2 BULLETIN 499, U. S. DEPARTMENT OF AGRICULTURE. 



paper 1 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 -i 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. 2 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. In Jour. Agr. Research, v. 6, no. 19, p. 721-739, pi. H and 96-97. 

 191G. 



2 Schreiner, O., and Shorey, E. C. Chemical nature of soil organic matter. U. S. Dept. 

 Agr., Bur. Soils Bui. 74, 48 p., 1 pi. 1910. 



Lathrop, E. C. Protein decomposition in soils. In Soil Science, v. 1, no. 6, p. 509- 

 532. 1916. 



