April 17, 1908] 



SCIENCE 



627 



plant food may be derived there is no doubt 

 but that the amount present, even in the 

 poorest soils, is very large when measured in 

 units of yearly crop demands. But the 

 amount is not so large, either absolutely or in 

 the rate of renewal by rock weathering, as to 

 be inexhaustible if only it could be made con- 

 tinuously available at the rate required for 

 good yields. Indeed the untruth of an as- 

 sertion that by establishing good mechanical 

 facilities for a high rate of removal of coal 

 and thorough sanitary conditions for the 

 miners, the supply of coal in a given mine will 

 be indefinitely maintained, is no more certain 

 than the contention that by a proper rotation 

 of crops and the maintenance of good tilth 

 and proper sanitary conditions uniformly 

 large yields may be indefinitely maintained 

 on any and all fields without a return to the 

 Boil of the plant food removed. We have in 

 Wisconsin residuary soils whose absolute con- 

 tent of potassium is only 4.6 tons, of calcium 

 5.3 tons, of magnesium 3 tons, of nitrogen .9 

 ton and of phosphorus .8 ton per acre-foot 

 of field. But it is very important to recognize 

 that by very far the larger proportion of these 

 plant food elements existing in the soil is, 

 properly speaking, no more to be regarded as 

 plant food for the crop growing upon the 

 ground than it is food for the cattle feeding 

 upon pasture grass, hence there is never pres- 

 ent in the soil of a field any such large 

 amounts of plant food proper as have been 

 stated. 



But, considering these amounts as capable 

 of being converted into plant food proper as 

 rapidly as large yields of crops would demand, 

 the whole amount of the phosphorus finds its 

 equivalent measure in the amount carried in 

 268, and the whole of the potassium in but 634, 

 20-bushel-per-acre crops of wheat, allowing 

 nothing for losses by leaching. And yet the 

 rate of surface erosion which will expose un- 

 cropped material from below as rapidly as one 

 foot in 4,000 years could supply phosphorus 

 only one fifteenth and potassium one sixth as 

 fast as would be demanded by the 20-bushel 

 crops of wheat. It follows, therefore, that for 

 the soil in question to have a productive 

 capacity of 20 bushels per acre per annum, 



indefinitely maintained without the applica- 

 tion of phosphorus and potassium, the rate of 

 surface erosion and of subsoil and rock 

 weathering must equal one foot in every 268 

 years for the phosphorus, and one in every 634 

 for the potassium. But these are rates seldom 

 if ever experienced in any agricultural region 

 and the mean rate of erosion for the whole 

 Mississippi Valley has been placed by geolo- 

 gists at not more than one foot in 4,000 to 

 6,000 years. 



Nature's method of developing and of main- 

 taining the productive capacity of fields has 

 always been that of returning to the soil the 

 whole crop, but, even so, nowhere has the 

 concentration of the mineral elements of plant 

 food been large as a result of soil formation 

 by rock weathering under plant growth. The 

 reverse rather has been the rule and very em- 

 phatically so with lime and magnesia. Only 

 organic matter, with its ash, has at times and 

 places accumulated to form peat and coal but 

 always under non-agricultural conditions. 

 There appears, therefore, no ground for a dif- 

 ference of opinion as to the point that it is 

 possible for a mere rotation of crops, coupled 

 with good tillage, and ample and timely 

 moisture supply, to indefinitely maintain high 

 yields where the whole crop above ground is 

 regularly and continually removed from the 

 field. 



Concentration of Soil Solutions. 



Notwithstanding the contention made in 

 Bulletin 22 of the Bureau of Soils, and reiter- 

 ated in later publications from the same ofiice, 

 that all soil solutions have essentially the same 

 concentration, it must nevertheless be admitted 

 that no observations yet published can be re- 

 garded as indicating even the approximate 

 concentration of a single soil solution as it 

 exists in the soil and functions in the 

 growth of plants under field, greenhouse or 

 pot culture conditions. It must further be 

 admitted that the water solutions which have 

 been recovered from soils do show a very wide 

 range, in both composition and concentration, 

 when judged by any standard admissible from 

 the cultural point of view. 



It is of the greatest importance in the con- 



