6 BULLETIN 6, U. 8S. DEPARTMENT OF AGRICULTURE. 
from nitrogen starvation when growing in acid humus. For such 
crops the neutralization of the acidity by lime is of vital impor- 
tance, for not until this is done can the nitrogen of the humus, how- 
ever abundant, be changed into nitrates. Whatever other direct 
injurious effect acidity may have on crops, the fact that it checks 
the nitrification of humus is of itself sufficiently important and sig- 
nificant to justify all the investigation that the subject has received. 
SOURCE OF NITROGEN FOR ACID-LAND PLANTS. 
There is another phase of the acidity question. Many plants thrive 
in soils which are acid and which therefore theoretically can produce 
no nitrates. There are three possible methods by which these plants 
may secure their nitrogen: 
(1) Although a sample of soil when tested as a whole shows an 
acid reaction, there may exist in it innumerable minute tracts, sur- 
rounding particles of lime, where the reaction is alkaline and where 
nitrates are in process of manufacture. It is to be hoped that inves- 
tigators will find some means to determine the possibility of such 
a method cf nitrogen nutrition in acid soils. 
(2) Many acid soils contain a large amount of nitrogen in the 
form of ammonia, and while hitherto scientific opinion has been much 
divided over the question whether ordinary crop plants can utilize 
ammonia nitrogen directly, without transformation by bacteria into 
nitrates, careful chemical investigation under such conditions as 
to eliminate the possibility of bacterial action should enable us to 
determine which of our crop plants can feed on ammonia nitrogen 
and which can not. Intelligent agriculture needs this information. 
(3) It is conceivable that a crop plant might utilize nitrogen that 
existed in organic form in the humus of the soil, having not yet 
reached the ammonia stage of decomposition. It is agreed by plant 
physiologists that ordinary plants, those bearing green foliage, are 
unable to do this. It is also agreed by plant physiologists that fungi 
not only can but habitually do use organic nitrogen. These two facts 
warrant the consideration of a remarkable partnership that exists. 
between certain leaf-bearing plants and certain fungi, a partnership 
the significance of which has only recently begun to be appreciated by 
botanists and is almost unknown in agricultural literature. 
The subject is well illustrated in the blueberry. The possibility of 
the culture of this wild berry has been under investigation for several 
years, the experiments having now reached a successful conclusion.* 
1Experiments in Blueberry Culture, United States Department of Agriculture, Bureau 
of Plant Industry, Bulletin 193, 1910; also Directions for Blueberry Culture, United 
States Department ef Agriculture, Bureau of Plant Industry, Circular 122, pages 5 to 11, 
1913. 
