Empirical determination of hydroponic solution buffer capacity

Abstract

Hydroponics is soilless crop growing method which uses nutrient solution instead of mineral soil and its popularity has grown over the years due to effective water, nutrients usage for leafy greens cultivation [1]. Nutrient solution (Hoagland’s solution) contains all essential micro and macro elements in their respective concentrations [2]. Hoagland’s solution used in hydroponics has limited but not quantified buffering capacity [3]. For pH mitigation nutrient solution must contain reasonable amount of protolytic base and acid pair [4]. There are 2 major salts in typical Hoagland’s solution which can act as buffering agents in hydroponic solution: NH4NO3 and KH2P04. Unfortunately, ammonia useful buffering range is above nutrient solution pH interval used in hydroponics, therefore, NH4+/NH3 buffer will not mitigate pH changes [5]. Mostly nutrient solution buffer capacity comes from second step of phosphate dissociation reaction (1) 𝐻2𝑃𝑂4−(𝑎𝑞)⇌𝐻𝑃𝑂42−+𝐻+(𝑎𝑞) 𝑝𝐾𝑎26.86−7.2 (1) Maximum buffer capacity is reached then desired pH value is equal to buffering agent dissociation constant and buffer effective range is pKa ± 1[4]. Usually, optimal pH level for hydroponic growth ranges from 5.5 to 6.5 [6] therefore, due to unequal absorption of ions from the nutrient solution during growth phases pH fluctuations accrues [7]. Ion absorption from the nutrient solution changes charge balance between the root inner membrane and the nutrient solution. As cation is absorbed from the nutrient solution H+ ions are released through the root cells to compensate charge difference; opposite reaction occurs then anion is absorbed – OH- ion are released for charge compensation [6]. Fluctuations outside optimal pH interval decreases efficiency of mineral nutrition, restricting plant performance, also irreversible changes may occur regarding plant physiology when rhizosphere is in contact with nonoptimal pH levels [8]. Plants grown in nonoptimal pH interval have lower yield: basil (Ocimum basilicum) and kale (Brassica olearacea), cultivated in hydroponic solution with slightly acidic pH had higher chlorophyll content and yields compared with leafy greens grown in slight alkaline pH [9]. Root induced changes in the rhizosphere may also affect plant mineral nutrition in various ways [10]. Ion unavailability due to salt precipitation occurs when pH levels is not in optimal range. For instance, at alkali pH iron, calcium, magnesium forms precipitate and becomes unavailable [11]. Hence, hydroponics growth potential can be maximized if hydrogen ion concentration is maintained at optimal interval but practically pH management should not require excessive technological solution and ideally done passively. Our objective was to empirically determinate nutrient solution buffer capacity using acid/ base titration and according to calculated buffering capacity suggest passive pH management strategy for hydroponic growth