330 University of California Publications in Agricultural Sciences [Vol. 3 



In this connection it should be emphasized that the beneficial effect 

 of a summer cover crop does not seem to be due so much to the raising 

 of the average humidity as it does to the buffer effect which it plays 

 when sudden extremes in climatic conditions are experienced. The 

 increase in the average humidity occasioned by the use of a summer 

 cover crop is probably considerably smaller than the difference which 

 may exist from one season to the next. It does not seem so important 

 that the average humidity has been increased somewhat by its use as 

 that when sudden hot, dry spells are experienced their effect is 

 modified by the use of such a crop. This would seem also to explain 

 the effect of the straw mulch which of course does not affect the atmo- 

 spheric humidity to any extent. 



If the limiting factor causing these abnormal water relations be 

 high soil temperatures then methods of orchard management which 

 will reduce such temperatures may be expected to result in heavier 

 crops. Such practices as mulching and the growing of intercrops are 

 known to reduce the soil temperatures. Moreover, such practices in 

 many cases have resulted in notably heavier yields. The junior author 

 had under observation a twenty-acre orchard in the Oroville district 

 in the 1917 season. This tract was planted out to purple vetch in the 

 late fall and was not plowed until the following June. It was heavily 

 irrigated during April and May. Although situated in a most exposed 

 position this orchard bore a much better crop than any other orchard 

 in this district, notwithstanding the extremely heavy fall of fruits 

 experienced in this season. It is possible that the heavy crops borne 

 at the Kellogg place are partly attributable to a reduction in soil 

 temperature during the growing season. 



Some data have been published on the effect of straw mulches on 

 the setting of Navel oranges. Briggs, Jensen, and McLane 67 report 

 as follows : 



The set of fruit was very light throughout the Eiverside district in 1915, 

 owing apparently to cold weather following the bloom. In the Sunny Mountain 

 tract, where the mulched basins were first installed in 1913, the average number 

 of oranges per tree on the check trees in 1915 was 116, while on the mulched- 

 basin trees the average number of oranges per tree was 281, or two and one half 

 times as many as on the check trees. 



Similar results are reported from other tracts. It should be remem- 

 bered, however, that the trees used in this work were not healthy but 

 were badly mottled, and the increased setting may be attributable to 



67 The Mulched-Basin System of Irrigated Citrus Culture, U. S. Dept. Agr., 

 Bull. 499 (1917), p. 30. 



