166 SOILS AND FERTILIZERS 



Another effect of excess nitrogen supply is to delay the 

 ripening of crops. This is often seen in orchards that receive 

 clean cultivation throughout the summer. The large supply 

 of nitrogen thus made available, as well as the moisture re- 

 tained in the soil, serves to retard ripening and the immature 

 wood is likely to be injured by winter temperatures. In 

 regions having short, but usually hot seasons, cereals are 

 sometimes delayed in ripening until injured by frost. 



Sometimes the quality- of crops may be injured by an ex- 

 cess of nitrogen. Barley deteriorates in its malting qualities, 

 and peaches in flavor when too much nitrogen is supplied. 



The percentage of nitrogen may be increased in some crops 

 by supplying a large quantity of available nitrogen. Tim- 

 othy hay responds in this way, as do many vegetables, and the 

 straw and even the grain of cereals. 



Resistance to disease is often decreased when nitrogen is 

 abundant. This is familiarly exhibited in the ease with which 

 a crop of wheat or oats on very rich soil will succumb to rust. 

 There are numerous cases of this kind, probably due to a 

 change in the physiological resistance of the plant to the 

 diseases to which it is exposed. 



218. Availability of nitrogenous fertilizers. — It has been 

 pointed out that nitrates are the form in which nitrogen is 

 most acceptable to the larger number of agricultural plants, 

 and this being the case fertilizers having nitrates offer a very 

 readily available form of nitrogen. Ammonium salts not 

 being so readily appropriated by most plants require at 

 least partial conversion into nitrates. Ammonia is ab- 

 sorbed by soil, but in its absorbed condition readily 

 undergoes nitrification. However, there is apparently some 

 loss or conversion into an insoluble condition, for experiments 

 have generally shown that there is rarely quite as much nitro- 

 gen recovered by crops from sulfate of ammonia as from ni- 

 trate of soda. The organic nitrogenous fertilizers must un- 



