THE MONTHLY BULLETIN. 399 



Stable manure is also produced in limited quantities. At best, only 

 a portion of the growers will ever be able to use it excliisively as a 

 source of nitrogen. Nitrate of soda is at present the cheapest source 

 of nitrogen but seems to have factors which make it less desirable than 

 an organic source of nitrogen. 



It would seem then that a nitrogen fertilizer from an organic source, 

 inexhaustible in supply and reasonable in cost, is practically a neces- 

 sity to permanent success in the citrus industry. 



The use of legumes as means of maintaining soil fertility dates back 

 to the early Roman days. Experience had taught those ancient "agri- 

 colae" that certain pod-bearing plants enriched the soil in which they 

 grew and it seems to have been common practice with them to rotate 

 their crops in such a way that legumes figured in them frequently. It 

 was finally discovered that the beneficial effects of this group of plants 

 on the soil were due to a gain in nitrogen. In 1885, W. 0. Atwater 

 demonstrated that the added nitrogen came from the air. In 1888, 

 Hellriegel and Wilfarth located the seat of the nitrogen fixation in 

 the nodules on the roots of the legumes. In 1889, M. W. Beijerinch 

 found that the causal agents in this fixation were Bacillus radicicola. 



With a knowledge of these facts, much work has been done in regard 

 to the value and amount of nitrogen thus added from the air. 



The realization of the importance of nitrogen as a constituent of 

 citrus fertilizers, together with the need of increasing the organic con- 

 tent of the soil, led the University of California to inaugurate an exper- 

 iment at Riverside to determine the value of legumes as winter green 

 manure crops. 



This work was begun in 1909 and the results tabulated in this paper 

 cover the work of the last four years. 



The experiment consists of two divisions. In the first part are grown 

 various legumes, such as vetch, peas and clover, in tenth acre plats, 

 with check jilats on which a non-legume, such as barley or rye, is grown. 

 One check plat lies next to each legume plat. 



We are now growing nine different varieties, so the experiment con- 

 sists of nine legume plats and eight of the non-legume plats as checks. 

 The seeds of th&se legumes are all sown during the early part of Sep- 

 tember and are treated in a uniform manner throughout the winter. 



The nine legumes used are as follows : common vetch, purple vetch, 

 Vicia ervilia, burr clover, Melilotus clover, Canadian field peas, Tan- 

 gier peas, lentils and Fenugreek. 



The non-legumes used in the dift'erent years have been barley, alfil- 

 aria and rye. 



At the ordinary time for plowing under such green manure crops 

 in citrus orchards, we measure oft* squares twenty by twenty feet in 

 each of the plats and determine the actual weight of green tops pro- 

 duced on that area. From this we can easily compute the tonnage of 

 green tops these various legumes have produced per acre. 



The tops from these squares are then spread out over the area cut 

 and a fertilizer is uniformly applied over the entire area occupied by 

 this experiment. This fertilizer consists of five hundred and forty 

 pounds of finely ground raw phosphate rock, and three hundred and 



