Monday, November 13, 2006

Impacts of Dairy Effluent Irrigation on Soil (Mollisol) and Plant Properties in a Tropical Island Environment.

Rowena B. Valencia-Gica, Russell S. Yost, Guy S. Porter, Rosalin Pattnaik, and Veronica A. Wilcox. Dept of Tropical Plant and Soil Sciences, Univ of Hawaii at Manoa, 3190 Maile Way, St. John 102, Honolulu, HI 96822

Dairy producers accumulate large quantities of effluent in lagoons. These lagoons occasionally overflow causing the nutrients and other contaminants in effluent to pollute the land and water bodies. Alternative uses of effluent are urgently needed for a sustainable and environment-friendly dairy production. This study determined the sustainable effluent application rate and assessed the effects of effluent irrigation on plant and soil (Cumulic Haplustoll) properties. Four grasses—bana (Pennisetum purpureum K.), signal (Brachiaria decumbens S.), star (Cynodon nlemfuensis V.), and suerte (Paspalum atratum S.)—were subsurface drip irrigated with dairy effluent at two rates based on potential evapotranspiration (ETp) at the site (Waianae, Hawaii)—2.0 ETp (7-44 mm d-1) and 0.5 ETp (2-11 mm d-1). Treatments were arranged in an augmented completely randomized design. Forage was harvested, and soil and soil solution were collected every month. Pennisetum purpureum and P. atratum had the highest dry matter yields (~60 and 46 mT ha-1 y-1, respectively). Nutrient removal of grasses was 30-279%, 14-100% and 2-18% of applied effluent N, P and K, respectively. Extractable soil P (86-159 mg kg-1 at 0-15 cm; 81-123 mg kg-1 at 15-30 cm) and soil solution total P (2-9 mg L-1) did not significantly increase after two years of irrigation. Soil pH increased from 7.4-7.8 (July 2003) to 8.1-8.9 (July 2006). Electrical conductivity (0.5-1.3 dS m-1 in July 2006) was below 4.0 dS m-1   (U.S. Salinity Laboratory's critical level for classifying saline soils). Soil exchangeable sodium percentage (ESP) remained below 15% (critical ESP for classifying sodic soils). Results suggest that irrigating high yielding tropical grasses with effluent at 2.0 ETp may be acceptable. Additional monitoring is needed to determine the longer-term impacts of effluent application on soil and plant properties. Information generated will allow dairy producers to reduce feed costs while minimizing pollution associated with effluent application—thus, creating a win-win option. 

Handout (.pdf format, 158.0 kb)