82 



SOIL CONDITIONS AND PLANT GROWTH 



weather generally, the nitrates do not accumulate but wash out, and 

 can be detected in the drainage water. The soil thus loses nitrogen 

 compounds, and in course of time the loss becomes very considerable. 

 At Rothamsted a little plot of arable land ~ acre in extent has been 

 kept free from vegetation by hoeing, but not otherwise disturbed, since 

 1 870 ; it has now lost one-third of its original stock of nitrogen. The 

 plot has been converted mto a lysimeter by isolating it from the sur- 

 rounding ground by cement partitions and then underdraining : the 

 drainage water is collected daily and analysed. At the end of thirty- 

 five years the amounts of nitrogen found as nitrate in the drainage 

 waters were added up and found to be only no Ib. less than the total 

 loss of nitrogen from the soil (Table XXXIV.). 



TABLE XXXIV. CHANGES IN NITROGEN CONTENT OF A SOIL KEPT FREE FROM 

 VEGETATION FOR THIRTY-FIVE YEARS, BUT EXPOSED TO RAIN AND WEATHER. 

 MILLER (200). 



The obvious uncertainty attaching to so prolonged an experiment 

 is reduced in this case by the fact that the determinations have for the 

 last twenty years of the period been made by the same analyst. 

 Miller found that the rate of loss of nitrogen (estimated by the quanti- 

 ties of nitrates in the drainage water) was about 40 Ib. per annum in 

 the earlier years, and fell below 30 Ib. per annum in the later years. 

 The experiment is not fine enough to justify any discussion of the 

 missing no Ib., but it shows that the loss of nitrogen is mainly due 

 to leaching out of nitrates. 



It is unfortunate that this highly important experiment has not 

 been repeated with other types of soil, because there is evidence that 

 a richer soil would lose more nitrogen than is accounted for by the 

 nitrates formed, the rest presumably escaping as gas. 



3. When the conditions are made wholly normal by allowing vege- 

 tation to grow, some of the nitrate is taken up by the plant and only 

 a part is washed away, the division depending on the favourableness of 

 the conditions for plant growth. The absorption of nitrate by the 

 plant is much greater, and the amount of nitrate in the drainage water 

 is therefore much less, on the Rothamsted wheat plots, where ample 



