previous

Table of Contents | Landscape Clinic | Environmental Studies

next

Appendix G: Calculation of Amount of Nitrogen Overfertilization

The average concentration of nitrogen as nitrate and ammonium in the groundwater over the month we sampled was 157 mM and 23 mM, respectively. To find how much nitrogen entering the groundwater this translates to we must know the amount of water entering the groundwater (groundwater recharge).

The ET_o for the month of March (the month we sampled in) was 3.46 inches. Multiplying this by the crop coefficient of Kikuyu (0.6) we get the evapotranspiration of the grass during this time. To get the groundwater recharge we used this equation:

Water applied through sprinklers + precipitation - ET of Kikuyu (0.6 ET_o)

During this time, the lawns were being watered 2.9 inches per week (giving 11.6 inches for a month) and precipitation was 1.2 inches. Therefore, the groundwater recharge was:

11.6 inches + 1.2 inches - 2.1 inches = 10.7 inches

This translates to 25,320 liters/1000 ft² of groundwater recharge.

25,320 liters/1000 ft² 180.5 mmol N/liter 10⁶ mol/mmol 14 g N/mol 2.204 10^-3 lb/g = 0.14 lb N/1000 ft² per month into the groundwater.

Assuming that the groundwater nutrient concentrations are this high for all twelve months of the year is most likely flawed logic, as growth rates are different during different times of the year, and the concentration in the second and third months after fertilization is probably somewhat lower than the figure for the first month. In addition, there may be a time when more than three months pass between fertilizations, allowing nutrient concentrations to fall even lower, so an estimate of nine months with concentrations as high as we found them is probably more accurate. If we assume this amount of nutrients is entering the groundwater for 9 months out of the year, we get that nitrogen overfertilization is occurring at the rate of 1.27 lb N/1000 ft² per year. As the grass we sampled may not be the most productive on campus (other areas that are cut shorter have higher growth rates and thus should have higher nitrogen usage), we rounded the figure down to 1 lb N/1000 ft² per year to ensure that all grass on campus would be getting the nitrogen that it needs.

Currently, facilities and maintenance plans to fertilize 4 times per year (with a 16-6-8 N-P-K fertilizer), which would equal a nitrogen fertilization of 2.56 lb N/1000 ft² per year. This means the grounds crew could reduce fertilization to 1.56 lb N/1000 ft² per year, which translates to using 2.4 lb fertilizer/1000 ft² for each of the four applications (rather than 4 lb fertilizer/1000 ft²). With a lawn size of 479,000 ft², the crew could use 4670 lb fertilizer/yr compared to the 7,664 lb/yr it is now using.

previous

Table of Contents | Landscape Clinic | Environmental Studies

next