Table 4.8 shows some major nutrients of the surface soil (0 – 10 cm depth) amenable to changes by the treatments applied for two consecutive years (2 cropping seasons year-1).
(i) pH: The pH of the soil varied among the treatments but the differences were not significant. It ranged from 5.12 to 5.42 in 2008, while it was from 5.10 to 5.54 in 2009. The soil pH under 10VGS+VM4 treatment was consistently higher than other treatments closely followed by VM6 treatments. With continuous cultivation of the land, the soil pH under NV, 10VGS and 20VGS consistently reduced in values. However, the effect of mulch was pronounced as the pH values of the plots with vetiver grass mulch increased. Integration of VGS and vetiver grass mulch increased the soil pH further with the highest pH (5.42 and 5.54 in 2008 and 2009, respectively) obtained on 10VGS+VM4 plots.
(ii) Soil organic carbon (SOC): The concentrations ranged from 9.7 g C kg-1 on NV to 18.8 g C kg-1 on VM6 plots in 2008, and from 8.2 g C kg-1 on NV to 22.6 g C kg-1 on VM6
plots in 2009 (Table 4.8). In 2008, the differences between SOC for NV and those for 10VGS, 20VGS, VM2, 10VGS+VM2 and 20VGS+VM2 treatments were not significant (P <
0.05) but were significantly lower than VM4, VM6, 20VGS+VM4 and 10VGS+VM4. Although VM4 and 20VGS+VM4 had greater and significant (P < 0.05) effects on SOC than NV, 10VGS, 20VGS, VM2, 10VGS+VM2 and 20VGS+VM2 treatments, their effects on SOC were significantly lower than VM6 and 10VGS+VM4. The results also showed that the SOC for VM6 and 10VGS+VM4 were not significantly different but significantly higher than other treatments. In spite of continuous cultivation of erosion plots in 2009 cropping seasons, SOC significantly improved on mulched plots than un-mulched plots. Meanwhile, the highest SOC was consistently recorded for VM6; the values did not differ significantly from that of 10VGS+VM4. The SOC for NV, 10VGS and 20VGS plots in 2009 decreased by 15.4%, 3.0% and 5.1% respectively, as compared with their respective SOCs in 2008. On the other hand, SOC in 2009, for VM2, VM4, VM6,20VGS+VM2, 20VGS+VM4, 10VGS +
VM2 and 10VGS+VM4 increased by 2.0%, 10.9%, 18.1%, 8.1%, 11.7%, 8.7% and 18.5%
respectively, as against their respective SOCs in 2008. Soil with high tonnage of vetiver grass mulch (4 and 6 t ha-1) had higher percentage increase in SOC during 2009 cropping seasons with the highest percentage increase observed in 10VGS+VM4, although the SOC
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Table 4.8: Effects of integrated use of vetiver grass strips and vetiver grass mulch on pH and some specific major nutrients of the surface soil in 2008 and 2009 cropping seasons
Treatments pH in
Organic Carbon
Total N
Avail P Exch K
Ca Mg Na
water g kg-1 mg kg-1 cmol kg-1
Soil nutrients after two cropping seasons in 2008
NV 5.12 ns 9.7a 0.91a 8.32a 0.21a 1.43a 0.87a 0.42ns
10VGS 5.17 10.1a 0.95a 8.42a 0.23a 1.50a 0.91a 0.44
20VGS 5.17 9.9a 0.93a 8.41a 0.23a 1.49a 0.90a 0.44
VM2 5.19 9.9a 0.93a 8.45a 0.23a 1.56a 0.95ab 0.45
VM4 5.38 14.7bc 1.41c 8.92bc 0.24ab 1.72b 1.18c 0.44
