Objectives were 1) to quantify acidic and basic effects on the root zone pH for eight vegetable and herb species grown in peat-based substrate and hydroponic nutrient solution and 2) to determine the applied NH4 +:NO3 – ratio expected to have a neutral pH reaction for each species during its vegetative growth phase. In one experiment, plants were grown for 33 days in substrate (70% peat:30% perlite by volume), and were fertilized with a nutrient solution containing 7.14 milli-equivalents (mEq)·L–1 N and NH4 +:NO3 – ratios ranging from 0:100 to 40:60. During the second experiment, the same species were grown in hydroponic nutrient solutions at 7.14 mEq·L–1 N with NH4 +:NO3 – ratios ranging from 0:100 to 30:70, and data were collected over a 6-day period. In substrate, species increased root zone pH when supplied 0:100 solution, except for cucumber, which did not change substrate pH. Increasing the NH4 +:NO3 – ratio to 40:60 increased acidity and decreased pH across species. Similar trends were observed in hydroponics, in which the most basic response occurred across species with 0:100, and the most acidic response occurred with 30:70. Arugula was the only species that increased root zone pH with all three NH4 +:NO3 – ratios in substrate and hydroponics. In substrate and hydroponics, mEq of acidity (negative) or basicity (positive) produced per gram dry weight gain per plant (mEq·g−1) correlated positively with mEq·g−1 net cation minus anion uptake, respectively, in which greater cation uptake resulted in acidity and greater anion uptake resulted in basicity. In hydroponics, the greatest net anion uptake occurred with 0:100, and increasing the NH4 +:NO3 – ratio increased total cation uptake across species. Cucumber had the most acidic effect and required less than 10% of N as NH4 +-N for a neutral pH over time, arugula was the most basic and required more than 20% NH4 +-N, and the remaining species had neutral percent NH4 +-N between 10% and 20% of N. Increasing the NH4 +:NO3 – ratio decreased Ca2+ uptake across all species in hydroponics, which could potentially impact tip burn and postharvest quality negatively. Controlling root zone pH in substrate and hydroponic culture requires regular pH monitoring in combination with NH4 +:NO3 – adjustments and other pH management strategies, such as injecting mineral acid to neutralize irrigation water alkalinity or adjusting the limestone incorporation rate for substrate.
Ryan W. Dickson and Paul R. Fisher
Ryan W. Dickson, Paul R. Fisher and William R. Argo
Floriculture species differ in their effect on substrate-pH and the resulting substrate micronutrient availability in container production. The objective was to quantify effects of floriculture plant species on substrate-pH. In a growth chamber factorial experiment, 15 floriculture species were grown in 70%:30% by volume peat:perlite substrate and fertilized with nutrient solutions containing 100 mg·L−1 N and NH4 +-N:NO3 −-N nitrogen ratios of 0:100, 20:80, or 40:60. The relationship between substrate-pH and milliequivalents (meq) of acid or base per unit volume of substrate was quantified by titration with hydrated dolomitic lime or HCl. After 33 days, species and solution type effects on substrate-pH and estimated meq of acid or base produced were evaluated. Final substrate-pH ranged from 4.83 for geranium in 40:60 solution to 6.58 for lisianthus in 0:100 solution, compared with an initial substrate-pH of 5.84. This change in substrate-pH corresponded with a net meq of acid or base produced per gram of tissue dry mass gain (NMEQ) ranging across solutions and species from 1.47 of base for lisianthus in the 0:100 solution to 2.10 of acid for coleus in the 40:60 solution. With the 0:100 solution, geranium produced the greatest NMEQ of acid (0.07), whereas lisianthus produced the greatest NMEQ of base (1.47). Because all N in the 0:100 solution was in the NO3 − anion form, meq of both anions and cations taken up by plant roots could be calculated based on tissue analysis. With the 0:100 solution, species that took up more anions than cations into plant tissue tended to have a more basic effect on substrate-pH, as would be expected to maintain electroneutrality. Data were used to estimate the percent NH4 +-N of total N in a nutrient solution that would be neutral (results in no substrate-pH change) for each species. This neutral percent NH4 +-N of total N ranged from ≈0% (geranium) to 35% (pentas). Species were separated into three clusters using k-means cluster analysis with variables related to NMEQ and anion or cation uptake. Species were clustered into groups that had acidic (geranium and coleus), intermediate (dusty miller, impatiens, marigold, new guinea impatiens, petunia, salvia, snapdragon, and verbena), or basic (lisianthus, pansy, pentas, vinca, and zinnia) effects on substrate-pH. Evaluating the tendency to increase or decrease substrate-pH across a range of floriculture species, and grouping of plants with similar pH effects, could help predict NH4 +:NO3 − ratios for a neutral pH effect and assist growers in managing substrate-pH for container production.
Ryan W. Dickson, Paul R. Fisher, Sonali R. Padhye and William R. Argo
Floriculture crop species that are inefficient at iron uptake are susceptible to developing iron deficiency symptoms in container production at high substrate pH. The objective of this study was to compare genotypes of iron-inefficient calibrachoa (Calibrachoa ×hybrid Cerv.) in terms of their susceptibility to showing iron deficiency symptoms when grown at high vs. low substrate pH. In a greenhouse factorial experiment, 24 genotypes of calibrachoa were grown in peat:perlite substrate at low pH (5.4) and high pH (7.1). Shoot dry weight, leaf SPAD chlorophyll index, flower index value, and shoot iron concentration were measured after 13 weeks at each substrate pH level. Of the 24 genotypes, analysis of variance (ANOVA) found that 19 genotypes had lower SPAD and 18 genotypes had reduced shoot dry weight at high substrate pH compared with SPAD and dry weight at low substrate pH. High substrate pH had less effect on flower index and shoot iron concentration than the pH effect on SPAD or shoot dry weight. No visual symptoms of iron deficiency were observed at low substrate pH. Genotypes were separated into three groups using k-means cluster analysis, based on the four measured variables (SPAD, dry weight, flower index, and iron concentration in shoot tissue). These four variables were each expressed as the percent reduction in measured responses at high vs. low substrate pH. Greater percent reduction values indicated increased sensitivity of genotypes to high substrate pH. The three clusters, which about represented high, medium, or low sensitivity to high substrate pH, averaged 59.7%, 42.8%, and 25.2% reduction in SPAD, 47.7%, 51.0%, and 39.5% reduction in shoot dry weight, and 32.2%, 9.2%, and 27.7% reduction in shoot iron, respectively. Flowering was not different between clusters when tested with ANOVA. The least pH-sensitive cluster included all four genotypes in the breeding series ‘Calipetite’. ‘Calipetite’ also had low shoot dry weight at low substrate pH, indicating low overall vigor. There were no differences between clusters in terms of their effect on substrate pH, which is one potential plant iron-efficiency mechanism in response to low iron availability. This experiment demonstrated an experimental and statistical approach for plant breeders to test sensitivity to substrate pH for iron-inefficient floriculture species.