Variability in surface properties of an Alfisol as function of types of land use

The knowledge of soil variability is crucial for a reliable estimation of the average of any given soil property for management purposes. A field trial was conducted to assess the variability of selected physical and chemical properties of an Alfisol with contrasting land use types (continuous arable cultivation, permanent crop cultivation, bush fallow and paddock). Plots measuring 20 m x 20 m (0.04 ha) were marked out and sampled for various soil properties and the variability of the measured properties was computed for each land use. The most variable properties under the continuous arable plot as measured by the coefficient of variation (CV) were organic carbon (28%) and exchangeable potassium (35%). Under permanent cultivation, the most variable properties were aggregate stability (40%) and volumetric moisture content (35%). For the bush fallow plot, the most variable properties were aggregate stability (26%), volumetric moisture content (45%), organic carbon (30%) and exchangeable potassium (55%). Under the paddock, the most variable properties were aggregate stability (37%), volumetric moisture content (41%), coarse sand (55%), fine sand (29%), silt (36%), clay (33%), organic carbon (28%), and exchangeable potassium (73%). While properties such as bulk density, total sand and pH exhibited low variability, available phosphorus exhibited high variability across all land uses selected. The number of samples required with an allowable error of 2.5% deviation from true mean within an area of 0.04 ha, was 74 for a CV of 10%, 286 for a CV of 20% and 628 for a CV of 30%. However, with an error of 20%, the sample numbers needed were just 3, 7 and 11 for a CV of 10%, 20%, and 30%, respectively. From the above results, care has to be taken to ensure that enough samples are collected from the field in estimating the average value of any given soil property. The number of samples needed, greatly depend on variability of soil properties, which in turn can be affected by previous land use types.