552 Journal of the Department of Agriculture. 



LOSS OF WATER-SOLUBLE PHOSPHATE IN 

 CERTAIN FERTILIZER MIXTURES. 



Xote fioiu tlu" r'heiiiical Laboratory, Elsenbiiro' School of A<iiifultnre, 



^Mulders Vlei, C'aiio. 



It i.s c(.iiuu()nly held that .superi)liosi)hate slioulil not hv juixcd witli 

 any fertilizer containing' lime, e.<>'. basic slag-, bone-dust, etc., because 

 on koeping the mixture tor some time an ai>pveciable decrease in the 

 percentage of water-soluble phosphate occurs. This decrease is what 

 is technically termed " reversion of the soluble phosphate," and it 

 implies that a chemical change has taken i)lace resulting in the 

 conversion of the soluble monocalcium phosi)hate (r'aHiP^O^) into the 

 less soluble dicalcium phosphate (C'a^HoPo^ ^s)- 



As a mixture of superphosphate and bone-meal is of great value 

 on many grain farms, a series of analyses was undertaken with a view 

 to determining the rate and amount of deterioration that lakes jdace 

 in the water-soluble phosphate in certain fertilizer mixtures. 



The following fertilizers were used in the preparation of the 

 mixtures: (1) High-grade superphosphate: (2) bone-meal; (3) Govern- 

 ment g'uano. 



The percentage of citric-soluble and waler-sohible phosphate was 

 determined in each of the above before the mixtures Avere nmde. 

 Throughout the experiment the mixtures were kept under dry storage 

 conditions in the laboratory ; periodic moisture determinations were 

 made in each of them, and the moisture content was found never to 

 exceed 5 per cent. 



Farmers who prepare such mixtures should not lose sight of the 

 importance of keeping them under normal storage conditions, as the 

 access of too much moisture would undoubtedly hasten the rate and 

 amount of loss of water-soluble phosphate. 



Three mixtures of bone-meal and su])erph()si)hate were prepared 

 in the proportions of 1:2, 1:1, and 2:1 respectively, and two 

 mixti/res of superphosphate, bone-meal, and (Tovernment guano in 

 the proportions of 1:3:1 and 1:1:A . An analysis of the citric and 

 water-soluble i)hosphate was made in each mixture immediately after 

 its preparation, and these analyses were continued at intervals of 

 seven days for a period of six weeks. 



From the results given on the following table it would seem that 

 the loss of water solubility in the i)liosphate is not very serious, more 

 especially if one takes into account the comparative availability of 

 the resulting precipitated phosphate and the advantage here of the 

 application of a mixture with lime to soils over the ap})lication of n 

 jiurely acid manure, 



