28 BULLETIN 490, U. S. DEPARTMENT OF AGRICULTURE. 



VICTORIA GROVE. 



The percentage of humus in the soil in the Victoria grove under 

 the various treatments is shown in Table XII. Here again the 

 humus did not increase perceptibly in the basins until October, 112 

 days after the initial samples were taken. The low initial humus 

 content in the basins is due in part to the removal of the surface soil 

 in making the basins. The manure and alfalfa were equally effective 

 in increasing the humus percentage up to the time of the last 

 sampling. 



Table XII. — Average percentage of soil humus to a depth of 3 feet when vari- 

 ous treatments were given in the Victoria grove, California, in 1915.. 



Soil treatment. 



June 22. 



Aug. 13. 



Sept. 13. 



Oct. 12. 



Alfalfa: 



0.073 

 .134 



.136 

 .150 



0.073 

 .106 



.130 



.112 



0.075 

 .147 



.136 

 .139 



0.084 





.155 



Manure: 



.153 





.160 







SUMMARY. 



Mottle-leaf of citrus trees is characterized by yellowish spots or 

 blotches on the leaves and by a reduction in the size of the leaves 

 and the density of the foliage. In its advanced stages it results in 

 a marked reduction in yield and in the size and quality of the fruit. 

 No fungus or bacterium has yet been found to be causally associated 

 with this trouble. 



Work carried out by the writers in 1914 and reported in another 

 paper showed that half of the mottling in about 120 orange groves 

 in the Riverside, Redlancls, Rialto, and Highland districts of Cali- 

 fornia was associated with a low humus content of the soil. Evi- 

 dence was also secured which indicated that inadequate irrigation 

 might be responsible in part for the development of mottle-leaf. 



The present paper is concerned with field experiments relating 

 to the irrigation of oranges and the use of organic fertilizers and 

 deals particularly with a new method of citrus culture, the mulched- 

 basin system, in which low dikes are thrown up so as to form large 

 shallow irrigation basins near each tree, and each basin is heavily 

 mulched with alfalfa hay, bean straw, manure, or some other organic 

 material. The basin makes it possible to supply each tree with water 

 in accordance with its needs, while the water carries with it to the 

 feeding- root system the plant food extracted from the disintegrating 

 mulch. 



The system of furrow irrigation and clean cultivation in common 

 use in citrus districts" in California tends to promote the formation 



