The relative availability of the organic phosphorus of sheep manure

Abstract

At Rothamsted nearly a century ago, experiments were started to study the use of various inorganic fertilizers as a means of maintaining soil fertility. Important among these was the study of phosphate fertilization, and early contributions were made to our knowledge of this subject. However, the problem of supplying available phosphorus adequate for plant growth still exists in varying degrees in different soils. Some soils possess a marked capacity to fix the soluble phosphorus in a form which plants cannot use. Research of the past two decades contributed many theories as to how the phosphorus is fixed. Even when the probable causes of fixation appear to be quite well understood, the problem of making the applied phosphorus available to the plants in a soil medium still remains unsolved. This problem was well recognized as early as 1902 when the results of 50 years of experimentation at Rothamsted were reported by Dyer (19). He had discovered that after 50 years annual application of superposphate to the soil, amounting to nearly 10 tons or acre that only 15 per cent of the phosphorus had actually been used by the crops and the remaining 85 per cent remained in the soil apparently as unavailable as the native soil phosphorus. In other words, the phosphorus actually used for plant growth cost approximately seven times the regular unit price of the fertilizer. On the other hand, 50 years’ application of manure at the annual rate of 14 tons per acre showed by analysis that approximately 43 per cent or nearly three times as much of the phosphorus had been used by the crops and the remainder was considered to be in a readily available form. Spencer and Stewart (59) and Stewart (63) suggested that organic forms of phosphorus fertilizer may be effective in avoiding fixation. Possibly also the organic forms of phosphorus may be especially suitable for supplying phosphorus to plants. Crop responses often observed where barnyard manures, green manures, or crop residues were applied to the soil likewise led to speculation that organic forms of fertilizer were highly successful for supplying phosphorus to plants. To verify these observations, controlled conditions where comparisons could be made between organic and inorganic forms of phosphorus were necessary. To prove conclusively that organic forms of phosphorus are assimilated as such plants must be grown in the absence of micro-organisms, thus forcing the plants to obtain their phosphorus from the organic. The final test for superiority of the organic phosphorus is the actual use of the organic forms in pot tests on soils low in available phosphorus and with a high phosphorus fixing power. This course of approach was chosen in this study to determine the value of organic phosphorus in plant nutrition. The value of animal manure in plant nutrition is well known. In an intensive grazing system, the return of manure to the soil is a very important consideration in maintaining soil fertility. Although the inorganic phosphorus of manure may be readily available for plant use, the organic phosphorus is more resistant to decomposition and is less available for plant use. This thesis is a report of work designed to determine the relative availability to plants of the organic phosphorus of sheep manure and the part it plays in the phosphorus cycle of plant nutrition.