Preliminary compositional nutrient diagnosis norms for cowpea (Vigna unguiculata (L.) Walp.) grown on desert calcareous soil

This study calculated the compositional nutrient diagnosis (CND) norms of cowpea (Vigna unguiculata (L.) Walp.), as well as identified significant nutrient interactions of this crop growing in an irrigated calcareous desert soil. Three genotypes were distributed in rows in a 2-ha field. The soil showed high heterogeneity in its chemical properties. For statistical analysis, 86 foliar composite samples from healthy plants were used. Preliminary CND norms were developed using a cumulative variance ratio function and the χ2 distribution function. Means and standard deviations of row-centered log ratios VX of five nutrients (N, P, K, Ca, and Mg) and a filling value R, which included all nutrients not chemically analyzed. Preliminary CND norms are: ${\mathrm{V}}_{\mathrm{N}}^{*}$ = 0.174 ± 0.095, ${\mathrm{V}}_{\mathrm{P}}^{*}$ = -2.172 ± 0.234, ${\mathrm{V}}_{\mathrm{K}}^{*}$ = -0.007 ± 0.267, ${\mathrm{V}}_{\mathrm{C}\mathrm{a}}^{*}$ = -0.022 ± 0.146, ${\mathrm{V}}_{\mathrm{M}\mathrm{g}}^{*}$ = -1.710 ± 0.132, and ${\mathrm{V}}_{{\mathrm{R}}_{5}}^{*}$ = 3.728 ± 0.084. These CND norms are associated with dry bean yields higher than 1.88 t ha-1, and are associated with the following foliar concentrations: 26.2 g N kg-1, 2.5 g P kg-1, 22.9 g K kg-1, 21.6 g Ca kg-1, and 4 g Mg kg-1. Cowpea plants growing in desert calcareous soils took up lower amounts of N, P, and K than those considered as optimum in a previous report. Six interactions were strongly indicated for cowpea through principal component analyses: positive for Ca–Mg, and negative for N–Ca, N–Mg, Ca–P, Mg–P, and K–P. Furthermore, two interactions were identified using simple correlations, negative N–P and positive K–Ca.