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 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 MoO 3 , resulting phosphomolybdic hetero-polyacid. Following the studies we observe that the highest potassium percent (K 2 O) 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 P 2 O 5 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”

Sorghum halepense, fosfor, potasiu,

Presentation: oral