248 
Journal of Agricultural Research 
Vol. XXXI, No. 3 
While the influence of soil conditions on the comparative efficiency 
of insoluble and soluble phosphates has been a subject of many investi¬ 
gations, the influence of the soil on the absolute efficiency of a soluble 
phosphate has received comparatively little attention. It has been 
known since the early days of soil science that the phosphorus of 
soluble phosphates is fixed by the soil and that some of the phosphorus 
is available after it is fixed. But we are comparatively ignorant of 
how much of the “fixed” phosphoric acid is available and how much 
remains unavailable in different soils. The following facts indicate 
that in many soils a considerable part of the soluble phosphorus is 
rendered at least temporarily unavailable: The small proportion of 
the applied phosphoric acid which is recovered in the crop; the loss 
in efficiency of phosphates remaining in unplanted soils; and the 
variation in the quantity of soluble phosphate required to produce a 
given crop increase on different soils. 
When fertilizers containing soluble forms of nitrogen or potassium 
are applied, not in excess, to soils responding to these ingredients, 
60 to 90 per cent of the quantities applied are commonly recovered 
in the crop (36, 37, 23). The recovery of phosphoric acid, however, 
frequently amounts to only 10 to 20 per cent (26, If). It might be 
held that the low recovery of phosphoric acid is due to only a part 
of the phosphorus being required as a plant nutrient, the remainder 
of the phosphorus being effective in increasing growth in other ways 
than in supplying available phosphorus to the plant. Some would 
hold, for instance, that soluble phosphates promote growth in some 
soils by reducing toxic, soluble aluminum in the soil (3, 2). It 
seems possible, however, that soluble aluminum is injurious only as 
it renders phosphate unavailable to the plant. The fact that rela¬ 
tively high recoveries of soluble phosphates may be made by plants 
growing in quartz sand indicate that the low recoveries from many 
soils are due to interaction between the phosphate and certain soil 
constituents (26). 
Pot experiments have shown that in some soils an appreciable loss 
in the efficiency of acid phosphate takes place when the phosphate 
is applied a few weeks in advance of planting, the loss occurring in 
the bare soil maintained at the optimum moisture content (8). It 
amounted in 30 days to approximately 40 per cent of the efficiency 
of acid phosphate applied immediately before planting. Since a 
crop does not abstract much phosphoric acid per acre until it has 
attained some size, it is evident that there may be an appreciable loss 
in the efficiency of acid phosphate applied under the usual conditions. 
A further indication that certain soils render soluble phosphates 
partially unavailable is the fact that very different quantities of acid 
phosphate may be needed to produce the same increased yield on 
two different soils, even in cases where the two soils give approxi¬ 
mately equal increases in yield with different maximum quantities 
of the phosphate. In some instances this may reasonably be attri¬ 
buted to different weather conditions, to different cultural methods, 
to the use of different nitrogenous fertilizers, and the like. Vari¬ 
ability in the response of a crop attributable to such conditions, 
however, can be largely eliminated in pot experiments. And a 
marked influence of the character of the soil on the quantity of 
phosphate required to produce a given crop increase is observed in 
pot experiments also. 
