RELATION OF INORGANIC SOIL COLLOIDS TO 



PLOWSOLE IN CITRUS GROVES IN SOUTHERN 



CALIFORNIA 



By Charles A Jensen 



Assistant in Plant Malnutrition, Biophysical Investigations, Bureau of Plant Industry, 

 United States Department of Agriculture 



The term "plowsole" as here used refers to the hard layer of soil 

 artificially formed in Citrus groves in southern California when the soil 

 is intensively cultivated during the irrigation season. Its upper limit 

 is the lower limit of the soil mulch established by cultivation, and it has 

 been found to vary in thickness from a few inches to 2 feet, though usu- 

 ally it does not extend below the first 18 inches of the soil. The hard- 

 ness of the plowsole varies from a perceptible crust to a layer difficult 

 to penetrate with a shovel. Immediately after irrigation it is usually 

 soft, and is then easily broken; as the soil dries, the plowsole layer hardens 

 again. 



If soil from groves, or from the surrounding native desert areas, is put 

 into pots in the laboratory, irrigated, and the surface layer stirred to 

 form a soil mulch, a hard crust will be formed under the mulch by the time 

 all the soil is dry. The soil just below the crust usually retains a crumbly 

 structure, and does not harden. This plowsole occurs as the result of a 

 single irrigation followed by a few surface cultivations. It thus appears 

 that the packing caused by teams and implements is not necessary for 

 its formation. 



Plowsole has been found to occur in all soil types investigated in the 

 Riverside, Redlands, and Corona areas. Coarse granitic soils, as well as 

 the heavier clay loam soils, are subject to plowsole formation. It has 

 not, however, been observed to form under the trees where the soil has not 

 been cultivated, especially when an organic mulch has been maintained; 

 nor has it been found in basins mulched with organic matter. When 

 the organic mulch is not maintained, the immediate surface soil crusts; 

 but the crust is formed simply by the drying out of the surface soil. 



In many groves the fibrous Citrus roots do not readily penetrate the 

 plowsole, though roots are often found in the hard layer. It appears 

 that in such cases of penetration the roots are formed during the rainy 

 season when the plowsole is soft and maintained in a moist condition. 

 Bet weer^ irrigations in the summer time the upper part of the plowsole 

 itself dries out below the moisture content necessary to maintain root 

 growth. 



Journal of Agricultural Research, Vol. XV, No. 9 



Washington, D. C. Dec. 3, 1918 



am Key No. G-i6s 



(505) 



