838 



Notes on Manures for December. 



[Dec, 



of the cases of apparent inertness of slag may be due to shortage 

 of potash. It has long been known that this happens on hght 

 soils, though here the increased yield of hay or of grazing vegeta- 

 tion may be too small to justify the expenditure incurred in 

 securing it ; but it has not before been suspected that a shortage 

 of potash might occur on heavy soils, and this possibility is there- 

 fore being tested at Eothamsted during the coming season. 



Does Superphosphate use up the free Lime in a Soil? — A 



correspondent asks the following question : — If a sample of 

 superphosphate contains a certain amount of combined lime can 

 it be assumed that an equal amount will be withdrawn from the 

 reserves in the soil when the supei-phosphate reverts, as it is 

 supposed to do, directly it is put on the land? 



The question is difficult to answer because the changes occur- 

 ring when superphosphate is added to soil, and again when it is 

 taken up by the plant, are very complex and cannot be set out in 

 any simple w^ay. Undoubtedly the superphosphate becomes in- 

 soluble and to this extent might be expected to combine with and 

 therefore withdraw from the soil a certain proportion of its 

 basic material including the free lime ; but the amount with- 

 drawn is really small, even if one supposed that the whole burden 

 falls on the lime. Onlv a fraction of the total lime is involved 

 in any case ; much of it is in the form of calcium sulphate which 

 does not react with lime ; the active part is the mono-calcium 

 phosphate, and the lime with which this can react does not 

 amount to more than 5 or 6 per cent, of the weight of the super- 

 phosphate, according as the sample is of 26 or 30 per cent, 

 strength; in other words, a dressing of 2| cwt. of superphosphate 

 would withdraw from the soil less than 15 to 20 lb. of lime per 

 acre. It is improbable, however, that all the burden falls on 

 the lime in the soil. 



Town Refuse as Manure: further Instances.* — A test of town 

 refuse has been made this season at Eothamsted. Three 

 plots were laid out on the mangold field : one was given London 

 stable manure at the rate of 10 tons per acre, and the other two 

 received town refuse sent from Ilampstead by the contractors. 

 In addition artificials were supplied at the rate of 3 cwt of super- 

 phosphate and a mixture of muriate and sulphate of ammonia 

 equivalent to 3 cwt. of sulphate of ammonia. No differences were 

 observed between the three plots at any time, and in the end the 

 yields of roots were equal within the errors of experiment, there 

 being 22 tons per acre on the straw manured plots and 23 tons 



* See this Journal, November, 1922, p. 685. 



