236 SOIL CONDITIONS AND PLANT GROWTH 



known as "alkali soils" from the circumstance that sodium 

 and potassium carbonates are often present. In wetter 

 climates the soluble substances tend to be washed out more 

 completely, but notable quantities often persist in heavy clay 

 soils, especially where the drainage is bad, and may produce 

 injurious effects on vegetation. 



I,n discussions of the' plant food in soils it is often assumed 

 that the only significant plant nutrients are nitrates, phos- 

 phates, sulphates and salts of potassium, calcium, magnesium 

 and iron, commonly called the "essential" plant foods. 

 Perfect plants can be raised in water cultures containing only 

 these substances. Reference to Fig. 12 (p. 68), however, 

 shows that productiveness is not always maintained indefinitely 

 simply by supplying these salts, and not nearly so well as 

 when the complex farmyard manure — excreta, litter, etc. — 

 is used. It is known also that certain other compounds 

 besides the "essential" foods may increase plant growth 

 (p. 56). Further, Russell and Petherbridge have shown 

 that on heating soil to 100° something is formed that 

 stimulates root production to a remarkable degree. Are these 

 effects the results of nutrition or of stimulation? Are the 

 manganese, fluorine, etc., compounds and the beneficial soil 

 constituents indispensable nutrients of which only traces are 

 required, or are they, as Armstrong expresses it, "condi- 

 mental "foods? 



It would be attractive to think that some of the vague 

 physiological conditions that trouble the grower are to the 

 plant what beri-beri and similar diseases are to the animal — 

 the result of withholding some essential or useful " accessory 

 substance". Bottomley^ considers that certain substances 

 obtained in the bacterial decomposition of peat are of this 

 nature. Maze has published some remarkable results ^ show- 



iW. B. Bottomley, Froc. Roy. Soc, 1914, 88, 237-247, and 1920, B 91, 

 83 ; Florence A. Mockeridge, Biochem. jfourn., 1920, 14, 432. The substances 

 are called auximones. See also J. F. Breazeale, yourn. Ag. Research, 1919, 

 l8, 267, who found that citrus seedlings grew better in peat extract than in 

 distilled water. 



'^ Mazd, Compt. Rend., 1915, 160, 211. 



