DOOEUBEB 18, 1908] 



SCIENCE 



861 



moved from the soil by a crop is not avail- 

 able when the crop is planted, but it must 

 be made available during the growing 

 season. 



Plant food is made available by chemical 

 and biochemical processes, of which amno- 

 nification and nitrification are among 

 those best understood. The products of 

 organic decomposition and nitrification, in- 

 cluding various organic acids, carbonic 

 acid, and nitric acid, are very efficient as 

 solvents for the mineral plant food. Thus, 

 in the conversion of organic nitrogen into 

 nitrate nitrogen for a hundred-bushel crop 

 of corn, the nitric acid formed is alone 

 sufficient to convert seven times as much 

 tricalcium phosphate into monocalcium 

 phosphate as would be required to supply 

 the phosphorus for the same crop ; but, of 

 course, it is not limited to this reaction. 

 The presence of calcium carbonate, or some 

 free base, and of oxygen, as in the aeration 

 of the soil by tillage, will assist greatly in 

 the decomposition of the soil and con- 

 sequent liberation of plant food. 



Some inorganic reactions, many organic 

 reactions, and most biochemical reactions 

 are not instantaneous, but long continued, 

 and the rate of reaction is influenced by 

 many factors, including temperature, con- 

 centration, aeration, and the presence of 

 catalytic agencies and bacterial food-sup- 

 plies. Under controlled conditions the 

 length of time required for many such re- 

 actions is now determinable; and any soil 

 investigation is incomplete which disre- 

 gards the presence or absence of active 

 decaying organic matter. It should be 

 understood, too, that this term is not 

 synonymous with humus. Partially de- 

 cayed peat has no such value as fresh farm 

 manure, clover or other green manures, 

 even though the peat may contain as large 

 or larger amounts of plant food, and pro- 

 duce similar physical effects. The one is 

 in a sense embalmed and very inactive, 



while definite and continued chemical ac- 

 tion is needed and is produced by the fresh 

 materials. 



Among the most unsatisfactory and mis- 

 leading investigations are those from which 

 the use of insoluble plant-food materials 

 has been condemned because they have 

 not responded when applied in the absence 

 of adequate supplies of active organic 

 matter. 



Under similar physical conditions the 

 amount of plant food made available dur- 

 ing the season varies chiefly with three 

 factors: namely, the presence of calcium 

 carbonate, the supply of decaying organic 

 matter, and the stock or store of fertility 

 contained in the soil. 



To supply the soil with decaying organic 

 matter, and with lime if needed, is a neces- 

 sary part of all extensive agi-icultural prac- 

 tise, and, with these provided for, the 

 question of the total stock of plant food 

 becomes of first importance. To illustrate 

 this importance we may well consider some 

 well known soils. 



The early Wisconsin brown silt loam 

 prairie, one of the commonest soil types in 

 the Illinois corn belt, contains in the 

 plowed soil of an acre (7 inches deep) 

 1,190 pounds of phosphorus and 36,250 

 pounds of potassium. For one hundred 

 bushels of corn each year the total supply 

 of phosphorus is sufficient for only seventy 

 years, while the potassium is sufficient for 

 more than seventeen centuries.* 



In the unglaciated yellow silt loam hill 

 land, the most abundant soil type in seven 

 counties of southern Illinois, the total sup- 

 ply of nitrogen to a depth of 40 inches 

 is sufficient for less than sixty such crops 

 of corn, while the total supply of potas- 

 sium to the same depth is sufficient for 

 more than ten thousand crops, assuming 

 in both cases that the grain is harvested 

 and the stalks left on the land. 



♦ Illinois Experiment Station Bulletin 123. 



