**3. Interaction of potassium with other nutrients**

Potassium concentration affects the sodium levels in the growing medium of plants. In general, antagonistic interaction exists in between potassium and sodium uptake by plants. However, the synergistic or antagonistic effect between them depends on the amount of each element present in the soil and on the plant type [12]. Under saline condition, plants preferably uptake sodium instead of potassium. While in sodium salinity condition, plant uses more selective high-affinity system for potassium uptake in order to maintain adequate potassium nutrition. Several studies showed the antagonistic effects of potassium and sodium in corn [13], rice [14], faba bean [15], and tomato [14]. It is also reported that adverse effect of sodium on plant growth is attributed to its antagonistic relationship with calcium, potassium, and zinc in plants [13].

It is inevitable that reduced potassium supply will inhibit plant growth and yield. Therefore, investigation on minimal requirements of potassium in plants maintaining their normal growth and development is necessarily important. Recent research reported that low potassium concentration in the nutrient solution significantly decreased the fruit potassium content in netted melon [16]. In leafy vegetables and tomato, sodium and magnesium content found to be increased significantly when potassium content was restricted to the culture solution [17].

Research results also showed that decrease in potassium levels increase the concentration of sodium and magnesium in tomato fruits [18, 19]. Sodium concentration in melon fruit increased with the decrease of potassium concentration in the nutrient solution and its concentration increase to 56% compared to standard concentration when plants were cultured without potassium fertilizer from anthesis to harvest [16].
