies of microscopic plants called bac- 

 teria which have the ability to absorb nitrogen and pass it along 

 to the plants to which they are attached. The irritation produced 

 by these colonies of bacteria causes the formation of small modules 

 or knots by which their existence upon the plants may be known. 



The plowing under of the plants that have secured their ni- 

 trogen elsewhere than from the soil adds to the latter the nitrogen 

 thus secured, and in addition benefits the soil in the w r ays men- 

 tioned above. 



It has been found by W. M. Ward, one of the leading or- 

 ange growers near Phoenix, that, after sowing the Yellow sweet- 

 clover seed one or two seasons in an orchard, enough seed will 

 mntnre under the trees, and in other places not reached by the 



