Fertility monitoring and management for pansies requires a balancing of the plants’ needs. Growers must be aware and manage root substrate pH and electrical conductivity (EC), as well as provide adequate, but not excessive, levels of all the essential elements for pansies.
Conducting root substrate testing either in-house or through a commercial lab will help ensure that your fertility program is on target. Ideally, testing should be conducted every two weeks and plotted to detect trends before any deficiency or toxicity symptoms appear. Tips on conducting PourThru Monitoring are available from the NC State University Web site at www.pourthruinfo.com .
The pH of your root substrate (medium) dramatically influences nutrient availability to plants for a soil-based substrate or an organic soilless substrate. Therefore, it is important to maintain the root substrate pH within a range that provides adequate availability of all essential elements. A substrate pH above 5.8 for pansies grown in a soilless root substrate can result in boron (see "Boron" section) or iron deficiencies. In addition, the incidence of black root rot caused by the fungus Thielaviopsis basicola also increases at substrate pHs greater than 5.8. A very low pH, below 4.8, can result in micronutrient toxicities.
High substrate and irrigation water pH can adversely affect nutrient availability and subsequent plant growth. However, the major factor regulating pH rise in substrate solutions is the degree of alkalinity of the irrigation water. If the irrigation water contains a high concentration of carbonates and bicarbonates, the substrate solution pH will rise to undesirable levels during plant production. The optimal pH varies by the root substrate used. The range for a soilless root substrate is 5.4-5.8 and for a soil-based substrate is 5.6-6.0 (See Table 1). Test your water to determine your alkalinity level, and take corrective actions like injecting acid to neutralize alkalinity if required.
Following are rapid corrective measures to take to adjust the root substrate pH in pots or benches already containing plants. Iron sulfate and hydrated lime may burn the foliage and should be applied only to the root substrate. Rinse the plant’s leaves if any solution comes in contact with them. Some plants may be sensitive, so test a small area or a few plants before treating a large area. Adjustment of pH is rapid, but effects are not long lasting; recheck the pH in a week and reapply solution if necessary.
To Lower pH (select one):
Option 1. Dissolve 1-2.5 pounds of iron sulfate [FeSO4·7H2O] in 100 gallons of water. Apply to the root substrate and rinse the foliage after application. Iron sulfate may increase the root substrate electrical conductivity (EC) level and may release toxic levels of minor elements from the root substrate’s exchange sites.
Option 2. Add sulfuric acid to clear irrigation water until the pH is approximately 4.5, and apply the solution to the root substrate as a drench. Rinse off foliage. Retest substrate pH, and reapply solution if needed until the pH is within the desired range.
To Raise pH (select one):
Option 1. Apply flowable limestone products as a substrate drench. Start with a rate of 1-2 quarts per 100 gallons of water. Lightly mist off any solution from the foliage after applying.
Option 2. Mix two pounds of potassium bicarbonate in 100 gallons of water, and apply as a substrate drench with at least 30 percent leaching. Lightly mist off any solution from the foliage after applying. This treatment supplies 933 ppm potassium but does not supply calcium or magnesium, which could be low when the substrate pH is acidic. To restore nutrient balance, apply a complete, basic-type fertilizer using moderate leaching one day after the potassium bicarbonate application.
Option 3. In a plastic bucket, mix one pound of hydrated lime with 3-5 gallons of warm water. Allow the mixture to settle, and pour off the clear solution into another plastic bucket. Apply the clear solution with a fertilizer injector set at 1:100 or 1:128. Hydrated lime is corrosive, so avoid contact with skin and metal. It may displace ammonium from the root exchange sites of the root substrate into the soil solution and cause root injury. Avoid using hydrated lime if high levels of ammonium fertilizer are present in the root substrate. Lightly mist off any solution from the foliage after applying.
