Soil fertility determinants of spear grass abundance along a cultivation chronosequence in Buikwe District, Uganda

Abstract

The conventional soil fertility evaluation methods are invariably non-small scale farmer friendly, owing to the costs involved, the complexity associated with the interpretation of the laboratory results and the patience required to access the necessary recommendations. As such, soil fertility assessment, which should be a routine practice for sustainable production in agricultural systems, is invariably out of reach of the farming communities especially in sub-Saharan Africa. Many plants especially weeds have been associated by small scale farmers with declining soil fertility. However, the reliability of this perception has not yet been extensively verified. Thus, an ethno-ecological study was conducted on fields under cultivation with intermittent fallows on Nitisols in Buikwe District, Central Uganda to assess the influence of cultivation chronosequence and residue burning on the occurrence of spear grass and soil properties in an annual cropping system; and to determine the specific soil related nutrient factors associated with the occurrence of spear grass in annual cropping system. Point samples on 25 m2 grids were collected from each of 82 farmer fields and eight secondary forests. On each established grid, a series of 1 m2 quadrants were used to determine spear grass population. Of the 82 farmer fields, 41 were fields on which farmers burnt plant residues at field preparation, and the other 41 were fields without recent burning history. There was no evidence of spear grass populations in the first six years of field cultivation. During the same period, mean values for total nitrogen (0.36 to 0.23%), exchangeable potassium (12.4 to 3.5 mg kg-1) and organic matter (4.7 to 4.0%) were within the favourable range for optimal crop growth. Beyond six years of cultivation mean values of the above parameters xii decreased significantly into unfavourable ranges, including those in the un-burnt fields. In addition, during the same period (after six years), evidence of spear grass infestation in the fields emerged but mostly in soils with low levels of N and K. This is consistent with the farmer’s perception that spear grass infestation and its subsequent proliferation are favoured by declining soil fertility. There was also, a strong association of spear grass population density with high levels of soil exchangeable magnesium and calcium, suggesting that these cations bear influence on the emergence of the plant in fields. It is difficult to discern whether spear grass as a species prefers habitation of low fertility soils or is simply more competitive on low fertility soils than other plant species including crops. This is an issue that deserves further research involving deliberate enrichment of the soil with fertilizers to eliminate the competitive aspect.