III.] PLANTS REQUIRE COMBINED NITROGEN 53 



nitrogen which they derive from the soil. In the water- 

 culture experiments, for example, we see how incapable 

 of growing the plant becomes if the nitrate is omitted 

 from the culture liquid ; and the same thing occurs if the 

 conditions are made more normal by growing the plants 

 in an artificial nitrogen-free soil. Most careful experi- 

 ments have also been made, in which plants are grown in 

 prepared soil and the nitrogen present in the seed and 

 soil at the beginning is compared with the nitrogen 

 contained at the end in the plant and soil, with the 

 result that no gain from the atmosphere is detectable. 

 These experiments, however, are very difficult, and 

 subject to a large experimental error ; moreover, it has 

 been argued that the artificial conditions result in such 

 a diminished vigour that the plant has no longer the 

 vitality necessary to fix the atmospheric nitrogen. 



Such arguments are, however, disposed of by another 

 type of experiment, in which a series of plants growing 

 in sand supplied with the necessary mineral constituents 

 are given varying amounts of nitrate, with the result 

 that, up to a certain limit, the growth is strictly propor- 

 tional to the amount of nitrogen supplied. The same 

 kind of trial has been made in the field ; for example, at 

 Rothamsted one plot of mangolds was given a small 

 amount of ammonium salts to supply enough nitrogen 

 to start the plant into full vigour; Table IV., however, 



Table IV.— Produce of Mangold Roots, Rothamsted. 

 27 Years' Average. 



