When I ran a hydroponics retail store, customers would come to me with all their questions and concerns. One topic that came up constantly was about fertilizers and nutrients. Novice and experienced gardeners alike are always interested in what nutrients do what, when they are needed in higher quantities or should be scaled back, and which organic options can provide the desired nutrients to their plants in the correct amounts.

In this article, I will focus on phosphorus, a nutrient all living creatures require for life. This overview should hopefully answer any lingering questions about what phosphorus does for your plants and provide the insight and understanding to take control of your garden and make smart, informed decisions when buying and using fertilizers.

What Is Phosphorus?

Phosphorus (P) is a primary or macronutrient, meaning plants need it in large quantities to grow and thrive. It is an essential component of DNA and necessary in the process of photosynthesis. It provides the energy transfer within plant cells. Phosphorus in plant fertilizers comes in the form of phosphoric acid or phosphates.

Phosphorus is listed on all plant fertilizer labels in the N-P-K section as a guaranteed analysis. It is a mobile element that can translocate throughout a plant. Phosphorus is associated with seed, flower and fruit production as well as root growth.

The highest concentration of phosphorus can be found in the root tips. Most plants have a symbiotic relationship with mycorrhizal fungi—fungi that live in the soil and root zone and can help plants absorb more phosphorus than the plant could on its own in its limited root space. Myccorhizal fungi concentrate phosphorus and other nutrients in the root zone and in exchange plants provide the fungi with starches, sugars and amino acids.

When To Use Phosphorus

Phosphorus is largely associated with the flowering phase of plant growth. Fast-growing annuals require large amounts of phosphorus for producing large flowers and fruit. This is why flowering fertilizers and bloom booster supplements have a high relative content of phosphorus.

These supplements should be introduced to your plant’s feeding regimen once flowering begins outdoors or two weeks after turning your indoor lights to 12 and 12 to induce flowering. It can be discontinued one week prior to harvest to prevent the buildup of salts in your plants.

Phosphorus is also required by seeds to germinate and for clones to root, as it is an essential element for plants to grow roots. Many plant nutrient supplements for this stage of plant development also have a high relative content of phosphorus.

Phosphorus Deficiency in the Garden

Plants require large amounts of phosphorus to grow and thrive. Without enough phosphorus, plants will develop a phosphorus deficiency and plant growth will become stunted. Leaves will be small, blue-green and are sometimes covered in blotches. Stems and veins may turn purple, beginning with the bottom older leaves. Leaf tips will turn dark and curl downwards, also beginning with the lower older leaves. Flowering plants will have few buds or flowers. Flowering and fruit set will be delayed with small flowers and fruits. The overall yield will be drastically reduced.

To treat a phosphorus deficiency, first adjust the pH levels of your growing medium and/or reservoir to 5.5 to 6.2. When soil becomes too acidic, it allows for a buildup of iron and zinc that will prevent the plant from absorbing the available phosphorus. Once the pH is adjusted, continue to water your plants with a balanced fertilizer. If the phosphorus deficiency continues, a phosphorus supplement such as bone meal may be added to boost phosphorus levels.

Phosphorus Toxicity in the Garden

Phosphorus toxicities are extremely rare as fast-growing plants consume a lot of this nutrient. Phosphorus toxicity interferes with a plant’s ability to absorb many micronutrients and trace elements, so the symptoms of phosphorus toxicity manifest as deficiencies of zinc, magnesium, iron, calcium and copper.

To treat plants affected by phosphorus toxicity, you must stabilize the pH of the growing medium and reservoir. Then flush the medium with a clearing solution or mild nutrient solution. This should remove the excess salts from your growing medium and leech excess salts out of the plant itself. Once this is done you can begin to water your plants with a complete, high-quality fertilizer.

Organic Sources of Phosphorus for Your Plants

Bat Guano : Bat guano is a great source of soluble organic phosphorus that is readily available for your plants. Bat guano in its natural form can have a strong odor, but there are liquid versions that are relatively odorless.

Bone Meal and Fish Bone Meal : Bone meal and fish bone meal are both great sources of organic phosphorus. The levels of phosphorus vary depending on the type of bones, the age of the bones and whether they are raw or steamed and cooked.

Rock Phosphate : Rock phosphate is another source of organic phosphorus. It is the raw material that phosphoric acid and fertilizer phosphate are derived from. It is not ideal, as the phosphorus is not readily available and takes time to break down.

Manure : Various manures are excellent sources of organic phosphorus. The content of phosphorus depends on the type and age of the manure. Manure, being animal waste and strong smelling, is not really meant for indoor garden use.

I hope this general overview of phosphorus, how it works, when it is needed, how it affects plants in high and low doses, and the brief list of organic fertilizers high in phosphorus will guide you in your quest to better understand this primary, essential nutrient.