We tested the effects of two organic materials (OMs) of varying chemical characteristics that is, farmyard manure (FYM) and
Soil acidity and phosphorus deficiencies limit crop production in many tropical soils [
There are a number of PR deposits of variable reactivity in eastern Africa which, however, differ greatly in their suitability as sources of P in P-deficient soils [
The study was conducted from April to July 2008 at Moi University, using soils collected at two sites in western Kenya which were selected on the basis of contrasting characteristics (Table
Initial surface (0–15 cm) soil properties.
Parameter | Bukura | Kakamega |
---|---|---|
pH (H2O) (1 : 2.5) | 4.80 | 5.10 |
Exchangeable acidity (cmolc kg−1) | 0.88 | 0.35 |
Exchangeable Al (cmolc kg−1) | 0.63 | 0.13 |
Ca | 1.94 | 2.1 |
Mg | 1.01 | 1.8 |
K | 0.12 | 0.2 |
ECEC | 3.95 | 4.85 |
Al saturation (%) | 22 | 7.2 |
Organic C (%) | 3.2 | 2.7 |
Total N (%) | 0.3 | 0.3 |
C : N ratio | 10.6 | 9.0 |
Total P (%) | 0.04 | 0.03 |
Olsen P (mg kg−1) | 5.6 | 2.5 |
P sorbed at (0.2 mg kg−1) | 260 | 45 |
| ||
Texture (%) | ||
|
52 | 54 |
|
18 | 28 |
|
30 | 18 |
Soil classification (FAO System) | Orthic ferralsol | Ferralic cambisols |
Surface soil (0–15 cm) samples were randomly taken from each site and thoroughly mixed by hand to produce one homogenous sample per site. Two hundred gram samples of air-dried soil (<2 mm) from each site were weighed into plastic polythene bags which were kept in upright positions in a laboratory. Finely ground (<1 mm) tithonia, FYM (obtained from cattle), BPR, MPR, or TSP were added to the soils according to the treatments given in Table
The experimental treatments.
Treatment | P source | P rate (kg ha−1) | ||
---|---|---|---|---|
From organics | From inorganics | Total P | ||
(1) Control | — | 0 | 0 | 0 |
(2) Tithonia (60 kg P ha−1) | Tithonia | 60 | 0 | 60 |
(3) FYM (60 kg P ha−1) | FYM | 60 | 0 | 60 |
(4) MPR (60 kg P ha−1) | MPR | 0 | 60 | 60 |
(5) TSP (60 kg P ha−1) | TSP | 0 | 60 | 60 |
(6) BPR (60 kg P ha−1) | BPR | 0 | 60 | 60 |
(7) Tithonia (20 kg P ha−1) + MPR (40 kg P ha−1) | Tithonia and MPR | 20 | 40 | 60 |
(8) Tithonia (20 kg P ha−1) + (TSP 40 kg P ha−1) | Tithonia and TSP | 20 | 40 | 60 |
(9) Tithonia (20 kg P ha−1) + BPR (40 kg P ha−1) | Tithonia and BPR | 20 | 40 | 60 |
(10) FYM (20 kg P ha−1) + MPR (40 kg P ha−1) | FYM and MPR | 20 | 40 | 60 |
(11) FYM (20 kg P ha−1) + TSP (40 kg P ha−1) | FYM and TSP | 20 | 40 | 60 |
(12) FYM (20 kg P ha−1) + BPR (40 kg P ha−1) | FYM and BPR | 20 | 40 | 60 |
(13) Tithonia (20 kg P ha−1) | Tithonia | 20 | 0 | 20 |
(14) FYM (20 kg P ha−1) | FYM | 20 | 0 | 20 |
(15) MPR (40 kg P ha−1) | MPR | 0 | 40 | 40 |
(16) TSP (40 kg P ha−1) | TSP | 0 | 40 | 40 |
(17) BPR (40 kg P ha−1) | BPR | 0 | 40 | 40 |
FYM: farmyard manure; TSP: triple superphosphate; MPR: Minjingu phosphate rock; BPR: Busumbu phosphate rock.
