Fertilization Management Methods and Technical Points for Pelargoniums

Pelargoniums are plants of the genus Pelargonium (Pelargonium), with more than 250 species in the genus, as well as various hybrids. The main types include zonal pelargoniums (English: Zonal Pelargoniums, Latin scientific name Pelargonium x hortorum), ivy-leaved pelargoniums (also known as peltate pelargoniums, English: Ivy-Leaved Pelargoniums, these plants originate from P. peltatum), and regal pelargoniums (English name: Regal Pelargoniums, Latin scientific name Pelargonium x domesticum), etc.

Pelargonium production requires an adequate supply of nutrient elements and a slightly acidic pH value. This article mainly provides an overview of the nutrition issues in pelargonium production separately for zonal pelargoniums, ivy-leaved pelargoniums, and regal pelargoniums.

The pH value, EC value, and nutrient levels suitable for pelargonium growth can be seen in Table 1. The measurement methods in the text are all values from saturated extract. Different types of pelargoniums also require different indicator parameters. The function of each nutrient element, deficiency symptoms, toxicity symptoms, and fertilization methods will be introduced in detail below. For example, if pelargoniums are fertilized and then watered, nutrient elements will be leached by 20 percent, and the EC value will decrease. If subsurface irrigation is used or the number of waterings is reduced, nutrient elements can be reduced by 25 percent to 50 percent while achieving a similar effect to the former. The deficiency symptoms and toxicity symptoms of each element for zonal pelargoniums can be seen in Table 2.

pH value: The suitable pH value varies depending on the type of pelargonium and the cultivation substrate used. For zonal pelargoniums, if soilless cultivation is used, the substrate pH value of 5.8 to 6.2 is more suitable; if soil cultivation is used, the substrate pH value of 6.0 to 6.5 is more suitable. For ivy-leaved pelargoniums and regal pelargoniums, the suitable pH range is 0.3 units lower than that for zonal pelargoniums. If the soil pH value needs to be adjusted, acidic fertilizers or acid injection can be applied to lower the pH value; applying dolomitic lime or hydrated lime can raise the pH value.

Electrical conductivity (EC) value: For zonal pelargoniums and regal pelargoniums, the suitable EC value is 1.5 to 2.5 mS/cm, and the EC value for ivy-leaved pelargoniums is slightly lower than this value. Producers can lower the substrate EC value by leaching with clear water, or by reducing the amount of fertilizer applied. If the EC value is too low, the amount of fertilizer applied needs to be increased, especially increasing the EC value of the substrate around the roots.

Nitrogen (N): The role of nitrogen is to synthesize amino acids, proteins, enzymes, and nucleic acids. When nitrogen is deficient, plants will show slow growth, stunting, premature aging, chlorosis and yellowing of lower leaves, and premature leaf drop. When nitrogen deficiency is severe, plant development slows and flowering is delayed. Pelargoniums are relatively sensitive to ammonium nitrogen (NH4+) toxicity, manifested as curling of old leaves, leaf discoloration, or necrosis. When fertilizing, having nitrate nitrogen (NO3-) account for more than 75 percent of the total nitrogen will avoid the occurrence of ammonium nitrogen toxicity symptoms. For zonal pelargoniums and ivy-leaved pelargoniums, the nitrogen concentration can be between 200 ppm and 250 ppm. Regal pelargoniums require less nitrogen. The nitrogen sources in calcium nitrate, potassium nitrate, ammonium nitrate, 20-10-20, or 15-5-25 fertilizers are relatively beneficial to the plants.

Phosphorus (P): The role of phosphorus in plants is to provide energy transformation (ADP), nucleic acids, enzymes, cell membrane composition, etc. It plays an important role in the development of plant roots and flowers, and also stimulates rapid plant growth. The primary symptom of phosphorus deficiency is stunting and leaves turning dark green. When phosphorus deficiency is relatively severe, the lower leaves of the plant will turn purplish red, then wilt and turn yellow, and may even completely necrose.

Excessive phosphorus can also slow plant growth and lead to deficiencies of various elements, such as iron, zinc, copper, and manganese. The fertilization concentration of phosphorus can be between 5 ppm and 20 ppm. Phosphorus can promote stem growth, so its application amount should be limited to prevent excessive vegetative growth. Before sowing, fertilizers containing superphosphate can be added to the substrate, and liquid fertilizers containing phosphorus such as ammonium phosphate, potassium phosphate, 20-10-20, 15-5-25, etc. can also be applied. Note that when calculating the phosphorus content, the content on the package bag is all the amount of P2O5, and the phosphorus element content can be further multiplied by 0.437.

