How much nitrogen leaches from containers?

Many years of nitrogen fertilizer application has led to nitrate-contaminated groundwater in large portions of the Central Valley of California. In 2014, this contamination led the Central Valley Regional Water Quality Control Board to require plant producers to submit Nitrogen Management Plans to their water quality coalitions. In 2019, the original Nitrogen Management Plan was replaced by the newer Irrigation and Nitrogen Management Plan (INMP) that also included irrigation management information. The INMP is a balance sheet that reports nitrogen inputs from fertilizer, irrigation water, and container media and output from the harvested product. Total nitrogen output is subtracted from total nitrogen input to estimate total potentially-leachable nitrogen. The idea behind potentially-leachable nitrogen is that any applied nitrogen not removed in the harvested product has the possibility to leach into groundwater. Estimating harvested nitrogen is straightforward for a crop like almonds because 136 pounds of nitrogen is removed for every one ton of almonds harvested. As nursery growers will recognize, estimating harvested nitrogen is not as straightforward for nursery crops due to the complex production system and variety of plant taxa and sizes grown at a single location. Possible fates of applied nitrogen to nursery crops include plant uptake, leachate from the container media, remaining in the container media, or possibly emitted as nitrogen gas from denitrification. We initiated an experiment to determine the fate of fertilizer nitrogen and answer the real question the Central Valley Regional Water Quality Control Board was asking, “How much nitrogen leaches from container-grown plant nurseries?”

We collaborated with a nursery in the Central Valley of California to document nitrogen input and output during production of Lagerstroemia indica “Whitt II” plants grown in a Douglas fir bark media incorporated with Osmocote Plus (15-9-12) and Apex polymer-coated sulfur-coated urea (9-2-0). Plants were transplanted from a #1 container into a #3 container in the beginning of May. On the third day after planting, the growing media was topdressed with 20-9-9 fertilizer. We measured all nitrogen inputs including: well-water applied as irrigation, total nitrogen in the growing media, and topdress fertilizer. Nitrogen outputs included: shoot uptake, remaining in growing media at harvest time, nitrogen gas emitted, and soluble nitrogen in leachate/runoff. To capture runoff nitrogen, we lined half of the growing beds in the test area with polyethylene sheeting sandwiched between sediment fabric before covering all the beds with gravel. The total nitrogen that infiltrated into the growing bed soil was the difference in the nitrogen in the runoff from the lined and unlined growing beds. After approximately three months, we harvested the plants when the grower was ready to ship them for retail sale. We cut the shoots off of the harvested plants at the crown and measured the total nitrogen in the shoots and growing media separately.

We determined that 61% of the applied nitrogen was in the plant or media when the plants were ready for shipping. Five percent of applied nitrogen was taken up by the plant shoots and 56% remained in the media as controlled-release fertilizer or as organic nitrogen in plant roots or immobilized by microbes. Maintaining a fertilizer nitrogen reserve in the growing media ensures that the plants will remain healthy and attractive while waiting purchase by home gardeners or landscapers. Six percent of the applied nitrogen was in the growing bed runoff water, predominantly as nitrate. Irrigation runoff water capturing and recycling is common in California nursery production and the nitrogen in runoff water could reduce future fertilizer application costs. In agreement with other nitrogen balance research from container-plant production systems, 28% of applied nitrogen was lost as gaseous nitrogen emissions from denitrification.

The question that the INMP calculations were supposed to answer is how much nitrogen is leaching into the soil and potentially contaminating groundwater? If harvested nitrogen from a Lagerstroemia indica production system was documented in the INMP by a grower, then 61% of applied nitrogen would be used as output in the calculations. This results in 39% of applied nitrogen deemed potentially-leachable by the INMP worksheet, when our research results recorded that a mere 3% was potentially-leachable. These results indicate that the INMP worksheets and calculations overestimated the amount of potentially-leachable nitrogen from container-plant production.

The results of this research may not apply to every container-plant production system at every nursery in every geographic region. Even though this research was performed on a plant that is an average nitrogen user, it is for a single woody-ornamental taxon and may not apply to the variety of herbaceous perennial or slower-growing woody ornamental taxa. As growers recognize, fertilization practices are not standardized in the industry and applying fertigation in overhead irrigation may result in much greater potentially-leachable nitrogen. Furthermore, this research was performed at a nursery on a clay loam soil with a very low infiltration rate and the amount of potentially-leachable nitrogen could be much higher in areas with coarser soil textures. Three percent of applied nitrogen is equal to 20 lbs. per acre and coupled with the small total area of nursery production compared to other crops in the Central Valley, it is unlikely that nursery production is a significant contributor to nitrate contamination of groundwater in this area. However, areas with a high density of nursery producers could result in localized nitrate contamination of groundwater.

These results are becoming more significant because the San Diego and Central Coast Water Quality Control Boards are considering nitrogen management reporting plans and these areas have concentrations of large numbers of nursery growers. Nurseries would have to conduct studies like our research to develop nitrogen input and output values to accurately fill out the INMP worksheet. Due to the large variety of plant taxa and sizes grown, a significant and possibly debilitating cost could be incurred. Therefore, instead of requiring Irrigation and Nitrogen Management Plans for nurseries, California’s Regional Water Quality Control Boards should facilitate implementation of irrigation and nitrogen best management practices at nurseries to reduce potentially-leachable nitrogen. Numerous best management practice guides exist and consultation with University of California Cooperative Extension Advisors could further facilitate implementation.

For more information refer to: Pitton, B.J.L., Oki, L.R., Sisneroz, J., and Evans, R.Y. (2022). A nursery system nitrogen balance for production of a containerized woody ornamental plant. Scientia Horticulturae 291, 110569.