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*Corresponding author: Shaimaa_Hassan@agr.asu.edu.eg DOI :10.21608/ejss.2018.2752.1153 ©2018 National Information and Documentation Center (NIDOC) P (K) is an essential element for plant growth that maintains water balance within its cells. Different forms of K were investigated for their effects on potato (Solanum tuberosum, cv. Spunta) plants grown under water stress conditions as a kind of adaptation to the climatic changes and water shortage under Egyptian soil conditions. Also, knowledge of different K fractions in the studied clay soil is important to achieve the sustainability in agriculture. Therefore, a field experiment was conducted during the two tested seasons of years 2014 and 2015 at the Faculty of Agriculture, Ain Shams University, Qalubia governorate, Egypt. Two sources of K were tested under three levels of irrigation water in a split-plot design. Results indicated that K fractions in soil were in order of total K > non-exchangeable/ fixed > exchangeable > water soluble. The 50% irrigation level of irrigation requirements (IR) combined with K-humate as ground application increased water soluble and exchangeable K in the studied soil. While there was hardly no changes observed due to the applied treatments on fixed and total amounts of K in the soil. Regarding the studied vegetative growth and yield parameters of the growing potato plants, the treatment 100% of IR combined with K-humate as soil application caused significant increases. The specific gravity of tuber, as an important indicator of potato tuber quality, recorded the highest value by applying 50% of IR combined with K-humate as ground addition, compared to mineral addition which came in the second order. Regarding the nutrient concentrations in potato haulm, results revealed that the 50% of IR combined with K-humate gave the highest content of N, while the same irrigation level with K-humate plus foliar spray gave the highest content of P and K, during the two tested seasons. In tubers, 50% of IR combined with K-humate as soil application gave the highest concentration of N and P during the two tested seasons, while 75% of IR in the first season and 50% of IR in the second season combined with K-humate plus foliar spray gave the highest values of K content. The calculated water use efficiency (WUE) showed that the highest value was obtained by 50% irrigation level of IR combined with soil application of K-humate. 15

egyptian journal of soil science

Comparison between organic and mineral sources of potassium and their effects on potassium fractions in clay soil and productivity of potato plants under water stress conditions