5o6 Journal of Agricultural Research vo1.xv,No.5> 



In other groves it was found that the fibrous roots grew freely into the 

 uncultivated surface soil under the trees, but not at all into the plowsole 

 layer formed in the area frequently cultivated. 



In addition to checking the root growth and limiting the soil layer 

 available for the root development of the shallow-rooted Citrus trees, the 

 plowsole seriously checks the penetration of the irrigation water. 



The condition may be temporarily relieved by breaking up the plow- 

 sole layer with a subsoiler, but the plowsole forms again when cultiva- 

 tion is resumed. The best remedy against the formation of plowsole 

 seems to be a surface organic mulch, with no cultivation during the 

 irrigation season beyond that necessary to keep weeds under control. 



The present report deals with some of the properties of plowsole as 

 found in citrus groves or formed artificially in the laboratory. The 

 work was done in Riverside, Cal., on soil samples from that area. 



COMPOSITION OF WATER EXTRACT OF PLOWSOLE 



Soil samples were collected from eight orange groves in which plow- 

 sole occurred. Samples at three different depths were collected, includ- 

 ing (i) the soil mulch, usually 3 to 5 inches deep; (2) the plowsole crust; 

 (3) the subsoil under the plowsole. 



Each sample was rolled, passed throgh a 2-mm. sieve, extracted with 

 distilled water in the ratio of i part of soil to 2 parts of water, and the 

 extract analyzed. The water extracts of the soil samples from four of 

 the groves were filtered through Chamberland porous filter tubes, and the 

 water extracts of the samples from the other four groves were filtered 

 through filter paper. 



Table I shows the average amounts, in parts per million on the basis 

 of dry soil, of iron, calcium, magnesium, and silica found in the respective 

 soil layers. Each figure is the average in samples from four groves. 



The elements determined are those usually considered to have a 

 cementing action in soil, and with one exception there was evidently no 

 accumulation of these in the plowsole. Mineral carbonates were present 

 in the soils included in these analyses only in traces or very small amounts. 



The comparatively large amount of iron in the water extracts of the 

 plowsole filtered through filter paper is due to one exceptionally high 

 determination — viz, 9 p. p. m. The average amount of iron in this 

 layer in the three other groves was 1.07 p. p. m., and this amount evi- 

 dently more nearly represents the true condition in respect to iron. In 

 the large number of iron determinations made in water extrac^ of soils 

 in the area considered, the highest amount found was 2.7 p. p. m., except- 

 ing the one mentioned above. The average amounts of iron in eight 

 other water extracts of soil mulch, plowsole, and subsoil were, respec- 

 tively, 0.88, 0.58, 0.87 p. p. m. on the dry soil. It would therefore seem 

 that 1.07 p. p. m. of iron in the plowsole is a more reliable figure than 

 3.05 parts, as shown in Table I, when the water extracts were filtered 



