THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS. 



333 



is assumed to be thus demonstrable by many French chemists. 

 Moreover, a number of King's results, tabulated in curves, 

 exhibit a remarkable general parallelism of the curves showing 

 totals of plant-food extracted by water, and actual crop pro- 

 duction. This is the more remarkable since it is known to be, 

 not pure water, but such as is more or less impregnated with 

 carbonic acid at least, that is actually active in soil-solution 

 and plant-nutrition. The farther development of this method 

 may, it would seem, lead to definite conclusions at least in re- 

 spect to the immediate productive capacity of cultivated, and 

 perhaps also of virgin soils. But it is not likely to give any 

 definite clew as to the durability of such lands. 



ASCERTAINMENT OF THE IMMEDIATE PLANT-FOOD REQUIRE- 

 MENTS OF CULTIVATED SOILS BY PHYSIOLOGICAL 

 TESTS. PHYSIOLOGICAL SOIL-ANALYSIS. 



As has already been stated, the quantitative analysis of culti- 

 vated soils by means of strong acids affords a presumptive in- 

 sight into their immediate productiveness, and the kind of 

 fertilizer needed to improve it, only in case of the extreme 

 deficiency of one or several of the chiefly important plant- 

 foods. The limits of deficiency of these in virgin soils have 

 been discussed above; but since in cultivated soils amounts of 

 soluble plant-food so small as to be beyond the limits of ordi- 

 nary analytical determinations, when distributed through an 

 acre-foot of soil may, when rightly applied, nevertheless pro- 

 duce very decided effects, the indications thus obtainable are 

 not absolute. Thus a dressing of 150 Ibs. of Chile saltpeter, 

 containing only about 24 Ibs. of nitrogen, is capable of causing 

 the production of a full crop of wheat where otherwise, even 

 under favorable physical conditions, only a fraction of a crop 

 would have been harvested; provided, that all the other re- 

 quisite ingredients were present to a sufficient extent and in 

 available form. Yet the amount of nitrogen thus added would 

 amount, in one acre- foot of soil to only .0008%, say eight ten- 

 thousandths of one percent; which, with the amounts of sub- 

 stance usually employed in soil analysis, would be an unweigh- 

 able quantity, and might easily be overlooked. 



Since the amounts of potash and phosphoric acid actually 



