Dakota Gardener: The myth of high phosphorus fertilizers for more flowers

In my youth, I worked for a well-known Minnesota garden center. My co-workers and I would be frequently asked why a customer’s dahlia, geranium or other prized ornamental plant wasn’t blooming. Without fail, we would sell the customer a high phosphorus fertilizer to stimulate flower production.

In my current position as a North Dakota State University Extension horticulture specialist, my job is to give objective, evidence-based recommendations to help home gardeners. Thankfully, I am not expected to promote product sales but rather to debunk erroneous and wasteful consumer myths such as the use of high phosphorus levels to promote more blooms.

To back up a little, plants do need nitrogen (N), phosphorus (P) and potassium (K) for growth and development. If you read a fertilizer bag, you will see the ratio listed in order of N-P-K. High phosphorus fertilizers have a high middle number compared to nitrogen which is the first number. For high phosphorus fertilizers, the ratio can be one-part nitrogen to as high as five-parts phosphorus (expressed as phosphates).

After my garden center stint, I pursued a couple of advanced plant science degrees. In my floriculture classes, I was stunned to discover that commercial greenhouses apply fertilizers that have low phosphorus ratios in comparison to nitrogen. In other words, greenhouses may apply fertilizers with a nitrogen to phosphorus ratio of 3-to-1 or even 5-to-1 depending upon the crop.

The preferred ratio is reversed because studies show that high phosphorus rates do not increase the number of flowers. Even a balanced fertilizer like 10-10-10 with equal parts of nitrogen and phosphorus is considered a waste of phosphorus.

Why is there such a disparity between consumer and commercial products? High phosphorus recommendations came out of older research to ensure productivity of high intensity farm crops rather than home landscapes. Unfortunately, fertilizer companies do not have an incentive to dispel this myth.

In northern states, the soil in most gardens contains sufficient levels of phosphorus. Unless a soil test shows otherwise, there is no need to add high phosphorus fertilizers.

The buildup of high phosphorus levels in home landscapes can be detrimental in several ways. Excess phosphorus in the soil can leach into lakes and rivers thereby promoting algae growth. Minnesota and other states banned the use of phosphorus in turfgrass fertilizers in established lawns absent a soil test documenting a deficiency to protect waterways.

I frequently see soil tests for ornamental and vegetable gardens that have an excess of phosphorus. These high levels are detrimental because a surplus ties up important plant micronutrients such as iron and zinc. Symptoms can include stunted plants, yellowing leaves, and an overall failure of the garden to thrive.

Unfortunately, phosphorus levels fall very slowly within a garden. It may take years for excessive phosphorus levels to decrease enough for the garden to recover.

In addition, we have to consider that phosphorus is a finite resource. Already, the cost of mined phosphate rock has drastically increased while the quality is decreasing. To preserve this scarce natural resource, we need to avoid waste and only apply when necessary.