3/24/2014 - By Matthew Shultz

The second letter on the bag of NPK = PHOSPHORUS. Phosphorus is a widely misunderstood “macro or major” nutrient. For those of you who fell asleep when going through the Kreb cycle in organic chemistry, it is involved in energy production, as in ATP’s. And it is also a constituent of some amino acids. But the major role Phosphorus plays is in root production. More than 80% of the P a plant takes up is for rooting.

It has also been said, that 80% of the Phosphorus a tree takes up is needed in its first 2 years, while establishing a root system. Hence it being the higher number in a soluble “starter fertilizer”, by starter we mean getting their roots started. Phosphoric acid is often the source of P in these formulations. When a grower is stepping up a liner to a 1 gallon or a 1 to a 3 gallon, coming in with a drench of a fungicide such as Subdue plus a high middle soluble Phosphorus is a good call. Not only does it have great margins, but it will sequester disease and promote roots.

Phosphorus and Magnesium tend to precipitate out when in solution together. That is why a high middle soluble or even a 20-20-20 have little Magnesium in it. While on the other hand, if you see a 6-0-6 stock mix, rest assured it has Magnesium . Growers can tank mix 20-20-20 with Epsom Salts as a spray or drench however;  they just need to put it out immediately after making the tank.

Phosphorus tends to not leach like Nitrogen or Potassium. You will often see a lower middle number in granular fertilizers as the levels will build up through time. When the correct threshold of Phosphorus is reached in field or containerized soils, it can be backed off upon. On the other hand in tree fields, if P levels are too low, roots will go searching for it. So for a dense fibrous rootball, proper Phosphorus is needed. Before a tree field is planted, often triple super phosphate will be tilled in like dolomite before planting.

When reading a soil report, you will often see a weak bray P and a strong bray P. A strong bray reading typically is what is the total soil Phosphorus reading. The weak bray is actual available P. Being a cation, such as a phosphate, Aluminum and Magnesium play a role in Phosphorus availability. One last tidbit: lower grades of Phosphorus such as Triple Super P often have Fluoride as a byproduct. So Fluoride sensitive plants like Dracaena ‘Janet Craig’ or ‘Warneckii’ could get tip burn when using a low food grade P. So recommend a plus product on these as they use premium grade Phosphorus.

Thanks to “big sugar” and other large agronomic industries, Phosphorus runoff is getting a bad rap. Who needs the Keys and the Everglades anyways! Some nursery wells are already being checked. Most wells tested for Phosphorus levels from ornamental nurseries tend to be in a decent range. Expect to hear more on this along with Nitrate monitoring.

By Matt Shultz, Horticultural Tech Support Specialist and Sales Representative, Harrell’s LLC and Marshall Horsman, Technical Services Manager, Harrell’s LLC.