Extensive Green Roof Pepper Production: Fertilizers and Nutrient Management

Extensive green roof substrates are typically shallow, low in organic matter, and designed for rapid drainage. Architectural load limitations preclude the use of heavy mulches and high organic matter inputs, leading to the adaptation of concentrated, fast-release synthetic fertilizers. Fertilizers markedly influence food crop revenue generation on green roofs, although costs differ widely among products. The lack of information regarding the relative yields and costs associated with different fertilizer options and comparisons of inorganic and organic products are addressed in this work. Organic and conventional synthetic fertilizer treatments [liquid organic fertilizer (8N–0P–0K) (low fertility), bonemeal (6N–9P–0K, bloodmeal 12N–0P–0K, combined bonemeal/bloodmeal, controlled-release synthetic fertilizer 14N–14P–14K, and granular 12N–12P–12K + Ca(NO 3 ) 2 15.5N–0P–0K (fast release)] at manufacturer recommended rates for general use were compared to assess their impacts on fruit yield, relative leaf chlorophyll content, and plant vigor ratings for ‘Sweet Hungarian’ pepper ( Capsicum annuum L.) over three growing seasons (2013, 2014, and 2015) on an extensive green roof with an expanded clay substrate. Fast-release fertilizer provided the highest yield per N cost and controlled-release fertilizer had the highest yield, but at considerably higher cost compared with that of all other treatments. Bloodmeal fruit weights were highest among organic treatments and similar to the fast-release treatment. These results indicate that the most cost-effective fertilizer based on the fruit weight yields obtained was the fast-release, although several organic options provided similar yields at a slightly higher cost. Because overall fruit weights were lower with organic fertilizer than with conventional fertilizer products, organic fertilizer rate recommendations must address conditions specific to extensive green roofs. Leaf chlorophyll was highly correlated with fruit yield differences among treatments, but not with overall yield differences between years. The results also indicated that plant vigor ratings better explained variations among pepper fruit number and weight compared with leaf chlorophyll. Fertilizer also influenced resulting green roof fertility properties over the study period, with calcium, magnesium, and K decreasing from initial 2013 levels, consistent with plant uptake needs. Additionally, factors that influence nutrient availability (e.g., cation exchange capacity and pH) were influenced by fertilizer applications and also may be partially impacted by expanded clay reuse, suggesting that green roof substrates change in response to weathering and production systems, and may be a consideration when developing fertilizer management systems. These results can help assess yield and cost tradeoffs among organic and synthetic fertilizer options for vegetable crops grown in extensive green roof environments and provide a better understanding of fertility management options and tradeoffs for expanded clay green roof substrates.