1 62 CHEMICAL COMPOSITION OF THE SOIL [chap. 



must ascertain by experiment the specific requirements 

 of each crop, and adjust the manure to them, taking 

 also into account the character of the soil and the 

 style of farming, whether high or low. 



According to their habits of growth, different crops 

 possess very different powers of feeding themselves 

 upon the stock of plant food in the soil — the specific 

 requirements of each is generally the particular element 

 the crop finds some difficulty in obtaining for itself. 

 For instance, wheat is sown without much previous 

 preparation of the soil, and makes the greater part of its 

 growth during the cooler portion of the year when 

 bacterial activity in the soil is low ; as a consequence, the 

 stock of nitrogen compounds in the soil is being but 

 slowly converted into ammonia and nitrates available to 

 the plant, and this particular crop becomes specially 

 dependent on a supply of some active form of nitrogen 

 as manure. On the other hand, the wheat plant 

 possesses a very extensive root system and has a long 

 period of growth, so that it searches the soil pretty 

 thoroughly and is well able to pick up the mineral plant 

 foods — potash, phosphoric acid, etc. — which it requires. 

 Hence, on the normally fertile soil, wheat requires no 

 mineral manures, but will respond to active nitrogenous 

 manure should the land not be in very high condition. 

 Barley supplies an instructive contrast — it is a spring- 

 sown crop, and for it the land gets a more thorough 

 preparation than for wheat. Nitrification and other 

 bacterial processes rendering available the nitrogenous 

 compounds of the soil are active in the recently stirred 

 land as it is warming up in April, May, or early June, 

 so that the barley crop requires little manurial nitrogen, 

 though it takes away from the soil as much of this 

 element as does the wheat crop. Being, however, 

 shallow-rooted and possessing but a short period of 