The soils and the OMs were analyzed using the following methods; organic C was determined by Walkley and Black sulphuric acid-dichromate digestion followed by back titration with ferrous ammonium sulphate [
Tithonia contained higher amounts of C, N, Ca, Mg, and K than FYM, but its total P content and pH were lower (Table
Average chemical composition of tithonia and farmyard manure used in the study over the three seasons.
m.c. | % C | % N | C : N ratio | % P | % Ca | % Mg | % K | pH | % Lig | % Poly | |
---|---|---|---|---|---|---|---|---|---|---|---|
Tithonia | 80% | 42 | 3.1 | 13.5 | 0.30 | 2.0 | 0.6 | 4.1 | 6.5 | 13 | 3.17 |
FYM | 30% | 36 | 1.8 | 20.0 | 0.40 | 0.9 | 0.5 | 2.2 | 7.7 | 21 | 0.84 |
FYM: farmyard manure; lig.: lignin; poly.: polyphenol; m.c.: moisture content.
Results for soil pH as affected by the treatments for the Bukura and Kakamega soils are presented in Tables
Effect of organic and inorganic materials on soil pH, exchangeable acidity and exchangeable Al for the Bukura soils in the incubation study.
Treatment | pH | Exchangeable acidity (cmol kg−1) | Exchangeable Al (cmol kg−1) | ||||||
---|---|---|---|---|---|---|---|---|---|
4 WAI | 16 WAI | Δ pH | 4 WAI | 16 WAI | Δ ex. acidity | 4 WAI | 16 WAI | Δ ex. Al | |
(1) Control | 4.63 | 4.24 | −0.39 | 0.89 | 0.87 | −0.02 | 0.63 | 0.62 | −0.01 |
(2) Tithonia (60 kg P ha−1) | 5.43 | 4.28 | −1.15 | 0.31 | 0.48 | 0.17 | 0.16 | 0.17 | 0.01 |
(3) FYM (60 kg P ha−1) | 4.78 | 4.26 | −0.52 | 0.66 | 0.77 | 0.11 | 0.42 | 0.49 | 0.07 |
(4) MPR (60 kg P ha−1) | 4.84 | 4.31 | −0.53 | 0.68 | 0.81 | 0.13 | 0.45 | 0.61 | 0.16 |
(5) TSP (60 kg P ha−1) | 4.68 | 4.25 | −0.43 | 0.79 | 0.84 | 0.05 | 0.67 | 0.59 | −0.08 |
(6) BPR (60 kg P ha−1) | 4.76 | 4.28 | −0.48 | 0.91 | 0.83 | −0.08 | 0.54 | 0.61 | 0.07 |
(7) Tithonia (20 kg P ha−1) + MPR (40 kg P ha−1) | 5.67 | 4.38 | −1.29 | 0.52 | 0.61 | 0.09 | 0.21 | 0.37 | 0.16 |
(8) Tithonia (20 kg P ha−1) + (TSP 40 kg P ha−1) | 5.39 | 4.44 | −0.96 | 0.53 | 0.65 | 0.12 | 0.25 | 0.47 | 0.22 |
(9) Tithonia (20 kg P ha−1) + BPR (40 kg P ha−1) | 5.25 | 4.28 | −0.97 | 0.41 | 0.65 | 0.24 | 0.21 | 0.53 | 0.32 |
(10) FYM (20 kg P ha−1) + MPR (40 kg P ha−1) | 4.84 | 4.25 | −0.59 | 0.61 | 0.79 | 0.18 | 0.41 | 0.60 | 0.19 |
(11) FYM (20 kg P ha−1) + TSP (40 kg P ha−1) | 4.81 | 4.18 | −0.63 | 0.77 | 0.79 | 0.02 | 0.47 | 0.65 | 0.18 |
(12) FYM (20 kg P ha−1) + BPR (40 kg P ha−1) | 4.72 | 4.28 | −0.44 | 0.77 | 0.83 | 0.06 | 0.55 | 0.65 | 0.10 |
(13) Tithonia (20 kg P ha−1) | 4.75 | 4.23 | −0.52 | 0.66 | 0.82 | 0.16 | 0.32 | 0.55 | 0.23 |
(14) FYM (20 kg P ha−1) | 4.76 | 4.29 | −0.47 | 0.71 | 0.83 | 0.12 | 0.48 | 0.65 | 0.17 |
(15) MPR (40 kg P ha−1) | 4.82 | 4.35 | −0.47 | 0.73 | 0.81 | 0.08 | 0.47 | 0.60 | 0.13 |
(16) TSP (40 kg P ha−1) | 4.73 | 4.18 | −0.55 | 0.83 | 0.90 | 0.07 | 0.57 | 0.71 | 0.14 |
(17) BPR (40 kg P ha−1) | 4.70 | 4.17 | −0.53 | 0.84 | 0.95 | 0.11 | 0.58 | 0.68 | 0.10 |
SED | 0.14 | N.S. | 0.11 | 0.07 | 0.10 | 0.07 | |||
CV % | 3.50 | 18.9 | 11.5 | 27.5 | 11.50 |
WAI: weeks after incubation; FYM: farmyard manure; TSP: triple superphosphate; MPR: Minjingu phosphate rock; BPR: Busumbu phosphate rock; N.S.: not significant; SED: standard error of difference between means; Ex: exchangeable.
