168 PHOSPHATES 



periodic dressings with lime or lime-containing manures, e.g., basic 

 slag. This is particularly the case with light sandy soils. Super- 

 phosphates are the quickest in action of all the phosphatic manures 

 and, for this reason, are very largely used. Whenever it is necessary 

 to supply a crop with a rapid supply of phosphoric acid, to tide it over 

 a particularly critical period where the conditions are unfavourable for 

 growth, e.g., during drought, or during the attacks of insect pests, 

 superphosphates are the most suitable manures to use. 



Abundant supplies of phosphates encourage root development and 

 early formation of seed. 



As a rule, superphosphates should be applied to the soil some time 

 before the plant is ready to assimilate them, in order that the acidity 

 of the manure may be neutralised by the bases in the soil. 



As to quantities per acre this, as with all manures, must depend 

 upon the fertility of the soil and the special needs of the crop. For 

 cereals, small dressings 1 or 2 cwt. per acre are sufficient, but for 

 turnips 3 or 4 cwt. or, in Scotland, up to 7 or 8 cwt. of superphosphate 

 per acre, best applied a month or two before sowing, are often used 

 with success. 



The objections to mixing superphosphates with nitrate of soda or 

 nitrate of lime because of the liberation of nitric acid, or with lime, 

 wood ashes, or basic slag or even with bone meal, because of the " re- 

 version ; ' of the phosphoric acid, have already been discussed. It may, 

 however, be mixed without harm with sulphate of ammonia, or with 

 sulphate of potash. 



(b) Basic slag. This cheap source of phosphoric acid should in 

 all cases be applied some time before the crop, and, indeed, is often 

 found to exert a considerable influence during the second and third 

 years after its application. Of great importance is its fineness of grist. 

 Its effects are most pronounced on damp, peaty soils, rich in organic 

 matter and poor in lime. On pastures and meadows it is especially 

 suitable and often exerts an effect similar to a nitrogenous manure. 

 This is probably due to two causes (1) the effect of its free lime in 

 promoting nitrification of the organic matter of the soil and (2) its 

 favouring influence upon the growth of leguminous plants which are 

 thereby enabled to increase nitrogen-fixation from the air. 



Basic slag, on account of its cheapness, slowness of action, and the 

 almost invariably beneficial effects of free lime on soils, may be used 

 with advantage, in comparatively large dressings up to 6 or 8 cwt. 

 per acre. 



It must not be mixed with sulphate of ammonia but may be used 

 along with nitrate of soda or potash salts. The slight tendency, which 

 such mixtures show to form little hard balls, may be lessened by the 

 addition of sawdust or peat. 



(c) Insoluble phosphates. These manures should be finely divided 

 and in any case are slow in action. Bones should be used as meal or 

 dust, since "quarter-inch bones" and "half-inch bones," which were 

 formerly much used, remain unchanged for many years in some soils. 

 Mineral phosphates are of little use unless extremely finely divided 

 and even then are slow in action. Indeed this class of manure tends 



