6 SCIENCE BULLETIN, No. 9. 



place until the crop is approaching maturity. "With us nitrification is active 

 and progressive during the early growth of the wheat plant, and nitrogenous 

 manuring is unnecessary, all that is required being the application of a 

 fertiliser which promotes the development of the root-system, a quality which 

 appears to be possessed in a high degree by superphosphate, thus ensuring the 

 young plants a vigorous start. 



It has been further shown by J. W. Paterson and P. R. Scott* that 

 superphosphate appreciably increases the nitrification of ammonia, indicat- 

 ing that, in some cases, the addition of phosphates may help to nourish the 

 nitrifying organisms as well as the crop. 



We will now review shortly some of the recent work which has shown that 

 the growth of plants is affected by causes other than lack of plant-food, or 

 unfavourable mechanical soil conditions, and which encourages us to look to 

 other remedies for unfertile conditions. 



We shall see, incidentally, that fertilisers may have an action upon the 

 growth of the plant which is altogether independent of its power of supplying 

 plant-food, and which until recent years has been quite overlooked. 



Toxic Substances in Soils. 



That substances are formed in the soil, either as the result of the decom- 

 position (chemically, or by means of micro-organisms) of crop-residues, or 

 excreted by the growing plant, seems to be abundantly proved. f 



O. Schreiner was the first to show the toxic effect of dihydroxystearic acid 

 and to isolate this substance from soils on which wheat failed to grow. J 



Further experiments by the United States Bureau of Soils have shown 

 that quite a large number of organic substances exercise a toxic action on 

 plant growth. 



F. Fletcheijl describes experiments showing the extraordinary influence 

 of the neighbourhood of sorghum and of maize upon the growth of "sesamuni 

 indicum." This is not due to the removal of moisture or of plant-food 

 by the maize crop, as both these essentials were abundantly supplied to the 

 sesamum, but must, he concludes, be attributed to the excretion of a toxin 

 by the roots of the maize plants. Fletcher believes this to be a salt of 

 dihydroxystearic acid. 



Among the numerous toxic organic compounds which Schreiner and his 

 fellow-workers have found to be present in the soil, three or four have been 

 more particularly studied in relation to their action upon plants provided with 

 varying quantities of the recognised fertilising ingredients. 



Schreiner and Skinner^ have shown that in water cultures with wheat, 

 dihydroxj'-stearic acid is least harmful when the plant is provided with fer- 

 tilising substances relatively rich in nitrogen (such as nitrates), and that in 



* Journal of the Department of Agriculture, Victoria, vol. 10, p. 393. 



t Schreiner and Shorey, Bull. 74, Bureau of Soils, U.S.A. 



jBull. 53, Bureau of Soils, U.S.A. 



Schreiner and Reed, Bull. 47, Bureau of Soils, U.S.A. 



ii Journ. Agric. Science, IV, p. 245. 



IT Bull. 70, U.S.A. Bureau of Soils. 