Effect of organic and inorganic materials on soil pH, exchangeable acidity, and exchangeable Al for the Kakamega soils in the incubation study.
Treatment | pH | Exchangeable acidity (cmol kg−1) | Exchangeable Al (cmol kg−1) | ||||||
---|---|---|---|---|---|---|---|---|---|
4 WAI | 16 WAI | Δ pH | 4 WAI | 16 WAI | Δ ex. acidity | 4 WAI | 16 WAI | Δ ex. Al | |
(1) Control | 5.16 | 4.67 | −0.49 | 0.24 | 0.28 | 0.04 | 0.08 | 0.09 | 0.01 |
(2) Tithonia (60 kg P ha−1) | 5.41 | 4.35 | −1.06 | 0.21 | 0.23 | 0.02 | 0.01 | 0.00 | −0.01 |
(3) FYM (60 kg P ha−1) | 5.36 | 4.71 | −0.65 | 0.21 | 0.24 | 0.03 | 0.01 | 0.00 | −0.01 |
(4) MPR (60 kg P ha−1) | 5.34 | 4.75 | −0.59 | 0.24 | 0.27 | 0.03 | 0.03 | 0.00 | −0.03 |
(5) TSP (60 kg P ha−1) | 5.20 | 4.73 | −0.47 | 0.25 | 0.26 | 0.01 | 0.06 | 0.06 | 0.00 |
(6) BPR (60 kg P ha−1) | 5.20 | 4.73 | −0.47 | 0.32 | 0.29 | −0.03 | 0.10 | 0.08 | −0.02 |
(7) Tithonia (20 kg P ha−1) + MPR (40 kg P ha−1) | 5.77 | 4.75 | −1.02 | 0.21 | 0.21 | 0.00 | 0.03 | 0.00 | −0.03 |
(8) Tithonia (20 kg P ha−1) + (TSP 40 kg P ha−1) | 5.32 | 4.61 | −0.71 | 0.25 | 0.25 | 0.00 | 0.04 | 0.00 | −0.04 |
(9) Tithonia (20 kg P ha−1) + BPR (40 kg P ha−1) | 5.39 | 4.38 | −1.01 | 0.24 | 0.31 | 0.07 | 0.05 | 0.00 | −0.05 |
(10) FYM (20 kg P ha−1) + MPR (40 kg P ha−1) | 5.36 | 4.82 | −0.54 | 0.23 | 0.25 | 0.02 | 0.03 | 0.00 | −0.03 |
(11) FYM (20 kg P ha−1) + TSP (40 kg P ha−1) | 5.33 | 4.74 | −0.59 | 0.24 | 0.27 | 0.03 | 0.02 | 0.03 | 0.01 |
(12) FYM (20 kg P ha−1) + BPR (40 kg P ha−1) | 5.36 | 4.61 | −0.75 | 0.24 | 0.28 | 0.04 | 0.02 | 0.04 | 0.02 |
(13) Tithonia (20 kg P ha−1) | 5.28 | 4.33 | −0.95 | 0.23 | 0.24 | 0.01 | 0.07 | 0.00 | −0.07 |
(14) FYM (20 kg P ha−1) | 5.21 | 4.68 | −0.53 | 0.24 | 0.26 | 0.02 | 0.02 | 0.00 | −0.02 |
(15) MPR (40 kg P ha−1) | 5.33 | 4.78 | −0.55 | 0.23 | 0.27 | 0.04 | 0.04 | 0.00 | −0.04 |
(16) TSP (40 kg P ha−1) | 5.20 | 4.70 | −0.50 | 0.25 | 0.29 | 0.04 | 0.07 | 0.00 | −0.07 |
(17) BPR (40 kg P ha−1) | 5.21 | 4.68 | −0.53 | 0.28 | 0.31 | 0.03 | 0.08 | 0.08 | 0.00 |
SED | 0.10 | N.S. | N.S. | 0.014 | N.S. | 0.018 | |||
CV % | 2.0 | 5.6 | 40.00 |
WAI: weeks after incubation; FYM: farmyard manure; TSP: triple superphosphate; MPR: Minjingu phosphate rock; BPR: Busumbu phosphate rock; N.S.: not significant; SED: standard error of difference between means; Ex: exchangeable.
