2i6 Journal of Agricultural Research voi. xii.no. s 



There was a consistent difference in the specific gravity of the oranges 

 from the Umed trees and from the unUmed, but the difference was small. 

 The average specific gravity of the oranges from the mulched and limed 

 trees was 1.0012, and of those from the mulched trees, but not limed, 

 was 1.0307. The specific gravity was calculated from the grading 

 results, and not by individual determinations of separate fruits. The 

 error in sizing by the grading machine would probably be compensated 

 by the number of fruits included, as the entire crop was used in the 

 calculation. 



On the whole, the evidence obtained from these experiments does not 

 show that the humus content of the soil correlates with orange pro- 

 duction. Neither does the information obtained justify the statement 

 that the humifying activity in the soil correlates with fruit production; 

 but the results obtained would indicate that this point might be worthy 

 of further study. 



It appears from the results here presented that the most important 

 function of organic matter as influencing orange production is not that 

 of merely furnishing humus; that humus in itself is not the most impor- 

 tant product of organic degradation for orange production. It appears 

 from other work (8) that a more important function of organic matter 

 is to make the plant food in the soil minerals more readily available. 

 It was found that organic substances, especially in a freshly decom- 

 posing condition, dissolved plant food elements in appreciable amounts, 

 even when the organic solvents contained practically no electrolytes. 

 This function and that of promoting the biological activities of the soil 

 seem to be more important rdles of organic matter in the soil than merely 

 to furnish humus. 



SUMMARY 



This report presents a study on (a) the changes in humus content in 

 soils in basins mulched with different organic substances, (b) the effect 

 of lime on humus content in soils in mulched basins, (c) the relation of 

 humus content in the soil to orange production. 



By "humus" is meant the brown- to black-colored organic extract 

 obtained from soil leached with i per cent hydrochloric acid to the 

 absence of calcium and the soil residue boiled for two minutes in a 7.5 

 per cent sodium-hydrate solution. 



Humus determinations in mulched basins in citrus groves showed a 

 fluctuation in the percentage of humus from time to time. 



The average percentage of humus increased more in basins on clay 

 loam soil than in basins on lighter soil tubes, with manure and alfalfa 

 as mulching m^aterials. 



Usually the percentage of humus in basins increased more when manure 

 was used as mulch than when alfalfa was used as mulch. This seemed, 



