Animal‐Based Organic Amendments and Their Potential for Excessive Nitrogen Leaching and Phosphorus Loading

Abstract

Nitrate contamination of groundwater has been attributed to application of poultry litter to amend soil organic matter on raspberry farms in British Columbia, Canada. A field study assessed the risk of organic amendments that cause nitrate leaching and nutrient accumulation. The amendments, poultry litter, municipal compost, horse manure, and three types of dairy manure solids, were applied at 25 or 50 Mg dry matter ha–1 to match farm rates before new plantings. Plots were either unseeded or seeded with a grass mix to simulate berry establishment with grass alley crop. Soil was sampled at 0‐ to 15‐, 15‐ to 30‐, and 30‐ to 60‐cm depths 15 times over 14 mo and analyzed for concentrations of NH4+ and NO3–, and at 0 to 15 cm for total C, N, and P at 6 mo after application. Three grass harvests were analyzed for yield and N offtake. Poultry litter supplied more than twice the total mineral N and total P compared to the dairy solids and horse manure, while the compost was intermediate. At 25 Mg ha–1, poultry litter increased concentrations of soil (0–15 cm) NH4+ and NO3– (>150 and >300 mg kg–1, respectively), compared with small short‐term increases by compost (∼70 and 100 mg kg–1, respectively). There was almost no increase from dairy and horse amendments. Grass N offtake reduced soil NO3– concentrations but did not eliminate the risk of leaching from poultry litter. We conclude that dairy solids and horse manure present less risk of nitrate leaching and P loading than poultry litter or municipal compost.Core Ideas Poultry manure and compost amendments greatly increased soil NH4+, NO3– and P. Grass ‘alley’ crop did no sufficiently mitigate soil mineral N from poultry manure. Little risk of nitrate leaching or P loading with dairy solids or horse amendments.