The importance of phosphorus in horticulture has changed throughout the years. Before the use of soilless media, field soil was used to grow potted plants. Phosphorus application rates were high because phosphorus becomes immobile in soil as it easily binds to the cation exchange sites on soil particles and forms insoluble compounds with aluminum, calcium, magnesium and iron. 

When soilless growing media were developed, this thinking came forward, but soilless media have a lower cation exchange capacity and very low levels of aluminum, calcium, magnesium and iron. Thus, for crops grown in soilless media, most of the applied phosphorus is available for plant uptake or is leached during irrigation.

Undesirable Effects

One benefit associated with an excessive application of phosphorus is increased flowering in plants. There is evidence to support this claim and therefore several fertilizers, including 5-40-17, 9-45-15, 10-30-20, etc., are available to help promote extra flowering. However, more recent research shows that excessive phosphorus applications (greater than 40 ppm P) cause undesirable stretching in plant stems.

Fertilizers with an equal or higher ratio of phosphorus (in the form of P2O5) to nitrogen induce stretching (e.g. 15-15-15, 20-20-20, etc.). Plants become too tall and spindly, leading to increased breakage and undesirable appearance. 

Too Little Phosphorus? 

Many growers have switched to lower phosphorus fertilizers such as 13-2-13, 17-5-17, 18-3-18, 20-3-19, etc., and some also use no phosphorus fertilizers such as 14-0-14 or 15-0-15. This will help reduce stretching and create more compact plants. However, phosphorus is still a required nutrient for plants. It is possible to reduce phosphorus application rates to a point that phosphorus deficiency can occur and plant health is compromised.

Figure 1 shows the effect of the absence of phosphorus on the growth of a tomato plant (right) compared to another tomato plant with normal levels of phosphorus (left)."

Figure 2 shows almost the same effect. The flat of pansies on the right did not have phosphorus incorporated into the starter fertilizer charge whereas the flat on the left did."

Both Figures 1 and 2 show that if a plant does not receive any phosphorus, it will restrict plant growth, but to the point that plants do not grow. The pansies without phosphorus seen in Figure 2 are much further behind schedule and of inferior quality to those with normal levels of phosphorus. The bottom line is that a low to no phosphorus fertilizer program can be detrimental.

Impact of phosphorus on tomato growth
Figure 1. Notice the impact of phosphorus on tomato growth. The plant on the left is grown with normal levels of phosphorus, while the one on the right is shorter and smaller due to phosphorus deficiency. Source:, Jim Richards, UC Davis.
Pansies poor fertilization
Figure 2. The pansy tray on the left has phosphorus included in the starter fertilizer charge while the flat on the right does not. No additional phosphorus was applied. The pansies without phosphorus are so stunted and behind schedule that they will not be saleable. Source: Premier Tech

Monitoring Phosphorus Levels

Phosphorus levels in the growing medium should be 5-20 ppm P (12-47 ppm P2O5). To know how much phosphorus is provided by low-phosphorus, water-soluble fertilizers, see Table 1. Many low-phosphorus fertilizers provide sufficient phosphorus when applied at rates of at least 100 ppm nitrogen. However, if fertilizer application rates are lower or if fertilizer is applied infrequently, phosphorus deficiency could occur in a crop.

Table 1. Phosphorus (P) provided by low-phosphorus fertilizers based on the nitrogen (N) application rate.
Fertilizer ppm Phosphorus (P) applied* per nitrogen application rate
50 ppm N 100 ppm N 200 ppm N 300 ppm N
13-2-13 3.4 ppm 6.7 ppm 13.4 ppm 20.2 ppm
15-0-15 0 ppm 0 ppm 0 ppm 0 ppm
17-5-17 6.4 ppm 12.9 ppm 25.7 ppm 38.6 ppm
18-3-18 3.6 ppm 7.3 ppm 14.6 ppm 21.8 ppm
20-3-19 3.3 ppm 6.6 ppm 13.1 ppm 19.7 ppm
20-10-20 10.9 ppm 21.9 ppm 43.7 ppm 65.6 ppm

Fertilizing with a low phosphorus fertilizer has advantages, but it is possible to go too far and provide insufficient levels of phosphorus, which stops plant growth. When using low-phosphorus fertilizers, it is best to monitor the phosphorus levels in the growing medium and plant tissue to minimize deficiencies.