Variabilidade em análises de fertilidade do solo

Abstract

The analysis of soil fertility is the subsidy for adequate fertilization, and therefore, it is necessary that this be accurate and reliable. Studying the variability of the methods used at the Instituto Agronômico de Campinas for soil analysis was the general purpose of this work. Eight laboratories registered in the IAC Proficiency Essay, with index of excellence superior to 90% between the years of 2011 and 2015, were selected and the results of the analysis of 20 soil samples, repeated three times, used to estimate the absolute and relative standard deviation of repeatability and reproducibility and the Mandel's k and h statistics, according to ISO 5725-2. The methods were considered repeatable and reproducible when the coefficients of variation of reproducibility (CVR) were less than 20%; with good repeatability and reproducibility when CVR were less than 10%; little repetitive and reproductive when presented CVR superior to 20% for some analyzed content; and without repeatability and reproducibility when they presented CVR always superior to 20%. The efficiency of the methods used for soil analysis was dependent on the content of the analyzed element, and the lower the nutrient concentration, the higher the coefficient of variation of repeatability and reproducibility of its determination. The determinations of the potassium, sulfur, aluminium, copper and zinc contents were considered little repeatable and little reproducible due to the imprecision of results involving the determinations of these elements in low concentrations in the soil. The determination of the boron content was considered without repeatability and without reproducibility. All other determinations of basic, micronutrient or granulometric analysis presented good repeatability and reproducibility. Except for the analysis of the micronutrient contents, in the same analytical set, laboratories that were repetitive, were also reproductive. The laboratories that presented excellent repeatability in the soil basic analysis also presented excellent repeatability in the analysis of micronutrient contents.