“Innovation” is the keyword I’ll be focusing on in this column, as the study I’m highlighting from the Southern Nursery Association Conference delves into a topic that may assist a variety of urban farmers, a growing population in this era of hyper-local produce.

“Modifying Green Roof Substrates for Nutrient Retention in Urban Farming Systems” by Ian Howard, Andrew Ristvey and John Lea-Cox, Department of Plant Science and Landscape Architecture, University of Maryland Extension

“Unused roofs represent up to 32% of the area in urban centers.” This is the first sentence of this study from a team at the University of Maryland Extension. The information comes from an older study (conducted in 2011) but it still points out a meaningful underutilization of free space in cities. There have been trends to change that – “green roofs” are becoming more common, with plants growing there serving as stormwater control systems, and more urban customers seeking locally grown produce. Rooftop farms tend to modify green roof substrates with added organic matter, enhancing water and nutrient retention, but these substrates need to be carefully managed because of the possibility of fertilizer and irrigation runoff during a growing season. Urban farmers utilizing vacant lots have similar issues, due to growth taking place in raised beds or in compromised urban soils.

What’s an urban farmer to do? The researchers noted one of the principle elements of the study was reducing the concentration of phosphorus (P) from rooftop leachates, and so they looked at alumina (aluminum oxide). Alumina saturated with P and applied to woody and flowering container plants when they were first potted in commercial settings showed fewer symptoms of P deficiencies later in production than those given just a single dose of Osmocote at planting. Another element was looking at how effective the addition of biochar combined with compost was in reducing ammonium nitrogen leaches. The goal of this study was to help urban farmers protect their nearby bodies of water and formulate better substrate and nutrient management practices.

Phase 1: The first phase of this study was designed to track the amount of leached P and nitrogen (N) from 15 different substrates. Each one had varying proportions of the mineral aggregate base of M2 Green Roof media, SmartLeaf municipal compost, mushroom compost, alumina and biochar, all placed in tubes to a depth of 14.6 cm. Each tube received 2.5 cm of water every 20 minutes to simulate rainfall events, up to 20 cm; they were allowed to drain and the leachate was collected after every “rain.” After the first “rain,” the tubes were fertilized with P and N. This was repeated 24 hours later. The leachates were analyzed, and four were chosen for Phase 2 of the study: #3 (80% M2B, 20% mushroom compost); #5 (75% M2B, 5% alumina, 20% mushroom compost); #9 (70% M2B, 10% biochar, 20% mushroom compost); and #13 (65% M2B, 5% alumina, 10% biochar, 20% mushroom compost). Mushroom compost demonstrated the most nutrient load.

Phase 2: This second phase used 16 30-liter tubs filled with 15 cm of the four different substrates, designed to simulate a green roof installation. This part of the study is still ongoing, as the tubs will be planted with successive vegetable crops over the course of one year – starting with Genovese basil, then Newham lettuce and then Lunchbox Red peppers. The tubs are inside the University of Maryland Greenhouse complex and will be irrigated using the same “rainfall” amounts described above – and their leachates will likewise be tested for N and P concentrations. The plants will be measured for vigor and yield to rate their performance in each substrate.

While we’re waiting on the results from Phase 2, let’s look at the takeaways from Phase 1. “In all substrates containing SmartLeaf or mushroom compost, the total dissolved-P leachate content … was significantly lower from substrates containing alumina than from substrates without alumina,” the study reads. Those substrates also retained more P after the fertilizer was applied and eight 2.5 cm “rainfalls” occurred. The amount of dissolve-P in the SmartLeaf/mushroom compost mixes was not significantly affected by any proportion of biochar.

We’ll have to follow this study to see what happens with the plants in the greenhouse during this year. The good news is the preliminary findings from Phase 1 show some promise for urban growers.