Measuring Potassium Uptake and Growth Response of Annual Ryegrass at Multiple K Rates in K-Fixing and Non-K-Fixing Soils.

The fixation of potassium in non-exchangeable forms complicates soil K testing and management in certain soils. Previous work in our lab has documented varying rates of K fixation in the San Joaquin Valley of California, and has demonstrated that vermiculite in the fine sand and silt fractions of the soil is largely responsible for the observed fixation. In order to better correlate laboratory measures of soil K status to plant response, a greenhouse pot study was conducted growing annual ryegrass (Lolium multiflorum) at four rates of K application (0, 50, 250, and 1000 mg kg^-1 soil) in 12 soil materials, representing a range of mineralogies and initial ammonium acetate-extractable K (NH₄OAc –K), sodium tetraphenylboron extractable K (TPB-K) and K fixation potential (Kfix) levels. Grass was harvested at 1 cm above the soil surface at three week intervals, with a total of three clippings. Oven dry weight and tissue K were measured for each clipping. At low levels of K application, total K uptake typically exceeded NH₄OAc –K plus added K, and was better represented by TPB-K plus added K. TPB-K was overall the best predictor of response to K treatment. K fixation potential was a significant factor affecting uptake and growth response for some, but not all, soils. Maximum growth was achieved when uptake exceeded about 500 mg kg^-1.