Averaged across the three inorganic P sources, the soil pH followed the trend Tithonia > FYM > no OM at both sites. Averaged across the OMs, MPR gave a significantly higher soil pH than TSP and BPR at both sites at 4 WAI. There was a decline in soil pH in all the treatments at 16 WAI compared to 4 WAI for both soil types. Averaged across all the treatments, the pH of the Bukura and Kakamega soils declined by 0.67 and 0.64 units, respectively. In general, the acidification over time was more pronounced with the tithonia treatments at both sites.
At 4 WAI, tithonia when applied alone or in combination with the inorganic P sources significantly reduced the exchangeable acidity with respect to the control for the Bukura soil (Table
There were no significant treatment effects on exchangeable acidity for the Kakamega soil at 4 WAI (Table
The exchangeable Al trends among the treatments were generally similar to those of exchangeable acidity for the Bukura soil, at both sampling times (Table
There was a strong significant negative correlation between the soil pH with both the exchangeable acidity (
All the applied inputs generally increased the Olsen P levels compared with the control for both soil types at 4 WAI (Table
Effect of organic and inorganic P amendments on Olsen P (mg P kg−1) at Bukura and Kakamega in the laboratory incubation study.
Treatment | Bukura | Kakamega | ||||
---|---|---|---|---|---|---|
4 WAI | 16 WAI | Δ Olsen P | 4 WAI | 16 WAI | Δ Olsen P | |
(1) Control | 7.3 | 8.9 | 1.6 | 3.2 | 4.3 | 1.1 |
(2) Tithonia (60 kg P ha−1) | 13.1 | 14.7 | 1.6 | 8.2 | 9.6 | 1.4 |
(3) FYM (60 kg P ha−1) | 16.0 | 16.5 | 0.5 | 9.5 | 10.0 | 0.5 |
(4) MPR (60 kg P ha−1) | 13.4 | 16.3 | 2.9 | 6.9 | 8.0 | 1.1 |
(5) TSP (60 kg P ha−1) | 18.2 | 17.7 | −0.5 | 9.8 | 10.1 | 0.3 |
(6) BPR (60 kg P ha−1) | 11.0 | 11.5 | 0.5 | 4.5 | 6.0 | 1.5 |
(7) Tithonia (20 kg P ha−1) + MPR (40 kg P ha−1) | 14.1 | 13.9 | −0.2 | 7.9 | 6.3 | −1.6 |
(8) Tithonia (20 kg P ha−1) + (TSP 40 kg P ha−1) | 17.4 | 15.8 | −1.6 | 8.9 | 8.6 | −0.3 |
(9) Tithonia (20 kg P ha−1) + BPR (40 kg P ha−1) | 12.4 | 12.6 | 0.2 | 4.4 | 5.1 | 0.7 |
(10) FYM (20 kg P ha−1) + MPR (40 kg P ha−1) | 15.0 | 15.7 | 0.7 | 7.4 | 9.3 | 1.9 |
(11) FYM (20 kg P ha−1) + TSP (40 kg P ha−1) | 14.5 | 17.7 | 3.2 | 8.0 | 6.0 | −2.0 |
(12) FYM (20 kg P ha−1) + BPR (40 kg P ha−1) | 13.1 | 16.1 | 3.0 | 5.9 | 5.8 | −0.1 |
(13) Tithonia (20 kg P ha−1) | 11.2 | 13.9 | 2.7 | 4.6 | 7.3 | 2.7 |
(14) FYM (20 kg P ha−1) | 12.7 | 15.6 | 2.9 | 5.7 | 7.5 | 1.8 |
(15) MPR (40 kg P ha−1) | 12.5 | 14.9 | 2.4 | 6.3 | 6.6 | 0.3 |
(16) TSP (40 kg P ha−1) | 14.2 | 16.6 | 2.4 | 6.7 | 7.3 | 0.6 |
(17) BPR (40 kg P ha−1) | 9.8 | 9.6 | −0.2 | 4.4 | 5.6 | 1.2 |
SED | 0.9 | 1.3 | 0.7 | 0.7 | ||
CV % | 9 | 11 | 10 | 10 |
WAI: weeks after incubation; FYM: farmyard manure; TSP: triple superphosphate; MPR: Minjingu phosphate rock;
