22 



SCIENCE 



[N. S. Vol. LII. No. 1332 



sure foundation of facts supported by an 

 abundance of experimental data now available 

 from tbe fields and laboratories of tbe Univer- 

 sity of Illinois operated under bis direction. 

 The Illinois system recognizes clearly tbat 

 there are six positive factors of crop produc- 

 tion. These factors of crop production may 

 be briefly indicated by single terms as the seed, 

 temperature, moisture, light, a home for the 

 plant, and food for its use. These factors are 

 aE. of equal importance in the production of 

 crops. ISTot all of the factors, however, are 

 susceptible of equal control. It is imjwssible, 

 for example, to change the temperature condi- 

 tions of winter so as to make that season suit- 

 able for crop production, and the practical 

 means available for modifying the tempera- 

 ture conditions of the soil during the growing 

 period of the crop are very limited. Of all the 

 factors of crop production, the food factor is 

 completely within the control of the farmer. 

 It is fully possible for him to completely 

 change, in an economic way, the amount of 

 food available for the plant within the soil. 

 It is frequently true, also, that the food supply 

 is the limiting factor of crop production, espe- 

 cially under humid conditions such as prevail 

 in Illinois. 



The Illinois system of permanent soil fer- 

 tility, therefore, deals in a large measure with 

 this factor of crop production. There are 10 

 essential elements of plant food and these are 

 carbon, oxygen, hydrogen, iron, sulfur, cal- 

 cium, magnesium, potassium, nitrogen and 

 phosphorus. All of these ten elements of plant 

 food are of equal importance in crop produc- 

 tion, for, in the absence of any single one, the 

 plant can not function normally and produce 

 a good yield of its kind. Not all of the plant 

 foods, however, are within the control of the 

 farmer, while some of them are obtained from 

 natural sources in sufficient quantities so that 

 they never limit crop yields. 



Carbon and oxygen are obtained by the plant 

 from the small amount of carbon dioxide of 

 the atmosphere, and this supply is constantly 

 being replenished from natural sources. Hy- 

 drogen is obtained by the plant from the soil 

 moisture which in turn is being constantly re- 



plenished by the rainfall under humid condi- 

 tions. The plant consists largely of these 

 elements of plant food, their compounds form- 

 ing approximately 95 per cent, of all plant 

 parts. These three elements of plant food are 

 constantly being replenished in the soil or air 

 from natural sources, and the farmer, there- 

 fore, need not concern himself further with 

 them. 



Iron is used by the plant in such extremely 

 minute quantities, and the supply in the soil 

 is so large, that it need never be added to the 

 soil as a plant food. While the plant food re- 

 quirements for sulfur are comparable in many 

 respects with those for phosphorus, stdfur is 

 constantly being added to the soil from nat- 

 ural sources in quantities more than sufficient 

 to meet the needs of the plant for food. For 

 these reasons, these two elements need no spe- 

 cial consideration in a permanent system of 

 soil fertility. 



There remain, therefore, five elements of 

 plant food which must receive careful consid- 

 eration by the farmer in any system of perma- 

 nent soil fertility which may be proposed. 

 These elements are calcivun, magnesium, potas- 

 sium, phosphorus and nitrogen. A system 

 which assumes to be permanent must provide 

 for the return to the soil of those elements of 

 plant food removed by the crop, ujiless they are 

 present in the soil in unusual quantities suffi- 

 cient to provide for the maximum production 

 of crops for indefinite periods of time. The 

 Illinois system considers and makes such pro- 

 vision for these five elements of plant food. 



The inorganic plant foods, calcium, mag- 

 nesium, potassiima and phosphorus, are re- 

 moved by the plants in comparatively large 

 quantities. An ordinary rotation of wheat, 

 corn, oats and clover would remove, for the 

 maximum production of crops, 77 pounds of 

 phosphorus, 320 pounds of potassium, 68 

 pounds of magnesium and 168 pounds of cal- 

 cium, and these substances are obtained by the 

 plant from the soil and there is no other pos- 

 sible source unless materials containing them 

 are added to the soil. It is, therefore, of 

 fundamental importance to know the amounts 

 of these materials which occur in the soil and 



