POTATOKS liT, 



and more rapidly distributi'd within tlic soil, tliu> inducinL' a 

 more extended development of feeding umi. 'YUr n\vi'ivj.i' 

 produce by the mineral and nitrogenous manures together, over 

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

 the estimated average produce of Great Britain under ()r(hnary 

 cultivation, and much more than that of Ireland. 



The plots receiving farmyard manure, containing al)()iit lMmi 

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

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

 86 lb. of nitrogen. In fact, only a small proportion of tin- 

 nitrogen of farmyard manure is rapidly available, that (hie t<» 

 undigested matter being more slowly available, and tliat in the 

 litter remaining for a long time inactive. Farmyard nianuri" 

 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 availal)le supply of ash-constituents, and 

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

 nitrogenous substance — starch, in the tubers. Thii>. the 

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

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

 genous and mineral manures together al)out '2~A)0 lb., 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 J ton per acre. 



Since we know that a free supply of potash is essential t<» 

 the j^roduction of any carl>oliydrate like starch, it might have 

 been expected that a bigger crcjp an<l an increased ])roduetion 

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

 mineral manure containing potash, than from l*lot 1> which 

 receives superphosphate only. There is, however, j)ractieally no 

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

 nitrogen and the exhaustion of the soil of its available supplies 

 (jf this constituent, the small crojis grown could always obtain 



