PLANTS AND NITRATES 295 



number of the organisms still active, but in the main the 

 process of nitrification is at a standstill and no nitrates are 

 being produced. Now the generally received opinion is that 

 such normal plants as constitute our crops take in their nitrogen 

 only in the form of nitrate, so that the freedom of their growth 

 is entirely dependent on the rate at which the nitrifying 

 organisms in the soil first do the work of manufacturing 

 nitrates. This is probably too hard and fast an opinion. 

 Without doubt most plants feed on nitrates for choice, and soils 

 contain very much more nitrates than ammonia, because as 

 fast as the latter is set free by the bacteria which split up other 

 nitrogenous compounds in the soil into ammonia, it is seized 

 upon by the nitrifying bacteria and converted into nitrates. 

 But there are not wanting experiments to show that many 

 plants, especially cereals, are capable of utilising the nitrogen 

 of ammonium compounds directly, and later experimenters 

 (Hutchinson and Miller, J. Agric. Sci., 3 (1909), 179) have 

 succeeded in growing plants with ammonium-salts as the sole 

 source of nitrogen under absolutely sterile conditions, excluding 

 all bacteria which could change the ammonia into other 

 compounds. Most plants, however, prefer nitrates to ammonia 

 as their source of nitrogen, and the reduced yield on the acid 

 soils may be partly due to the fact that the grass is driven to 

 feed on ammonium compounds instead of the more usual 

 nitrates. The cause of the infertility of the acid soils cannot, 

 however, be regarded as completely established ; the competi- 

 tion of the fungi for the manures is doubtless a factor in the 

 result, but that would hardly seem to account for the total 

 failure of barley on the Woburn plots. It was surmised that 

 the fungi might produce substances poisonous to the growth of 

 the higher plants, but experiments to test this point have so 

 far yielded a negative result. 



Another example of these secondary actions between 

 fertilisers and the soil, which are not immediately apparent, is 

 afforded by nitrate of soda. The relation of nitrate of soda to 

 the plant may be regarded as the simplest possible ; we 



