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



The accumulation of the soluble soil constituents near the surface 

 is strikingly in evidence during the summer months in almost any 

 heavily fertilized grove under furrow irrigation. The sides of the 

 furrows and the surface of the ridges between the furrows will be 

 found coated with a brown crust which closely resembles an ordinary 

 " alkali " crust as seen in saline districts. This crust has been found 

 by Breazeale to be so rich in soluble salts that it will kill young seed- 

 ling oranges or lemons, the seedlings sometimes dying overnight. 

 The soils of the area do not normally contain enough soluble matter 

 to bear any resemblance to "alkali" soils; but these surface crusts 

 illustrate very strikingly the tendency under this system of fertili- 

 zation and irrigation of the soluble constituents to move into and 

 accumulate in the surface soil. 



PLOWSOLE. 



Furrow irrigation and intensive surface cultivation often promote 

 the formation of a " plowsole," or incipient hardpan, under the soil 

 mulch, varying in thickness from 3 inches to 2 feet. This layer 

 of soil beneath the mulch frequently becomes so hard as to polish 

 under the point of the plow when the irrigation furrows are opened. 

 A long time is required for irrigation water to penetrate it, and it 

 offers a serious obstacle to the development of the roots of citrus 

 trees. Deep plowing loosens the plowsole temporarily, but destroys 

 the roots to the depth of plowing, and the plowsole soon forms again. 



ORGANIC MULCHING. 



The study of the cause of mottle-leaf led to experiments having 

 as their object the development of a method of fertilizing and irrigat- 

 ing citrus trees which would avoid the objections of the present sys- 

 tem. The use of surface mulches of organic material, such as straw, 

 manure, damaged alfalfa, etc., as a substitute for the intensive culti- 

 vation was suggested in 1912 by one of the writers as a possible solu- 

 tion of the problem. 1 A trial of the organic surface mulch was 

 inaugurated in 1913 on a small field plat of lemons on the Limoneria 

 Ranch near Santa Paula. Cal. Under date of February 24, 1915, 

 Mr. James Culbertson, assistant manager of the Limoneira Co., re- 

 ported upon this experiment as follows: 



You will doubtless remember our discussion ... a year ago last December 

 ■when the possible value of mulching as a corrective for chlorosis was the topic. 

 Upon reaching home I had three short rows thoroughly mulched, one with 

 sawdust, one with bean straw, and the third with fairly well rotted manure. 

 They were not in the worst affected areas. Later in the spring, in May, when 



1 At a meeting of the California Lemon Club, at which this idea was advanced, Prof. 

 H. S. Fawcett, of the University of California Citrus Experiment Station, reported an 

 orange grove on an island near the eastern coast of Florida that for many years had 

 been successfully operated under the mulching system without cultivation ; and he has 

 since observed the organic mulch in use in Cuba. 