Electrical Conductivity (EC)
Soluble salts refer to the total dissolved salts in the root substrate at any given time and are measured in terms of electrical conductivity (EC). Nitrogen and potassium are the main fertilizer materials contributing to the EC concentration of the root substrate; in some areas, high sodium in the irrigation water can also impact the EC.
Excess salts accumulate when leaching during irrigation is insufficient, when too much fertilizer is applied or when the irrigation water contains a high amount of dissolved elements. Excessively high EC readings are associated with poor plant growth. Plant symptoms often begin on the lower leaves as leaf chlorosis and progress to necrotic leaf tip margins. If the root substrate is allowed to dry down, plants may also exhibit wilting
symptoms because of root tip dieback, which further inhibits water and fertilizer uptake. High EC has also been linked to an increased incidence of Pythium root rot.
At the opposite end of the spectrum, when the EC content of the root substrate is too low, plant growth is stunted from lack of fertilizer. Symptoms of low EC typically are lower leaf yellowing (nitrogen deficiency) or lower leaf purpling (phosphorus deficiency).
To promote good growth, maintain EC levels between 0.75-1.5 mS/cm (see Table 1). See fertilization tips under the next section, "Nitrogen," for periods of excessive leaching due to high rains.
Plants with nitrogen deficiency exhibit slow growth, stunting, lack of lateral shoot growth and, with advanced conditions, lower leaves initially turn greenish purple to yellow (chlorosis). Leaf abscission occurs after prolonged deficiency conditions. Excess levels of nitrogen will result in a darker green color, reduced plant growth and delayed flowering.
A fertilization rate of 125 ppm nitrogen on a constant liquid fertilization or 175-200 ppm nitrogen constant liquid fertilization with excessive leaching (outdoor production) is recommended. See Table 1 for specific fertilization recommendations based on growth stage and fertilization practices.
Some growers find the addition of slow-release fertilizer in larger pots or tubs to be beneficial, especially in the South where heavy rainfall removes large amounts of nutrients. Another common practice used after a heavy rainfall is to immediately fertilize pansies with around 200-300 ppm nitrogen, which helps restore the nutrient charge. Even though the substrate is already thoroughly saturated from the rain, this keeps the plants from stalling due to lack of fertilizer.
Phosphorus deficiencies can occur and are first expressed as stunting with the leaves turning darker green, followed by the lower leaves becoming reddish purple. Lack of phosphorus, root rot, wet substrate and cool temperatures can cause this reddish purple coloration. So if you see the lower leaves turning purple, the first step is to check the roots.
Most substrate mixes have a preplant phosphorus charge, which should provide ample levels of phosphorus to the plants. Because pansies prefer acidic conditions, most growers use acidic fertilizers like 20-10-20. Remember that 20-10-20 applied at the rate of 200 ppm nitrogen will supply 44 ppm of phosphorus – more than what is required! In fact, phosphorus applications should be limited to keep the plants compact: Do not apply more than 5-10 ppm phosphorus (see Table 1).
Potassium deficiency, while rarely observed, first develops on the medium to older leaves as a darker green color, which progresses to marginal leaf necrosis (browning). Plants also develop short, compact internodes with few lateral shoots.
A fertilization rate of 104 ppm potassium constant liquid fertilization or 145-166 ppm potassium constant liquid fertilization with excessive leaching (outdoor production) will provide ample potassium levels (see Table 1). Potassium fertilization rates greater than 200 ppm can have an antagonistic effect on calcium or magnesium uptake by the plant. Supplying the plants with a K:Ca:Mg ratio (ppm) of 4:2:1 will limit any antagonisms.
Calcium deficiency symptoms are initially expressed as stunted plant growth and death (blackening) of terminal buds and roots. Young leaves are straplike with necrotic spots, which become complete necrosis over time.
A fertilization rate of 50-100 ppm calcium constant liquid fertilization will provide ample levels of this nutrient (see Table 1). In many locations, there is sufficient calcium in the irrigation water, but in areas of the Southeast and Northeast, additions are required. This is especially important if you are using a fertilizer that does not contain calcium, like 20-10-20.
