2 BULLETIN 499, U. S. DEPARTMENT OF AGRICULTURE. 
paper? dealing with a field and laboratory study of the relation of 
mottle-leaf to soil conditions. In that paper it was shown by statis- 
_ tical methods that the percentage of mottling in 120 orange groves 
studied was inversely correlated to the extent of 0.67 with the 
humus content of the soil, as represented by the organic matter ex- 
tracted with 4 per cent ammonium hydroxid and determined colori- 
metrically by comparison with a freshly prepared gravimetric stand- 
ard. In other words, one-half of the mottling is associated with and 
is apparently due to a deficiency in the humus content. The in- 
vestigations of Schreiner and his colleagues, Shorey, Lathrop, and 
Walters, have demonstrated that the organic matter extracted from 
soil by an alkaline solution is a varying mixture of numerous organic 
compounds having widely different properties.? If the humus com- 
ponents which are active and perhaps essential in maintaining the 
nutrition of the orange trees could have been separated from the 
mixture, a still higher inverse correlation would probably have been 
obtained between the amount of these essential or active constituents 
and the percentage of mottling. The analytical difficulties involved, 
however, combined with the incompleteness of our knowledge regard- 
ing the active constituents of the humus, made such a procedure im- 
practicable. 
Mottle-leaf, however, is apparently not associated wholly with the 
low humus content of the soil. Groves which have been fertilized 
chiefly with manure and green cover crops are by no means always 
free from mottle-leaf. The evidence collected in the above-mentioned 
field survey indicated that the furrow system of irrigation and the 
intensive surface cultivation in vogue in these districts may be in 
part responsible for the deterioration of the groves. The present 
bulletin deals with an investigation of this phase of the problem. 
Special attention is given to a new method of citrus culture, which 
the writers have termed the mulched-basin system, in which the trees 
are irrigated by means of basins heavily mulched with organic ma- 
terial, the soluble organic compounds resulting from the decompo- 
sition of the mulch being carried by the irrigation water into the soil. 
The soil-moisture conditions and the yields under the mulched-basin 
system and the furrow-irrigation system are compared, and the 
results of determinations of the rate of increase of the humus content 
of the soil under the mulched-basin system are presented. 
1 Briggs, L. J., Jensen, C. A., and McLane, J. W. Mottle-leaf of citrus trees in relation 
to soil conditions. Jn Jour. Agr. Research, y. 6, no. 19, p. 721-739, pl. H and 96—97. 
1916. 
2 Schreiner, O., and Shorey, E. C. Chemical nature of soil organic matter. U. S. Dept. 
Agr., Bur. Soils Bul. 74, 48 p., 1 pl. 1910. 
Lathrop, E. C. Protein decomposition in soils. Jn Soil Science, v. 1, no. 6, p. 509- 
Hole 4 O1G: 