BPR: Busumbu phosphate rock; SED: standard error of difference between means.
The combined application of the OMs, that is, tithonia or FYM, with TSP or the PRs did not result in synergy, whereby the available P increased more than the sum of the increase from either of the P sources applied singly. This is illustrated in Figures
Increase in Olsen P above the control treatment as affected by tithonia and TSP at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when tithonia (at 20 kg P ha−1) was applied in combination with TSP (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when tithonia, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when TSP was applied alone at 40 kg P ha−1.
Increase in Olsen P above the control treatment as affected by tithonia and MPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when tithonia (at 20 kg P ha−1) was applied in combination with MPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when tithonia, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when MPR was applied alone at 40 kg P ha−1.
Increase in Olsen P above the control treatment as affected by tithonia and BPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when tithonia (at 20 kg P ha−1) was applied in combination with BPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when tithonia, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when BPR was applied alone at 40 kg P ha−1.
Increase in Olsen P above the control treatment as affected by FYM and MPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when FYM (at 20 kg P ha−1) was applied in combination with MPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when FYM, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when MPR was applied alone at 40 kg P ha−1.
Increase in Olsen P above the control treatment as affected by FYM and TSP at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when FYM (at 20 kg P ha−1) was applied in combination with TSP (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when FYM, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when TSP was applied alone at 40 kg P ha−1.
Increase in Olsen P above the control treatment as affected by FYM and BPR at Bukura. Note: “combined appl.” refers to the observed increase in Olsen P above the control obtained when FYM (at 20 kg P ha−1) was applied in combination with BPR (at 40 kg P ha−1), while “individual appl.” refers to the increase in Olsen P above the control obtained when FYM, applied alone at 20 kg P ha−1, was added to the increase in Olsen P above the control obtained when BPR was applied alone at 40 kg P ha−1.
