Biological and Chemical transformation of Phosphorus and its Availability to Plants as Influenced by Humic Acid in NWFP (Pakistan) Soils

Abstract

The work of this report is concerned with the study of some aspects of P-cycling in NWFP soils. in the first phase preliminary study related to physicochemical properties, C-mineralization and P-absorption of agriculturally important soil series were carried out in laboratory. In order to monitor P-changes in greater depth laboratory incubation study was conducted in the second phase to study the effect of FYM on both native and added P. CO2 evolution and P-turnover rates showed that moderate to high level of C but very little P, was turned over. C- mineralization rate which range from 65.0 to 110.0 mgkg-1 soil week-1 indicate the presence of biologically available C. 40% of the series studied showed P-turnover, while the rest exhibit PO4 presence corelated with lime, clay, organic matter and total salts content. The rate of change of extractable P ranges from -0.2 to 0.3 mgPkg-1 soil week-1. Due to either loss off P or very low P mineralization rates, the effect of soil pH on P was not pronounced. The generally low value of initial P and its significant correlation with P-turnover suggest that p-mineralization may be slower in these soils. High value of absorption maxima (149 to 250 mgkg-1 soil0 calculated from Langmuir plots highlight the great ability these soils have for removing phosphate from solution. significant corelation CaCO3 and clay content with P- absorption indicated that these two factors are greatly instrumental in the absorption phenomena. Significant but +ve correlation of pH with P absorption explain that P-fixation is a serious problem in alkaline calcareous system. It is a general belief, that organic matter influences the available-P content thru its microbially decomposed products. A laboratory incubation study was conducted to investigate the effect of 5 level of FYM on the bias of humic acid content (0.1,0.,2,0.4,0.8 and 1.0 mgkg-1soil of humic acid). The result showed that addition of FYM increased that availability of P, but did not increase that amount of mineralization-P from the applied-P with time. The mineralized-P either transform into compounds not extractable or assimilated by microorganism and result in the build-up of total-P retained in the soil. Studies have shown that Pakistan’s low rank weathered coal have sizeable amount of humic acid. This humic acid can be utilized as a fertilizer and soil conditioner to boost up agricultural production. Information in this respect is, however, limited. Increasing the availability of native and added-P by humic acid, may be better understood, if the effect of their application on native and added inorganic-P is known. Incubation of calcareous soil (% CaCO3 = 23.0) with34 mgkg-1 soil added-P reinforced with 5 levels (0.1-1.0 mgkg-1 soil) of humic acid, resulted in increasing mineralized-P. it was noted that mineralized-P increased with the increase in humic acid from 0.4 to 1.0 mgkg-1 soil. Humic acid @ 0.1 mgkg-1 (eq=200 gha -1) alone mineralized P equalling to that of 1.0 mgkg-1 soil. Similarly immobilized-P was lower in humic acid applied @0.1 and 1.0 mgkg-1 soil alone. Percent-P recovery and available-P was also greater in these two-treatment applied alone or reinforced with P. humic acid @200gha -1 seem to be more conducive for P availability and suppress P fixation through various mechanism, both chemical and biological. Marked effect of added-P suggests that humic acid greater than 200 gha -1 may not be necessary to reinforce with P fertilizer for increasing its efficiency. Humic acid applied greater than 200 gha-1 caused P starvation. Added inorganic-P and that release from organic forms may be followed by sorption, precipitation a microbial assimilation. Humic acid @ greater than 200 gha-1 induce P-immobilization on reinforcement with P necessitated to further study the respose of maize=e (zae mays L.Cr. Shaheen) crop to evaluate the relative efficiency of added-P reinforced with 200 gha-1 humic acid in term of yield and yield components. Since it was expected that in addition to biological and chemical P-immobilization or both might play an important role, in crop respose to applied-P, absorption potential was also measured before and after humic acid application, the experiment was conducted at Agricultural University Research Farm, during 1996 and ssp (single super phosphate) @ 50,100 and 150 kg P2O5 ha-1 was applied alone and reinforced with humic acid in different combinations. The result showed that all levels oof P reinforced with humic acid proved ineffective in increasing the grain yield and yield components significantly, expect humic acid applied alone. Humic acid @ 200 gha-1 increase grain yield and all yield components significantly and was also comparable with all P level applied alone, in term yield and yield components. 100 kgP2O5 ha-1 gave the highest yield, but statically not better than humic acid applied alone. Fertilization increased the P-bonding constant, thus increasing P absorption and decreased P recovery. The decrease in adsorption maxima, after fertilization, decreased P utilization by maize crop. As a result, there was decreasing effect on P concentration of plant by humic acid reinformed with P. the decrease in P uptake seem to be the major cause of low yield. To conclude, the effect of humic acid applied alone @200 gha -1was more effective in making the soil environment conductive for plant nutrients availability and may be considered beneficial for the subsequent crop. The organic matter status of the soil could be built up gradually by use of even small quantity of humic acid, it can be also advocated that the cumulative effect would be far more than both, direct and residual on soil physical, chemical and biological properties.