Research Journal of Agricultural Science, 43 (4), 2011 STUDIES REGARDING PHOSPHORUS AND POTASSIUM CONTENT OF JOHNSON GRASS IN DIFFERENT DEVELOPMENT STAGES Ramona STEF, Ioana GROZEA, A. F. CARABET, Ana -Maria VIRTEIU, Snejana DAMIANOV Banat's University of Agricultural Science and Veterinary Medicine Timipiara Calea Aradului no.119, 300645 Timipiara, Romania E-mail:chirita ramona@yahoo.com Abstract: The Johnson grass is adapted to a wide variety of pedo - climatic, climatic and biological conditions. In Romania Sorghumm halepense still considered a problematic weed (Sarpe N., 2004) in some crop cultures. The Johnson grass damaging potential consist in his high competitiviy to the crop plants in order to get nourishment, water and light (McWhorter, 1973). Based on these informations we have established as objective the quantification of phosphorus and potassium in Johnson grass in different growth stages, between BBCH 10 -BBCH 69 (emerging - blossom ending). In this interval the quantity of phosphorus and potassium was determined from rhizomes, stalk, leaves and panicle. The Johnson grass samples where gathered in zigzag way, the samples were placed in plastic bags and brought in the agrochemical laboratories of Agricultural Sciences am Veterinary Medicine of Banats University Timisoara. The chemical analyses were based on methods found in various papers (Radulov Isidora, 2004) where the potassium quantity is assessed trough emission spectrometry method where the Key words: Sorghum halepense, fosfor, potasiu, intensity of K atoms emission under excitation on air - acetylene flame is measured. In order to determine the phosphorus quantity we used the colorimetric method based on the fact that the orthophosphoric acid anions present in the analysing solution react with molybdenum trioxide MoO3, resulting phosphomolybdic hetero-polyacid. Following the studies we observe that the highest potassium percent (K20) 3,95% was present in growth stage BBCH 18 (8 leaves developed), the recorded values lay between 0,8 - 3,95%. The results obtained assessing P205 quantity do not exert significant fluctuation (0.3-1.0 mg), the highest value was recorded in growth stages BBCH 39 when the vegetative organs of Sorghum halepense achieved maximum development. Among analysed organs the inflorescence proved to have the highest potassium quantity (1,92%). The highest phosphorus quantity was achieved in rhizomes, sustaining the statement "the rhizomes are the energy deposit of species Sorghum halepense" INTRODUCTION The invasive species are known as the main biodiversity threat. The high economical impact in agriculture places them in the first position among the damaging biotic factors. The Johnson grass is very competitive and invasive, is the most damaging weed of Romania in crop cultures, causing yield lose up to 80% of entire production, even 100% at a high infestation degree of maize crop. To the production Competitive capacity of Johnson grass adds production of allelopathic substance (dourine) which adversely affects germination of other plant species. In light of this information has undertaken this study aimed to determine the amount of phosphorus and potassium, present in Johnson grass plants in different growth stages. Research conducted by SINGH and col., 1973; NEWTON and col., 1985; BESSIN andcol., 1990; BROWN $i col, 1993, showed that invasive species Sorghum halepense compared to other grasses, have the highest phosphorus content. Studies on the content of nitrogen, phosphorus and potassium of the species Sorghum halepense made by PINTILIE in 1984 and in 2006 by 196