28 BULLETIN 499, 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: | 
mt basis 22335. oo Ss Fee se os ees eee ee eee 0.073 0.073 0.075 0.084 
DHaimen Plus NN GC Sepa acne ee eee ee eee ee - 134 - 106 147 155 
Manure: | 
Tn ASINS “477 Seas a 5 2 sea aie eae eee ee eee 136 130 - 136 - 153 
Same; plus limescse2=. 2 :22222--32- SES Ne oe eee - 150 | 112 139 - 160 
e 
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, Redlands, 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 
