Limiting Fertilizer in Landscape & Garden Design
In the Western U.S., nitrogen deposition (pollution) is the result of emissions from transportation, agricultural production, and industrial activities like electricity production (California State). Dry deposition is of greater magnitude than wet deposition due to the arid summer climate (Bytnerowicz and Fenn). However, fog deposition can also be significant along the Coast during summer months (Bytnerowicz and Fenn). Nitrogen pollution comes in various forms, including: nitric acid vapor, particulate nitrate, ammonium, nitrogen dioxide, nitric oxide, peroxyacetyl nitrate, and ammonia (Bytnerowicz and Fenn).
Nitrogen (N) deposition (resulting in high soil nitrogen levels) has been shown to have detrimental effects on ecosystem health and diversity by giving an advantage to exotics that better utilize N as a nutrient (Galloway and Schlesinger). In addition, N deposition accumulates in surface and ground waters where, ultimately, it leads to excessive algae growth and oxygen depletion (hypoxia) in recipient waters (Galloway and Schlesinger). In terrestrial ecosystems, excessive nitrogen can wreck havoc on soil chemistry, diminishing other nutrients such as calcium, magnesium, and potassium, leading to a decline in overall soil fertility (“EarthTrends”). The agricultural and landscape practice of applying fertilizers accounts for the largest amount of human-generated nitrogen in the environment, some 86% (“EarthTrends”). This practice has increased exponentially since the 1950s to the point that half of all fertilizer ever produced have been applied since 1984 (“EarthTrends”). Unfortunately, only about half of the fertilizer applied ever makes it into plant biomass, the rest either evaporates or is washed into watercourses (“EarthTrends”).
In light of the above overview of how most landscapes are awash in N, and in light of studies showing how increased levels of atmospheric CO2 enhance plant nitrogen uptake (Hu), general applications of fertilizer in designed landscapes should be avoided. This cautious approach is further needed because in most California soils phosphorus and potassium are naturally present in adequate amounts (“California Gardens”), reducing the need for supplements of these nutrients. Therefore an assessment of soil nutrients should be made for landscape sites to determine what, if any, additional nutrients are needed.
Generally N pollution is a problem in California, with the southern part of the State having the highest levels of deposition. Therefore, our landscape specifications limit fertilization to specific circumstances where soil test have indicated nutrient deficiencies. In place of specifying general fertilizing schedules, we specify various beneficial microbes and establishing a sustainable nutrient cycle with plant litter.
California State 2003. Pier Program Project Summary: Assessing Nitrogen Deposition Models and Habitat Impacts in California. California Energy Commission.
Bytnerowicz, A. and Fenn, M. E. 1996. Nitrogen deposition in California forest: a review. Environmental Pollution, Vol. 92(2). 127-146.
Galloway, J. N. and Schlesinger, W. H. 2006. Human Alteration of the Nitrogen Cycle: Implications for Plant Growth, Food Supply, Climate, Water Quality, and Human Health. Environmental Science Seminar Series. American Meteorological Society, Washington, DC.
“EarthTrends.” The Environmental Information Portal. 1998-99. Nutrient Overload: Unbalancing The Global Nitrogen Cycle. World Resources Institute.
Hu, Shuijin 2001. Nitrogen limitation of microbial decomposition in a grassland under elevated CO2. Nature, January/409. 188-191.
“California Gardens” 2005. Organic Material in the Garden. California Gardens.com.