Suitability of Modified Dilute Double Acid ÐMehlich 1 (0.1NHCL + 0.025NH2SO4) and Olsen (0.5MNaHCO3PH 8.5) Methods for Testing of Available Phosphorous for Kenyan Soils

Abstract

A study was conducted to investigate the suitability of using modified Mehlich 1 and sodium bicarbonate Olsen methods for testing of available P for Kenyan soils. The soils were from 52 sites within the high and medium agricultural potential rainfall areas. Maize grain yields were used to calibrate the soil available P. The two P extraction methods were used at soil extracting ratio of 1:5. Regression analysis of maize grain yields (kg/ha) from plots with no fertilizer P application against extractable P by the two methods was done for the soil PH range: 4.0- 7.8, PH𕟭.5, 5.6≤PH £6.4 and 6.5 ≤PH£ 7.8. Similar regression analysis was done for HB-600S and HB500S maize varieties and agro ecological zones (UM, LH, UH, LM and CL). Further regression was done taking into account of different prevailing agro-ecological zones. The results revealed that maize grain yields (kg/ha) were significantly (P𕟨.015) correlated (r= 0.335) to soil available P extractable by Olsen method and not modified dilute double acid Mehlich 1 method at soil PH range of 4.0-7.8. These results showed that available P can be estimated by the sodium bicarbonate method although the correlation was very small (r =0.335) but significant. The very small significant correlation implied need for further regression with more soil data. When regression analysis was done at soil PH 𕟭.5 the correlations for both methods were not significant. This meant that both test methods were not suitable for estimating available P at this soil PH range and another suitable method should be found for soils within this PH range. At 5.6≤PH£ 6.4, both methods were significantly, (P𕟨.05) suitable with improved correlations (r = 0.488 and r= 0.490). This meant that at this soil PH range, both methods were suitable and can be used interchangeably although Olsen method gave lower values of available P than the double acid method. At 6.5≤PH £7.8, both methods were not significant and hence not suitable for estimating available P. When maize varieties HB 600 and HB 500 series were
taken into account. It was found that both methods were significantly (P𕟨.05) suitable for estimating soil available P for planting maize HB 600 series. Only Olsen P method was significantly suitable (P≤ 0.011) for estimating soil available P for planting maize HB 500 & 600 series. When agro ecological zones were taken into account, both methods were found significantly (P𕟨.05) suitable for estimating available P in LH zones only. It was concluded that Olsen method is significantly suitable for testing of available P at all soil PH range while modified Mehlich 1 method is not. The two methods are not suitable at PH≤ 5.5 and another method must be found. At 5.6≤ PH≤ 6.4, both methods can be used interchangeably. At 6.5≤PH𕟯.8 both methods are not suitable. This was attributed to over extraction by the modified dilute double acid and laboratory errors associated with Olsen method particularly on increase in pH of extracting solution when not kept in plastic containers. This leads to over extraction and hence poor correlation with crop yields. Olsen P method is suitable for estimating available P for planting maize HB 600 and HB 500 series. Both methods are not suitable for estimating available P in UM; LM and CL zones but are suitable in LH zones. In overall Olsen method appeared to be slightly suitable method for estimating available P in Kenyan soils and can be used interchangeably with modified Mehlich 1 at soil PH, 5.6≤ PH𕟮.4. However the small significant (p𕟨.015, r= 0.335) correlation meant more research was required to correlate other soil P testing methods for Kenyan soils.

Keywords:Soil pH, P-testing, crop response and correlations

Discovery and Innovation Vol. 19 (3) 2007: pp. 195-201