10 Oct., 1918.] The Residual Effect of Superphosphate. 615 



form unavailable to the rootlets of the wheat plant. Water soluble 

 (mono-calcie) phosphoric acid is the form in which this plant food is 

 readily available for the use of plants, and is, therefore, the most valuable 

 to the fanner. On reference to the guaranteed analysis on the tags 

 attached to bags containing superphosphate, it will be seen that by far 

 the greatest part of the phosphoric acid which the fertilizer contains is 

 soluble in water, and consequently immediately available to the plants. 

 Its ready availability is demonstrated by the increased vigour and 

 growth of the crop to which it is applied, practically from the appear- 

 ance of the plants over ground. This point is well illustrated by the 

 growth on the unmanured plot No. 4 (Fig. I.) as compared with that on 

 the adjoining plot, which received only ^ cwt. of superphosphate per 

 acre. Both plots were sown with Yandilla King wheat on 19th June, 

 and the photograph was taken on 17th iSeptember following. 



When applied to the soil, water soluble phosphoric acid quickly 

 reverts (combines with lime, &c., in the soil) to forms less soluble, and 



t 



Fig. 4. — Green Manure Trials — Feeding off Cape Barley with Sheep. 



therefore less available to plants, but probably this reversion does not 

 occur till the phosphoric acid has become dissolved in the soil moisture. 

 Assuming such to be the case, the plant food would be distributed through 

 the soil, and in such a fine state of subdivision that, although reverted, 

 it could still be utilized by the plants. In any case, the variation in 

 growth shoAvn in Fig. No. II., which is brought into sharp relief by 

 Fig. No. III., and the weights recorded in Table No. 1, clearly indicates 

 that there is a considerable amount of plant food still fairly readily 

 available after the fertilized crop has been harvested. 



Cash Value of Residual Benefit. 



Close to the field where the results specified above were obtained is 

 another experimental field, in which different crops are annually grown 

 and fed off with sheep. The sheep are carefully weighed " in " and 

 " out " of the plots, in order to estimate the grazing value of the crop 

 on which they are pastured. No wheat is grown for this purpose, but 



