LOSSES OF PLANT FOOD FROM PLANTS 551 



of August 30 probably being due to variation in yield of the plots 

 harvested. Concerning this investigation, Lawes and Gilbert 

 made the following statements: 



"It might be supposed that there was some error in these estimates of the 

 amounts of the crop and of its constituents over a given area, and they ad- 

 mittedly involve some difficulty and uncertainty; as, for example, the possi- 

 bility of loss by fallen leaves, etc. But the fact that the phosphoric acid, which 

 would probably for the most part exist in a less soluble and less migratory 

 condition, is shown by the figures to increase gradually in amount per acre from 

 the first period to the last, tends to confirm the contrary results relating to the 

 lime and potash; and, assuming them to be correct, the supposition is that a 

 quantity of surplus lime and potash had been accumulated in, or excreted by, 

 the roots." 



There is no evidence to substantiate the "supposition" that 

 potassium and calcium are excreted by the roots or that they 

 tend to leave the tops and accumulate in the roots, but there is now 

 complete proof of loss by leaching. 



While these results obtained by Lawes and Gilbert many years 

 ago show losses only of elements which are not constituent parts 

 of the living tissue or structure of the plant, it should be kept in 

 mind that phosphorus may have been the limiting element, and 

 that the bean plant probably took up through its symbiotic rela- 

 tionship with the nitrogen-fixing bacteria no more nitrogen than 

 was needed for the normal growth of the plant; also that where 

 an excess of available nitrogen or phosphorus is furnished by the 

 soil, any growing plant may take up and tolerate more than it can 

 use in tissue building (because of some other limiting factor) and 

 that such excess of any element beyond the needs of the plant 

 may perhaps be removed in the process of leaching by rains. 



An investigation by Wilfarth, Romer, and Wimmer was reported 

 in 1905 (Landwirtschaftlichen Versuchs-Stationeri), in which bar- 

 ley, wheat, and potatoes were harvested at different periods of 

 growth and both the tops and roots were analyzed. The results 

 for barley are shown in Table 114. 



From these experiments it will be seen that the dry matter and 

 phosphorus reached their maxima in the third harvest and declined 

 but slightly thereafter; whereas the largest amounts of nitrogen and 

 potassium were found at the time of the second harvest (June 17) 



