| dc.description.abstract | Based on a survey conducted among 198 households in the Central Kenyan
Highlands in which 33% of the farmers identified soil conditions as a leading
constraint to crop productivity, 12 households in Kiambu District were randomly
selected for more detailed investigation of soil management practices and
analytical comparison of soils identified by farmers as being either productive or
non-productive. Soil texture analysis was conducted and the parameter was used as
a test of physical similarity, after which chemical analyses were conducted on soils
with similar texture.
The criteria used by fanners to distinguish productive soils versus non-productive
soils were crop performance (100% of the fanners), soil tilth (50% of the farmers),
soil moisture retention (50% of the farmers), soil colour (42% of the farmers) and
presence of certain indicator animal and plant species (25% of the fanners). The
fanners attributed the low fertility status of some patches of their farms to
inadequate use of organic and inorganic fertilisers (100% of the fanners), removal
of crop residues (100% of the farmers) continuous cropping (83% of the farmers),
lack of crop rotation (66% of the farmers) and soil erosion (42~"O of the farmers).
Sand and clay contents were not significantly different between the two soil
categories but productive soils had significantly higher silt content than nonproductive
soils (P < 0.001). Productive soils had significantly higher soil pH,
ECEC. exchangeable cations, extractable phosphorus, total kjeldahl N, organic C
and P (P<O.OOl) than non-productive soils. Soil microbial biomass C and Nand
net N mineralisation (AMN) were significantly higher in productive soils than in
non-productive soils (P < 0.01). Diurnal soil respiration (DSR) was significantly
greater in productive soils than in non-productive soils (P < 0.05) and was
positively correlated with net N mineralisation (r = 0.45) and microbial biomass
nitrogen (r = 0.67). From these chemical analyses, productive soils had higher
organic matter content and soil nutrients than non-productive soils. Nutrient
depletion in the non-productive soils was attributed to soil organic matter decline.
Productive soils had significantly greater particulate organic matter (53-200 urn)
and light traction organic matter (> 1.37 g cm') C, Nand P contents than nonproductive
soils (P < 0.05).
A mild chemical oxidation procedure with potassium pennanganate (KMn04)
proposed as a test of soil organic matter (SOM) quality was compared to more
conventional SOM physical fractionation approaches. Potassium pennanganate
oxidisable C for productive soils was also significantly greater than that for nonproductive
soils (P < 0.001) and correlated highly with particulate organic matter (r
= 0.69), light SOM fractions (r = 0.48 to 0.61), microbial biomass C (r = 0.46) and
net N mineralisation (r = 0.50) showing that the KMn04 oxidation technique is a
reliable and rapid technique and could be used for assessment of soil fertility
status. Correlation of total soil C, Nand P with other soil quality indicators such as
AMN and DSR were higher than correlation of SOM fractions with total soil
nutrients, AMN and DSR, suggesting that detailed C fractionation procedures are
not necessary for routine soil quality diagnosis of humic Nitisols. An index
developed to measure the rate of change in the soil C dynamics of a system relative
to a relatively more stable system, the Carbon Management Index (CMI) showed
that productive soils had higher CMIs than non-productive soils (P < 0.001).
Fanners' perceptions of soil quality were substantiated through soil chemical
analyses and SOM fractions provided qualitative information on these differences.
The study shows that fanners have knowledge of their soils, they are aware of the
causes of the low soil fertility and in some cases, they had possible solutions to soil
fertility depletion. However, application of these solutions is limited by social and
economic constraints. Future research on soil fertility management should consider
indigenous knowledge and fanners' practices as a starting point for any new
interventions. | en |