VM6 5.41 18.8d 1.82d 9.20c 0.26b 2.68d 2.23e 0.45
20VGS+ VM2 5.25 12.3ab 1.17b 8.62ab 0.24ab 1.63ab 1.09bc 0.44 20VGS+ VM4 5.40 15.4c 1.48c 9.12bc 0.25b 2.13c 1.36d 0.44 10VGS+ VM2 5.26 12.6ab 1.20b 8.62ab 0.24ab 1.68ab 1.12bc 0.45 10VGS+ VM4 5.42 18.4d 1.78d 9.20c 0.26b 2.64d 2.14e 0.45
Soil nutrients after two cropping seasons in 2009
NV 5.10 ns 8.2a 0.76a 7.56a 0.21a 1.39a 0.85a 0.42ns
10VGS 5.16 9.8a 0.92b 8.36b 0.23a 1.43a 0.90a 0.45
20VGS 5.14 9.3a 0.87b 8.17b 0.22a 1.40a 0.87a 0.44
VM2 5.20 10.1a 0.95b 8.49bc 0.23ab 1.59ab 0.96a 0.44
VM4 5.49 16.3c 1.57d 9.18de 0.28cd 1.93c 1.23b 0.44
VM6 5.52 22.2d 2.16e 9.56e 0.30d 3.12e 2.32d 0.45
20VGS+ VM2 5.36 13.1b 1.25c 8.84cd 0.26bc 1.74bc 1.15b 0.44 20VGS+ VM4 5.51 17.2c 1.66d 9.41e 0.28cd 2.48d 1.42c 0.45 10VGS+ VM2 5.38 13.4b 1.28c 8.88cd 0.26bc 1.82c 1.18b 0.44 10VGS+ VM4 5.54 21.8d 2.12e 9.58e 0.30d 3.11e 2.27d 0.44
Means within a column in each year, followed by different letter(s) differ at P < 0.05 according to Duncan multiple range test (DMRT). ns represents no significant differences among the treatments
10VGS– vetiver grass strips spaced at 10 m interval; 20VGS – vetiver grass strips spaced at 20 m interval;
VM2 – vetiver grass mulch applied at 2 t ha-1; VM4 – vetiver grass mulch applied at 4 t ha-1; VM6 – vetiver grass mulch applied at 6 t ha-1; 20VGS + VM2 – vetiver grass strips spaced at 20 m interval and vetiver grass mulch applied at 2 t ha-1; 20VGS + VM4 – vetiver grass strips spaced at 20 m interval and vetiver grass mulch applied at 4 t ha-1; 10VGS + VM2 – vetiver grass strips spaced at 10 m interval and vetiver grass mulch applied at 2 t ha-1; and 10VGS + VM4 – vetiver grass strips spaced at 10 m interval and vetiver grass mulch applied at 4 t ha-1.
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(22.2 g C kg-1 soil) on VM6 is slightly higher than the 10VGS+VM4 (21.7 g C kg-1 soil) during the year.
(iii) Total nitrogen (N): The total Nitrogen followed similar trends observed in SOC, and it varied significantly (P < 0.05) among the treatments (Table 4.8). It however ranged from 0.91 g N kg-1 soil under NV to 1.82 g N kg-1 soil under VM6 in 2008, while it ranged from 0.76 g N kg-1 soil under NV to 2.16 g N kg-1 soil under VM6 in 2009. In comparison to the baseline N status prior to the trial, the total N in 2008 reduced in values due to continuous cultivation on NV, 10VGS, 20VGS, VM2, 20VGS+VM2 and 10VGS+VM2 by 28.9%, 25.8%, 27.3%, 27.3%, 8.6% and 6.3%, respectively. Whereas the soil total N under VM4, VM6, 20VGS+VM4 and 20VGS+VM4 increased by 10.2%, 42.2%, 15.6% and 39.1%, respectively in the same year. In 2009, as against the baseline N, total N of the surface soil of NV, 10VGS and 20VGS reduced further by 40.6%, 28.1% and 32.0%, respectively while the reduction in total N of the soil reduced in values under VM2 (25.8%), 20VGS+VM2 (2.3%) and 10VGS+VM2 (0%). There were appreciable increases in the soil total N on VM4, VM6, 20VGS+VM4 and 20VGS+VM4 with increases of 22.%, 68.8%, 29.7% and 65.6%, respectively as against the initial 1.28 g N kg-1 before imposing the treatments in 2008.
