Potassium (K) is a primary macronutrient for overall plant growth, yield potential, product quality and stress resistance of crops. Potato (Solanum tuberosum L.) crops require a high amount of potassium to achieve the ideal yield and quality. Therefore, the determination of optimum K rate and efficient source for potato is necessary because K affects crop physiological processes, dry matter production, cooking, and processing requirements. Through modeling on the pooled data extracted from 62 studies, the highest tuber yields might be obtained at an exchangeable soil K level of 200 mg kg⁻¹ approximately, dependent on soil pH, texture, and organic matter. Through modeling on the data of 48 studies, it also revealed that application of potassium sulfate (K₂SO₄) and potassium chloride (KCl) at rates of 200 kg ha⁻¹ and potassium nitrate (KNO₃) at a rate of 100 kg ha⁻¹ might achieve the ideal yield, implying the importance of K sources in potato production. However, these values (either soil exchangeable K content, or fertilizer rates) might not be applicable in a specific growing environment for a specific potato variety. It seems that there is no discrimination among split, pre-plant or in-season application of K, although pre-plant fertilization might be a trustworthy strategy for economic tuber yield. Owing to the luxury consumption of K by potato crop, a combination of factors, including soil exchangeable K level, petiole K concentration, crop removal amount, soil conditions, management practices, climatic conditions, and potato variety, should be considered in order to make rational K fertilizer recommendations.

We studied the effects of a 2-day exposure to aluminum (Al), copper (Cu) and zinc (Zn) stress on root exudates and root apices of aseptically grown poplar (Populus tremula L.) cuttings. Aluminum induced root exudation of oxalate and citrate, Cu induced root exudation of oxalate, malate and formate, and Zn induced root exudation of formate. The threshold treatment concentrations were 100 μM for Cu and 500 μM for Al and Zn, corresponding to about 30 μM Cu²⁺, 140 μM Al³⁺ and 290 μM Zn²⁺. Simultaneous with the increase in organic acid anion exudation, sulfate and the nutrient cations K⁺, Ca²⁺ and Mg²⁺ were released into the solutions. Significant positive correlations between the organic acid anions and the cations indicate possible co-release. Toxicity symptoms of the poplar roots included browning of the root apices, which occurred at Cu concentrations of 50 μM and above, at Zn concentrations of 500 μM and above, and at an Al concentration of 1000 μM, and callose formation, which was observed solely in response to Al concentrations of 500 μM or higher. The results indicate that the composition of the exuded organic acid anions from poplar roots and the toxicity symptoms are specific to each of the applied heavy metals.