Site-specific soil pH management across spatially variable soils

Abstract

Knowledge and management of soil pH, particularly soil acidity across spatially variable soils is important, although this is greatly ignored by farmers in the Limpopo Province of South Africa. The significance of understanding spatial variability of soil acidity is the implementation of best and site-specific management strategies because when soil acidity is poorly managed, toxicity and nutrient deficiency problems in the soil are inevitable. The objective of the study was to evaluate in-field spatial variability of soil pH, and compare the efficiency of managing soil pH through site-specific method vs. uniform lime application. The study was conducted in 3 site years (23o50’ S; 29 o40’ E and 23o59’ S and 28o52’ E) with site year I, and II adjacent to each other in the semi-arid regions of the Limpopo Province, South Africa. Soil samples were taken in four replicates within a 1 m radius from geo-referenced locations in 3 study sites to sampling depths of 0-20 cm on a regular grid of 30m using differential Global Positioning System (DGPS). Soils were analyzed for pH, and SMP buffer pH for lime recommendations. Lime requirement to achieve a soil pH of 6.5 for a 20 cm plough layer per hectare was calculated using CalciumCarbonateequivalent, efficiency factor (fineness factor), and neutralizing index of the liming materials. The spatial maps for SMP buffer pH and lime requirement maps were produced with surfer version 8.0 (Surfer Version 8, Golden Software, Golden, CO). The soil pH datasets from systematic unaligned randomly sampled soils on a 30-m grid were interpolated using inverse distance weighing (IDW) in Surfer software version 8.0 (Surfer Version 8, Golden Software, Golden, CO). Soil pH varied from strongly acidic to slightly acidic with minimum values of 4.22, 3.93, and 4.74 and maximum values of 6.11, 7.00, and 6.82 in site I, II, and II respectively. In Site I, II, and III, the areas of the field that had soil pH values of less than 6.0 were 99.43, 82.61, and 62.89% of the field. When lime was recommended for application using a conventional method of uniform lime application based on an average value derived from samples collected in the whole field, the results of the study showed a waste of lime in excess of lime recommended for individual grids. An excess amount of lime as high as 10, 30, and 7 tons/ha recommended on sites I, II and III respectively under uniform application. These recommendations were in excess on field areas that needed little or no lime applications. Again, the fields showed under applications of lime as much as 30, 35, and 13 tons/ha in site I, II, and III respectively for uniform liming applications. This under- and over recommendations of lime based on average soil pH values suggests that uniform soil acidity management strategy is not an appropriate strategy to be adopted in these fields. Again, in both of these sites as shown in the maps, the areas that required high amount of lime and those that require little or no lime are clearly defined, such that the fields can be divided into lime application zones. When a field is divided into lime application zones, management of soil acidity becomes easier because instead of applying variable rates of lime for every grid, lime rates are applied per zone. These zones could be areas in a field that require, (i) high rates of lime, (ii) low rates of lime, and (iii) areas that requires no lime at all. Agricultural fields that exhibit spatial variability of soil acidity must not be managed or treated as uniform when lime is applied in the field.

Keywords: Lime requirement, Site specific management and Soil pH.