Reduce phosphorous during finishing to control growth

Departments - Production Pointers

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

January 23, 2020

Photo courtesy of Christopher J. Currey

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.

P2O5 versus P

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.

Reducing phosphorous during finishing

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.

How to provide less phosphorous

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.

Take-home message

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 ( is an Associate Professor in the Department of Horticulture at Iowa State University.