Phosphorus

Phosphorus is an essential plant nutrient, second only to nitrogen in plant health. In fact, phosphorus is found within every living cell on earth. It is part of our DNA and every cell wall.

Phosphorus helps plants use and store energy. It is also used make oils, sugars, and starches, within each plant. Most important to the home gardener, phosphorus supports flower and root growth. Much the way nitrogen supports vegetative growth, and potassium supports crop quality and size, phosphorus is the reproduction nutrient, the in-between stage between growing and fruiting. It supports root, flower, seed, and fruit growth. Phosphorus is also part of photosynthesis.

Sources of phosphorus

Despite being so important, phosphorus is rarely found in a form plants can use. This is because phosphorus is highly reactive. When elemental phosphorus is exposed to oxygen, it actually glows! [This is where we get the word ‘phosphorescence’!] Mostly, phosphorus exists as an acid- or salt-version of its former self, called phosphates. Scientists recently discovered that phosphorus is created when a star goes supernova. Here on Earth, organic sources of phosphorus include animal manure, urine, guano, compost. blood meal, and bone meal. Most of the phosphorus found in bags of fertilizer is mined in China, Russia, and the midwest, with 50% of the world’s supply found within the borders of Arab nations. [Perhaps phosphorus will become the new petrol...] Phosphorus is commonly applied around seeds at planting time in a process called banding.

Worldwide, the demand for phosphorus is growing twice the rate as the human population, mostly for agricultural use as fertilizers and pesticides. Phosphates are also used as nerve agents and in detergents. [I prefer soap nuts.] Experts predict a phosphorus shortage by 2040, and a complete end of mineable phosphorus in 345 years. Other experts claim this will happen much sooner. Yet another reason for not applying fertilizers your soil does not need.

So, before we run out, how do you know if your plants have enough (or too much) phosphorus?

Nutrient mobility

Phosphorus is a mobile nutrient, which means that it can be moved around, within a plant, after it has been absorbed. Other nutrients, such as iron and calcium, are described as ‘immobile’ because they generally stay where they were first dropped off by the vascular system. The reason this matters is that it helps in identifying deficiencies and toxicities. Mobile nutrient deficiencies are normally seen in older leaves first. This is because mobile nutrients are pulled out of older leaves to provide support for new leaves. With immobile nutrients, the opposite is true. Older leaves have already gotten their nutrients and hold them in place. The new leaves do not have access and so exhibit deficiency symptoms.

Phosphorus deficiency and toxicity

Phosphorus deficiency is practically unheard of in California home gardens. The optimal range, in parts per million, is 4 to 14. My soil test results reported a value of 84.3 - nearly 10 times what my plants need! The problem wasn’t phosphorus deficiency, but accessibility. Without enough iron in the soil, my plants could not access all that phosphorus (and several other important nutrients). In addition to being rare, phosphorus deficiency can be difficult to identify. In the early stages of growth, a deficiency may appear as nothing more than sluggish growth or mild stunting.

​Since phosphorus is an important part of genetic information transfer, deficiencies ultimately result in smaller and fewer leaves, and fruit set failure. This deficiency also causes a procedural imbalance between photosynthesis (carbohydrate production) and carbohydrate storage. This imbalance leads to too many carbs in the leaves, which makes them darker and more purple or red than normal, especially on the underside, with a shiny almost metallic appearance on the top surface. These symptoms cannot be relied upon as a diagnostic tool, because the same symptoms may indicate several other conditions. Soil testing and plant material testing are the only way to know for sure.

The leaves of phosphorus deficient corn turn reddish purple (عمرو بن كلثوم) CC BY-SA 2.5

Too much phosphorus can interfere with a plant’s ability to absorb copper and zinc, but this condition is extremely rare in garden environments. It can be seen in containerized plants, or those being grown hydroponically. Zinc and copper deficiencies appear as chlorosis, twig dieback, and bronzing.

Testing for phosphorus

Soil tests are invaluable in learning about what is in your soil. The reason for using a local reputable lab lies mostly in the tests for phosphorus. There are two tests generally used when calculating phosphorus levels: the Bray P1 test and the Olsen sodium bicarbonate test. Their effectiveness lies in soil pH. The land west of the Rockies tends to have alkaline soil, which is better suited to the Olsen test. More acidic midwest and eastern seaboard soils give a more accurate reading when the Bray test is used. If you send your soil samples to the other side of the country, your results may be less accurate (and less useful).

So, before you add any more phosphorus to your soil, take the time to find out if it is actually needed.

Finally, did you know that the rough surface used to strike a match is made with glue, ground up glass, and phosphorus? Now you know.