496 SCIENCE PROGRESS 



Dr. Scott Robertson's monograph appears, therefore, at an opportune 

 time. He has tested, over a period of several years, a number of different 

 types of slag, and has also included various mineral or rock phosphates. 

 The experiments have been done at a number of centres in Essex, on boulder 

 clay, London clay, and chalk soils. Inspection of the tables and diagrams 

 clearly shows the beneficial effects on the yield of hay of all the forms of 

 phosphate used. There were, however, two centres where the increase was 

 not significant. In one case the soil was comparatively rich in phosphate 

 to begin with, and deficient in potash, and in the other the potash content 

 was also low, and the conclusion is drawn that the lack of potash was acting 

 as a limiting factor to production. In cases where no such limitation comes 

 in, the highly soluble slags gave better results in dry seasons than less soluble 

 slags or mineral phosphates, but in wet seasons there was less difference. 

 In view of the low average rainfall in Essex, it seems reasonable to conclude 

 that over the greater part of the country rock phosphate — especially that 

 from North Africa — will be a suitable substitute for the disappearing high- 

 grade basic slags. There is one probable restriction to this conclusion. 

 Dr. Scott Robertson, during his frequent inspections of the experimental 

 plots, concluded that rock phosphates are slower than basic slag in their 

 action during spring and early summer. Hence for conditions when the 

 harvest is early, higher soluble slags should be used, but if the crop grows 

 until the latter end of July the dift'erence largely disappears. 



Besides this increase in bulk of the herbage, the change in its quality 

 has also been examined. The characteristic effect of phosphates in increasing 

 the amount of clovers present is accompanied by an increase in the better 

 type of grasses, probably due to a direct fertilising action. Tlois latter effect 

 was noticed at one centre where the clover did not markedly respond until 

 the following year. There are considerable fluctuations in the amount of 

 clover from year to year, the causes for which are not clear, and could profit- 

 ably receive further study. 



A very interesting set of moisture and temperature determinations shows 

 the beneficial indirect effect of phosphates in preventing the soil from crack- 

 ing, and in keeping down the temperature during dry, hot spells. Some 

 observations on the effect of phosphates on the accumulation of nitrates in 

 the soil, both in the field and in pots, are also given, which indicate that 

 at certain periods during the season there is a greater accumulation of 

 nitrates in the slag plots than in the untreated. 



The monograph concludes with a section on the action of basic slag on 

 the acidity of the soil, in which the failure of the clover plant at Cockle Park 

 is discussed and contrasted with the failure on some pastures in Essex during 

 the dry season. In the former case phosphate appears to be the limiting 

 factor, and in the latter case the principal cause is the lack of available 

 potash. 



The energy and zeal of Dr. Scott Robertson will be appreciated by all 

 those familiar with the large amount of work inseparable from the efficient 

 conduct of scattered field experiments, and the information given in the 

 monograph should prove most useful to agricultural advisers and farmers 

 generally. 



B. A. Keen. 



Agricultural Economics. By James E. Boyle, Ph.D., Extension Professor of 

 Rural Economy, College of Agriculture, Cornell University. [Pp. 

 ix + 448.] (London: J. B. Lippincott Co., 1921. Price 12s. 6d. net.) 



As Malthus realised more than a century ago, the food supply of a country 

 determines both the extent of its population and the happiness of the people 

 comprising it. The food supply itself is determined by agriculture and by 

 other industries such as transport and storage closely connected with it 



