732 Journal of Agricultural Research vol. vi,n . 19 



of soil required to include 1 gm. of humus. The coefficient of correlation 

 between this quantity and the percentage of mottled leaves for all the 

 orange groves included in the main group was found to be 0.67 ±0.03. 

 The association would be represented by the square of this quantity, or 

 0.45. In other words, approximately one-half of the mottling can be 

 accounted for by the low humus content of the soil. This conclusion is 

 reached from a consideration of the data by impartial statistical methods 

 and is free from personal bias. 



The failure of the trees in some cases to respond to manure appears to 

 be due to the methods of cultivation and irrigation employed. It has 

 been the general practice in California orange culture to maintain a 

 deep dust mulch in the groves by cultivating frequently during the sum- 

 mer months. In fact, the cultivation which is carried on between irri- 

 gation periods, combined with the opening and closing of irrigation 

 furrows, results in working the surface soil on an average of nearly once 

 a week during the summer months. It is quite impossible for any effec- 

 tive root development to take place in this surface layer under such con- 

 ditions. The roots are destroyed by the constant cultivation; and 

 owing to the frequent stirring, the soil during the greater part of the 

 time is entirely too dry for root development. Yet this is the part of the 

 soil to which manure is applied, the usual practice being to disk the 

 manure into the mulch. Even when the manure or a cover Crop is 

 plowed under, the plowing is often so shallow that the material turned 

 under is within reach of the teeth of the cultivator. The result is, there- 

 fore, that the organic matter is partly disintegrated and lost without 

 ever coming in contact with the feeding roots of the tree. Under such 

 conditions it is not surprising that little benefit has resulted from the 

 use of manure. It would be difficult to conceive a more effective method 

 for the rapid destruction of the organic matter than the repeated stirring, 

 moistening, and drying to which it is subjected in this deep surface 

 mulch. 



The difference in humus content between the soils of the badly mottled 

 groves and those relatively free from mottling is about 0.1 per cent. 

 This difference may at first sight appear small; but when expressed in 

 terms of the weight of the soil, its magnitude becomes apparent. An 

 acre of soil 3 feet in depth weighs approximately 10,000,000 pounds, so 

 that a humus content of one-tenth of 1 per cent is equivalent to 10,000 

 pounds of humus. Data regarding the amount of humus formed from 

 a ton of organic matter are not at present available, but manure would 

 probably not often yield more than 10 per cent, or 200 pounds of "humus", 

 or "matiere noire", per ton. On the basis of this assumption it would 

 require an application of at least 50 tons of manure per acre to bring the 

 humus content of the badly mottled groves up to that of the groves 

 relatively free from mottling. 



