26 FERTILISERS CONTAINING NITROGEN [chap. ii. 



statement made in the previous chapter, that they only 

 obtain the nitrogen they require in a combined form by 

 means of their roots. The form that the demonstration 

 has taken may be seen in the water culture experiment 

 which has already been illustrated ; in the absence of 

 combined nitrogen, the development of the plant is very 

 small. The same is true for cultures in sand, which re- 

 produce more closely the natural conditions, and many 

 experiments have been performed with the greatest 

 care with plants thus growing in artificial soils supplied 

 with a known amount of nitrogen. When the plants 

 have come to the full term of their growth, the nitrogen 

 they contain is found to be exactly balanced by the 

 amount of the same element which has been removed 

 from the soil. Among these experiments, a most 

 elaborate series were carried out at Rothamsted in 

 1857-58, and were generally regarded as definitely 

 settling the question against the fixation of nitrogen 

 by the plant itself. 



The experiments were made with wheat, barley, 

 oats, clover, beans, peas, and buckwheat, and the trials 

 were repeated, in the one case with no manure in the 

 pots, and in the other with the supply of a small 

 quantity of sulphate of ammonia. The soils employed 

 were made up fr6m either ignited pumice or ignited 

 soil, and the glass shades under which the plants were 

 grown rested in the groove of a stoneware vessel, mercury 

 being used as a lute. The air, previously passed 

 through sulphuric acid and sodium carbonate solution 

 and washed, was forced into the apparatus, so as to 

 always maintain a greater pressure inside than out, 

 thus minimising all danger of unwashed air leaking in; 

 carbonic acid was also introduced as required. Under 

 these rigorous conditions the following results were 

 obtained : — 



