Controlling pH in soilless substrate is critical to managing nutrient availability in container production (Peterson, 1981). This can be a challenge considering that several factors interact and affect pH over time, including substrate components, limestone type and rate, applied nutrients and concentration, irrigation water alkalinity, and plant species (Argo and Biernbaum, 1996, 1997; Johnson et al., 2013). Floriculture species also differ in susceptibility to developing iron or manganese toxicity or deficiency symptoms if substrate-pH drifts too low or high during production (Argo and Fisher, 2002). A key grower decision is the selection of a water-soluble fertilizer formulation and concentration that stabilizes pH over time.
Fertilizer effects on substrate-pH are dominated by nitrogen form and concentration applied (Argo and Biernbaum, 1996; Barnes et al., 2014; Haynes, 1990; Huang et al., 2001; Johnson et al., 2013; Marschner, 2012). Fertilization with ammonium nitrogen (NH4+-N) produces an acidic reaction that decreases pH as a result of H+ efflux from roots during uptake and from nitrification. Nitrification can occur rapidly in container substrate above pH 5.5 and depends on factors that affect microbial activity such as pH, temperature, oxygen, moisture, crop duration, substrate components, nitrogen form, and concentration (Argo and Biernbaum, 1997; Lang and Elliot, 1991). Fertilization with nitrate nitrogen (NO3−-N) usually produces a basic reaction that increases pH, resulting from the efflux of hydroxyl (OH−) or bicarbonate (HCO3−) ions from roots (Haynes, 1990; Marschner, 2012). Ammonium typically has a greater impact on substrate-pH compared with NO3−-N when both forms are applied because NH4+-N uptake is energetically favored over NO3−-N uptake (Engels and Marschner, 1995; von Wiren et al., 2001). The effect of urea nitrogen (urea-N) on substrate-pH varies depending on the state of hydrolysis, nitrification, and the subsequent uptake of NH4+-N vs. NO3−-N (Verburg et al., 2003).
Floriculture species differ in their effect on substrate-pH, even when supplied with the same water-soluble fertilizer (Huang et al., 2001; Johnson et al., 2013). Huang et al. (2001) showed that seedlings of pansy (Viola ×wittrockiana Gams.), petunia (Petunia ×hybrid Vilm.-Andr.), and vinca (Catharanthus roseus G. Don.) increased the pH whereas celosia (Celosia cristata L.) and zinnia (Zinnia elegans Jacq.) decreased the pH in peat: perlite substrate. In both substrate (Johnson et al., 2013) and hydroponic nutrient solution (Dickson et al., 2016), geranium (Pelargonium ×hortorum Bailey L.H.) was acidic and decreased the pH compared with petunia which was basic and increased the pH, whereas impatiens (Impatiens wallerana Hook. F.) had intermediate effects to geranium and petunia.
A major process by which plants affect root zone pH is through imbalanced uptake of cation and anion nutrients (Haynes, 1990; Lea-Cox et al., 1996; Marschner, 2012; Rengel, 2003). Roots maintain charge balance either by equal uptake of cations and anions or by the efflux of ions equal to the net charge taken up by roots. Net cation uptake is balanced by efflux of H+ ions whereas net anion uptake is balanced by efflux of OH− or HCO3− from roots (Kirkby and Knight, 1977; Lea-Cox et al., 1996; Marschner, 2012).
Some agronomic crop species that are labeled “iron-efficient” acidify the root zone as a strategy to improve iron solubility and uptake when grown in calcareous soil (Marschner, 2012). Geranium is a floriculture species that is referred to as “iron-efficient” because of high susceptibility to iron or manganese toxicity at low pH and also has the tendency to decrease substrate-pH over time (Argo and Fisher, 2002; Johnson et al., 2013). Other floriculture species susceptible to iron or manganese toxicity may also decrease pH, possibly as a means to increase iron uptake.
Johnson et al. (2013) modeled the interaction between three floriculture species (geranium, impatiens, and petunia) fertilized with 18 water-soluble fertilizers differing in applied nitrogen forms (NH4+-N, NO3−-N, and urea-N) and concentration. The acidity or basicity of a fertilizer could be manipulated by adjusting nitrogen forms and concentrations to balance species root zone effects and stabilize pH. For example, the model predicted that geranium, impatiens, and petunia would require 0%, 10%, and 31% of total N as NH4+-N, respectively, with the remainder of N as NO3−-N. This ratio would be expected to maintain a stable pH over time when these species were grown with zero alkalinity irrigation water and without residual lime in the substrate.
Commercial bedding plant operations typically grow a wide range of plant species, often in the same greenhouse, that may differ in effects on substrate-pH. Species diversity is therefore an important consideration in pH management. However, it is also not feasible to provide separate fertilizer regimes to hundreds of cultivars grown at a single location, and grouping of plants is necessary. The objective was to quantify the effects of plant species on substrate-pH for floriculture species fertilized with different NH4:NO3 nitrogen ratios. Floriculture species common in container production were selected, where species also differed in reported susceptibility to iron or manganese deficiency or toxicity and optimum pH (Argo and Fisher, 2002; Whipker et al., 2003).
The objective of this study was to quantify effects of floriculture plant species on substrate-pH. In a factorial experiment conducted in a controlled environment growth chamber, 15 floriculture species were grown in peat:perlite substrate and were irrigated with modified 0.5× Hoagland’s nutrient solutions with NH4+:NO3− nitrogen ratios of 0:100, 20:80, and 40:60. After 33 d, substrate-pH was measured and meq of acid or base produced per L of substrate was estimated using an acid–base substrate titration (Johnson et al., 2010). Nitrogen is taken up in different forms that vary in their charge (including NH4+ and NO3−), and without isotope labeling (which was not used in this study), total N level in tissue cannot distinguish between the original fertilizer nutrient form. Dry tissue was harvested from plants irrigated with the 0:100 solution, and tissue nutrient data were used to estimate net uptake of anions minus cations, and cation or anion uptake ratio, assuming that all N was taken up in the anionic NO3− form. Species and solution NH4+:NO3− ratio were evaluated for effects on substrate-pH and meq of acid or base produced per L of substrate. Species were separated into clusters that corresponded to species overall acidic, intermediate, and basic effects on substrate-pH, and linear regression was used to predict species-specific NH4+:NO3− ratios expected to result in a stable pH. We hypothesized that species fertilized with 100% NO3−-N that had greater net anion minus cation uptake would result in greater basicity than species that favored cation uptake. We also hypothesized that floriculture species that are reportedly susceptible to iron or manganese toxicity at low pH may tend to decrease substrate-pH, possibly acidifying the root zone as an iron-efficiency mechanism (Marschner, 2012).
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