On the front of every fertilizer bottle are three numbers that tell you how much nitrogen (N), phosphorus (P) and potassium (K) it contains, otherwise known as the NPK. These are macronutrients and are essential for plant growth, but what exactly do they do for our plants and why are they so important? Let’s look at the second number, phosphorus, and why it is essential for your plants to thrive.

What is Phosphorus?

Phosphorus is a chemical element with the atomic number 15. It’s highly reactive and is found in ATP, DNA, RNA, phospholipids, and the Earth’s crust. The majority of mined phosphates are used in fertilizers, but are also ingredients in detergents, pesticides and nerve agents.

For plants, phosphorus is considered the second most essential nutrient after nitrogen and is responsible for a wide variety of biological functions. It plays a crucial role in helping plants capture and store energy from the sun during photosynthesis by converting ADP (adenosine diphosphate) into ATP (adenosine triphosphate).

Phosphorus is a vital component of ATP. ATP is a chemical found in all living things that stores and transfers energy. It is often referred to as “energy currency” and works like money in the bank by storing energy for future reactions or withdrawing energy to pay for reactions required by cells. ATP provides the energy to drive many different processes and biochemical reactions from seedling growth to harvest.

Phosphorus is critical for stimulating root production and growth, fruiting and flowering, seed formation, cell division and multiplication, and transporting nutrients. It is an important structural component of DNA and aids in genetic transfer, improves resistance to drought and cold temperatures, increases stem and stalk strength, and increases resistance to plant diseases. Without enough phosphorus, you’ll get a plant with stunted growth, delayed maturity, and a smaller yield.

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Spotting Phosphorus Deficiencies

Phosphorus deficiencies can be tricky to detect. The first sign of a deficiency is slow growth in the early vegetative stage. This can present as fewer, smaller leaves that are thin, curled and darker in color than normal.

In some cases, leaves, stems, and branches may have a purplish or reddish tint, however, leaf discoloration will usually only appear if there is a severe phosphorus deficiency in the soil. Keep in mind that discoloration can be a result of other environmental factors, so you won’t know if phosphorus deficiency is the cause without a soil test.

Fruit and seeds will also be smaller and take longer to mature, but by the time you notice this, it will be too late to do anything about it. Phosphorus is a highly mobile nutrient, so if there is a deficiency, the plant can take phosphorus from older growth and translocate it to new growth, flowers, and fruits.

Phosphorus toxicities are also a possibility and tend to lock up iron, manganese, and zinc, which can present as yellowing leaves and slowed growth.

Preventing and Treating Deficiencies

Phosphorus content in soil can vary considerably, ranging from 0.04 per cent on the Atlantic and Gulf coasts to 0.3 per cent in the northwestern United States. The amount of phosphorus in the soil is influenced by many factors, including the material the soil was derived from, climatic conditions, amount of weathering and erosion, as well as crop removal and fertilization.

The best way to prevent a phosphorus deficiency is to test your soil at a reputable lab and amend it if necessary. Sources of phosphorus for your garden include manure, bone meal, rock phosphate, and phosphate fertilizers. Chicken manure is particularly high in phosphorus.

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Phosphorus availability is another important factor to consider. Just because phosphorus shows up on a soil test doesn’t mean it is available to your plants. Soil pH, temperature, moisture, and compaction will affect absorption rates.

Ideal soil pH should be between 6.0-7.0 for maximum availability. Adding lime will raise the pH of your soil over time, if needed. Low soil temperature, excessive soil moisture, and soil compaction will decrease availability.

Root growth can have a major impact on phosphorus uptake. Because phosphorus is very immobile in soil, it is important that the plant’s roots can grow through the soil to find the phosphorus it needs. It is important to consider all factors that affect root growth if you don’t want to end up with a deficiency.

Even if your soil has high levels of phosphorus, compacted soil, herbicide damage, and root-feeding insects can decrease root growth and dramatically reduce a plant’s ability to uptake phosphorus. The soil environment in early spring, with its cold temperatures and water-logged soil, can cause roots to grow slowly. For this reason, many gardeners find that young seedlings can benefit from extra starter fertilization when planting even if phosphorus levels in the soil are high.

Increase Phosphorus Availability with Beneficial Microbes

Microorganisms are key to making sure your plants get enough phosphorus. Soil that contains a healthy population of beneficial bacteria and fungi will promote strong root growth so there is more surface area for nutrient uptake. These microbes also break down unavailable forms of phosphorus into plant available forms.

Phosphate solubilizing bacteria (PSB) have been introduced to the agricultural community as phosphate biofertilizer and have been proven to improve nutrient uptake and reduce the use of synthetic fertilizers and pesticides. Three bacteria strains in particular have proven beneficial: Pantoea agglomerans, Microbacterium laevaniformans and Pseudomonas putida. Researchers at Colorado State University have shown that multiple strains of bacteria solubilize phosphorus more effectively than any one strain alone.

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You can apply PSB by inoculating seeds before planting, through fertilization, and in hydroponic nutrient solutions. Compost and compost tea are additional ways to increase microbial communities in your soil, although if you want the PSB strains, you’ll need to buy a mycorrhizal inoculant with those specific strains included.

Phosphorus is essential to healthy, vibrant plants, and maximum yields. It drives energy production and storage, which is the foundation of just about all the chemical reactions and processes throughout a plant’s lifetime. With these basic techniques, you can guarantee your garden will always have an available source of phosphorus and thrive.