THE REQUIREMENTS OF PLANTS 



The effects of phosphates in raising the quality and feeding value 

 of the crop are very great. The most nutritious pastures in England 

 and the best dairy pastures in France are those richest in phosphates. 

 Paturel 1 has also shown that the best wines contain most P 2 O 5 (about 

 0-3 gram per litre), the second and lower qualities containing suc- 

 cessively less. Further, when the vintages for different years were 

 arranged in order of their P 2 O 5 content a list was obtained almost 

 identical with the order assigned by the wine merchants. 



This close connection between cell division and phosphate supply 

 may account for the large amount of phosphorus compounds stored 

 up in the seed for the use of the young plant, and also the relatively 

 large amounts of phosphate taken from the soil during the early life of 

 the plant. 



Potassium. Hellriegel has shown (Table XVI.) that equivalent 



TABLE XVI. EFFECT OF POTASSIUM SALTS ON THE GROWTH OF BARLEY. 

 HELLRIEGEL (131). 



amounts of the soluble compounds of potassium have practically all 

 the same nutritive value. 



The effect of potassium compounds is more localised than that of 

 phosphates, so that potash starvation can be more readily detected. 

 The colour of the leaf becomes abnormal ; the potash-starved grass plots 

 at Rothamsted have a poor, dull colour, as also have the mangold plots ; 

 the leaves also tend to die early at the tips. The most striking effect, 

 however, is the loss of efficiency in making starch, pointed out long ago 

 by Nobbe (215) ; either photosynthesis or translocation it is not yet 

 clear which is so dependent on potassium salts that the whole process 

 comes abruptly to an end without them. Mangolds, sugar beets, 

 potatoes, and other sugar- and starch-forming crops reduce their pro- 

 duction of sugar with decreasing potassium supply even before the leaf 

 area has been diminished. Thus, in the mangold experiments of 

 Table XII. (p. 35), 7255 Ib. of leaf give rise to 14,684 Ib. of root 



1 Bull. Soc. Nat. Agric., 1911, p. 977. 



2 Lupines, however, could not tolerate the acid conditions set up when the mono- 

 phosphate was supplied (p. 138 in (130)). 



4 



