By controlling phosphorous levels, plants can be finished without plant deficiencies showing.

Reducing phosphorous (P) during plug production has become a standard practice to control growth. However, this technique does not need to be limited to plugs. Rather, recent research is showing that reducing P during finishing for containerized crops can control unwanted stretch while producing marketable plants.

Before discussing P further, some clarity is required. On fertilizers, the concentration of P in fertilizer analyses is the concentration of phosphorous pentoxide (P2O5), not strictly P. Elemental P is actually 43.7% of the weight of P2O5. So, a 20-10-20 fertilizer is not 10% P by weight, as we commonly think; rather, it is 10% P2O5 and 4.37% P by weight. Similarly, a fertilizer solution providing 200 ppm nitrogen (N) is not providing 100 ppm P; it is actually providing 100 ppm P2O5 and 43.7 ppm P. By multiplying P2O5 concentrations by 0.437, the true or elemental P concentration is determined. Conversely, the P2O5 concentration can be determined by dividing P concentrations by 0.437.

When 150 to 250 ppm N is provided from fertilizers such as 15-5-15 or 20-10-20, the amount of P provided to plants ranges from approximately 22 to 55 ppm. This is more than enough P to produce healthy-looking plants, and these concentrations also maximize growth, including stem elongation.

By providing less P to plants, there is less stretch and stem elongation (Fig. 1); however, P concentrations must be enough to avoid unwanted symptoms of P deficiency, which may reduce marketability. For example, P concentrations between 5 and 10 ppm P can control growth and produce healthy-looking plants with no P deficiency symptoms for a number of annual bedding plants and flowering potted plants.

For many plants, P cannot be completely eliminated during finishing and 2.5 to 5 ppm P should be provided to sustain acceptable growth and appearances. However, we have seen some crops, including some vegetatively propagated crops and large dormant perennial plugs, grown to flowering with no P and have seen positive results. More than likely, there is sufficient P stored in the rooted cutting or perennial crown to suffice for the plant after transplanting and throughout finishing and forcing. For seed-propagated plants, the P content of a seedling plug is much less than a rooted liner and is not sufficient to carry the plant through production.

Though the focus on reducing P is for controlling growth, there are other potential benefits to reducing P during production. For example, P is one of the most easily leached nutrients from soilless substrates in production. By reducing the amount of P applied to crops, the amount of P entering waterways is also being limited.

There are several ways to reduce the amount of P provided to plants in the greenhouse. One of the easiest ways to do this is to provide less fertilizer. Since most water-soluble and controlled-release fertilizers are complete with all macronutrients, including P, using less fertilizer inherently provides less P to crops. However, in using less fertilizer, the amount of other nutrients provided to plants is less. This may be problematic, and insufficient amounts of other nutrients may be provided.

Selecting a fertilizer with less P relative to other nutrients is another way to provide less P to crops. For example, a 20-10-20 fertilizer provides half the amount of P compared to a 20-20-20 option. But there are other fertilizers that provide even less P relative to N. For example, fertilizers such as 21-5-20 and 13-2-13 provide lower amounts of P relative to other nutrients, so less P can be provided without reducing the amount of nitrogen or micronutrients provided. However, when using a complete fertilizer like these for your P source, the P is still tied to the fertilizer concentration.

The most flexible way to control P provision to containerized plants is to use a source of P independent from the primary fertilizer. First, select a fertilizer without P, such as 20-0-20 or 15-0-15. Then, use a separate fertilizer to supply P, such as monopotassium phosphate (MKP; 0-52-34). This can either be tank-mixed with the stock solution of your primary, P-free fertilizer or it can be injected separately through a second fertilizer injector.

By separating out P from the other macro- and micronutrients, you can limit the P provided to plants without potentially limiting other important nutrients.

At the N concentrations commonly used for finishing containerized crops, growth-saturating amounts of P are also being provided. By reducing P concentrations during finishing, growth can be controlled without P deficiency symptoms. There are several approaches to providing less P, and a potential solution for any producer should be among them.

The author (ccurrey@iastate.edu) is an Associate Professor in the Department of Horticulture at Iowa State University.