POTATOES 125 



and more rapidly distributed within the soil, thus inducing a 

 more extended development of feeding root. The average 

 produce by the mineral and nitrogenous manures together, over 

 twenty-six years of continuous growth, was very nearly that of 

 the estimated average produce of Great Britain under ordinary 

 cultivation, and much more than that of Ireland. 



The plots receiving farmyard manure, containing about 200 

 Ib. of nitrogen, gave less produce than the mixture of mineral 

 manure and ammonium -salts or nitrate of soda, supplying only 

 86 Ib. of nitrogen. In fact, only a small proportion of the 

 nitrogen of farmyard manure is rapidly available, that due to 

 undigested matter being more slowly available, and that in the 

 litter remaining for a long time inactive. Farmyard manure 

 is, however, often applied in very large quantities for potatoes, 

 the process being to a great extent one of forcing, after which 

 remains a great amount of unexhausted manure -residue within 

 the soil. 



The characteristic effect of nitrogenous manures, provided 

 there be a sufficient available supply of ash-constituents, and 

 especially of potash, is to increase the amount of the non- 

 nitrogenous substance starch, in the tubers. Thus, the 

 produce of starch per acre was about 650 Ib. without manure, 

 about 1350 Ib. with purely mineral manure, and with nitro- 

 genous and mineral manures together about 2500 Ib., or rather 

 more than 1 ton. In other words, the increased produce of 

 starch by the use of mineral and nitrogenous manures together 

 was more than f ton per acre. 



Since we know that a free supply of potash is essential to 

 the production of any carbohydrate like starch, it might have 

 been expected that a bigger crop and an increased production 

 of starch would be obtained from Plot 10, receiving a complete 

 mineral manure containing potash, than from Plot 9 which 

 receives superphosphate only. There is, however, practically no 

 difference in the yield from the two plots ; in the absence of 

 nitrogen and the exhaustion of the soil of its available supplies 

 of this constituent, the small crops grown could always obtain 



