Abstract:
The study was carried out in the southern Guinea savanna zone of Nigeria to develop a soil testing programme which leads to the establishment of a rational basis for phosphorus fertilizer recommendations to crops. To achieve this, correlation studies to determine the best extractant of available phosphorus, and calibration studies to develop site specific fertilizer recommendation practices were carried out. Thirty-six surface soil samples collected across Benue, Plateau and parts of Kogi States were used for the correlation studies. Eight soil test methods, Mehlich 1, Mehlich 2, Bray 1, Bray 2, Olsen’s, Modified Mehlich 1, Mississippi, and AB-DTPA were used to extract phosphorus from the soils. Pot experiments were set up in the greenhouse using maize as the test crop. Two treatments, P0 without phosphorus, and P1, with adequate amount of phosphorus, were used. Maize was grown for 35 days. Forage dry matter yields and phosphorus uptake were determined. Ten sites were selected for the field fertilizer rate studies which were carried out for two cropping seasons, 1988 and 1989. Six phosphorus fertilizer rates, 0, 20, 40, 60, 80 and 100 kg P2Os/ha, were used. Surface soil samples were collected before maize planting and fertilizer application. Available phosphorus was extracted with the eight soil test methods. The results showed that each soil test method extracted a wide range of phosphorus from the soils. The effectiveness of the methods in solubilising quantities of phosphorus from the soils was Bray 2 > Olsen’s > Mehlich 2 > Modified Mehlich 1 > Bray 1 > Mehlich 1 > AB-DTPA > Mississippi, with a mean extractable phosphorus of 5.54, 5.47, 4.50, 4.0, 3.57, 2.86, 1.74 and 1.22 mg kg-1, respectively. The herbage dry matter yields of maize at P0 varied among the soils and ranged from 1.03 to 13.72 g/pot. Analysis of variance showed that the yields varied significantly at the 0.1 % probability level with a high coefficient of variation of 21.1 %. The phosphorus uptake at P0 also varied very significantly with the soils and ranged from 1.29 to 24.0 mg/pot. Actual yield, relative yield, actual yield increase and phosphorus uptake were used to evaluate the effectiveness of the soil test methods. In general, the precision of prediction was relative yield > actual yield > phosphorus uptake > actual yield increase. The effectiveness of the extractants varied with the yield parameters. In all, Mehlich 1, Modified Mehlich 1 and Bray 1 were the best predictors, in that order. From the correlation studies it was found that all the eight soil test methods were suitable for solubilising available phosphorus from these soils. However, Mehlich 1 which was the most precise and effective and the least influenced by soil properties is recommended for soil testing in the southern Guinea savanna. The results of the field fertilizer rate studies showed that maize grain yields varied from treatment to treatment in each site, from site to site for each treatment, and from year to year for each site-treatment. In general, the soils in all the 19 site-years responded significantly to phosphorus fertilization although the degree of response varied from site to site and year to year. The critical levels of extractable phosphorus were determined by both CateNelson graphical and analysis of variance methods. The results were as follows: Mehlich 1, 7.25 and 7.29 mg kg1, Mehlich 2, 13.0 and 13.20 mg kg1, Bray 1, 8.0 and 7.83 mg kg”1, Bray 2, 8.0 and 6.76 mg kg’, Olsen’s, 7.25 and 6.94 mg kg1, Modified Mehlich 1, 6.75 and 6.36 mg kg1, Mississippi, 1.20 and 1.12 mg kg1, and AB-DTPA, 2.70 and 2.60 mg kg-1, for the two methods, respectively. The Mitscherlich-Bray equation, Log(A-y) = Log A-C,b, and its expanded form, Log(A-y) = LogA – C,b – Cx, were used for calibrating the responses of maize to phosphorus fertilization. The correlation factors, C,, obtained for Mehlich 1, Mehlich 2, Bray 1, Bray 2, Olsen’s, Modified Mehlich 1, Mississippi and AB-DTPA soil tests were 0.047, 0.040, 0.056, 0.048, 0.069, 0.045, 0.243 and 0.156, respectively. Also the efficiency factor, C, for band applied phosphorus fertilizer was calculated as 0.014. Based on the Mitscherlich-Bray relationships and a graphical approach, arbitrary values of percent sufficiencies, y, of 10 to 93% which cover a full range of soil deficiencies, and corresponding soil test values, b, phosphorus fertilizer recommendations, x, were obtained. Calibration curves and tables were produced for the various soil test methods for use in interpreting soil test data to farmers.
Description: A thesis submitted to the Postgraduate School, Ahmadu Bello University, Zaria, in partial fulfillment for the award of the Degree of Doctor of Philosophy in Soil Science. Department of Soil Science Faculty of Agriculture Ahmadu Bello University Zaria, Nigeria
STUDIES ON SOIL TEST CORRELATION, CALIBRATION AND INTERPRETATION FOR PHOSPHORUS FERTILIZER RECOMMENDATION ON MAIZE (Zea mays L.)
