Cyberconference: Subirrigation and nutrition for poinsettias -- Blom and Brown

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Monitoring quality of irrigation water

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Water quality for woody plants, Part 1

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Subirrigation: An irreversible trend

Irrigation considerations for container plants

Alkalinity control for irrigation water

The effects of limestone in the substrate, and bicarbonate and NH₄⁺/NO₃^- ratio in irrigation solution on subirrigated poinsettias

By Theo Blom and Wayne Brown
University of Guelph

Subirrigated pots are basically grown in a closed system: one starts with a substrate containing nutrients and a cutting, while at flowering there is a mature plant containing nutrients as well as substrate with nutrients. Provided the nutrient levels within the substrate are similar at flowering compared with planting, the difference in nutrient content (weight) of a mature plant and a cutting has to be provided by the (sub)irrigation solution. With each irrigation, plant(s) receive water with dissolved nutrients at a predetermined concentration. There is no leaching and whatever nutrients enter the substrate are either used by the plant or will remain in the substrate. Therefore, the supply of nutrients has to be managed, so they do not create any excess (salts), and the supply has to be tailored to the uptake. This means that nutrient uptake follows an "S" curve over time with a slow uptake at the beginning and at the end and a rapid uptake during the middle of the growing period.

Most of the active roots are within the bottom one-half of the pot, and it is the management of this part of the pot which has become important. In our subirrigation research, we wanted to look at the effects of pH- increasing (limestone and bicarbonate) and pH-decreasing (ammonium) materials on rootzone-pH and plant growth. This was done in a factorial experiment where the potting substrate was supplemented with three rates of limestone in combination with fertigation solutions containing two ratios of ammonium and nitrate (NH₄⁺ / NO₃^-) and three bicarbonate (HCO₃^-) concentrations. This experiment was done with both pot chrysanthemums as well as poinsettias over a period of three years. Only the results with poinsettias will be discussed in this paper.

Materials and Methods
Standard growing medium (50% peat, 35% perlite and 15% Turface by volume) were prepared and mixed with ½ X, X and 1½ X of the recommended rate of limestone (X = 7.0 kg of calcitic limestone per m³ of loose sphagnum peat moss (Grade : von Post H2-H5). The amount of limestone was based on the amount of peat moss in the substrate (50% by volume) only. In addition to the limestone, superphosphate (1.5 kg/m³), magnesium sulphate (0.5 kg/m³), ammonium nitrate (0.4 kg/m³) and microelements with a wetting agent (Soilwet) @340 g/m³ were added to the substrate. Rooted poinsettia cuttings (Gross Supjibi 'Red') were planted in 15-cm (6-inch) diameter pots filled with one of the three lime amended substrates and placed on subirrigation troughs. Each trough had its own tank with a nutrient solution which was recirculated. The nutrient solution was one of the six combinations as described in Table 1.

Table 1. Formulation of the six different fertilizer solutions combining two ratios of NH₄⁺ / NO₃^- and three bicarbonate concentrations.

Code AX BX CX AY BY CY

NH₄⁺ / NO₃^- ratio* 0.05 0.5

bicarbonate (mMol/L) 0 3 6 0 3 6

HCO₃^-** 0 3.0 6.0 0.0 3.0 6.0

H₂PO₄^- 0.50 0.50 0.50 0.50 0.50 0.50

Ca⁺⁺ 3.5 3.5 3.5 2.5 2.5 2.5

NH₄⁺ 0.75 0.75 0.75 5.0 5.0 5.0

NO₃^- 12.75 12.75 12.75 9.50 9.50 9.50

K⁺ 3.5 5.5 7.5 3.0 4.5 6.0

SO₄^2- 0.25 0.25 0.25 0.50 0.50 0.50

Mg²⁺ 1.25 1.75 2.25 1.0 1.75 2.5

Cl^- 0 0 0 4.0 4.0 4.0

* Ratio is expressed on mMol/L basis; 1.0 mMole bicarbonate / L = 60 ppm HCO₃^-.
** Bicarbonate and macronutrients expressed on mMol/L basis.

