It is relatively easy to set the initial target pH of a root substrate by matching lime type and rate with the acidity of the substrate components. The challenge lies in maintaining this target pH throughout crop production. Factors that impact pH over time include irrigation water alkalinity (Bailey, 1996); residual content and properties of liming materials (Huang et al., 2010; Rippy et al., 2007, 2016); acidification due to nitrification (Marschner, 1995); plant and microbe respiratory acidification (Marschner, 1995); acidic, neutral, or alkaline biotic effect of nutrient uptake (Pertusatti and Prado, 2007), which varies among plant species (Fisher et al., 2014a; Johnson et al., 2013); and the abiotic effect of fertilizer (Hignett, 1985).
There is interplay between fertilizer type and some of these pH controlling factors. Most fertilizer solutions have a low pH, thus they are abiotically (chemically) acidic, even when they are biotically (physiologically) neutral or basic. When fertilizers supply ammonium, rhizosphere biotic acidification can occur during microbial nitrification of ammonium to nitrate, where two protons are generated for each ammonium ion oxidized. Plant and microbe uptake of ions supplied by fertilizers have yet another biotic effect on substrate pH. During uptake of cationic nutrients, protons are released to the rhizosphere in exchange for uptake of positive cation charges (Havlin et al., 2014; Kafkafi, 2008; Marschner, 1995; Nelson, 2011; Zhu et al., 2009). Alkalinization occurs when microbes or plants take up protons along with anionic nutrients or release OH− or HCO3− to the rhizosphere in exchange for anionic nutrients (Pertusatti and Prado, 2007).
Plant species also interact with some of the factors controlling substrate pH, namely respiration and proportion of cationic to anionic nutrient ions taken up. Release of CO2 by roots and rhizosphere microorganisms during respiration has an acidifying effect on the rhizosphere through the generation of carbonic acid (Marschner, 1995). Root respiration differs among plant species and with growth conditions (Taiz and Zeiger, 2010) and thus the potential for acidification of the substrate pH from the release of CO2 also varies across plant species. Plant species also differ in the proportions of ions extracted from the soil solution. Since nitrogen (N) is the only nutrient that is plant available in both anion (nitrate) and cation (ammonium) forms and more N ions are typically taken up than other types combined (Taylor et al., 2010), the form of N taken up by plants has the largest effect on substrate pH. Although the form of N taken up by plants is influenced to a degree by availability, plant species do vary in their affinity for ammoniacal vs. nitrate forms of N (von Wirén et al., 1997). Plants adapted to acid soils generally favor ammonium uptake, whereas those found in calcareous soils favor nitrate uptake (Marschner, 1995). As an example, ammonium uptake often predominates in blueberries (Hanson, 2006). A large differential effect of species on substrate pH during germination and early seedling growth was reported by Huang et al. (2001). Johnson et al. (2013) found a strong species effect on substrate pH when growing three bedding plant species for 4 weeks.
A PABR is included on the labels or technical sheets of greenhouse fertilizers. Pierre (1933) established the early procedures for this rating, which were later refined by the AOAC (1970, 1999) and described by Johnson et al. (2010, 2013). The PABR encompasses both biotic and abiotic impacts of fertilizer on substrate pH. Although this rating system does not allow for effects of plant species, stage of maturity, or fertilizer concentration on substrate pH, it is universally used today. In many situations, it adequately forecasts pH shifts. But there are other situations where it fails. The aberrant pH shifts are usually more acidic than predicted by PABR, suggesting involvement of the abiotic fertilizer effect. In this study, it was hypothesized that the unpredicted acidification is due to application of fertilizer in excess of that used by the plant and microbes. Production scenarios leading to excess fertilizer accumulation in the substrate can include the following: 1) quantity of fertilizer applied is higher than that recommended for the crop; 2) a single fertilizer program applied to multiple species that is designed to meet requirements of the faster growing species will result in excess application to the slower species; and 3) failure to reduce fertilizer application later in crop production when a plant’s specific rate of growth and nutrient demand typically declines.
To test our hypothesis we 1) measured the differential effects of 13 plant species on substrate pH and 2) assessed the interactive effect of fertilizer concentration and PABR on substrate pH during plant growth.
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