Soil phosphorus content determined by Mehlich 3 and modified Mehlich 3 methods

During the last century multiple methods have been developed for the determination of the amount of plant available nutrient elements in the soil. One of the critical nutrient elements in soil is phosphorus. Most of the phosphorus exists in soil in the form of insoluble inorganic and organic compounds. Therefore the amount of plant available P is limited as soluble P. The goal in the development of methods is to find an extraction solution, which can extract the nutrients from soil in a similar amount as plant roots. Due to large variations in soil properties and plants abilities it is a very complicated task. 
Several methods for determination of plant available P in soils are P specific (Bray P1, Olsen, CAL). Many methods are developed using the same extractant for two (DL, Joret-Hebert) and three (AL, AAC-EDTA) element extraction simultaneously. To minimize the time of analysis and laboratory costs the focus of the research is directed to the development of multielement methods, which is usable for macro and micro nutrients analysis from the same extract.
One widely used multielement method is Mehlich 3, developed in 1983. This method is useful for determination of macro elements (P, K, Ca, Mg) and microelements (Zn, Fe, Cu, Mn, B) in soil. As used extractant (0,2M CH₃COOH, 0,25M NH₄NO₃, 0,015M NH₄F, 0,013M HNO₃, 0,001M EDTA) consists of nitrate (NO₃^-) ions, this method can not be  used to determine nitrate in soil. The modified multielement method was proposed by Yanai et al. in 2000, where the composition of the extraction solution is: 0,2M CH₃COOH, 0,25M NH₄Cl, 0,005M citric acid, and 0,05M HCl. As there are no nitrate ions in the composition of the solution, it could be used beside macro and micro nutrient elements also for nitrate content determination in soils. 
In our research soil sample sets with different pH, texture and carbon content were analyzed by Mehlich 3 and modified Mehlich 3 method. Correlations of analyzed P contents between methods and impact of soil properties to the correlations were investigated.