The risk of nitrogen oversupply in urban gardening
Problem
Urban agriculture is a spreading phenomenon. Different to professional small scale farms located at the urban fringe, urban agriculture and gardening is mainly practiced on allotments, house gardens and balconies and, more recently, by an increasing number of community gardeners and start-up entrepreneurs. On comparably small inner-city areas it makes use of unused spaces, combines multiple objectives in new ways, and develops new concepts and techniques. Part of this urban agriculture is the zero acreage cultivation, taking place on sealed surfaces in raised seed beds, containers or bags that allow establishing moveable gardens on sites made available for temporary use only. Practitioners often do not have a professional education in agriculture. They are usually non-profit oriented, and their produce is distributed along short supply chains.
Though, in particular in community gardens, organic farming practices are broadly applied, we hypothesize a lack of knowledge on sustainable nutrient supply. The benefits of informal learning by doing and knowledge sharing through new media contrast the risk for spreading false or insufficiently specified management recommendations. Most notably production in raised beds, containers or bags uses commercial growing substrates and or compost instead of fertile mineral soil as growing substrate, because the latter is limited in accessibility or difficult to organize. Also gardeners add organic fertilizer rather randomly. The probability of oversupply with nitrogen (Nt) is accordingly given. Theoretical calculation of the Nt-supply resulting from urban gardening media recommendations suggests indeed a more than twofold Nt-supply with nitrogen compared to professional conventional farming. We were interested in the question whether this might lead to a risk of excessive nitrogen mineralisation and unintended accumulation of mobile fractions (ammonia NH4+ and nitrate NO3-), that might result in unsustainable losses through leaching of nitrate or excessive accumulation in vegetables.
Pot trial
We carried out a pot trial in bags as used for urban gardening, organized as a randomized block trail with 10 replications. Growing bags were filled with 20 liters of substrate mix. Three levels of N-supply were set up: M1 (60 g Nt/bag; compost : commercial organic growing substrate = 2:1 Vol%; + 20g organic fertilizer pellets); M2 (45 g Nt/bag, compost : commercial organic growing substrate = 1:1 Vol%); M3 (30 g Nt/bag; compost : commercial organic growing substrate = 1:3 Vol%). Over a period of 10 weeks (07.08.2014 – 23.10.2014) soil was uniformly kept moistly. From each bag soil samples were taken from two layers and analysed on Nt, NH4+-N, NO3—N, Corg, K).
Results
Results show N-losses of 10-15% after 10 weeks. Nitrate concentrations in the topsoil had decreased markedly and were significantly below those of the lower soil layer, what indicates nitrate leaching, which however in case of water and nutrient uptake by plants would not have been occurred. In comparison of the mineralisation between the three levels, we found that especially nitrate levels in the lower soil layer of M1 were markedly higher than in M2 and M3. In total, average equivalent values of 154 kg Nmin-N/ha were found in M3, 186 Nmin-N/ha in M2, and 382 Nmin-N/ha in M3. In comparison: conventional recommendations for fertilization suggest a supply of rd. 170 kg Nmin/ha for potatoes.
Contribution to shortening food supply chains
Though these results are very preliminary, they indicate that more caution is advised regarding nutrient supply in urban agriculture practice. As observed for nitrogen dynamics, already the processes related to the use of substrates and organic fertilizers are hardly to predict or to control, the more thoughtful input of compost and fertilizers should be handled.
Environmental sustainability forms an important cornerstone for short food supply chains. The need for efficient use of nutrients and minimization of losses into water or air derives from the objective of closing cycles. Production of healthy food requires avoiding excessive accumulation of undesirable or even harmful substance like heavy metal or nitrate in the products. Better targeted information and more analysis is crucial on these issues.