Status of Soil Nitrate and pH after Continuous Application of Biosolids.

Research showed that Biosolids should be applied occasionally for crop production to avoid NO₃ accumulation and subsequent contamination of ground water. One winter wheat (Triticum aestivum L) field experiment was established at the Efaw Experiment site, OK in the fall of 1993 to assess effects of N rates from biosolids and ammonium nitrate (AN) fertilizer on Soil NO₃ and pH. Factorial combinations of the two sources and six N rates (0, 45, 90, 180, 270 and 540 kg N ha^-1 yr^-1) were evaluated for 15-yr.  The experimental design was a randomized complete block with three replications.   All N was broadcast and incorporated prior to planting.  Nitrate was measured on a Lachat flow-injection analyzer. Soil pH was determined using an Accumet excel SL series pH meter and a combination electrode. Results showed that after 15-yr of  continuous applications of  biosolids, the soil NO₃ level was < 10 ppm at soil depth of 40-120 cm even with the highest rate of N. Alternatively, with continuous AN application, NO₃ level was >10 ppm for rates ≥ 180 kg ha^-1. Soil pH was maintained at 6.4 units throughout the study period with the application of biosolids but decreased to 5.7 with the application of AN. Similarly, pH remained constant with increase in N rate in biosolids applied plots while it decreased at a rate of 0.27units with increase in N rate with the application of ammonium nitrate. The results suggest that both soil NO₃ and pH were not increased to a detrimental level with continuous application of biosolids at several rates in Norge loam soil of Oklahoma.