No. 6. DEPARTMENT OF AGRICULTURE. 187 



thought, that the quantities of matters yielded to solvents in a 

 short time by coarse sands and fine clays would be governed by the 

 difference in the respective sums total of the superficial areas of 

 their particles. The difference between sands and clays in the diam- 

 eters of their particles is, indeed, far greater than most suppose. 

 Thus Whitney found in an ounce of pine barren sand 1.7 billion par- 

 ticles, with 15.4 square feet of total surface; while a fine limestone 

 clay contained 690,000,000,000 particles, with 173 square feet of 

 total surface. But the matter is not to be so simply dealth with. 

 For, in the first place, the composition of the clays and sands of 

 the humid region is not the same. The sands are largely coarse 

 grains of nearly pure quartz or silica, and Lougbridge found that 

 particles^)ver 1-1000-iuch in diameter derived from a sandy soil, gave 

 up nothing whatever to acid. Clays, on the other hand, are largely 

 composed of very fine particles, consisting not only of kaolin, or 

 pure china clay, but also of a very great variety of mineral com- 

 pounds, including many that are quite readily decomposed by acid. 



It is found needful, in judging of the probable fertility of a soil 

 from its analysis, to take into account the total weight of material 

 useful to the crop, and for this i)urpose the weight of the soil 

 in the volume occupied by the principal root system is adopted as a 

 basis of computation. A cubic foot of air-dry loam, representing the 

 plow depth (eight inches) on the Experiment Station farm, weighs 

 sixty-eight pounds, while an equal volume of sand weighs 110 to 120 

 pounds, or nearly twice as much. Moreover, crop roots extend more 

 widely and deeply in sandy soils than in more compact lands. 



Hilgard found, indeed, by a very interesting experiment, that a 

 compact adobe soil through which roots extended but a short dis- 

 tance yielded, upon dilution with pure quartz sand, a much more 

 extended root system and a heavier crop up to the point when the 

 diluent sand exceeded 80 per cent, by weight of the soil mixture. 



In the preparation of soils for analysis, it is the general custom 

 to exclude all stones and gravel, in fact, everything that will not 

 pass a sieve with Uif shes of one-fiftieth inch (0.5 millimeter). 



The state of combination in which the acid-soluble constituents 

 exist in the soil greatly affect their availability, and while we are 

 unable from the results of the analysis by use of strong acid to 

 determine v,\t\\ certainty the exact nature of the original combina- 

 tions, we can by consideration of the proportions of the dissolved 

 constituents to one another gain upon this point some notions of 

 geneial value. In particular, it must be observed that if soil be 

 well supplied with lime carbonate, the potash, nitrogen and phos- 

 phoric acid are more available than when the lime carbonate is de- 

 ficient; wiiereas, with a large proportion of alumina, or lime hydrate, 

 the i)hosphoric acid is probably but slightly available. As the result 

 of extensive analyses of European soils, together with a study of 

 their several crop-producing powers, Maercker has compiled the fol- 

 lowing schedule of ratings for soils, according to their percentage 

 composition in terms of their principal plant food constituents. 



