Incorporating granular inorganic fertilizer into perennial grassland soils to improve water quality

ABSTRACT:

Commercially-available inorganic fertilizers are often used by beef and dairy producers to increase forage growth on perennial pastures and hay fields, but the usual practice of spreading fertilizer granules on the surface of these grasslands leaves the nutrients exposed for transport in runoff, and for rapid nitrogen (N) loss to the atmosphere as ammonia. We hypothesized that such problems could be minimized by using a knifing technique to move the fertilizer granules into the subsurface root zone of perennial forages, and conducted a study to determine the effect on runoff water quality. Plots were constructed on a hillside (8 to 10 percent slope) field with a silt loam soil covered by well-established bermudagrass [Cynodon dactylon (L.) Pers.] and mixed grass forage. Each plot had borders to isolate runoff and a downslope trough with sampling pit for collection. Inorganic 13-13-13 fertilizer granules were applied at 1.34 Mg ha⁻¹ (0.6 t ac⁻¹) by one of three methods: surface spreading, incorporation, or surface spreading on soil cuts. Each treatment had six replications and there were three control plots that were not fertilized. Runoff samples from simulated rainfall applied at 50 mm h⁻¹ (2 in hr⁻¹) and natural rainfall events showed that nutrient losses in runoff from incorporated fertilizer were statistically no higher than from control plots, but were usually at least 90 percent less than those from surface-applied fertilizer. Soil cuts slightly decreased nutrient losses from surface-applied fertilizer, but this effect was rarely significant. When compared to surface-applied fertilizer, the incorporated fertilizer not only improved water quality, but also showed a strong tendency to increase forage yield.