136 



quent and violent, there is a very large amount of nitric acid, either 

 free or combined, in the rain water. The relative values in different 

 climates are as follows : 



The island of Reunion. 



Caracas 



Rothamsted ._ -- 



Liebf rauenberg 



Weight of 

 nitrogen 



per meter 

 rainfall 

 per hec- 

 tare. 



Kilos. 

 6.93 



NITROGEN DIRECTLY ABSORBED BY THE SOIL. 



Schloesing has shown that the atmospheric ammonia has its in- 

 fluence upon the plant greatly multiplied by the direct absorption 

 of this ammonia from the air into the soil. The absorption is greatest 

 when the difference between the tension of the ammonia in the soil 

 and that in the atmosphere is at a maximum ; it is therefore greatest 

 when the soil is moist and when nitrification converts the ammonia 

 into nitrates as fast as it is absorbed. 'Wlien the earth is dry nitrifi- 

 cation is suspended, and the ammonia accumulates in the soil up to 

 a certain point, beyond which the rate of absorption gradually 

 diminishes. (Agr. Sci., Vol. IV, p. 292.) 



FIXATION OF NITROGEN BY PLANTS. 



Experiments as to the source whence the grains (Graminea^) and 

 the beans and peas (Leguminosa^) derive their nitrogen have been 

 made both in Germany and France by independent methods. Thus 

 Hellriegel and Wilfarth from 18S3 to 1887 experimented upon sam- 

 ples of these plants, each of which was placed in a pot of sterilized 

 quartz sand to which was added a nutrient solution, and the plants 

 were watered with distilled water so as to keep the conditions favor- 

 able to growth. The results were that oats and barley behaved alike ; 

 when they are not furnished with nitrates there is no developmei\t 

 beyond the reserve in the seed, and when they are fed with nitrates 

 the harvest of dry matter is directly proportioned to the quantity of 

 nitrate. For every milligram of nitrogen the increase of dry matter 

 is 93 milligrams for barley and 96 for oats, respectively. Steriliza- 

 tion of the soil and of the pots on the one hand, and the addition of 

 the microbes contained in the washings of cultivated soil on the other 

 hand, cause no variation in the above results. 



Peas behave quite differently from the preceding. Some plants 

 languish if they have no nitrates, but others suddenly acquire new 



