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Figure 2. Nitrate concentrations measured in leach - ate during Lysimeter Experiment 1 for unfertilized lysimeters and lysimeters treated with ammonium sulfate at one and one -third times the recommended application rate of 77 kg N ha -1. treatment presented in Figure 2, preferential flow was probably occurring in several of the lysim- eters, limiting interaction between fertilizer and soil and resulting in high nitrate concentrations in leachate. No increase in nitrate concentration was observed following fertilizer application in Experiment 2. The nitrate concentrations from the P- only treatment and the com- bination N and P treatment did not differ from the nitrate concentrations produced by the unfertilized treatment. Figure 3 shows the average dissolved phosphate con- centrations measured before fertilizer application and during Experiment 1 for each fertilizer rate. No phosphorus fertilizer was applied during this portion of the study, yet leachate concentrations were consistently measured over 1.0 mg L -1. During the sev- enth week, a small increase in average phosphate concentra- tion was observed in leachate from lysimeters treated with ammonium sulfate fertilizer. 2.5 One -third 2 —Ar —One � 1.5 E r CL 1 H a 0.5 01 5/29/04 Since no phosphorus fertilizer was applied during this portion of the study, P measured in leachate must have originated from residual soil P. Ap- plication of ammonium sulfate may have tem- porarily reduced the pH of the soil solution and increased the availability of calcium phosphate species. The increase in phosphate concentra- tion was short-lived and leachate concentrations returned to previous levels by the next irrigation event. Over the entire duration of the study, ammonium sulfate fertilizer applied at one and one -third the full application rate had very little influence on average phosphate concentrations in leachate. Average phosphate concentrations in leachate during Experiment 2 are presented in Figure 4. Lysimeters treated with a combination of am- monium sulfate and superphosphate fertilizers produced a three -fold increase in the average phosphate concentration following fertilizer application, illustrated by the spike in concentra- tion occuring on 10/05/04 for the NP treatment. These results further suggest that ammonium sulfate fertilizer was temporarily increasing the availability of phosphorus. In this case, ammo- 6/12/04 6/26/04 7/10104 7/24/04 8/7/04 8/21/04 Figure 3. Average phosphate concentrations measured in leachate prior to fertilizer treatment and during Experiment 1. Data points represent average phosphate concentrations for lysimeters of the same treatment following each irrigation event. The dashed line marks the beginning of Experiment 1 when ammonium sulfate fertilizer was applied to a por- tion of the lysimeters. Fertilized lysimeters were treated with either one or one -third times the recommended fertilizer rate of 77 kg N ha -1.