Modeling the Contribution of Salts to the Electrical Conductivity of Fertigation Solutions Used in Greenhouses in the Mediterranean Area

Abstract

Composition and concentration of soluble salts in the root-zone medium solution have an influence on plant growth, both by creating osmotic imbalance and via specific physiological toxicity of ions. Osmotic potential is related to electrical conductivity (EC) by mathematical models. The knowledge of the contribution of ions to the EC by means of theoretical estimations or regression models could link both effects of salts on nutrient uptake by plants and the development of new strategies for managing fertility and irrigation of horticultural crops. Solutions were obtained from nine commercial greenhouses (G), during the cultivation cycle of tomato plants var. Daniela. There were 273 samples, including irrigation water (IW), nutrient solutions (NS), and soil solutions (SS) obtained by suction cup. Solutions showed a wide range of EC and nutrient concentrations. Attending to nutrient concentration [nitrate (NO3 −), potassium (K+), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), chloride (Cl−), and sulfate (SO4 2−)], the individual contribution of EC (Ω−1 cm2 mol−1) was estimated by means of the theoretical Debye–Hückel–Onsager equation reference, obtaining the ion molar conductivities (IMC) of 0.071 NO3 −, 0.073 K+, 0.119 Ca2+, 0.106 Mg2+, 0.050 Na+, 0.076 Cl−, and 0.160 SO4 2−. On the other hand, the contribution coefficient for each parameter of EC (dS m−1) was estimated by means of multilinear regression, presenting the following coefficients: 0.033 NO3 −, 0.072 K+, 0.124 Ca2+, 0.068 Mg2+, 0.038 Na+, 0.046 Cl−, and 0.026 SO4 2−. The determination coefficient of the model was 0.93. To validate both models 194 samples of solutions (IW, NS, and SS) obtained from different horticultural crops cultivated in greenhouses under fertigation in the Mediterranean area were used. The determination coefficients of the validated simple regressions were 0.83 and 0.89 for the theoretical and the multilinear models.