A population consists of individuals and a correct sampling of the individuals estimates population characteristics adequately. However, to evaluate soil fertility, what would be the soil individual (sample units), and of what size? The objective of this study was to determine the dimension of the individual soil component of a particular population, in three sampling areas: under no-till or under conventional tillage before or after plowing, and to evaluate soil fertility and develop a soil sampling method, determining the number of single samples required to form a composite sample that would characterize a soil individual (sample unit). Single soil samples (5.4 cm diameter x 10 cm deep) were collected along the semi-diagonals of five hexagons outlined on the selected sampling areas. The sampling hexagons had 2 m long sides and from each of the six semi-diagonals, 10 single soil samples (55 per hexagon) were collected at the distances 12.5; 25.0; 37.5; 50.0; 75.0; 100.0; 112.5; 150.0 and 200.0 cm, measured from a simple central sample, to the center of the sampling spot. The single central sample was the same for the six semi-diagonals. Soil pH (H2O), available P and K, Ca2+ and Mg2+ concentrations and H + Al, equilibrium-P and organic matter were determined. The single samples were grouped successively from the hexagon center and formed nine sampling units: A (up to 18.75), B (up to 31.25), C (up to 43.75), D (up to 56.25), E (up to 81.25), F (up to 106.25), G (up to 118.75), H (up to 156.25) and I (up to 206.25 cm). Regression analysis of the means and standard deviations of the evaluated characteristics was performed considering the possible dimensions of the soil sample unit. Based on the results, the conclusion was drawn that 25 simple samples must be collected to characterize a soil individual (sample unit), for the formation of a representative composite sample in a 68.75 cm-sided hexagon with an area of 1.228 m².
sampling; sample unit; spatial variability
