NUTRITION 93 



exhaust or use up the mineral "nutrients" more rapidly 

 than a succession of the different kinds of crops, nor does 

 it introduce into the soil any permanent excrement toxic 

 to wheat. It does, however, tend to introduce with the 

 seed, stubble, roots, et cetera a number of kinds (at least 

 five) of parasitic fungi that cause diseases of the wheat 

 plants. These fungi destroy, blight, and rot off the roots 

 of the plants, and live internally in the straw and the 

 seeds. The accumulation of these fungi in fertile soils 

 brings about the condition of wheat-sickness, ''wheat- 

 tired" soil. The fungi attack the roots, leaves, stems, 

 young developing grains, and seedlings, and the value of 

 crop-rotation lies in growing a series of different kinds of 

 crops that do not transmit or bear each other's diseases. 

 Crop-rotation is not primarily to conserve the fertility of 

 the soil, but is a sanitary measure, tending to eradicate 

 contagious disease. The reproductive bodies {spores) of 

 these fungi are carried from field to field and persist in the 

 field for some time, but lose their vitality during the few 

 seasons when other crops are being cultivated. It is thus 

 seen that one farmer, by careless methods of agriculture, 

 may not only suffer a loss of yield of his own crops, but 

 may also infect his neighbors' fields. In addition to crop- 

 rotation, this trouble may be reduced or avoided by care- 

 fully sterilizing all seed, before sowing, by soaking them 

 in a weak solution of formaldehyde, and also by sterilizing 

 the soil in a similar manner. The validity of this theory 

 is based upon extended studies of wheat, oats, barley, 

 and flax: it doubtless holds true also for other crops. 



These three theories are all based upon thorough experi- 

 mental investigation, and it is probable that all three 

 contribute to a rational basis for the rotation of crops. 



