174 THE POT AS SIC FERTILISERS [chap. 



marked off the whole plot very distinctly, especially 

 after the first frosts had taken place. 



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. The photograph, Fig. 4, 

 shows two typical roots taken in 1902 from plots with 

 and without potash, both receiving the same large 

 dressing of nitrogenous manures. 



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 correspondingly sus- 

 ceptible 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 pro- 

 long 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 



