Faba bean yield and growth dynamics in response to soil potassium availability and sulfur application

The faba bean is one of the most important sources of plant protein for humans and animals. The relationships among crop growth dynamics, carbohydrate metabolism, and the plant yield of faba bean are not fully recognized. This study assessed the effect of elemental sulfur (S) application (0, 25, and 50 kg S ha−1) on the seed yield (SY), yield components, and growth dynamics parameters. Field experiments were carried out in soils with variable potassium (K) availability and fertilization as follows: i) low content and without K application (-K), and ii) high content and with additional K application at the rate of 133 kg K ha−1 (+K). Vicia faba L. var. mayor was the botanical strain of faba bean used in the experiments. During the growing season, plant samples were taken every 7-8 days, beginning at the two-leaves-unfolded stage. The sampling was completed when the seeds reached the stage of milk maturity (green seeds). A three-parameter logistic function was applied to assess the dynamics of dry matter accumulation. The function provided growth parameters (e.g., critical time points, the crop growth rate = CGR) that were used to explain the effect of soil available K and S application rates on SY. It was shown that the plant response to K treatment depended on the weather conditions during the vegetation season, whereas the plant response to S depended on the K fertilization system. Overall, the faba bean grown in the +K treatment plot accumulated more dry matter and was characterized by a higher CGR compared with the plot without K application (-K). In the −K treatment plot, application of S contributed to an improvement in plant growth parameters (e.g., CGR) which, in turn, increased the SY. The optimum S rate for this treatment was 25 kg S ha−1. In the +K treatment plot, the effect of S application on the growth parameters was similar. Despite this finding, S application led to a reduction of SY in comparison with a treatment without S. Among the studied growth parameters, total dry matter accumulation at the beginning of the intensive growth stage turned out to be the factor that considerably differentiated the treatments with respect to SY. This result confirms that faba beans should be provided with nutrients in the most effective way, i.e., at the immediate beginning of the vegetation period.