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pH and Fertility Review for Vegetatively-propagated Annuals

Providing the proper combination of pH and fertility requirements for vegetatively-propagated annuals can be quite a challenge for growers accustomed to the traditional "one-size-fits-all" approach to fertilizing bedding plants.  Table 1organizes the pH and fertility requirements of vegetatively-propagated annuals with the help of  Ball FloraPlant®, and Proven Winners® catalogs. The plant culture sections of the catalogs are in very close agreement on the pH and fertility requirements for almost all of the species.

  Low pH
Medium pH
High pH
Low fertility
150-200 ppm N
(Also, seed-propagated snaps)
New Guinea impatiens
Double impatiens
(Also seed-propagated impations)
Medium fertility
200-250 ppm N
Also, seed-propagated pansy and vinca)
Zonal Geranium
(Also, seed-propagated geranium, marigold and celosia).
High fertility
250-300 ppm N
(Also, seed-propagated petunia)

How to meet the requirements of the plants using a commercial soilless mix testing about pH 5.8 at planting is summarized in the following sections.

Fertilizer types. 15-15-15, 15-16-17, or 20-10-20 are good choices vegetative annuals needing low or medium pH, particularly those sensitive to iron deficiency. For species needing high pH and sensitive to low pH or Fe/Mn toxicity use Cal-Mag 15-5-15, 15-5-25 or 15-0-15. Make sure to alternate 15-0-15 with another fertilizer containing phosphorus.

Fertilizer strategy. Begin fertilizing vigorous types shortly after transplanting. Small, slow-growing species, however, should receive low rates (50-100 ppm N) or less frequent application until their root systems are well-established and the plants show signs of growth. The fertilizer rate should be cut in half at visible bud or about 2-3 weeks from marketing as a means of hardening the plants and/or enhancing their shelf-life.

Common nutrient problems

Excess soluble salts. High growth medium electrical conductivity (EC) can injure or inhibit the growth of young transplants. Use low rates (50-100 ppm N) for slowing-growing species in the one to two weeks following transplanting. Whenever a high EC problem occurs, check for root disease.

Iron/manganese toxicity. High pH species, especially zonal geranium, and all types of impatiens are the most susceptible plants to iron (Fe)/manganese (Mn) toxicity. This disorder is sometimes called "bronze speckle" due to the appearance of numerous small brown spots on the leaves. Growth medium pH should be maintained in the recommended range by adequate liming prior to planting, careful selection of fertilizers with low potential acidity, pH monitoring, and the use of liquid limestone preparations to raise pH after the plants are established in their containers. Some growers make a routine liquid limestone treatment once the plants are established after transplanting. Raising the pH limits the availability of Fe and Mn and prevents toxicity. Consult the "Iron Out" nutrient management fact sheet from the University of Florida for more information on this problem.

Iron deficiency. Iron deficiency symptoms generally show up as an interveinal chlorosis, normally starting at the shoot tips, but often they occur throughout the entire plant. Sometimes the leaves of some Fe deficient plants turn almost white such as the photo of this cleome. Calibrachoa, scaevola, snapdragons, and petunias are the vegetative annuals most susceptible to iron deficiency. Preventing Fe deficiency can be accomplished by controlling pH and using an iron chelate fertilizer:

pH control. Acid pH favors the availability of Fe to plants, therefore the target pH range for crops susceptible to Fe deficiency is fairly low, 5.5 to 6.0. Most commercial soilless media have pHs in this range and the use of an acid-forming fertilizer like 20-10-20 may be enough to keep the pH in this range. A major exception would be if the irrigation water is alkaline and then acid injection would be needed. If a grower mixes his/her own sphagnum peat-based growth medium dolomitic limestone should be added at a rate of no more than 5 lbs./yd. Too much limestone is a aggravating factor contributing to Fe deficiency.

Iron chelate. Fertilizing sensitive crops with Fe chelate fertilizer from time to time is probably the least complicated way of preventing Fe deficiency. Most greenhouse supply companies carry Sprint 330® (10% iron), Sprint 138® (6% iron), or similar iron chelate products. Sprint 138®, however, is the preferred chelate if it is available. Sprint is generally applied as a soil drench at the rate of 8 oz./100 gal. (½-¾ tsp. gal.). The chelate is also soluble enough to make a concentrated solution for injection and low rates can be mixed and injected with other fertilizers. At the rate recommended here, Fe chelate can be applied every 3 or 4 weeks if desired.

Prepared by Douglas Cox
University of Massachusetts
Amherst, MA 01003