(iv) Available phosphorus (Avail P): Available phosphorus varied among the treatments, ranging from 8.32 to 9.20 mg kg-1 in 2008 and 7.56 to 9.58 mg kg-1 in 2009 (Table 4.8). The soil under NV treatment had the least available P, and this was consistently lower than other treatments in both seasons. The integration of VM4 with 10VGS and 20VGS as 10VGS+VM4 and 20VGS+VM4, respectively had significant influence on soil available P than 10VGS+VM2 and 20VGS+VM2 but they were not significantly different from VM4
and VM6 treatments. Generally in 2008, soil available P on NV, 10VGS, 20VGS, VM2, 10VGS+ VM2 and 20VGS+VM2 did not differ significantly, while the available P on VM4, 20VGS+VM2, 20VGS+VM4 and 10VGS+VM2 treatments was not significantly different.
However, in 2009, in spite of continuous cultivation, the soil available P on vetiver grass mulched plots increased further, with highest increase (4.1%) as against 2008 seasons observed on 10VGS+VM4. On the other hand, there were reductions of 9.1%, 0.7% and 2.8% in soil available P on NV, 10VGS and 20VGS, respectively after two cropping
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seasons in 2009, as against 2008 cropping seasons. Even then, there were no significant differences among VM4, VM6,10VGS+VM4 and 20VGS+VM4 with regard to soil avail. P.
(v) Exchangeable potassium (K) ranged from 0.21 to 0.30 cmol kg-1 in four cropping seasons (Table 4.8). The soil samples from VM6 and 10VGS+VM4 contained highest concentrations of exchangeable K (0.26 and 0.30 cmol kg-1 each in 2008 and 2009, respectively). Although, there were no significant differences among VM4, VM6, 10VGS+VM2, 20VGS+VM2, 10VGS+VM4 and 20VGS+VM4 relatives to soil P concentration in 2008. However, the exchangeable K under integrated VGS and VM treatments (10VGS+VM2, 20VGS+VM2, 10VGS+VM4 and 20VGS+VM4) were consistently higher than when 10VGS, 20VGS, VM2 and VM4 were used alone in both seasons. In comparison, only the 10VGS+VM4 and 20VGS+VM4 treatments could match the available P value in VM6 while the concentrations of P under 10VGS+VM2 and 20VGS+VM2 were significantly lower than VM6 in 2009.
(vi) Calcium (Ca) concentration of the surface soil varied among different treatments. The concentration of Ca in the soil under VM6 treatment was consistently and significantly higher than all other treatments except 10VGS+VM4 in both 2008 and 2009 cropping seasons (Table 4.8). The mulched plots had higher Ca levels than the plots with vetiver grass strips alone and no-vetiver grass. While the Ca concentrations on mulched plots increased further in 2009 as against 2008 values, the concentration of Ca reduced on NV, 10VGS and 20VGS by 4.1%, 4.7% and 6.0%, respectively. On the other hand, the integrated VGS and VM plots (10VGS+VM2, 20VGS+VM2, 10VGS+VM4 and 20VGS+VM4) had higher Ca concentrations than when 10VGS, 20VGS, VM2 or VM4 was used alone in both seasons.
(vii) Magnesium (Mg) concentrations under NV (0.87 and 0.85 cmol kg-1 in 2008 and 2009 cropping seasons), like other soil nutrients, were consistently lower than other treatments in both seasons, even though the concentrations under NV did not differ from those 10VGS, 20VGS and VM2 (Table 4.8). On the other hand, the concentration of Mg under VM6 was consistently and significantly higher (P < 0.01) than other treatments in both 2008 and 2009 seasons, except 10VGS+VM4. Similar to the trends observed in other exchangeable bases, the concentrations of Mg on mulched plots of VM2, VM4, VM6, 10VGS+VM2,
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20VGS+VM2,10VGS+VM4 and 20VGS+VM4 treatments increased by 1.1%, 4.2%, 4.0%, 5.5%, 4.4%, 5.4% and 6.1%, respectively in 2009 as against 2008 cropping seasons.
(viii) Sodium (Na) concentrations of the surface soil among the treatments were not significantly different (Table 4.8). Although, the soil under NV consistently recorded lower Na (0.42 cmol kg-1) than other treatments in both seasons, there were no discernible trends observed in Na concentrations among the treatments in both seasons. Even then, its value ranged from 0.42 cmol kg-1 to 0.45 cmol kg-1 in both seasons.