Row-centred log ratios as nutrient indexes for saturated extracts of organic soils

Abstract

Composition of soil solution was investigated in the top layer of a Typic Mesisol across seven crop sequences. Each sequence started and ended with carrots (C) and included onions (O), barley (B) and weeded fallow (F) as follows: CCCC, CCBC, CBCC, CCOC, COBC, CBOC and CFFC. The soil showed adequate pH and electrical conductivity values. Soil solution composition was assessed by the saturated soil extraction procedure. The changing levels of nutrients in the saturated soil extracts over seasons, years and crop sequences were analyzed using absolute values of nutrient concentrations, nutrient percentages relative to total soluble salts (the nutrient intensity and balance concept) and row-centred log ratios of nutrient concentrations or activities (compositional data analysis). The row-centred log ratio is the log-transformed ratio of a given element to the geometric mean of soluble elements. Nutrient percentages were highly correlated with row-centred log ratios. The three relative nutrient expressions reduced seasonal variations as compared with raw nutrient concentrations by incorporating component proportions. Concentration of total soluble salts was significantly correlated (P < 0.01) with concentrations of NO3-N (r = 0.37), P (r = 0.80), K (r = 0.80), Ca (r = 0.96) and Mg (r = 0.92) in saturated soil extracts. Between-years homoscedasticity as well as reduced skewness and kurtosis for all nutrients were obtained using row-centred log ratios of ion activities. Other advantages of using row-centred log ratios are discussed. Key words: Saturated soil extract, compositional nutrient diagnosis, seasonal variability