It’s planting time and most fertilizer manufacturers and nurseries are pushing their high phosphorus products, usually under names like Plant Starter, Root Booster and the one I really like “MegaMass”. These fertilizers claim to “supply the high phosphorus needed for rapid root development”.
Who doesn’t want good roots on their newly planted babies? Are roots not the key to great plants?
Let’s have a look at this extremely common myth.
Why Do People Sell High Phosphorus Fertilizer?
Some poorly designed studies in the 1940s and 1950s found that phosphorus stimulated root growth. At the time, most plants were propagated in soil which absorbs excess phosphorus and makes it unavailable to plants. Adding higher amounts did overcome this problem in a limited way. More studies followed and it is now well understood that this is a very short term bandage.
Fast forward to today, when almost all propagation is done in a soilless mix and the problem of absorbed phosphorus is no longer an issue. With today’s horticultural practices, there is no advantage to using high phosphorus fertilizers – they do not produce larger root systems.
Even when these plants are transplanted into real soil, an excess amount of phosphorus does not stimulate root growth because it is quickly immobilized (no longer plant available) by soil.
After those early studies, fertilizer manufacturers started making high phosphorus fertilizers. NPK values of 10-52-10 became popular and they are still being promoted today as plant starters and root boosters. There is no science to support the use of these products.
What is the Ideal NPK?
Numerous studies have shown that an NPK ratio of 3-1-2 or 3-1-3 works well for most plants.
As an example, this study looked at the growth of Lantana and found a “high” phosphorus level did not produce better plants. What did they consider high? High for them was 3/4 of the nitrogen level. A 10-52-10 fertilizer has 5 times the nitrogen level. They would consider that toxic!
Michigan State University developed the “perfect” fertilizer for a wide variety of plants and their ratio is 5-1-5.
A study called “Do Plants Require Nutrients in Similar Proportions“, looked at a large number of plants and studies and concluded that a good average NPK ratio is 3-1-2.5.
No Support for a Balanced Fertilizer
Many people recommend using a balanced fertilizer, like 10-10-10. That is also promoted by fertilizer companies with no scientific support.
Anyone who makes this recommendation is just following marketing nonsense.
“Balanced Fertilizers (e.g. 10-10-10, 15-15-15, 20-20-20, etc.) induce stretching in plants. Plants become too tall and spindly, leading to increased breakage and undesirable appearance. ”
Reports suggest that global phosphorus reserves are dwindling and that “the global reserve will be depleted within the next 50 to 100 years.” I don’t know how true that is but what is clear is that excess phosphorus does find its way into streams and lakes where it causes pollution and algae growth.
Spreading excess phosphorus on soil is harming the environment. So don’t do it.
High Phosphorus Inhibits Mycorrhizal Fungi
Mycorrhizal fungi can be important for plants. It is well known that high phosphorus levels inhibit their association with plant roots.
Bloom boosters are also popular and these also contain high phosphorus levels. To be honest a bloom booster is just a plant starter that is sold under a different label.
Which Fertilizer Should You Use?
For seedlings and potted plants, in a soilless mix, use a fertilizer with a ratio of 3-1-2.
When you plant in real soil you normally do not need to add fertilizer. If you add compost and manure you definitely don’t need fertilizer, unless a soil test tells you differently.
For vegetable gardens you might add some extra nitrogen, but no phosphorus.
To understand the difference between a 3-1-2 and a 6-2-4 fertilizer, have a look at Fertilizer NPK Ratios – What do they Really Mean?