38 



There is abundant "experimental evidence to show that potash makes 

 the plant more resistant to the attacks of fungoid diseases. It has already 

 been explained how susceptible the use of nitrogenous manures renders the 

 mangolds on certain of the Rothamsted plots to the attack of a leaf spot 

 fungus — Uromyces betae. The attack is, however, much less severe on the 

 plots receiving an abundant supply of potash ; there, the plant remains 

 healthy even though the nitrogen is in excess. 



Just in the same way, the wheat on the potash-starved plots is always 

 subject to rust, even in a good season when very little is to be seen on the 

 other plots normally manured. The grass also on potash-starved plots is 

 attacked by various fungi; hence it may be taken as a general rule, that 

 crops which do not receive their full supply of potash will be correspond- 

 ingly susceptible to disease. 



It is not possible to say whether this is due to any specific alteration 

 in the composition of the cell contents or to a general lack of vigour, but 

 the latter is probable, because an excess of potash tends to prolong the 

 vegetative growth of the plant and to delay maturity. Plants receiving 

 potash are always a little the greener, especially late in the season, and this 

 is not always an advantage, as may be seen from the fact that the barleys 

 grown on the plots receiving potash at Rothamsted, show a somewhat 

 darker and less attractive colour than those grown without potash. That 

 potash tends to prolong growth may also be inferred from the fact that its 

 effect upon the yield is always most pronounced in dry seasons. 



In the dry season of 1893, the yield of barley (grown also with ammo- 

 nium salts and superphosphate) was increased by a dressing of potash 

 from 18.1 to 30.8 bushels per acre, whereas in the wet season of 1894 the 

 increase was only from 34.9 to 41.4 bushels per acre. 



Similarly with the wheat in the wet season the application of potash 

 only increased the yield of grain from 11.1 to 16.0 bushels, and the weight 

 from 54.6 to 57.8 lbs. per bushels ; whereas in the dry season the yield was 

 increased from 7.7 to 16.4 bushels (more than double), while the weight 

 was raised from 56.4 to 62.6 lbs. per bushel. That the bad results in the 

 dry year were due to a premature ripening of the plant, which was 

 deferred by the potash, is seen from the fact that with potash the ratio of 

 grain to straw was 98, whereas without potash it only reached 67.3, in 

 which case the migration of materials from the straw to the grain is clearly 

 incomplete. But though in such cases of grain crops the use of potash pro- 

 longs the development of the plant and defers maturity, apparently an 

 opposite effect is produced upon root crops. On the Rothamsted field, for 

 example, where potash is used, the mangold leaves will begin to turn 



