Ever increasing carbon dioxide in urban environments requires a proportional increase in leafage, as leaves are the primary and only organ of carbon capture and removal. Leafy plantations in expensive urban plots would remain limited unless their horticulture is highly profitable. Replicating advanced nations’ urban hydroponic success in Lahore, Pakistan a city of more than 11 Million people; would require local adjustments, as unlike most of those nations, Lahore has a warmer climate and records high temperatures even in the cooler months of January, February and March. This paper reports hydroponic trials that were conducted between January and March 2020 under the protection of a polyhouse. Hydroponic recipes that work in cooler climates or temperature-controlled glasshouses of more developed nations would not work in basic polyhouses constructed in Lahore’s warmer climate. The authors conducted three trials with different nutrient concentrations in a purpose built hydroponic polyhouse to uncover the right mix for Lahore’s climate. Local adaptation is essential to first build profitable urban hydroponics and then achieve the environmental goal of carbon capture through leaves grown in those hydroponic facilities.
Atmospheric carbon dioxide is captured and converted into sugars by leaves, and fast-growing leafy green vegetables such as household lettuce can make useful contributions to our struggle against global warming. Urban centers such as Lahore are hotbeds of C02 generation due to industrial activity, motorized traffic and other combustion-related activities. The government of Pakistan plans to capture and remove the carbon dioxide produced in Lahore and other urban centers by planting billions of trees under the ‘Ten Billion Tree Tsunami Programme’. This is a commendable environmental initiative, as billions of trees would have many trillions of leaves, with each leaf removing a fraction of carbon dioxide from our environment. However, there are three concerns with this programme, all of which can partially be addressed by hydroponic urban leafy green cultivation. First, this is a distant solution to an immediate problem, as trillions of leaves would achieve mass carbon sequestration once the newly planted trees have attained maturity and some trees can take decades to mature. Second, the trees planted in remote scrub or mangrove forests are quite far from the CO2-generating city of Lahore. Third, a very small fraction of the Billion planned trees can be planted inside Lahore, as existing landowners would be reluctant in a plantation drive that does not promise immediate economic returns. These three concerns encourage us to look for additional ways of achieving quick and profitable foliage cover within or near Lahore.
Although leafy greens such as Lactuca sativa are miniscule compared to Himalayan cedars or Sindhi mangroves planned under the Ten Billion Tree Tsunami Programme, they partially address the three concerns related to this programme. First, Lactuca sativa is a very fast-growing leafy green that starts capturing carbon produced in Lahore today. Second, hydroponic greenhouses or polyhouses can be set up within Lahore, even on unutilized rooftops, enabling sequestration of carbon right next to where it is being generated. Third, it can be taken up by existing landowners, as it has the ability to quickly generate a high return on low investment. If Lactuca sativa’s contribution to carbon sequestration is to be appreciated and utilized, then the right composition of macro- and micronutrients that can deliver the ideal growth of its leaf should be explored.
Concentrated nutrient solutions with high proportions of nitrogen, phosphorus, potassium and micronutrients can lead to fertilizer induced leaf burn due to high evapotranspiration. A diluted solution on the other hand can limit the growth of leaves by limiting the uptake of essential nutrients. A healthy leaf can sequester maximum carbon dioxide, but to let that leaf develop, an optimum nutrient mix should be provided to roots, which is neither diluted nor concentrated. To determine the right nutrient concentration for ideal leaf development, the authors conducted trials of the same Lactuca sativa plants in three different hydroponic troughs. One of these troughs had nutrient concentrations close to those prescribed by Brechner & Both (2013) (Parks & Murray, 2011) (Huett, 1994) (Paulus, et al., 2008) [2–5]. The nutrient concentration in the second trough was adjusted to 0.5 times (half) that of the first trough to make a diluted solution. At the third tough, the concentration was 1.5 times that of the first trough to make a concentrated solution. First trough’s nutrient formulation prescribed by multiple scientists would have shown the best results in laboratory settings, but in Lahore’s practical polyhouse setting, the author observed the best leaf growth in diluted solution.
Multiple nutrient formulations were studied from extension reports and journal articles published around the world. Although hydroponics is argued by some as an ancient technique first used in hanging gardens of Babylon, the first scientific discussion of nutrient formulation in hydroponics can be traced back to 1938 when a paper titled ‘The water-culture method for growing plants without soil’ was published by Hoagland & Arnon in 1938) . Both (Jones, 2005)  and (Sonneveld & Voogt, 2009) , in their respective books on hydroponics and plant nutrition credit (Hoagland & Arnon, 1938)  with the first formulation of nutrient solution. They argue that formulations developed in subsequent years were adaptations of this pioneering work. Authors working on different crops developed unique crop-specific formulations. (Parks & Murray, 2011)  recommended NPK concentrations of 116-22-201 Parts per million (PPM) for East Asian leafy vegetables such as Bok Choy. Roughly similar NPK concentrations of 125-31-215 PPM were reported for Lettuce by Brechner & Both (2013) . The original Hoagland solution had NPK concentrations of 210-31-235 PPM . This comparison informs us that later scientists working on leafy green nutrient concentrations drove the nitrogen concentration down quite significantly. Trials conducted for this research paper showed that nitrogen and other concentrations can be reduced even further in the case of polyhouse hydroponic farming in Lahore.
A comparatively low nitrogen concentration was also kept in hydroponic lettuce trials of Singh et al, 2019 . The NPK ratio of (5) N, (4.8 or 5.2) P, and (21.6) K was used by them  to grow different cultivars of lettuce in channels using nutrient film technology. They verified the utility of this relatively low nitrogen formulation by comparing the nutrition in their sample lettuce leaves with prior work on lettuce leaf nutrient ranges by Hartz et al. (2007) . They also proved the efficiency of their nutrient formulation; by looking for nutrients in leaf samples using TruSpec Elemental analyzer, by using Chlorophyll meters (SPAD 502), and by studying the dry weight of trial plants.
Nutrient formulation or calculation in the 21st century is automated. A software application developed by Daniel Fernandez titled ‘HydroBuddy’ helps researchers and hydroponic farmers calculate nutrient concentrations tailored to their needs. This software utilizes the Lettuce nutrient formulation developed by Cornell University’s College of Agriculture and Life Sciences and published by (Mattson & Peters, 2014)  and (Brechner & Both, 2013) . Nutrient formulations for Lactuca sativa can also be found in the works of (Peckenpaugh, 2004)  and (Resh, 2013) , which this software utilizes along with other formulations. The author of this research paper utilized this software to calculate an average concentration from this software. This average concentration set as the benchmark for later concentration alteration was based on estimates published by most of the abovementioned scholars.