Fertilizer type and concentration, plant species, water alkalinity, substrate components, and lime amendment dynamically affect substrate-pH during plant production. Fertilizer effect on substrate-pH occurs primarily through plant and microbial uptake of nutrients and the subsequent release of ions and/or root exudates along with other microbially induced processes such as nitrification (Marschner, 1995). Using nitrogen as an example, uptake of NO3-N can result in the release of OH– and HCO3– or coabsorption of a cation, whereas H+ ions are released with uptake of NH4-N. The microbial conversion of NH4-N to NO3-N (nitrification) also produces H+.
Pierre's method [“PM,” Pierre (1933)] is the standard used in the U.S. fertilizer industry to quantify the potential of a fertilizer to change soil acidity or alkalinity. The reported PM value for an acidic fertilizer, in units of calcium carbonate equivalents (CCE) of acidity per unit weight of fertilizer, refers to the CCE required to neutralize the acidity resulting from application of the fertilizer (Pierre, 1933). For basic reaction fertilizers, the CCE of basicity represents the basic residue in CCE left in the soil after application of the fertilizer.
Pierre's original methodology was modified and adopted by the Association of Official Agricultural Chemists (AOAC) and is recognized as the official method to estimate the equivalent acidity/basicity of complete fertilizers (Tisdale and Nelson, 1966). The final official procedure was published by AOAC in 1970 with a minor revision in 1999 (AOAC, 1970, 1999). In brief, the fertilizer mixture is ground and a 0.25- to 1.0-g dried sample is mixed with a sodium carbonate–sucrose solution and ashed in a furnace (at 575 to 600 °C) to remove carbon and nitrogen compounds. After cooling, HCl is added for lime digestion (AOAC, 1970; Hignett, 1985). Once digested, the sample is titrated to pH 4.3 using 0.5 N NaOH and compared with a similar titration of a blank sample of sodium carbonate–sucrose solution only (AOAC, 1970). The titrated difference between the blank sample and the fertilizer sample is the acidity or basicity of the fertilizer other than that contributed by nitrogen (Hignett, 1985). Table 1 shows the equivalent acidity or basicity factors for individual salt elements (Hignett, 1985), in which a positive value indicates the fertilizer sample is basic (non-acid-forming) and a negative value indicates the sample is acidic (Horat, 1939). For elements other than nitrogen or phosphorus, therefore, the acidity or basicity of a fertilizer salt is a direct measurement of its inorganic acid/base reaction in this titration and is independent of the soil or plant system.
Equivalent acidity (negative) or basicity (positive) values in units of kg of CaCO3 per kilogram of element using Pierre's method (Hignett, 1985).
Factors for nitrogen and phosphorus were based on empirical field studies by Pierre in the 1920s and 1930s that measured soil pH changes after fertilizer application, primarily using Cecil clay loam with an initial pH of 6 (Hignett, 1985). Pierre assumed that: 1) the acidifying effect is caused by all the sulfur and chlorine, one-third of the phosphorus, and one-half of the nitrogen contained in the fertilizer; 2) calcium, magnesium, sodium, and potassium are base-forming elements; and 3) ammonium nitrogen is completely nitrified; nitrate nitrogen combined with bases such as sodium or calcium will have a net basic effect (Pierre, 1933; Tisdale et al., 1999; Tisdale and Nelson, 1966).
These assumptions merit critical evaluation when applied to use of water-soluble fertilizers in soilless container crop production. Pierre's method assumes nitrogen in fertilizers is provided as nitrate or is converted to nitrate through nitrification (Pierre, 1933). Rapid nitrification does indeed occur in container substrate above pH 5.5, which is the typical growing range for most container crops (Argo and Biernbaum, 1997; Lang and Elliott, 1991). However, PM does not take into consideration acidification resulting from plant NH4+ uptake or acidity from nitrification, and therefore it underestimates the lime-equivalent values required to neutralize the acid formed by the various nutrient salts in the fertilizer (Tisdale et al., 1999; Tisdale and Nelson, 1966).
Andrews (1954) maintained that all ammonium converted to nitrate creates acidity equivalent to 3.57 kg CCE per kilogram of nitrogen (N) [calculated as mw of CaCO3/(2 × mw of N) to correct for valence differences]. Andrews also argued that 3.57 kg CCE base is leached from the soil along with every kg of NO3-N leached, and his analysis included multiple years of field trials and re-evaluation of Pierre's data. Andrews' assumptions result in differing acidity and basicity potentials for fertilizer salts than those of Pierre; he considered anhydrous ammonia to be twice as acidic as Pierre's estimate. Andrews' calculations indicate calcium nitrate is slightly acidic and potassium nitrate neutral, whereas Pierre's calculations indicate that they are both basic (Andrews, 1954; Tisdale and Nelson, 1966).
Pierre considered that NH4+ uptake was possible but the amount was insignificant under normal agricultural conditions (Pierre, 1933). However, ammonium uptake is energetically favored over nitrate uptake when both N forms are supplied (Engels and Marschner, 1995; von Wiren et al., 2001). Plant species is an important factor to consider in N preference. For instance, plant species adapted to soils that are acidic or with low redox potential tend to have a greater rate of uptake of ammonium, whereas plant species adapted to calcareous soils have greater uptake of nitrate–nitrogen (Marschner, 1995, p. 247). Ammonium follows an electrochemical gradient from apoplasm to symplasm without the requirement of metabolic energy. The exact path of NH4+ uptake is still unclear. It is believed that ammonium is taken up through an NH4+ uniport, H+-NH4+ symporter, diffusion, or ion channels through the plasma membrane (Engels and Marschner, 1995; von Wiren et al., 2001). In contrast to NH4+, NO3– requires active transport from the apoplasm into the symplasm in association with H+ fluxes through H+-NO3– symporters and H+-ATPase stimulation that maintains membrane electrical potential (Engels and Marschner, 1995). It is estimated that the absorption of 1 mol of NO3– consumes 1 to 3 mol of ATP (Touraine et al., 2001). For that 1 mol of NO3– to be used by the plant, it must first be reduced to NH4+ in a process requiring 347 kJ of energy (Lewis, 1986). Low NO3– concentration in the soil solution, low soil pH, and low soil temperature have been shown to favor NH4+ uptake over NO3– (Engels and Marschner, 1995).
The relative balance among nitrification, ammonium and nitrate uptake, and nutrient leaching, which in turn affect substrate acidity, is affected by factors such as species, irrigation method, soil microbial activity rate, plant evapotranspiration rate, and relative size of the plant and substrate volume. Pierre's assumption that only half of the N is actually acid-forming is therefore an approximation based on the experimental conditions under which his model was calibrated.
Although on a strict inorganic chemistry basis using PM, cations such as calcium, magnesium, sodium, and potassium have a basic residue (Table 1), these cations can also have acidic behavior in a soil/crop system. Displacement of protons on exchange sites by cations causes a decrease in substrate-pH (Rippy and Nelson, 2005). Cation uptake by plants is an acidic or neutral process accompanied by anion uptake, base uptake, or in association with a proton pump (Marschner, 1995). Similarly, anion uptake by plants is a basic reaction despite the acidic factors for sulfur and chlorine in Table 1. Overall, the substrate-pH effect of a fertilizer salt is a balance between inorganic chemistry and the dynamic plant/microbial/soil system.
The objectives of this study were 1) to evaluate protocols to quantify fertilizer acidity or basicity in soilless substrates; and 2) to use these protocols to compare the reported fertilizer acidity from PM with the experimentally measured acidity when plants were grown with an ammonium-based, water-soluble fertilizer in a peat/perlite substrate.
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