Potassium (K): Potassium is a catalyst for metabolism, enabling stomata to perform their function normally and improving disease resistance. When potassium is deficient, plants will show necrosis at the edges of lower leaves, and weak stems and petioles; excessive potassium will reduce plant uptake of calcium, magnesium, manganese, zinc, and ammonium nitrogen. The normal application concentration of potassium is 150 ppm to 250 ppm. To avoid potassium affecting the plant's uptake of calcium and magnesium, the K:Ca:Mg ratio in the fertilizer can be controlled at 4:2:1 (similar to poinsettia). Better sources of potassium are potassium nitrate, 20-10-20, or 15-5-25 fertilizers. Remember that when calculating the fertilization ratio, the content on the package bag refers to the percentage of K2O, so when calculating the potassium content, it needs to be multiplied by the parameter 0.83.

Calcium (Ca): Calcium is the main component of cell walls. The main symptom of calcium deficiency in plants is blackening and necrosis of the growing points of stems and roots. Calcium is an immobile element in plants and is absorbed by root tips. Excessive calcium will reduce plant uptake of potassium, magnesium, and boron. The recommended application concentration of calcium fertilizer is 50 ppm to 100 ppm, and the K:Ca:Mg ratio in the fertilizer is 4:2:1. Irrigation water (if the calcium content is relatively high), dolomitic lime, and calcium nitrate can also provide calcium.

Magnesium (Mg): Magnesium plays an important role in the molecular composition of chlorophyll and enzyme activation. Symptoms of magnesium deficiency in plants include loss of green color along the veins of old leaves, and leaves may curl upward; excessive magnesium will reduce plant uptake of calcium. A concentration of 25 ppm to 50 ppm is sufficient. Note that the K:Ca:Mg ratio in the fertilizer is 4:2:1. Dolomitic lime, magnesium sulfate, etc. can be used as magnesium sources. To correct magnesium deficiency, a magnesium sulfate solution can be used to drench the roots. Note that it must not be mixed with other fertilizers. It can be applied once a month to prevent the occurrence of magnesium deficiency.

Iron (Fe): Like magnesium, iron also plays an important role in the formation of chlorophyll. The symptom of iron deficiency in plants is loss of green color along the veins of new leaves, and in severe cases, necrosis of the growing point. Iron deficiency can occur when the soil substrate pH value is too high, roots are necrotic, or phosphorus, manganese, and copper contents are too high. Excessive iron content can lead to insufficient manganese uptake by plants. When the substrate pH value is too low (pH<5.5）时容易发生铁元素中毒症，植株表现为下部叶片失绿，出现坏死斑点。

Some experts have observed that when the soil pH value is<6.0、Fe浓度>1.0 ppm and the Fe:Mn ratio is greater than 3:1, plants tend to show iron toxicity (saturated extract analysis). Therefore, iron can be added to the substrate before or after planting pelargoniums, but do not overdose. Iron can also be applied to plants in the form of chelated iron or ferrous sulfate. Both iron toxicity symptoms and deficiency symptoms can be resolved by supplying an appropriate amount of iron and maintaining the substrate pH value at 5.8 to 6.5.

Example fertilization plan: Taking zonal pelargoniums as an example, a fertilization plan suitable for their nutritional needs is introduced. Each 1 liter of stock solution contains 67.4 grams of calcium nitrate, 52.4 grams of potassium nitrate, and 45 grams of 15-5-15 fertilizer. Apply magnesium sulfate once a month by spraying, at a concentration of 1.2 grams of fertilizer added to each 1 liter of water. In the above fertilization plan, the concentrations of each macronutrient are 214 ppm nitrate nitrogen, 21 ppm ammonium nitrogen, 10 ppm phosphorus, 246 ppm potassium, 138 ppm calcium, and 9 ppm magnesium (the magnesium concentration does not include the magnesium sulfate specially sprayed monthly). Be sure to conduct substrate testing frequently to monitor nutrient levels.

Water quality: Peat-based substrates are more sensitive to changes in chemical properties than cultivation substrates based mainly on natural soil. Well water has relatively high alkalinity, which will gradually cause the soil pH value to become alkaline. When pH > 7.0, the availability of iron decreases, causing iron deficiency (iron deficiency chlorosis). Therefore, the pH value and alkalinity of irrigation water should be tested, and acid can be injected into the irrigation water when necessary.