Evaluation of critical levels of P in some soils of South-West Nigeria, for maize production

Phosphorus is an essential element for maize production and soil test interpretation for P in the soil, is based on the critical range of this nutrient in the soil. This study re-evaluated the soil test interpretation guide for P in soils, for maize production, by carrying out a calibration study. The calibration study was carried out, to determine the optimal soil P concentrations that maximize maize yields. The effectiveness of three different soil P testing methods that are suitable for soil series in the south western Nigeria was also evaluated, and a critical P-value for fertilizer recommendations for maize was developed. Accordingly, ten (10) different soil series in some south western states in Nigeria was used, namely Iwo, Ondo, Itagunmode, Olorunda, Egbeda, Badagry, Alagba, Owode, Ibeshe amd Ipaja series. The field experiment was conducted on ten soil series on farmers farms located at the different stations of the Institute of Agricultural Research and Training, in the south western part of Nigeria. Phosphorus was applied at 0, 30, 60, 90 and 120 kg P2O5 /ha as Single superphosphate replicated three times, laid out in a randomized complete block design with three replications on 10 experimental sites. Muriate of potash (MOP) was applied at 60kg K2O/ha and 60kg N/ha of Urea as basal dose. Three (3) different methods were used- Bray and Kurtz, method, Mehlich III and CaCl method of extraction. With Ca2Cl2 , the highest value with an optimum of 9.15mg/kg was observed for Ibeshe series with the least value, at an optimum of 7.57 mg/kg for Ipaja series. With Bray 1 P, the highest value with an optimum of 11.55mg/kg was observed for Alagba series with the least value, at an optimum of 9.10 mg/kg for Ipaja series. With Mehlich III, the highest value with an optimum of 12.72 mg/kg was observed for Badagry series with the least value, at an optimum of 10.41 mg/kg for Ipaja series. Bray I was a more reliable method for estimating the available P on the acid soils as it showed the highest correlation r = 0.81 with the relative yield of maize and, therefore, was the most effective method for fertilizer rate recommendation. However, finally it can be concluded that the variability signified that soil test should be both soil and extractant specific, for a proper soil test interpretation for P in soils.