PHOSPHOROUS FORMS, ADSORPTION DYNAMICS AND RELATED SOIL PROPERTIES AS INFLUENCED BY AMENDMENTS, AND ITS REQUIREMENTS BY BREAD WHEAT VARIETIES (Triticum aestivum L.), IN ACID SOILS OF CHEHA AND DINSHO DISTRICTS, ETHIOPIA

Abstract

Low availability of P is among the major problems limiting crop production in the highlands of Ethiopia, including the study area, where soil acidity is a major problem. Information on chemical forms and rate of P adsorption, which can be used as a basis for P fertilizer recommendation, is not available in Cheha and Dinsho districts, souhthern highlands of Ethiopia. The current study was, therefore, conducted to evaluate the amount and distribution of P forms as influenced by lime, vermicompost (VC), and inorganic P applications; mechanisms that govern phosphate adsorption kinetics and quantify internal and external P requirements of two wheat varieties. Twelve representative soil samples were collected from the surface layers (0-15 cm) of cultivated fields selected after conducting a preliminary soil survey and field observation. Relevant laboratory analyses, including different forms of extractable P, were carried out following standard laboratory procedures. Pseudo first-order, pseudo second-order, parabolic diffusion, Elovich, and fractional power equations were used to study phosphate adsorption kinetics. An incubation experiment was conducted to evaluate the effects of lime, vermicompost, and mineral P fertilizer on the distributions of P fractions and other selected soils chemical properties. A pot experiment was also conducted to evaluate the external and internal P requirements of Damphe and Picaflor wheat varieties. Relevant statistical analyses of the data generated were also carried out. The studied soils were in the range of very strongly acidic to strongly acidic (4.65 to 5.45). Mehlich-3 P had positive association with pH, CEC and OM, while it was negatively correlated with exAc and exAl (p ≤ 0.01). Oxyhydroxides of iron and aluminum were dominating the exchange sites of the studied soils in Dinsho district [Feox (64.61 mmolkg-1), Fed (309.82 mmolkg-1) Alox (84.45 mmolkg-1) and Ald (123.62mmolkg-1)] and Cheha District [Feox (82.85 mmolkg-1), Fed (512.36 mmolkg-1), Alox (137.15 mmolkg-1) Ald (156.35mmolkg-1)]. In terms of the relative abundance of P forms, Res-P and HCl-P were overriding other P forms in Cheha and Dinsho district respectively, followed by NaOH-Po, and NaOH-Pi, whereas the total sum of readily available P forms NH4Cl, NaHCO3-Pi and NaHCO3-Po didn’t exceed 4% of the total P. The sum of P fractions ranged from 606.34 mg kg-1 in SC1 soil to 1145.73 mg kg-1 in SD3 soil where the Res-P and HC1-P fractions dominated the soil P pool respectively. The study results also showed that pseudo second-order equation provide the best fit to experimental data for different initial phosphate concentrations. Initial P concentration and contact time between the phosphate ions and the adsorbent soil influenced the rate and amount of P adsorbed at equilibrium. Soils of Cheha district had higher kinetics of adsorption and greater amount of adsorbed P at a specified equilibration period and concentration. Statistically considerable (p ≤ 0.01) association between adsorption kinetic parameters and soil chemical properties has been exhibited. The Freundlich model could be considered as the best model for the description of the P adsorption characteristics of the soils in this particular study. The Freundlich coefficient Kf ranged from 123.32 to 315.31mg P kg-1. In this study, the SC1 soil had the highest Kf (315.31 mg P kg-1) value; followed by the SC3 soil (264.75 mg P kg-1) and SD5 had the lowest Kf (123.32 mg P kg-1) value. The value of SPRf ranged from 94.04 to 154.02 mg P kg–1 for the soils of Cheha district. Likewise, the SPRf for the xix Dinsho district soils varied from 50.50 to 95.33 mg P kg–1, and was less thanthat of the soils of Cheha district. The SPRf (0.2 mg L-1) levels for the studied soils that were obtained after 24 hrs equilibration periods were not similar. Strong significant (p ≤ 0.01) relationship was observed between the Freundlich adsorption parameters and soil physicochemical properties. Combined applications of lime, vermicompost and mineral P significantly increased (p<0.001) total phosphorus, available phosphorus, labile inorganic phosphorus fractions and decreased free metal oxides, potential sorption capacities in the studied soils. The external P requirements of 0.24 and 0.2 mg P L-1 for Damphe in SC1 and SD2 soils, and 0.3 and 0.22 mg P L-1 for Picaflor in SC1 and SD2 soils were recorded respectively. Whilst, Damphe had internal P requirements of 0.37 and 0.33 mg P L-1 in SC1 and SD2, Picaflor had requirements of 0.38 and 0.37 mg P L-1 correspondingly. Given the impact of low available P and high acidity on growth of crops in soils of the study area, low input P management strategy using amendments such as compost, lime and selecting acid tolerant cultivars have to be implemented. The combined applications of amendments effectively fixed aluminum and iron instead of phosphorus, thus rendering phosphorus available by keeping the inorganic phosphorus in a bio-available labile phosphorus pool compared to sole application of the amendment and control. The pseudo- second- order kinetic model was found to be the overwhelming kinetic model to explain the adsorption dynamics of the soils in the study area. The presence of high correlation between the adsorption parameters and the soil properties suggested that adsorption coefficients did play a prominent role in explaining P adsorption characteristics of the soils in terms of the thought-out soils properties. The blanket P fertilizer rate recommendations in Ethiopia (20 kg P ha-1) without the knowledge of external P requirement of each crop might have decreased yield in the study areas as it could have resulted in under-application of fertilizer P. The findings suggested that the combined applications of amendments altered soil chemical properties in a way that enhanced the availability of phosphorus in the studied soils. The two varieties responded differently to the same soil solution P levels in the soils. Thus it may be inferred that the external and internal P requirements of crops as well as varieties should be tested in the field to find a reasonable amount of fertilizer to be applied to a particular soil.