While providing some calcium to pansies is good, avoid adding too much. Boron deficiency induced by excess calcium applications, specifically with the use of calcium nitrate (Ca(NO3)2), can easily occur.
Initially, magnesium deficiency symptoms develop as an interveinal chlorosis (yellowing) of older, lower leaves. Upward curl of the leaves is possible, and with advanced conditions, the leaves rapidly turn chlorotic to necrotic (dead, brown tissue).
A fertilization rate of 25-50 ppm magnesium constant liquid fertilization will provide ample levels of this element (see Table 1). Many areas have sufficient magnesium in their irrigation water, but this is not always the case in the Southeast and Northeast, where additions are required. This is especially important if you are using a fertilizer that does not contain magnesium, like 20-10-20.
If your fertilizer does not contain enough magnesium, another option is monthly applications of magnesium sulfate (Epsom Salts) (MgSO4·7H2O). Apply at the rate of 1-2 pounds per 100 gallons of water, and do not mix with other fertilizers to avoid possible precipitates.
Boron deficiency can be a serious problem with pansies. Symptoms are initially expressed on the new leaves and stems, with the young growth being thick-textured and straplike. With advanced conditions, death of the growing point can occur, resulting in axillary shoot growth. It is important to correct boron deficiency when symptoms first appear because growing point death or distorted leaves cannot be reversed. If deficiency symptoms are severe, it is rarely economical to try to reverse the damage. Dispose of the crop.
Excessive levels of calcium can have an antagonistic effect on boron availability, and growing the crop at substrate pHs above 6.2 can tie up boron. Make sure your fertilizer or irrigation water contains ample levels of boron. Limit excessive calcium applications by avoiding calcium nitrate-based fertilizers, and maintain the pH within the acceptable range of 5.4-5.8 to ensure that boron is readily available to the plant.
Problems with iron deficiency are usually associated with the pH being too high. Symptoms appear as a distinct interveinal chlorosis of the younger leaves, progressing to complete lighter yellow leaves, and then tip dieback under severe conditions. Deficiencies can also occur with root death, overirrigation, poor drainage of the root substrate, insect damage (fungus gnat larvae) to the roots or when excessive levels of lime are applied.
Apply iron as part of your normal fertilization program. Maintain the root substrate pH between 5.4 and 5.8 for a soilless root substrate, and 5.6 and 6.0 for a soil-based substrate to maximize iron availability. Take the corrective actions listed under the "pH" section if modifications are required.
Which Fertilizer to Use?
Based on the nutritional needs of pansies, there are a number of fertilization strategies that will work. If your system is working, don’t change it. But if you want to change, there are four factors to consider when selecting a pansy fertilizer.
1. Pansies prefer a lower pH. Is the fertilizer acidic?
2. Most acidic fertilizers are high in ammoniacal nitrogen and phosphorus. Height control of pansies during the summer is difficult enough without supplying too heavy a dose of either of these. Does the fertilizer have less than 30 percent ammoniacal nitrogen and a low amount of phosphorus?
3. Does the mix supply all of the essential elements? Does your irrigation water supply enough calcium and magnesium, or does it need to come from the fertilizer?
4. Where are the microelements?
No single fertilizer fulfills all of the above requirements. While all of the special pansy blends are excellent fertilizers, they are basic and can cause a pH increase – even though pansies prefer an acidic pH.
What to do? Rotate fertilizers! Use pansy fertilizers with 15-2-20, 15-3-30 or 13-2-13 ratios, which provide adequate levels of calcium, magnesium and micros, but have low amounts of phosphorus and ammoniacal nitrogen. Then use a fertilizer like 21-5-20, which is acidic and has a low amount of phosphorus. Monitor the pH to make sure it stays within the acceptable range of 5.4-5.8. This type of fertilization program will keep your pansies on track.