Soil sampling is designed to ensure obtaining reliable information about the existence, concentration and distribution of the analyzed substances in the investigated area. The caatinga is usually characterized as tree and shrub formations, mostly with high fertility soils, but suffering major physical limitations. This study aimed at determining the minimum number of single samples that theoretically should be used to form a composite sample representative of the depths 0 to 10, 10 to 20, and 20 to 30 for the chemical characteristics: pH, P, K+, Ca2+ and Mg2+, as well as show that the average fertility estimated from the arithmetic mean of the results of single samples did not differ statistically from that estimated from the chemical analysis of the composite sample and statistically evaluate the effects of collection instruments (auger, cut-shovel and cup-auger) at a depth of 0 to 10 in the main indices of soil fertility. The study was conducted in a caatinga area near the city of Arcoverde, State of Pernambuco-PE. Samples were collected from 40 plots of 250 m2. The minimum number of simple samples that should be used to form a representative composite sample for fertility indices increases with decreasing angular error. According to the results, there was less spatial variability to the pH at all depths analyzed. Below 0.2 m, the number samples to be at maximum for proper estimations of P and K+levels in soils. A smaller number of samples can be taken for the variables, pH, Ca²+ and Mg²+. Significant differences in rates of fertility (pH, P and Mg2+) were observed among the instruments. The order of variability was different for all instruments. There were no differences in fertility rates for the average of single samples over the average of composite samples consisting of three simple subsamples.
Key words: Soil analysis, collection instrument, variability.
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