Gustavo A. Martínez-Rodríguez

Phosphorus in runoff from two highly weathered soils of the tropics

Ramírez-Ávila, J. R., Sotomayor-Ramírez, D., Martínez-Rodríguez, G. A. and Pérez-Alegría, L. R. 2011. Phosphorus in runoff from two highly weathered soils of the tropics. Can. J. Soil Sci. 91: 267-277. Agricultural fields with high soil phosphorus (P) content are important contributors to surface water degradation. Two consecutive simulated rainfall events were conducted on two Ultisols previously amended with inorganic P fertilizer or broiler litter. Soil test P (Bray 1 and Olsen) levels evaluated ranged from 1 to 350 mg kg-1. Surface runoff concentrations of total P (TP) and dissolved P (DP) generated by a 30-min runoff event were quantified. Runoff DP concentrations ranged from 0.08 to 3.98 mg L-1 in fertilizer P-amended soils and from 0.08 to 4.93 mg L-1 in broiler litter-amended soils. A single exponential model adequately described the relationships between soil test P and DP concentrations in runoff. For each soil, the soil test P-DP concentration relationships were positively influenced by soil organic matter and negatively influenced by antecedent soil moisture (P<0.05). For both soils, the soil test P-DP concentration relationships were positively influenced by groundcover percentage and negatively influenced by slope. Environmental soil test P critical levels corresponding to a runoff threshold of 1 mg L-1 DP, ranged between 176 and 296 mg kg-1 (Olsen) and 143 to 276 mg kg-1 (Bray 1) in soils amended with fertilizer-P. In broiler litter-amended soils, threshold values were 88 and 111 mg kg-1 using Olsen and Bray 1, respectively. Differences in surface runoff-P concentrations due to amendment sources, antecedent soil moisture content, soil organic matter, groundcover and slope suggest that these factors need to be considered in P management decisions at the farm level.

Grass filter strips evaluation for reducing sediment and nutrient exportation from grasslands under manure applications in Puerto Rico

Sediment and nutrients losses from agricultural lands have been identified as a significant environmental problem. Vegetative filter strips are commonly used as a best management practice (BMP) to control agricultural pollution. This study was conducted to test the hypothesis that grass filter strips were effective in reducing sediment and nutrient exportations in runoff from grazed pastures amended with irrigated dairy manure. The experiment was carried out under conventional management practices in a dairy farm in Puerto Rico. Natural runoff events were diverted into runoff collection devices placed at 0, 10, and 20 m within a grass filter barrier. Collected samples were analyzed for Total Suspended Solids (TSS), Total Kjeldahl Nitrogen (TKN), Dissolved Phosphorus (DP), and Total Phosphorus (TP). TSS concentrations in runoff entering the filter strips were minimal, indicating that TSS losses are not significant from pasture fields exhibiting high vegetative coverage. The higher TP and TKN concentrations were

Caribbean Phosphorus Index Validation and Management Practices Evaluation on Fields under Manure Applications

Phosphorus (P) losses from agricultural soils are a major cause of fresh water quality impairment. The objective of this study was to validate, at field level, the Caribbean Phosphorus Index (CAPI). The CAPI is a tool developed for the identification of agricultural soils with high risk of P losses to surface waters. P runoff concentrations measured under natural rainfall condition on various fields of dairy and poultry farms in Puerto Rico were compared with risk levels determined using CAPI. Average Olsen soil test P (STP) values were 41 mg P kg-1 and 152 mg P kg-1 for the dairy and the poultry farms, respectively. Average total phosphorus (TP) and dissolved P (DP) concentration losses in runoff were 2.29 mg TP L-1, and 1.79 mg DP L-1 for the dairy farm, and 5.87 mg TP L-1 and 4.82 mg DP L-1 for the poultry farm. The CAPI ranked both dairy farm fields as fields having a Medium potential for P movement and poultry farm fields with Medium and High potential. The current CAPI version underestimated the impact of nutrients on the surrounding waters because runoff concentrations greater than 1 mg L-1 from fields less than 30 m from a surface water body should be ranked in the Very High category. A modification of the CAPI excluding the soil erosion factor from the CAPI matrix yielded results that were more attuned to the observed runoff concentration losses. This exclusion is justified from evidence indicating that soil erosion is minimal in fields exhibiting abundant (>80%) grass cover.