The application of both FYM and tithonia generally increased the soil pH at 4 WAI with tithonia-treated soils having a higher pH than the FYM-treated soils at this time. The soil pH, however, declined by 16 WAI with tithonia-treated soils showing the highest pH reductions. The increase in soil pH due to application of OMs at 4 WAI in this study is consistent with results reported by several other workers (e.g., [
Addition of tithonia, FYM, and MPR had the effect of reducing both the exchangeable acidity and exchangeable Al, but the magnitude of the reduction varied with each of these materials. Tithonia appeared to be more effective in reducing exchangeable Al, but not exchangeable acidity, compared to FYM. The reduction in exchangeable acidity can partially be attributed to an initial increase in soil pH that was observed with the OMs. Several other workers have measured an increase in soil pH with concomitant decrease in exchangeable Al during decomposition of organic residues in soils [
The Al complexing effect of tithonia is likely to have been stronger than that of FYM given that FYM gave higher soil pH (5.17) than tithonia but still ended up with a higher level of exchangeable Al (0.35 cmol kg−1). Tithonia was applied as a green manure and was thus likely to produce large quantities of organic acids, which would be involved in complexation reactions [
Addition of P from both organic and inorganic sources generally resulted in increase in the Olsen P relative to the control. The magnitude of the increase in the Olsen P depended on the soil type, time of soil sampling, P source, and rate of P application. On average, addition of P inputs generally resulted in larger increases in Olsen P for the Bukura soil than the Kakamega one. Similar site-specific differences in extractable soil P, in response to applied P fertilizers, were found by [
TSP gave the highest amount of Olsen P compared to the PRs, tithonia, or FYM, applied at the same total P rate at all times. This is ascribed to the higher solubility of TSP compared to the PRs whose dissolution is usually low and slow [
The significant increase in Olsen P above the control by MPR indicates that the soil conditions at both sites were conducive to its dissolution. Some of the factors known to increase the dissolution and subsequent release of P in PRs include low soil pH, low exchangeable Ca, and low P [
The interaction between the OMs and inorganic P sources was significant only on a few occasions. In such instances, it was observed that combining the PRs with tithonia or FYM gave higher Olsen P values than when the PRs were combined with urea. However, when the TSP was combined with tithonia or FYM, it gave lower amounts of Olsen P than when it was combined with urea. This may suggest that tithonia and FYM were enhancing the dissolution of PRs, but retarding the availability of P from TSP. However, closer examination of the data reveals that tithonia and FYM were unlikely to have enhanced the dissolution of the PRs and that combining these two OMs with the PRs has no advantage in terms of increasing the Olsen P compared to their application with urea. There was therefore no synergistic effect in terms of increased Olsen P, when PRs were applied in combination with organic materials. In general, the combined application of organic and inorganic P sources generally resulted in observed increases in Olsen P intermediate to those of sole applications of the organic or inorganic P sources.
The likely reason why the PRs when combined with tithonia and FYM gave higher Olsen P levels compared to their combination with urea is because both tithonia and FYM were generally more effective in increasing the Olsen P compared to the PRs, and therefore, a portion of the insoluble PRs (20 kg P ha−1) was substituted for by the more available tithonia or FYM in the combinations. However, when combined with urea all the 60 kg P ha−1 was from the low soluble PRs and thus the lower Olsen P levels. On the other hand, TSP when combined with urea, gave higher Olsen P levels compared to its combination with tithonia or FYM. In this case, TSP was more effective in increasing the Olsen P compared to tithonia and FYM whose P is mostly in organic forms initially, and hence, substituting a portion of it (20 kg P ha−1) in the combination with tithonia or FYM yielded less Olsen P than when it (TSP) was applied at the full rate of 60 kg P ha−1 with urea.
The findings of the present study are in contrast to others (e.g., [
Tithonia and farmyard manure were more effective in increasing the soil pH and reducing exchangeable acidity and Al than the inorganic P sources (MPR, BPR, and TSP) in the early stages of incubation suggesting that these OMs can substitute for lime. Addition of P from both organic and inorganic sources generally resulted in an increase in the Olsen P, relative to the control, whose magnitude depended on the soil type, time of soil sampling, P source, and rate of P application. The effectiveness of the inorganic P sources in increasing P availability followed the order, TSP > MPR > BPR, while among the OMs, FYM was more effective than tithonia. There was no synergistic effect, in terms of increased Olsen P, when inorganic P sources were applied in combination with OMs. In general, the combined application of organic and inorganic P sources resulted in observed increases in Olsen P intermediate to those of sole applications of the organic or inorganic P sources. The combination of OMs with inorganic P fertilizers may, however, have other benefits associated with integrated soil fertility management.
The authors thank Moi University for financial assistance and for providing laboratory facilities, Mary Emong’ole for conducting laboratory analyses, and Laban Mulunda of Bukura Agricultural College for assistance with collection and preparation of the soil samples.