Spatial variability of soil fertility in a block design Variabilidad espacial de fertilidad química del suelo en un dise o de bloques Variabilidade espacial da fertilidade química do solo num delineamento em blocos

In the characterization of the initial state of a long-term experiment (ELD) under no-till, we hypothesized that there are horizontal and vertical fertility gradients in the topsoil. The objective of this paper is to describe and to quantify those variations in a block design wheat fertilization experiment in the field of the National University of the South (UNS), Argentina. In a slightly inclined field 4 blocks were located across the slope. Soil depth was 80 cm at the top and increased downward to 100 cm. The soil was classified as a Petrocalcic Paleustoll, thermic coarse loam. Composite samples of each block were taken at the depths of 0-6, 0-12, and 0-20 cm. They were air-dried and sieved through 2 000 μm. Determinations were: pH; electrical conductivity (CE); cation exchange capacity (CIC); exchangeable cations: calcium (Caint), magnesium (Mgint), potassium (Kint) and sodium (Naint); extractable nutrients: phosphorus (PBK), sulphur (S-SO42-), boron (Bext), iron (Feext), manganese (Mnext), copper (Cuext) and zinc (Znext). In samples sieved through 500 μm total organic carbon (COT) and nitrogen (Not) were determined. Walkley & Black (W-B) was used for analysis of easily oxidizable organic carbon (COX24), lower H2SO4 concentrations of 12 and 18 N were used for further determinations (COX12 and COX18). Results were analyzed using descriptive statistics, t test for paired means and regressions. Soil organic carbon variables and oxidation factor for W-B were consistent with low intensity use of native grasses in the plot selected as a starting point for the ELD. The range of pH values (7.3-8.0) reflected base saturation, with ideal levels of Caint, Mgint and Kint. Nutrient availability was high for S-SO42-, Bext, Feext and Mnext, moderate for Cuext and Znext and low for PBK. The coefficient of variation (CV) was between 7 and 11 % for ten variables. The most stable were pH, Mgint, CIC, C:N and Cuext (CV < 7 %). Others (COX12, Not, Mnext and Znext) showed greater variability (CV 11-20 %) and PBK (CV 33.9 %). The statistical treatment of the space variability was adapted to the block design and sampling depths. In 12 out of 20 parameters, sampling depth and/or the position of the blocks explained over 30 % of the observed variations. The changes in depth of PBK, Kint and cation micronutrients (with the exception of the Cuext) can be associated to their scarce mobility in the soil. The position in the slope with potential effects on available water and biomass production, would explain the gradients in CIC, Caint and organic carbon. The opposite trend found