February 2005

Organize Crops by Nutrition By Paul Fisher and William Argo

You could receive a perfectly healthy liner or plug, but without correct fertilization crop quality (and profit) will quickly go downhill. Because the wide range of plant species we grow differ in nutritional needs it is often difficult to grow multiple crops with a one-size-fits-all fertilizer program. Our research has shown that it is helpful to understand varietal differences and to separate crops during production according to these needs.

Defining the Groups

Iron and manganese are more soluble and available for uptake by plant roots at low media pH. When pH is below 6, some “iron-efficient” plants such as marigold, seed and zonal geranium, and New Guinea impatiens are able to take up so much iron or manganese that toxicity symptoms occur. Iron-efficient plants such as geraniums have strategies to increase iron solubility and uptake, for example by exuding acid to drop pH around the root zone, increasing root enzyme activity, or exuding chelating agents through the roots.

“Iron-inefficient” plants, for example petunia and calibrachoa, lack iron efficiency strategies. These species often show iron deficiency symptoms when media pH is above 6.4. You may be familiar with petunia or calibrachoa having chlorotic new growth. In most cases, plants with these symptoms do not need more nitrogen, potassium or phosphorus. Rather, they are lacking iron because the high pH means that iron is insoluble in the growing medium and therefore unavailable for uptake by roots.

We have found it helpful to organize plants into three groups that vary in their efficiency at taking up micronutrients. These groups range from the Geranium Group (iron-efficient) through an intermediate General Group to the iron-inefficient Petunia Group. The three nutritional groups alert you to which problems are likely to arise with each species and can help you simplify fertilizing multiple species.

At the University of New Hampshire, we have run trials to help separate plants into these groups. Combined with industry experience, those results are summarized in Figure 1, page 58.

Our list of species is incomplete, so you might want to mimic our experiment to separate other varieties into the three groups. We grew plants in a peat/perlite (70 percent/30 percent by volume) medium at pH 5, 6 and 7 using different lime incorporation rates. In addition, we drenched plants at the high lime treatment with 45 ppm of iron from iron-EDDHA (0.1 oz. per gal.). If you only have a commercial blended medium available, a high pH treatment could be created by repeatedly drenching media with flowable lime (1-gal. flowable lime diluted at 1:50) until the pH increases to 7. A low pH Á treatment could also be created by repeatedly drenching media with ferrous iron sulfate at 2 lb. per 100 gal. until the pH drops to 5. We used a SPAD chlorophyll meter (available at Spectrum Technologies) to evaluate leaf color, but you could achieve a similar result by a visual green color scale. We also measured plant growth and checked the plants for signs of micronutrient toxicity at low pH or following the iron drench. Micronutrient toxicity typically appears as small necrotic (dead) leaf spots. Plants that show iron deficiency symptoms and reduced growth at high pH tend to be categorized in the Petunia Group. Species with increased growth at high pH or micronutrient toxicity either at low pH or following the iron drench are in the Geranium Group. Less pH or micronutrient-sensitive varieties are in the General Group.

Managing Multiple Crops

If you are able to manage crops separately, then Figure 2, page 59, shows ideal conditions to grow each group and techniques for correcting pH- and iron-related problems. This is a summary of fertilizer and corrective actions, and trialing under your conditions is essential before applying these methods on a commercial scale.

Train staff to recognize likely problems with each group of plants — micronutrient deficiency at high pH for the iron-inefficient Petunia Group and toxicity for the iron-efficient Geranium Group at low pH.

Maintain a moderate media pH of 6.0-6.2 when growing a range of species in the same container (e.g., a combination basket or pot) or using the same fertilizer injector and medium.

When planting combination baskets, do not try to grow every possible mix of species. For example, it may not be the best idea to mix red seed geraniums (very susceptible to iron toxicity) with white calibrachoa (very susceptible to iron deficiency). Ask yourself, “Do you have to have a geranium and a calibrachoa, or do you need a red upright plant and a white trailing plant?” There are many species that fit your plant color or form requirements without having conflicting pH requirements.

If you must have specific combinations that do not grow well together, grow plants for combination baskets or pots in separate 4-inch pots, and combine in the final container two weeks before sale. This minimizes nutrition problems and allows plants to be grown under several different fertilizer regimes. The Geranium Group can be more susceptible to toxicity early in production when it is actively growing and fertilizer applications are more frequent or heavier. Á Geraniums can be more easily combined with the Petunia Group when plants are mature.

Organize greenhouse zones so that plants within the same nutritional group can be managed with one injector or fertilizer tank.

If you have the ability to run different fertilizers, use a more acidic-reaction fertilizer (i.e., more ammonium) for the Petunia Group and a more basic-reaction (nitrate-based) fertilizer for the Geranium Group. Check the fertilizer label to determine if it is an acidic- or basic-reaction fertilizer.

If you have high alkalinity and can adjust acidification of irrigation water, acidify for the Petunia Group and not for the Geranium Group.

Some growers will incorporate extra lime (for example 50 percent more) or apply flowable lime at 1:100 into growing medium for the Geranium Group before planting to raise pH above 6.0.

If you use one injector for everything, test pH every 1-2 weeks and be prepared to apply flowable lime to the Geranium Group if media pH is below 6.0, or supplement iron for the Petunia Group if pH is above 6.2.

Managing media EC and media pH go hand in hand. Not only will high media EC suppress media pH, but a high EC is an indication that high levels of fertilizer salts are present, increasing the risk of toxicity for the Geranium Group grown at low pH. In contrast, a low EC indicates that low levels of fertilizer salts are present, increasing the risk of deficiency symptoms in the Petunia Group.

Caution: When applying any chemical, including micronutrients, to a plant, test the application on a small number of plants to check for phytotoxicity before applying to the entire crop. Always read and follow label instructions.