46 



SOIL CONDITIONS AND PLANT GROWTH 



Sulphur is probably an essential food constituent, but is wanted 

 only in small quantities. Sulphates are present in rain and in soil, but 

 further additions in manure have been found by Dymond (93) to be 

 useful for heavy crops rich in protein, although they were not needed 

 for cereals or permanent pastures. 



Silicon does not seem to be essential, but it occurs to so large an 

 extent in some plants that it is not likely to be wholly useless. Wolff 

 and Kreutzhage (315) found that soluble silicates increased the yield 

 of oats in water cultures and also the proportion of grain, behaving 

 in their opinion much like phosphates. Certain of the Rothamsted 

 plots are treated with sodium silicate, and marked crop increases are 

 obtained on the phosphate-starved plots (Table XXL). Hall and 



Table XXI. — Effect of Silicates on the Growth of Barley, 1864-1904. 



Rothamsted. 



Morison (119) conclude that silicates act by causing an increased as- 

 similation of phosphoric acid by the plant, the seat of .^ction being in 

 the plant and not in the soil. 



Absence of Injurious Substances. — We have seen that many 

 salts have a toxic effect if given alone to the plant, but for our pur- 

 pose we need consider only those causing injury in presence of other 

 compounds. Two cases arise in practice : some substances are in- 

 jurious in small quantities, others only in excess. 



Substances Injurious in Small Quantities : Acids. — Cultivated plants 

 will usually not grow in too acid or too alkaline a medium, but prefer 

 something more nearly neutral. In water cultures it is necessary to 

 begin with a faintly acid solution because of the formation, as growth 

 proceeds, of sodium and potassium carbonates (see p. 1 19) : in soils, how- 

 ever, certain changes set in that not only obviate the need for acidity 

 but necessitate the presence of calcium carbonate. 



The unsuitability of the atmosphere of industrial towns has been 



