10 MASS. EXPERIMENT STATION BULLETIN 306 



0.002 N n.jSO.j is used as the solvent. According to Truog sandy soils should 

 yield a minimum of 50 pounds per acre of readily available phosphorus for general 

 farming. By this criterion all these soils are below the minimum limit for phos- 

 phorus and should respond strongly to applications of phosphates, but with the 

 exception of W-9, no marked response to this element was obtained in these 

 experiments. However, VV-1 soil contained only a trace of readily available phos- 

 phorus, and gave a negative response to phosphates. But in this case, as also 

 it may be with W-2, the soils showed a comparatively high fixing capacity for 

 phosphates. Apparently the chemical fixing capacity of soils for phosphates is 

 fully as important as the amount of readily available phosphorus in determining 

 their response to phosphatic fertilizers. It is interesting to note, and is probably 

 significant from a diagnostic standpoint, that of the four Worcester soils studied 

 in detail the two that showed a positive response to phosphorus also showed the 

 lowest fixing power. 



Phosphates soluble in N/5 H2SO4 were also determined by a modification of 

 the method used for the readily available phosphates, and are indicated in Table 7 

 as pounds of difficultly available phosphorus per acre. This represents the native 

 phosphorus that may be expected to become available through a long period by 

 the natural weathering agencies. These figures, together with such chemical 

 analyses as have been made indicate that the soils of Massachusetts are well 

 supplied with native phosphates. In 194 miscellaneous samples (3) from different 

 parts of the state the average content of P3O5 soluble in strong acid was 0.214 

 per cent; of 17 samples from Worcester County 0.25 per cent, or about 2200 

 pounds of phosphorus per acre of soil to plow depth. The total phosphorus will 

 exceed this figure by about 25 per cent. But this large quantity of native phos- 

 phorus is, like the immense store of native potassium in our pasture soils, for the 

 most part only slightly available to plants. 



Midgeley (4) and Weiser (7) studied certain factors affecting the penetration 

 of phosphates into soils. They showed that the phosphoric acid of superphosphate 

 penetrated soils more slowly than did that of certain other carriers, notably sodium 

 phosphate. These writers (5) called attention to "positional" and "chemical" 

 fixation of phosphoric acid by soils. Positionally fixed phosphoric acid is in a 

 readily available form, but largely unavailable to plants because it remains in 

 the surface layers of the soil and, therefore, out of reach of most of the roots. 

 Chemically fixed phosphoric acid is unavailable to plants by reason of its having 

 entered into difficultly soluble chemical combinations. That both positional and 

 chemical fixation of phosphates seem to have been high in Amherst pasture soil is 

 indicated by the figures of Table 8. For example, on p!ot-2P 243.3 pounds per acre 

 of phosphorus were applied in the course of 8 years, but only 108 pounds were 

 found in a readily available form in the first 4 inches. With a reasonable allow- 

 ance for available phosphorus originally in the soil and that removed by the 

 crop, it appears that about 31 per cent of the applied phosphorus was positionally 

 fixed, and 69 per cent chemically fixed. This latter figure compares favorably 

 with the 75 per cent of chemical fixation given for soil H-15 in Table 7. 



It may be noted, however, that even with the comparatively large amounts 

 of available phosphorus from the 2P and 3P treatments, the growth of vegetation 

 was low, averaging only 403 and 329 pounds for these treatments respectiveh-, 

 for the years 1931 and 1932; but when nitrogen was added to the phosphorus 

 treatment the yields were more than doubled. It appears that even if sufficient 

 available phosphorus is present in our upland pasture soils, deficiencies of other 



