Why Your Garden Needs Phosphorus

Phosphorus is the energy element, essential to the chemistry of life. It is part of a molecule called ATP (adenosine tri-phosphate) that serves as the energy currency of plants. Energy from photosynthesis and respiration is temporarily stored in the high-energy phosphate bonds. When the phosphate bonds are broken, energy is released to activate a series of chemical reactions in plants.

If a plant doesn’t receive adequate phosphorus, its energy needs can’t be fully met. The plant’s new growth will be stunted, both at the roots and at the shoots, and as phosphorus deficiency worsens, the plant will eventually shut down and die. An adequate amount of phosphorus is required throughout the entire life cycle of a plant, and lack of available phosphorus is often the limiting factor for plant growth.

Phosphorus is not very mobile in soil. It is easily adsorbed or locked up with other minerals, making it unavailable to plants. Very little plant-available phosphorus is actually dissolved in the soil solution. In fact, adsorbed phosphates on soil particles are often hundreds to thousands of times greater than phosphates in the soil solution. As a plant takes up the phosphates from the soil solution, the adsorbed phosphates slowly take their place, but they are sometimes not fast enough to meet the energy needs of the plants. Plants must have a steady supply of phosphorus from seed to harvest.

Boost Phosphorus with Mycorrhizal Fungi

In nature, mycorrhizal fungi help plants take up phosphorus. The fungi penetrate root cells and send out hyphae (thread-like structures), seeking out water and precious phosphorus in the surrounding soil and organic matter. The fungi exude enzymes and organic acids to make the phosphorus soluble, then feed it to the roots of the plant.

In exchange, the plant exudes sugars to feed the mycorrhizal fungal. Usually, it’s a good trade. In fact, if there is a phosphorus deficiency in the soil, plants will exude signal molecules to attract mycorrhizal fungi. On the other hand, if there is plenty of water-soluble phosphorus, plants will exude enzymes to repel mycorrhizal fungi, treating them as a pathogen.

Organic gardeners using relatively insoluble forms of phosphorus such as bone meal and rock phosphate should consider inoculating roots with mycorrhizal fungi. But for even better results, adding phosphorus-solubilizing bacteria along with the mycorrhizae is a powerful combination.

The beneficial bacteria hitch a ride on the fungal strands and swim to places in the soil solution the fungi can’t reach. The bacteria then exude enzymes to release phosphorus from the surrounding soil and organic matter and make it more available to the mycorrhizae. Phosphorus-solubilizing bacteria feed the fungi, and the fungi feed the plants.

Water-soluble Forms of Phosphorus

One of the benefits of hydroponics is the availability of water-soluble phosphorus. Even so, the phosphates must be kept separate from calcium ions in concentrated form. That’s why hydroponic nutrients often come in two-part and three-part formulas. All of the calcium is in one bottle, and all of the phosphates are in the other bottle.

If the two were combined in concentrated form, the calcium would react with the phosphates to form calcium phosphate, which is 95% insoluble. Both the calcium and the phosphates would lock up, precipitate out of solution and become unavailable to the plants. But once diluted in enough water, the calcium and phosphates remain soluble in solution to be easily absorbed.

Mono-ammonium phosphate (MAP), a highly water-soluble form of phosphorus, is often included in starter fertilizers used by commercial growers. The additional phosphorus energizes the rooting process, and the ammonium is quickly assimilated through the developing roots to produce growth hormones and enzymes.

Adding a little extra phosphorus during the first 2-3 weeks of vegetative growth can have a dramatic effect on root strike and the establishment of plants. Lab tests have shown up to a 20% increase in root mass with nutrient formulas supplemented with a moderate increase of MAP during the early vegetative growth stage.

Phosphorus During Fruiting and Flowering

Another key stage for phosphorus application is during the fruiting and flowering stage. Most hydroponic bloom formulas provide phosphorus in the form of mono-potassium phosphate, providing adequate phosphorus throughout this phase. But sometimes plants need a boost. For example, during the transition from grow to bloom, a great deal of energy is diverted to flower production, and the plant may not be able to keep up with the extra energy demand. A little supplemental phosphorus during the early flowering stage can give a plant the energy boost it needs, promoting earlier flowering and more flowering sites.

During heavy fruit and flower production, plants continue to require higher levels of phosphorus to help provide energy for the developing fruit, but higher levels of potassium are also important for increased carbohydrate metabolism. That’s why there are many P-K boost formulas on the market.

Generally speaking, phosphorus and potassium are both important during the fruiting and flowering stage, but increased phosphorus is particularly beneficial during the early flowering stage, while increased potassium is particularly beneficial during heavy fruiting and flowering. If you want to fine-tune the nutritional needs of your plants, it’s best to spoon-feed phosphorus and potassium separately whenever possible.

Watch Your Temperatures

Phosphorus uptake is temperature-dependent. If your nutrient solution is too cold, plants won’t take up adequate amounts of phosphorus. Plant growth will stall, and the stems and undersides of the leaves may even start to turn purple. When using phosphorus fertilizers, carefully follow dosing directions and try to make sure the nutrient solution stays above 58°F. For best results, maintain the nutrient temperature between 68 and 75°F.

Don’t Overdo It

It is possible for plants to develop phosphorus toxicities. If excess phosphorus starts to build up in the reservoir, plants may take up too much all at once. There are no direct symptoms of phosphorus toxicity—it shows up first as a zinc deficiency, then as an iron or magnesium deficiency. One reason phosphorus levels become too high is the addition of phosphoric acid to the solution. Many hydroponic growers use phosphoric acid to lower the pH of their nutrient solution, but since phosphoric acid is actually a phosphorus fertilizer, it can quickly build up to toxic levels if too much is used. Moderation is the key.

Phosphorus additives are beneficial when they are used in the correct amounts at the correct times. Learn to spoon-feed your crops to give them exactly what they need when they need it. A little extra phosphorus can energize the rooting process and stimulate the flowering process, but too much at the wrong time can have adverse effects. Manage your phosphorus fertilizers wisely and your plants will reward you with heavy yields of vibrant flowers and tasty fruits.

By Harley Smith