130 PLANT DISEASE 



tons of lime per acre, with the result that the 

 soil was rendered very loose and friable. 



It was then sown to oats, and one of the very 

 finest crops I have ever seen was produced. 



One naturally asks what was the change 

 that took place to enable such a crop to be 

 grown. No doubt all the phosphoric acid, 

 potash, and iron that were in the soil were 

 rendered available for plant food, but where 

 did the nitrogen come from ? 



It would not be likely to be locked up in 

 such a soil, and if it was one would think the 

 lime would expel it from the soil ; at least, lime 

 applied to stable manure drives off the am- 

 monia, and any available nitrogen had no doubt 

 been taken out by such crops as it had 

 previously produced. 



So that the aeration and oxidation which 

 no doubt took place, leads one to the natural 

 deduction that the oxidised iron in the soil 

 took up the ammonia, with the resulting 

 nitrification. 



The fact remains that nitrogen was an essen- 

 tial constituent of fertile soils for thousands 

 of years before nitrogenous manures were even 

 dreamt of ; then where did it come from ? 



