DAIRY MANURE INFLUENCES ON PHOSPHORUS RETENTION CAPACITY OF SPODOSOLS

Land areas used for dairy farming can result in accumulation of manure on soils that could produce nutrient-rich surface and subsurface runo ff and cause accelerated lake eutrophication. This research was conduc ted an Spodosols that mere differentially impacted by manure; the stud y included sites with different levels of total P (TP), from 2300 mg k g(-1) in the soil highly impacted by intensive dairy farming to 18 mg kg(-1) in an unimpacted area. The P retention characteristics of these soils were determined by using both single-point (1000 mg P kg(-1) or 100 mg P L-1) and traditional Langmuir isotherms, Phosphorus sorption values using a single high P solution had approximately a 1:1 relatio nship with values obtained for the maximum retention capacity, S-max, obtained from Langmuir isotherms (r(2) = 0.98). The surface A and E ho rizons of manure-impacted soils had essentially no sorbing rapacity wh ile the Bh (spodic) and Bw horizons had mean S-max values 430 and 385 mg kg(-1), respectively. The P sorbing capacity of the Bh and Bm horiz ons mere attributed to high Al concentrations in these horizons. Highe r P concentrations in the surface A horizon resulted in greater P conc entrations in solutions equilibrated with the Bh- and Bw-horizon mater ials, which suggests a potential for vertical P movement through the s oil profile, The spodic horizon of the less-impacted soils may not hav e been exposed to sufficiently high solution P concentrations to accum ulate significant P in the soluble fraction.