In order to maintain electro-neutrality within the nutrient solution, the higher bicarbonate concentrations (B- and C-series) were counterbalanced by both higher K⁺ and Mg²⁺ concentrations, while the Ca⁺⁺ concentration remained the same for a given NH₄⁺ / NO₃^- ratio. The high NH₄⁺ / NO₃^- ratio series (Y) were produced by adding NH₄Cl to the solution. The EC of the solutions with the highest bicarbonate (6.0 mMol/L) levels were 0.6 mS/cm higher than the lowest bicarbonate levels (0 mMol/L), while the EC for the different NH₄⁺ / NO₃^- ratios at a given bicarbonate concentration was the same. Every two to four weeks the solutions were renewed. Each trough contained 10 plants of each of the three limestone-amended substrates. The experiment was repeated in the fall of 1995.
Samples from the substrate were taken on two occasions, namely at the end of October and at flowering (beginning of December) from three different heights within the pot, i.e. bottom 4 cm; mid-range between 4-8 cm (1.6-3.2 inches) and the top > 8 cm (3.2 inches). These samples were analyzed using the saturated medium extract method for pH, electrical conductivity (EC) and the major macro nutrients. Plant measurements were also taken at these times and consisted of fresh and dry weights, leaf surface area, and height. Tissue analysis was done on leaf and bract tissue separately.
Results
As the data for the fall of 1995 was similar to that of 1994, we present only the data of the second year.
Substrate
The effect of the three factors (limestone supplementation, bicarbonate concentration and NH₄⁺ / NO₃^- in the solution) on substrate-pH was found primarily in the bottom part of the substrate (rootzone) (Figure 1). The substrate-pH in the bottom of the pot varied between 4.3 and 8.0 for the different NH₄⁺ / NO₃^- and bicarbonate combinations, while it varied between 5.2 and 5.6 in the upper layer. The interactive effects of all three factors in the bottom layer is illustrated in Figure 2.

The effect of limestone, bicarbonate concentration and NH₄⁺ / NO₃^- ratio on substrate pH.

Interactive effects of limestone, bicarbonate concentration and NH₄⁺ / NO₃^- ratio on pH of the bottom 4 cm (1.6 inches) of substrate.

Substrate-pH increased linearly with the amount of limestone, namely 6.3, 6.6 and 6.9 for the overall mean for ½ , 1 and 1½ X, respectively. These values were 0.5 -1.0 unit higher than at the start of the experiment (5.4, 5.8 and 6.4 for the ½ , 1 and 1½ X limestone rates, respectively). The effects of bicarbonate concentration depended on both the amount of limestone and the NH₄⁺ / NO₃^- ratio. As more limestone was added to the substrate, the smaller the effect of the bicarbonate concentration, especially when the NH₄⁺ / NO₃^- ratio was low. The strongest effect of the bicarbonate was found when the NH₄⁺ / NO₃^- ratio was high (Figure 2). Only substrates, which received a high NH₄⁺ / NO₃^- ratio with 0 or 3mMol bicarbonate had substrate values lower than the initial pH.
Plant growth
The overall means of the plant growth, measured in aerial plant (stems, leaves and bracts) dry weight, was not affected by the different limestone levels. Increased bicarbonate concentration decreased the overall means of the plant dry weight from 32.2 g (1.1 oz.) for 0 mMol/L bicarbonate to 28.1 g (1 oz.) for 6 mMol/L bicarbonate (Figure 3). Vegetative plant growth (stems + leaves) benefitted from high NH₄⁺ / NO₃^- ratios, while generative plant growth (bract) was not affected by the NH₄⁺ / NO₃^- ratio.

The effect of limestone, bicarbonate concentration and NH₄⁺ / NO₃^- ratio on total plant dry weight.

There was a negative correlation between substrate-pH and plant growth (Figure 4), irrespective of how the pH was achieved. The plant dry weight decreased by 1.5 g (0.05 oz.) or about 4-5% for every pH-unit increase.

Correlation of substrate pH on plant growth.

