Long-term effects of land application of poultry manure on crop production, and soil and water quality under a corn-soybean rotation system in Iowa
In recent years, the poultry industry has seen a steady growth in Iowa. With the increase in the broiler and egg industry, the public is concerned about the potential threat to environmental quality (surface and subsurface water and air quality) from increased volumes of poultry manure. Although the effect of poultry manure on crop production has been studied, its effects on water quality, under corn and soybean rotation, in Iowa have not been extensively studied. Therefore, a long-term study was initiated in 1998 to investigate the impacts of poultry manure on crop production and environmental quality (soil and water quality). This thesis will present the results of this eight year study (1998 to 2005) on land application of poultry manure on (i) crop yields and crop quality; (ii) subsurface drain water quality, and (iii) soil quality. Eleven experimental field plots of sizes varying from 0.14 to 0.4 ha were used in this study. Each field plot was drained by a single subsurface drain line passing through the center of the plot. These subsurface drain lines were intercepted at the end of the plot to collect water samples for water quality analyses. Corn and soybeans were planted in the same plot with corn on half of the plot and soybean on the other half. Only the corn side received the N fertilizer (poultry manure or Urea Ammonium Nitrate, UAN). Subsurface drain water samples were collected weekly during the growing season (March-October) and analyzed for NO3-N and PO4-P concentration. Soil samples were collected before planting and after harvesting each year and analyzed for NO3-N and PO4-P at 5 different depths of the soil profile. Corn stalk samples were also collected after harvest at the height of 20-25 cm from the ground and analyzed for nitrogen concentration. Two application rates of poultry manure (168 kg-N/ha and 336 kg-N/ha) and one application rate of UAN (168 kg-N/ha) were applied in split plot design with unbalance replications. A check plot, not receiving any fertilizer (neither poultry manure nor UAN) during the study period, was also used to collect data on crop yields, and soil and water quality for comparison purposes. Statistically, data on corn and soybean yields were analyzed separately for all treatments. The overall results of this study for the eight year period (1998-2005) showed that applications at higher rates resulted in high NO3-N and PO4-P losses in comparison to lower application rates of poultry manure or UAN (168 kg-N/ha). Also, the results of this study showed that higher N application rates from poultry manure did increase corn yields significantly when compared to lower N application rates from either poultry manure or UAN. Plots receiving poultry manure at the same rates as those of UAN (168 kg-N/ha) resulted in significantly higher corn yields. This shows that poultry manure gives a better N fertilizer value for corn yields.