igo6.] How LONG DOES Lime last in the Soil? 327 



some of the grass plots are similarly acid through the use of 

 ammonium salts, and though sterility has not resulted, both the 

 amount and the character of the herbage have been very 

 injuriously affected. The action of sulphate of ammonia is 

 quite intelligible on ordinary chemical grounds, but the facts 

 that farmyard manure and, again, nitrate of soda, protect the 

 carbonate of lime in the soil from washing out require a little 

 further consideration. 



At first sight it would seem that the use of farmyard manure 

 ought to result in an increased consumption of carbonate of 

 lime ; the organic matter in its decay produces certain ill- 

 defined bodies we know as humic acids, also the nitrogenous 

 matter must be " nitrified " before it reaches the plant, i.e., it is 

 actually converted into nitric acid and requires a base like lime 

 for its neutralization if the process is to continue. Nitrification 

 in particular must be taken into account ; it is a process going 

 on in all healthy soils, v/here practically the whole of the nitrogen 

 reaching the crop has first to be nitrified and combined with a 

 base from the soil, so that if we assume the average crop to 

 contain 50 to 100 lb. of nitrogen per acre, then from 200 

 to 400 lb. of carbonate of lime will have been used up previously 

 for this process. Yet many soils which contain a mere 

 trace of carbonate of lime, retain their healthy condition 

 year after year under ordinary farming. Nitrification goes on 

 in these soils, acids are produced, but the soil does not becom.e 

 actually acid and infertile, provided no manure like sulphate of 

 ammonia is applied in large quantities. It would be out of 

 place here to go into the details of the solution of this problem, 

 which has long been one of the difficulties in interpreting the 

 chemistry of the soil, but the essential fact is that the plant, 

 when growing under normal conditions and feeding upon salts 

 dissolved in the soil water, does itself excrete or leave behind 

 in the soil an amount of base sufficient to make up for the 

 annual losses by nitrification. Thus when nitrate of soda, a 

 ready-made nitrate removing no base from the soil, is used for 

 manure, the plant which feeds upon it, by taking up more of the 

 nitric acid than of the soda, actually increases the amount of base 

 in the soil, so that the annual loss of carbonate of lime by drain- 

 age is diminished, as was found on the Rothamsted plots. Again, 



