Is too much phosphorus a bad thing?

Legume crops, fruit trees, and lettuce need high levels of phosphorus and often benefit from phosphorus-rich fertilizers. By contrast most houseplants do not need high phosphorus fertilizer unless the soil is already deficient. https://pixabay.com/en/legumes-power-chickpeas-jewish-665788/

Phosphorus is an essential mineral nutrient required for vital life functions in plants.

Phosphorus makes up the molecules that allow the plant to turn sunlight into chemical energy for all its life functions, typically making up between 0.1 - 1 % of its tissue. The phosphates (type of phosphorus) your plant can absorb are usually either H₂PO₄^- or  HPO₄^2-.

These phosphates produce anhydrides and esters which are involved in energy storage and energy transfer in the plant. The most important of these substances are ADP and ATP. Without ADP and ATP, the plant would not be able to photosynthesize. 

Phosphate also provides the building blocks for plant components like phospholipids (in the membrane of plants cells), nucleic acids (that carry genetic material between cells), nucleotides, coenzymes, and phosphoproteins.  

As with anything, however, you can have too much of a good thing. Excess phosphorus will kill your plant. 

In the wild, excess phosphorus is very rare. The element is rarely present in the soils at the required levels. Usually, it is human intervention in the soil that causes it whether in the form of excess phosphate fertilizer run-off from agriculture or from adding too much fertilizer directly. Excessive phosphorus especially becomes a risk with indoor gardening, where small soil volume plus the need to supply nutrients for optimal growth makes it easy to add too much.

The main effect of excess phosphorus is that the plant is unable to take up other essential nutrients. This will first affect zinc and copper but it will also interfere with the normal uptake of calcium.

How much phosphorus is too much?

Different species require different levels of phosphorus for peak health. They also have varying thresholds for phosphorus toxicity. Generally however, most species will suffer if they have phosphorus levels in their leaf tissue of over 1%.

Plants have some capacity to deal with an excess of phosphorus. Excess phosphates are stored in the vacuole. From there it is supplied to the cytoplasm. If this excess reaches critical levels however, the plant will be damaged. Excess phosphates accumulate in older leaves and cause toxicity. This encourages more nitrogen to be taken up, which delays the formation of reproductive organs - the flowers. For fruit plants, harvests will be reduced in size and quality. 

What are the signs of phosphorus toxicity in plants? 

The main sign of phosphorus toxicity in plants will be leaf discoloration. The leaf between the veins will turn yellow or dark.

Most studies of phosphorus toxicity have been conducted on economically valuable crop plants, however, these symptoms will also apply to houseplants.

In cucumber, leaf discoloration tends to happen when phosphorus made up 1.50% of leaves. The upper level for phosphorus in tomato leaves is 0.60%.In soybeans, you get a reddish-brown color on the margins of leaves, a reddening on leaf veins and between the leaf veins, followed by darkening.

Plants that have excess phosphorus will also display symptoms of calcium deficiency. This is because too much phosphorus interferes with the normal uptake of calcium. Symptoms are the browning and dying off of new growth at the tips of leaves and roots, disease susceptibility, and poor quality fruit and seed production.

These symptoms of phosphorus toxicity are very similar to symptoms for other nutrient toxicities. You will need to make a judgment on whether your plant is suffering from phosphorus or some other mineral toxicity based on the nutrients that you have been supplying your soil.

Excess phosphorus is not just damaging for plants but for the environment as a whole. This lake algae is a result of excess phosphorus and other nutrients - run off from horticultural activities. MPCA Photos https://www.flickr.com/photos/mpcaphotos/5269202787 

What are the causes of phosphorus toxicity in plants? 

Phosphorus toxicity in houseplants tends to affect houseplants that produce flowers. Many gardeners will try to encourage blooms by adding too much phosphorus for flowering plants like azaleas, roses, or rhododendrons. Additional phosphorus is only needed if the soil is already deficient in this element. Yet it is often the case that soils contain enough phosphorus already. Unlike nitrogen, phosphates in soil are highly immobile. Phosphates are much less likely to leach out or be taken up and used by plants so you often don't need to be adding high phosphorus fertilizers to it.

Brand new store-bought plants are also at risk of over-fertilization because the soil might already contain slow release fertilizer that will last a few weeks or months. 

How do I make sure I don't give too much phosphorus to my plants? 

Unless you have a laboratory to hand, it is difficult to say precisely how much your particular houseplant species needs. This is why you must always err on the side of caution when applying fertilizers. As a rule, unless you know that your soil is deficient in phosphorus, don't use a high phosphorus fertilizer. 

For small plants, use fertilizers with a low NPK ratio. The NPK ratio is found on all bottles of fertilizer. It indicates the percentage of nitrogen, phosphorus, and potassium inside the fertilizer solution. A balanced 5:5:5 fertilizer that contains 5% nitrogen, 5% phosphorus, and 5% potassium is suitable for smaller plants.

A fertilizer with a higher NPK ratio of 15:15:15 would be better for larger plants since it contains more of the three macronutrients. Always read the manufacturer’s instructions for how much fertilizer you should add to your plant’s water. 

Remember only to fertilize regularly just before and during the growth and/or flowering period for your plants. In the non-growing months, keep fertilizing to a bare minimum. 

Organic fertilizers, either bought or made at home, can reduce the risk of over-fertilizing with phosphorus. Organic ingredients tend to have lower concentrations of the mineral than synthetic versions. See the Southside Plants recipe for organic, homemade phosphorus fertilizer.

For your most prized plants, perform a soil nutrient test before applying fertilizer  By testing how much of each nutrient is present in the soil, you can determine exactly how much fertilizer and what kind of fertilizer you should be applying. 

Here are some of our favorite phosphorus fertilizers (as an Amazon associate we earn from qualifying purchases)