i8 



Yield of Wheat. 



development of the root system. Little is known directly of the 

 manurial requirements of spring- wheat, but probably, like barley 

 and oats, it is less dependent upon nitrogen but more on phos- 

 phoric acid than autumn wheat. 



The wheat plant in climates like that of England continues to 

 take some food from the soil — nitrogen and phosphoric acid, for 

 example — almost . up to the time of harvest. Assimilation also 

 continues as long as any part of the stem or leaves is green. 



The process of seed formation consists in transferring pre- 

 viously stored starch, protein, &c. , from stem and leaf to the 

 seed, but the material transferred has much the same composition 

 in the earlier and later stages of the process. That wheat which 

 has been prematurely ripened through excessive drought or an 

 attack of rust is exceptionally rich in nitrogen is probably due to 

 the loss of carbohydrate from the grain by continued respiration, 

 and not to gluten entering the grain first, to be followed by carbo- 

 hydrates only in the later stages of filling. 



The ripening- process appears to be mainly one of desiccation. 



(4) Continuous Wheat Growing. — At Rothamsted, where 

 wheat has been growing on the same land for more than sixty 

 years, there is little evidence of any secular decline in the yield 

 due to the constant repetition of the crop, provided that sufficient 

 fertilisers are supplied. On the unmanured plot and in cases in 

 which complete fertilisers are given, the production tends to reach 

 a constant level when long-period averages are taken to exclude 

 the fluctuations due to season. That the Rothamsted yields are 

 never so high as are sometimes attained under ordinary farming- 

 conditions is due to the type of soil and certain difficulties in 

 cleaning and preparing the land when wheat follows wheat so 

 rapidly. 



(5) It is not, however, possible to analyse completely all the 

 factors involved. Type of soil in relation to climate is very 

 important. Thus the Rothamsted plots, even those receiving 

 abnormally larg-e dressings of manure, have only on two occasions 

 (1864 and 1894) approached 50 bushels, which on a good brick 

 earth would be by no means an exceptional crop even when grown, 

 as usual in England, with but little manure. To each type of 

 soil there is a limiting yield beyond which the crop will not go. 

 But the limit is not the same for all varieties ; it is not unusual 

 to find that one variety may do much better than another under 

 one set of conditions, but not so well under others. There is still 

 a good deal of work to be done in inquiring into the soil conditions 

 and reducing to precise terms such vague expressions as " a good 

 wheat soil." For example, on soils not very dissimilar, with the 

 same rainfall and management, a heavy wheat crop will stand in 

 one case, while on the other soil it will invariably go down, and 



