538 TRANSACTIONS OF SECTION M. 



Phosphatic fertilisers show the same kind of effect, but less frequently. In 

 the Aberdeen experiments increases in the dressing of superphosphate up to the 

 extraordinary dressing of 10 cwt. per acre still gave increases in the turnip crops, 

 while in the Cambridge experiments on the fen soils increases in superphosphate 

 up to 6 and 8 cwt. gave marked increases in mangolds and potatoes. 



2. But, when for any reason such as climate, supply of other nutrients, or 

 some soil condition, the crop has reached its limit of gi-owth, then the extra fer- 

 tiliser has no effect ; not until the limiting factor is removed can it begin to act. 

 In our own experiments swedes did not respond to increased dressings of manure, 

 because the climate does not allow of more growth than about 12 tons to the 

 acre ; so that, unlike the Aberdeen results, the extra dressings of manure were 

 without effect. In the Irish experimente already quoted, increasing dressings 

 of superphosphate had no effect on the yield of potatoes so long as only 1 cwt. 

 of sulphate of ammonia was given. 



3 cwt. of super 



Standard Dressing of Nitrogen and Potash + 



Tons Cwt. 

 10 16 



4 cwt. of super 



Tons Cwt. 

 11 2 



5 cwt. of super 



Tons Cwt. 

 11 3 



Whitney considers that this is the general rule in the United States, and, in 

 summarising the results of several thousand fertiliser experiments on wheat, 

 cotton,^ and potatoes, finds little indication of any significant difference in pro- 

 ductivity due to different amounts of fertiliser used.'" 



There is no real discrepancy between the two cases. What happens in the 

 first is that there is more tillering of the cereals, so that the number of individual 

 leaves and stems keeps on increasing, as the dressings of fertiliser increase. The 

 effect of phosphates on the root-crops is probably to facilitate swelling of the 

 roots, or, in the case of potatoes, to increase the number of tubers, either of 

 which would probably facilitate the deposition of storage products from the sap. 

 In these experiments there is no indication of any end-point, and apparently the 

 more the crop is fed the larger would be the yield. But the process does come 

 to an end. The final limit is reached by the inability of the plant to stand up 

 any longer or to grow any bigger. Wlren the corn-crop gets beyond a certain 

 size it is almost invariably beaten down by the wind and rain, so that the 

 difficulty of getting it in becomes considerable. Heavy dressings of nitrogenous 

 manures also predispose the crop to fungoid disease; attacks apparently being 

 facilitated by the thinning of the cell-walls and the change in composition of the 

 cell-sap. 



The way for further progress is then to seek new varieties that can stand up 

 and resist disease. And here a good deal has been done. Biffen has shown how 

 desirable properties may be transferred from one wheat to another, and his inves- 

 tigations are revealing the limits within which it is possible to construct a variety 

 of wheat according to the growers' specification. Similar work is badly wanted 

 for other crops. Fortunately our great seedsmen are fully alive to the possi- 

 bilities in this direction, and have already done much useful work. It is not 

 only in the case of cereals and potatoes that new varieties can be sought; there 

 is great scope also for new varieties of all other crops. The striking superiority 

 of wild white clover over tTie ordinary cultivated varieties, and the great 

 differences demonstrated at Woburn between varieties of rape and lucerne, 

 show that there is a considerable future for this sort of work. It need not 

 stop with varieties of crops at present in cultivation : the net might be 

 thrown further afield. Elliot boldly introduced some imconventional con- 

 stituents into his mixture with considerable success. Swiss pastures look 

 strange mixtures to English agriculturists, accustomed to recognise only grasses 

 and clovers as pasture crops, and yet the Swiss agriculturists assure us of the 

 value of some of the other plants. When I see a light-land farmer spending 

 time and money in trying to make a fodder-crop grow, and time and money 



" U.S. Dept. 0,/ Agric, Bureau of Soils, Bull. 62, 65, 66. 



