68 SOILS. 



whenever at liberty. Each herd of these animals has its definite dung- 

 ing place at some convenient point. That such herds have existed in 

 the region from time immemorial is obvious from historical as well as 

 collateral evidence ; and as their manure accumulated, its nitrification 

 would progress rapidly under the prevailing arid conditions. The com- 

 mon salt would naturally be derived from the urine and excrements, 

 and the alkaline salts which exist throughout this region as the products 

 of soil decomposition, would be quite sufficient to account for the 

 alkaline bases in the caliche. On the other hand, the presence of 

 iodine points to seaweeds as the organic source. 



Intensity of Nitrification in Arid Climates. Of the efficacy 

 of nitrification under arid conditions abundant evidence may 

 be found within the State of California. In the alkali lands 

 of southern California the nitrates of soda, lime and magnesia 

 are almost universally present ; they form at times as much as 

 one-fifth and even more of the entire mass of alkali salts, and 

 in one case the total amount in the soil has been found to reach 

 two tons per acre, with an average of twelve hundred pounds 

 over ten acres. In the plains of the San Joaquin Valley, spots 

 strongly impregnated with niter are found, especially under the 

 shadows of isolated oak trees, where the cattle have been in the 

 habit of congregating for a long time; a case quite analogous 

 to that supposed by Kunlcc to exist in the Chilean locality. 

 Of course it is only in arid climates that the accumulation of 

 nitrates can usually occur; f<r in the region of summer rains 

 the nitrates formed during the warm season will inevitably be 

 washed into the subdrainage, unless restrained by absorption 

 by the roots of vegetation. The heavy losses occasionally 

 occurring from this cause in the course of a rainy winter on 

 summer-fallowed land have been amply demonstrated by many 

 investigations. 



POTASH MINERALS. P>y far the most abundant occurrence 

 of potash in the earth's crust is that in silicates and notably in 

 orthoclase or potash feldspar, which contributes so largely to 

 soil-formation. But in the absence of any economically suc- 

 cessful artificial method for producing potash compounds from 

 feldspars on a commercial scale, almost the entire supply of 

 potash salts was, until a comparatively late period, derived from 

 plant ashes, viz., the " potashes " of commerce. At the same 



