There are many questions and confusion surrounding products made with phosphorus. Phosphites are labeled and sold as both fungicides and fertilizers. If we’re to maximize our success with our crops there needs to be a clear understanding of just what the nutrients we provide do for our plants and which one we have selected.

Most containers for plant nutrient will list on their label an N-P-K rating. This refers to the per cent of nitrogen, phosphorus, and potassium in that product. I consider these macronutrients the meat and potatoes of a plant’s meal. Micronutrients, on the other hand, could be considered the vegetables or vitamins of the meal.

Nitrogen (N) is primarily used by plants for the growth of foliage while potassium (K) helps plants keep growing fast, producing stronger stems and branches, and as an aid for environmental stress. This article is about the one in the middle—phosphorus (P).

To aid in understanding various chemical terms associated with phosphorus, it would be useful to define a few of its forms first. In an attempt to simplify terminology, most manufacturers report and label their phosphate as P2O5, yet this chemical symbol and formula is technically known as diphosphorus pentoxide.

Another name for this compound is phosphoric anhydride because its reaction with water will produce phosphoric acid. By itself, diphosphorus pentoxide is toxic and considerable care is needed when handling, yet after reacting with water, it becomes phosphoric acid which is the form available for plant uptake.

Confusion between various forms of phosphorus

There is a big difference between phosphoric acid (H3PO4) and phosphorous acid (H3PO3). PO3 will oxidize eventually or be converted by microorganisms to become PO4. The point is that phosphoric acid is often used to make fertilizers in the form of a phosphate (a salt of phosphoric acid). However, phosphorous acid (often referred to as phosphonic acid) is used primarily to make fungicides.

The most significant use of phosphorous acid is the production of phosphites, used in water treatment, and phosphonates, such as glyphosate, which is a weed or plant killer. Phosphite salts, such as potassium phosphate, are fungicides. The similarity between these chemical names often cause significant confusion. The confusion is substantial in that phosphonic acid not only moves through the soil very slowly, but lacks evidence that it can be used as a fertilizer.

Organic phosphorous must be converted to phosphoric acid to be available to plants. This is typically done through soil bacteria (when not done during the manufacturing process). Because the natural conversion is slow, there is less chance of damaging the plant through accidental overfeeding.

Organic phosphate fertilizers do not necessarily come from bone sources, as they also come from mineral sources like rock dust or colloidal phosphate. Bone sources such as steamed bone meal or fish bone meal are popular organic forms, while mineral phosphorus sources are cheaper and last longer in the soil.

Benefits of Phosphorus

Phosphorus is essential for all living organisms. Growers use phosphoric acid products to maximize many aspects of plant growth. From the production of healthy root systems, flowers, and seed-to-fruit development, phosphorus is a key nutrient used to give the grower increased production and quality. Plants need phosphorus for normal growth and maturity.

The main use of phosphorus in plants is for the formation of adenosine triphosphate, the energy storing molecule of the plant. It also plays a role in photosynthesis, cell division, respiration, energy transfer, and storage.

Because it is important in cell division and development of new tissue, it contributes substantially to cell enlargement and growth. It also helps a plant convert other nutrients into usable building blocks with which to grow.

Phosphorus Deficiency

In plants, phosphorus is considered second only to nitrogen as the most essential nutrient to ensure health and function. Genetic processes such as cell division and plant growth are impaired when phosphorus is present in inadequate levels.

Plants deficient in phosphorus will likely mature at a slower rate than plants with adequate amounts. Stunted growth, along with smaller leaf size and fewer leaves, are often induced by a phosphorus deficiency. This deficiency can create an imbalance in the storage of carbohydrates.

Photosynthesis, the main function of plant cells, usually remains at a normal rate under this deficiency, but phosphorus use within the cell usually slows. This imbalance of rates in phosphorus-deficient plants leads to the buildup of excess carbohydrate within the plant. This buildup is often observed as the darkening (usually purplish) of leaves.

How Does Phosphorus Work?

Plant cells usually uptake nutrients at much higher concentrations than are present in the surrounding soil. Movement of nutrients within the plant depends largely upon transport through cell membranes, which require energy to oppose the forces of osmosis. Phosphorous compounds provide this energy.

Phosphorus enters the plant through root hairs and the outermost layers of root cells. Phosphates move quickly through plants and animals; however, the processes that move them through the soil are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.

Sources of Phosphorus

Organic nutrients are increasing in popularity in the same ratio as most of the other practices connected with organic growing. Phosphorus can be acquired from inorganic but natural sources such as rock phosphorus, or organic sources like bone meal. Soil pH needs to be below 7.0 for the phosphorus in bone meal to be available to plant roots.

Many organic nutrients, such as bat guano and fish or chicken fertilizer, are high in phosphorus and have more rapid uptake profiles. Sea minerals and liquid seaweed are other organic sources of readily available phosphorus.

See: Soup Up Your Soil with Molasses, Guano, and Kelp

There are two primary categories for phosphorus availability: water soluble and citrate soluble. (You can find additional information on how phosphorus works, available sources, and how to improve plant uptake by following the link at the end of this article.)

Organic phosphorous fertilizers have been used for a very long time as the primary source for crops. Even with the advent of new phosphorous fertilizer technology, organic sources from animal manures—including composts—are still very important.

From a fertilizer nutrient perspective, the primary difference is the availability of phosphorus from the fertilizer. As with most of the fertilizer products, especially those with varying analysis, additional chemical analysis should be done to determine an availability coefficient as a portion of the reported total phosphorus.

See: Organic Nutrients for a Sustainable Tomorrow

Phosphorus contained in organic sources is a combination of inorganic and organic. Essentially, everything inorganic is in the orthophosphate form. The combination of the organic and inorganic phosphorous ratios in various organic sources and the soil environment will affect the availability for organic phosphorus.

Most animal manure research interpretations indicate that approximately 60 to 80 per cent of the total phosphorus is available to crops in the first year. Due to the chemical composition of other organic sources, such as bone meal, lesser amounts of plant available phosphorus compared to total are expected.

An important consideration in nutrient safety is the quality and control of the manufacturer’s process for producing the phosphorus in the fertilizer. These are vital issues controlled by proper composting, especially in animal based fertilizers.

Do you really need more phosphorus?

As with most fertilizer regimens, providing the optimum level of every nutrient is the goal. An excess can be as damaging to a crop as a deficiency.

Know what the phosphorus level in your soil is before making a substantial increase in the level. As noted earlier in this article, total phosphorus is the combination of water-soluble and citrate (or mild acid) available. A more accurate total might be reflected if your test included at least water and mild acid solutions to extract the phosphorous.

Providing the right amount of various nutrients to your crop is essential to maximize your yield and the organoleptic quality of the food you grow. Understanding some of the major contributions that the various nutrients provide to your plants is the first key towards success, then knowing when to add them and just how much will be next.

Gardening is always joyful to those of us that take these challenges gladly, and making the effort to learn, note, and compare just what type and level of nutrients our plants are getting will take us through to better crops.

For additional tips on better understanding the many forms of phosphorus and to view the references for this article, click here.