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.
Source: ndsu.edu
Photo Credit: pexels-olga-divnaya
Categories: North Dakota, Rural Lifestyle