Substrate pH must be carefully managed to control nutrient availability in container substrates (Peterson, 1981). A number of factors that include substrate components, limestone type and rate, the irrigation water source (IWS), plant species, and water-soluble fertilizer (WSF) interact to affect the nutrient supply in container substrate throughout crop production (Argo and Biernbaum, 1996, 1997). A key grower decision is the selection of the WSF formulation and concentration to maintain a stable substrate pH over time. This is particularly challenging given the range in iron efficiency of floriculture crop species, which affect both tendencies to either raise or lower pH and susceptibility to either iron deficiency at high pH or iron/manganese toxicity at low pH (Argo and Fisher, 2002).
With agronomic crops, some species reduce susceptibility to high pH-induced iron chlorosis through an ability to lower (acidify) the rhizosphere pH through root exudation of H+ and organic acid (citrate, malate) when grown in calcareous soils (pH greater than 7.8). In contrast, species that do not lower the rhizosphere pH are much more susceptible to lime-induced iron chlorosis (Marschner, 1995). Among cultivars of the same species, there may be considerable differences in the susceptibility of high pH-induced iron chlorosis because of differences in their ability to lower the rhizosphere pH (Froehlich and Fehr, 1981; Saxena and Sheldrake, 1980).
In floriculture crops, much less is known of species or cultivar effects on substrate pH and the resulting differences in nutrient uptake. In laboratory experiments on germinating seedlings, Bailey et al. (1996) found that substrate pH varied from 4.5 with Lycopersicon esculentum Mill. (tomato) to 7.5 with Zinnia hybrida Roem. & Usteri (zinnia) under the same conditions. In greenhouse experiments, Argo and Biernbaum (1997) found that the average substrate pH of 10 potted plant species given the same WSF [20N–4.3P–16.6K Peatlite Special (Scotts, Marysville, OH)] ranged from 5.1 with Saintpaulia ionantha Wendl. (African violets) to 6.5 with Gerbera jamesonii Bolus ex Hook. (gerbera). In addition, WSF concentration also influenced substrate pH. For example, the substrate pH of gerbera decreased from 7.1 to 5.8 as the N concentration of the WSF increased from 50 to 200 mg·L−1, whereas the substrate pH decreased from 5.2 to 4.8 with African violets over the same range of WSF concentrations.
Johnson et al. (2013) showed that the majority of the WSF effect on substrate pH could be modeled based on the concentration of different N forms (NH4+, NO3–, or urea) despite a wide range in concentration of other ions. By analyzing the observed pH change for impatiens, petunia, and pelargonium, with a number of WSF formulations, parameters were developed for each N form and plant species. Estimated milliequivalents (meq) of acid (negative values) or base (positive values) per mmol of each N form applied were NH4-N –0.6678, –0.6143, and –0.8123; NO3-N 0.0713, 0.2746, and –0.1296; and urea-N –0.2038, –0.1445, –and 0.2711 for impatiens, petunia, and pelargonium, respectively. The parameters for meq of acidity or basicity per mmol showed that NH4-N tended to be strongly acidic, urea-N somewhat acidic, and NO3-N weakly basic to slightly acidic. Parameter values were generally in line with expected effects of N forms on substrate pH. However, the pelargonium parameter for NO3-N was slightly negative, indicating acidity even with NO3-N as the sole N source, presumably because of the acidic effect of other cations such as calcium, magnesium, and potassium or from a greater overall tendency for pelargonium to acidify the root zone. Strong acidification of NH4-N could occur from both the charge balance effects of cation uptake by plant roots and nitrification by soil microbes with uptake of NH4-N favored over other N forms (Engels and Marschner, 1995; Lang and Elliott, 1991; von Wiren et al., 2001). Acidification from urea-N occurs through the net effect of hydrolysis (basic), nitrification (acidic), and nitrate uptake (basic) processes (Verburg et al., 2003). Basic effects of NO3-N occur through the charge balance effect (exudation of a base) from anion uptake by plant roots (Argo and Biernbaum, 1997; Marschner, 1995). The need for species-specific parameters resulted from an observed difference in substrate pH in the order from pelargonium (lowest pH), to impatiens, to petunia (highest pH) when grown using a WSF at a given NH4+:NO3–:urea-N ratio.
Using these parameters, the CCE of fertilizer acidity or basicity could be calculated for soilless production with WSF using a simple “N CCE” formula based on N form, N concentration, and plant species. The N CCE therefore provides an alternative to the potential acidity or basicity calculations from Pierre (1933), which are widely used in the horticultural industry but which were calibrated to field soils and solid fertilizer application.
For example, consider a WSF that provides 40 mg·L−1 N from NH4-N, and 60 mg·L−1 N from NO3-N, equaling 2.9 and 4.3 meq N from each N form, respectively (which corresponds with the “Acidic WSF” in Table 1). Parameter values (negative values assigned for acidity, positive values for basicity) from Johnson et al. (2013) for pelargonium were PNH4.S = –0.8123 meq of acidity per meq N from NH4-N and PNO3.S = –0.1296 meq of acidity per meq N from NO3-N. From Eq. , the CCEWSF therefore equals –0.8123*2.9 + –0.1296*4.3 = –2.3 + –0.6 = –2.9 meq of acidity per liter of WSF. The irrigation water had 130 mg·L−1 CaCO3 of alkalinity, which divided by 50 mg·L−1 CaCO3 per meq equals 2.6 meq of base. The net acidity or basicity of the FS (CCEFS) from Eq.  for the Acidic WSF with pelargonium therefore equaled –2.9 + 2.6 = –0.3 meq of acidity. This excess acidity (a negative CCEFS) would expect to lead to a decrease in substrate pH over time, whereas an excess of basicity of a FS (for example, the positive CCEFS for petunia in combination with the basic FS in Table 1) would be expected to increase substrate pH over time. Increasingly negative or positive CCEFS indicates greater expected acidic or negative effects of the FS.
Total macronutrient concentrations of the nutrient solutions used in the two experiments resulting from different water-soluble fertilizers (WSFs) and water sources.z
Although the N CCE model from Johnson et al. (2013) is a promising tool to assist growers in WSF selection, further validation is required. The objective of this study was to evaluate whether the N CCE model correctly predicted the trend in pH change under a range of conditions typically found in horticulture production. The first experiment focused on the effect of plant species at one N concentration and water alkalinity, where impatiens, petunia, and pelargonium, along with six other bedding plant species, were grown with WSF that had NH4+:NO3– ratios of 40:60, 20:80, or 3:97 at 100 mg·L−1 N. A second experiment focused on the effect of N concentration and water alkalinity with impatiens only with 50 to 200 mg·L−1 N concentrations, NH4+:NO3– ratios of 75:25 or 3:97, and water alkalinity at 0 to 136 mg·L−1 CaCO3.
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