Effects of Excessive Soil Phosphorus Accumulation on Loblolly Pine (Pinus taeda L.) Seedlings

Many landowners in East Texas apply poultry litter to pastures as a source of nitrogen (N) for forage crops. After many years of repeated poultry litter application, soils can accumulate extremely high extractable phosphorus (P) concentrations, sometimes over 1000 mg kg-1 of plant available P. Landowners report the conversion of these pastures to loblolly pine (Pinus taeda L.) plantations is often problematic, with poor seedling survival and abnormal tree growth. This study was conducted to assess the effects of excessive soil phosphorus on loblolly pine seedlings. An outdoor pot study was conducted using bare root seedlings and triple superphosphate treatments to simulate different soil P concentrations of those from poultry litter applications. The treatments were a control, 250 mg kg-1, 500 mg kg-1, 750 mg kg-1, 1000 mg kg-1, and 1250 mg kg-1of plant available P with eight replications. Seedlings were grown for one growing season and measured periodically for survival, height and diameter growth, foliage color using a Munsell color transformation, and at the end of the study sampled for dry biomass (above and below ground), survivability, foliar nutrient content, and ground needle color Munsell color transformation comparison. Growth trends in the study were positive for growth relationship to P treatment level increase. However, at the end of the first growing season seedlings presented deficiency symptoms like needle tissue chlorosis and branch tip necrosis in the high P treatments. Trends in foliar nutrient content were an increase in zinc concentrations and a decrease in iron concentrations as phosphorus treatment level increased. Excess foliar phosphorus ratioed to iron and zinc emphasized a dilution effect or possible phosphorus:iron competitive interaction, especially with iron. Munsell color comparison between living and ground color showed a higher variability in ground color range in the 3-dimensional color space, as well as a higher significance between color and deficiency symptoms. Munsell color proved to be a useful tool to analyze the relationship between color and variable plant health