Determination of the most effective index to evaluate soil potassium availability and k-fertilizer recomnendation

Many research works have been done on potassium nutrition of plants growing in soils especially in Iran. Answers of some critical questions about plant potassium nutrition such as: How plants prepare their needs to K?, How it supply to plant roots and how they uptake K?, How different soil K forms change with time?, What is the best soil K extractant?, and many other questions, are almost clear, but lack of an accurate and reliable method to assess K availability in different soil and different conditions still remained. The usual method of soil K assessment based solely on the extraction of potassium in soil using normal and neutral ammonium acetate solution has not enough sensitivity to different soil and climate conditions, and the recommendations based on it sometimes cannot confirm the real plants need. According these gaps, this project was designed to compare different methods of estimating plant available K in soils and K fertilizer recommendations to select the best method. A field experiment conducted on wheat in two different locations (Golestan & Alborz provinces) to determine the effect of different levels of potassium fertilizer on grain and straw yield. The experiments were done in a randomized complete block design (RCBD) with four treatments and three replications in Karaj and Golestan was conducted during the 2010-2011. Different four K treatments were applied before cultivation from potassium sulphate: Karaj : T1 = 0, T2 = 120, T3 = 240 and T4 = 360 kg/ha Golestan : T1 = 0, T2 = 150, T3 = 300 and T4 = 450 kg/ha Determination of yield and plant traits after harvesting showed that, according soil texture and other conditions, wheat yield and traits were better in Golestan than in Karaj. The results showed that the application of potassium fertilizer in different levels could not significantly affect (in 5% level) plant yield and traits (thousand seed weight, seed yield, total yield, K concentration in straw and grain) in Karaj and Golestan. Highest mean concentration of potassium in straw and grain, 2.11% and 0.63 % respectively, was from T3 treatment in Karaj experiment. In this treatment, the total amount of potassium uptake by grain and straw, average 121.5 kg/ha, available potassium in the soil after harvest, average 247 mg/kg and the amount of potassium released by soil were in the highest level. In Golestan, the highest grain and straw K concentration, 2.35 and 0.64%, respectively were from T4 treatment. Available soil potassium after harvest and potassium uptake by the plant in T4, with a mean of 127 mg/kg and 243 kg/ha respectively, were higher than other treatments. But the highest average amount of released soil potassium achieved in control treatment. Results showed that the highest level of available soil potassium after harvest, achieved in T3 in Karaj experiment, but in T4 in Gorgan experiment (average of 247.5 and 127.3 mg/kg).It should be mentioned that, however due to a decrease in experiment locations during work, achieving a reliable indicator for accurate assessment of the plant available potassium status in soil and recommendation of K fertilizer based on that, was not accessible, but at least it showed that the amount of soil available K can't give an appropriate index to distinguish deficiency or sufficiency level of soil potassium for plants lonely. Key words: Available potassium, potassium recomnendation index, wheat