If you are shopping for fertilizer, you will see three numbers on the bag, N-P-K, which stands for Nitrogen (N) – Phosphorus (P) – Potassium (K). What is the secret recipe for the right fertilizer combination for your lawn or garden this year? There is no way to know unless you take a soil sample and order a soil test.
But let’s chat specifically about one number on the bag that gets most of the attention and is heavily regulated….PHOSPHORUS. Your phosphorus purchase should not be made by guessing. Why? Because too little and you compromise your plant and too much and you burn your plant while harming the environment. What is this nutrient phosphorus and why does it have such polarizing outcomes?
Phosphorus (P) is responsible for the plant’s cell formation, genetic reproduction, root health, bloom, and it transforms solar energy into chemical energy. A proper amount is essential for optimal health. Once you have received your soil test report customized for your crop, you can take your results to any lawn and garden center and match the needs of your soil and crop to the corresponding attributes in fertilizer products. But what does it mean when your soil is too low or too high in phosphorus.
If you are deficient in phosphorus, the health of your plant is compromised. Your plants require phosphorus to survive and depriving plants of this nutrient is harmful. If you have not diagnosed this from a soil test, there can also be visual signs to look for. You will see stunted growth and weak appearance. Even the leaves can turn dark green or purplish.
In addition, depending on where you live, you may have regulatory restrictions regarding when you can and cannot apply a phosphorus-based product. Please always consult your local extension office regarding the regulations. Most phosphorus regulations apply to warmer, high-heat temperatures, areas that are naturally high in phosphorus, or areas near sensitive watersheds. Your solution? Only apply the amount of phosphorus that your soil test recommends whether that’s an organic fertilizer like bone meal or manure, or a synthetic version. Either way, the guaranteed analysis on the bag will indicate the amount of phosphorus you will be applying.
If your soil is too high in phosphorus, the health of your plant is compromised, but the environment around you is also susceptible to residual affects. Unlike other plant nutrients, phosphorus is more stable and can build up in soil. Too much phosphorus and the plant begins to burn and it causes other deficiencies such as Iron(Fe) and Zinc(Zn) simply because they are trapped and unusable. T.L. Provin and J.L. Pitt said it best when they wrote an article for Texas A&M AgriLife Extension called Phosphorus – Too Much and the Plants May Suffer:
Phosphorus can become water-soluble and mobile, entering surface waters and causing algae and other undesirable plants to grow. This reduces water quality and desirable fish and aquatic plants.
How do high phosphorus conditions happen? Most of the time, it is from homeowners over-applying phosphorus based fertilizers and organic manures. The notion that organic fertilizers or the “higher the number on the bag,” the better is just not the case. I say this all the time, too much of a good thing, is a bad thing. Your solution to high phosphorus? Avoid the use of phosphorus based products altogether until your tests show a need for it. And for the other deficiencies it causes such as Iron (Fe) and Zinc (Zn), you will need to add those nutrients in a foliar form to by-pass the trapped conditions caused by phosphorus. Sadly, it is nearly impossible to find an organic product without phosphorus (a zero in the middle of the N-P-K analysis) due to the nature of its source.
If you do find that you are too low or too high in phosphorus, I recommend that you do an annual soil test to make sure you are addressing the levels in a way that is healthy for the plant but also the best practice for environmental stewardship.