THE MICRO-ORGANIC POPULATION OF THE SOIL 297 



growth of barley: up to the time of formation of the head 

 the rate of absorption progressively increases till finally the 

 amounts of nitrogen and potassium in the plant reach a 

 maximum. During the period of translocation of material 

 into the developing heads, however, there is not only a 

 decreased rate of absorption from the soil but a substantial 

 loss of nitrogen and potassium from the aerial parts of the 

 plant. Towards the end of this period some of the lost 

 materials are regained. In the third period ripening is com- 

 pleted, absorption ceases and losses are resumed. These 

 losses are experimentally traced from the aerial parts alone, 

 and it is impossible to assert that they take place from the 

 whole plant owing to the difficulty of completely extracting 

 the roots from the soil. The presumption is, however, that 

 such losses do take place. 1 



It is interesting, and may be significant, that Russell and 

 Appleyard (241^) observed a marked increase in CO 2 content 

 of the atmosphere of cropped soils during the period of ripen- 

 ing, when these losses from the plant are presumably taking 

 place. 



We have now to ascertain how these various effects react 

 on the soil micro-organisms. Several attempts have been 

 made to correlate bacterial numbers with the nature of the 

 crop, but the data hitherto obtained are inadequate for satis- 

 factory discussion. There is, however, considerable evidence 

 to show that nitrate accumulates more readily on uncropped 

 than on cropped soils, even after allowance is made for the 

 quantity absorbed by the plant. Table LXXX. gives some 

 of the results obtained at Rothamsted. 



To some extent climatic factors come into play, the cropped 

 land frequently being somewhat cooler and drier than the fal- 

 low. But this does not hold universally, and the phenomenon 

 has been observed under such widely different conditions that 



1 Le Clerc, U.S. Dept. of Agric. Year Book, 1908, 389-402 ; also Andre, 

 Compt. Rend., 1910, 151, 1378-82, support the view that the losses occur by 

 washing from the leaf rather than from the root. For mass of data suggesting 

 root losses see Wilfarth, Romer, and Wimmer, 3096. 