Tissue analysis
At flowering, bracts and leaves were sampled separately for each plot, dried and analyzed for N, P, K, Ca, Mg, Fe, Mn, Zn, and Cu.
Bracts versus leaves. The mineral content of N, Ca, Mg, Fe and Mn were higher in the leaf- than in the bract-tissue (4.3 vs. 2.7% for N; 1.36 vs. 0.50% for Ca; 0.67 vs. 0.37% for Mg; 63 vs 31 ppm for Fe; and 96 vs. 26 ppm for Mn).
Leaves. Limestone decreased N, Mg, Mn, Zn, and Cu but increased Ca level in the leaf-tissue. Bicarbonate decreased N, Ca, Fe, Mn, Zn and Cu while increased NH₄⁺ / NO₃^- ratio increased Ca, Fe, Mn, Zn and Cu.
Bracts. Limestone did not affect mineral content in the bract tissue. Bicarbonate decreases Ca, Fe, Mn, Zn and Mn, while higher NH₄⁺ / NO₃^- ratio increases Ca, Fe, Mn, Zn and Cu.
These results are an indication how the pH directly affects the minor element content of both leaves and bracts.
Solution Analysis
Tanks for the recirculating solutions were refilled every two to four weeks with fresh solutions and samples were taken before ('old') and after ('new') refilling (Table 2). Over the entire production period (five refills), the solution-pH decreased by 0.2, while the EC increased by 0.1 mS/cm when the overall means were taken. It is not clear why the pH in the solution decreases. There was no evidence that the pH decrease was due to the nitrification of NH₄⁺ into NO₃^- . In previous years, there was some evidence that NH₄⁺ changed into NO₃^- in the subirrigation solution, thereby producing hydrogen but not during this experiment.

Table 2. Changes in nutrient composition from a fresh ('New') to a two- to four-weeks recirculated ('Old') solution. Data are the mean of five sets of data.

AX BX CX AY BY CY

New Old New Old New Old New Old New Old New Old

pH 4.41 4.00 7.25 7.52 7.38 7.84 4.23 4.14 7.17 6.45 7.34 6.79

EC 1.27 1.38 1.35 1.49 1.55 1.65 1.37 1.50 1.49 1.57 1.67 1.74

NO₃-N 105 107 101 107 91 97 74 74 70 73 64 66

Ca²⁺ 92 94 84 82 72 67 65 64 58 59 50 53

HCO₃^- 11 15 86 66 160 125 3 3 82 31 123 53

Note: EC is expressed in mS/cm; values for NO₃^-N, Ca²⁺ and bicarbonate (HCO₃^-) are expressed in ppm and normalized to EC=1.0 mS/cm for comparison.

The increase in EC is likely due to evaporation of water in the tanks and the solution picking up some residue in the troughs when irrigating. The normalized calcium concentrations did not change appreciable with recirculating but were generally lower with an increased bicarbonate level. Bicarbonate levels were generally not attained at the prescribed levels but also decreased between a fresh and a recirculated solution. The decrease was likely due to evaporation into the atmosphere and/or reaction with NH₄⁺ .
Summary
Substrate-pH in the lower section of the substrate within the pot was highly dependent on bicarbonate and NH₄⁺ / NO₃^- ratio within the solution. While higher bicarbonate increased, higher NH₄⁺ / NO₃^- decreased the substrate-pH in the bottom layer of the pot. High bicarbonate decreased, while higher NH₄⁺ / NO₃^- increased plant growth, especially at the low limestone rate. Although the limestone rate did not affect plant growth, high substrate-pH had a negative effect on plant growth. These results have shown that bicarbonate and the NH₄⁺ / NO₃^- ratio can affect the substrate-pH in the bottom part of the substrate and also influence plant growth and minor element uptake in both the leaf and bract tissue.
These results also indicate that high bicarbonate levels can be compensated with higher ratios of NH₄⁺ / NO₃^- . Solution - pH and -EC were fairly constant in the recirculating solution. Conversion of NH₄⁺ / NO₃^- within the recirculating solution did not take place to a measurable level.