Phosphorus is one of the ‘big three’ major elements that’s vital to the growth and health of plants. It assists in converting the sun’s energy and other chemicals, such as nitrogen, into usable food for plants. A phosphorus deficiency is definitely something that every indoor grower wants to avoid as it invariably leads to sick-looking, stunted plants that produce smaller, lower quality fruits and flowers. Not good! So is the answer to bigger yields simply to pack on the P? Well, it’s not quite as simple as that, so we asked Geary Coogler from BS Horticulture for some expert advice. Here’s what he had to say…

WORDS: Geary Coogler

What’s all the noise about phosphorus these days – this idea that plants do not need the levels of phosphorus that are generally advised? The amounts recommended by agronomists and plant physiologists are accurate; the problem comes in interpretations made by the marketing departments of some companies or in the minds of self-purporting experts. Nutrient recommendations and applications are made with numerous variables in mind based on medium composition, plant variety, pH, temperature, moisture, nutrient interactions, plant requirements, economics, etc., and not just pulled from the air nor based on a layperson’s understanding of karmic forces or scientific data.

A FIRST LOOK AT PHOSPHORUS

So how do we understand the relationship between plants and phosphorus? We start with the basics – these include many processes and other elements as well. Every element has its own weight different from all the others: one atom of nitrogen weighs less than one atom of oxygen which weighs less than one atom of magnesium which weighs less than one atom of phosphorus and so on. Molecules are combinations of atoms that are expressed in combined weights of all the elements in the molecule. Fertility components can be “elemental” (based on the pure form of the nutrient, such as calcium) or “molecular” (based on a combination of atoms, such as nitrates, sulfates, or phosphates).

This is how the plant takes up the nutrient components. It can also be how it is measured on labels and reports. Few, if any, nutritional elements are taken up by the plant as applied and must either change form, change ionization properties, or disassociate; this is especially true of phosphorus as it requires a special pathway (known as an H+-HPO42- symporter) that takes it up as a phosphate ion after activation.

All applied nutritional components are under competitive pressure in the root zone from not only the plant, but the environment as well, including temperature, pH, interaction with other elements, and other life forms. Most elements are more concentrated in certain areas of the plant based on the plant itself: for example, leaf tissue (mesophyll) will have as much iron and manganese as it does sulfur and magnesium, while phosphorus is present in larger amounts in root and flower tissues (especially seeds). It’s important to note that the only way to have a complete picture of the composition of the plant is to analyze the entire plant: roots, stems, leaves, shoots, flowers and seeds.

A DEEPER UNDERSTANDING OF PHOSPHORUS

Phosphorus is used by the plant in the formation of such things as sugar phosphates (stores and transfers energy), nucleic acids, nucleotides, coenzymes, phospholipids (membranes), phytic acid, and high energy phosphate bonds (ADP, ATP). The main entry point into assimilation pathways of phosphate occurs during the formation of ATP (adenosine triphosphate), the energy currency of the cell.

ATP is the energy for almost every process in the plant, from uptake of nutrients, conversion of nutrient complexes such as nitrate to release the nitrogen, to production of DNA and cell division. Photosynthesis is a well known general process which produces ATP through photophosphorylation. Respiration produces ATP through an oxidative process known as oxidative phosphorylation. Power used in homes and industry is measured in Watts, which gives a value for the amount of energy needed to make things work; ATP is used by biochemists to indicate the energy needed to make biological processes occur.

The phosphate group is the energy and, once incorporated into ATP, can be converted to energy or transferred by many different processes to form all the phosphorylated compounds found in a plant. These groups may also form other energetic compounds that function the same basic way in specific processes. The entire pathway and its many routes are known as “phosphate assimilation.” Phosphate is required to transport most elements into the roots, through cell membranes, and to change the nutrient into usable forms; without it, the plant would starve or, rather, not grow.

There are many different elements that compose plant tissues. Some elements like sodium can be more specific to certain plants, like cacti and grasses, while others (like nitrogen, carbon, phosphorus and potassium) are required by all life forms. Concentrations of elements in plant tissues are expressed in terms of “adequate levels,” which means that enough are present to ensure availability when needed for the many processes and metabolites present in plants. There are levels that are considered high, especially in nitrogen and the heavy metals, which can cause problems, sometimes to the plant but mostly to those animals and life forms feeding off of the plant tissues. Table 1 gives a fairly accurate yet general idea of those elements needed and the concentration they are used in. It is apparent by examining the table that, while some elements are equal in percent composition, there are differences in the actual number of atoms. This goes back to the first point made here, that each atom has its own unique mass; weights are different. Hydrogen, carbon and oxygen are considered critical nutrient elements for the plant, but are obtained through water or the air and not applicable to this conversation on applied fertilizers.

NUTRIENT LABELS: AS EASY AS N-P-K?

Let’s talk about labels on fertilizer bottles: how do you interpret them and what do they mean? There are as many fertilizer label requirements as there are countries and, in the United States, as many states. Labels are used to represent to the grower the contents of the nutrients and other constituents of a mixture, slurry, or homogenous blend of nutrient or nutrients. In most incidences, these labels are politically acceptable, not necessarily scientifically acceptable, and sometimes based on archaic methods of measuring. In the case of phosphorus, labels are based on a by-product of burning the compound in enriched air. Science, unhindered by politics, deals with getting as close as possible to an accurate reflection of true events. There are several ways to represent the content of these fertilizers, not one most accurate way, and several politically accepted ways. These are Mass/Mass (m/m) or Mass/Volume (m/v); in North America, and some other countries world-wide, this is done Mass/Mass as grams of element per kilogram of fertilizer. (The other is Mass/Volume or grams/liter.)

On all North American labels that are registered, elements are given as a percent of composition in terms of weight: for every kilogram (or pound) of fertilizer material there is X% by weight of the identified nutritive element. In general, the biggest or first three numbers that appear on the front or back (or both) of the label represent nitrogen to phosphorus to potassium (N-P-K): for example 10-10-10. The additional elements may be listed under the Guaranteed Analysis section of the label, if the company wants to guarantee those elements, in the same percentage format. N-P-K elements are macronutrients and considered major elements, but macronutrients include other elements as well (see Table 1).

Currently, nutritional elements are classified as either macro or micro elements based on the relative amount used by the plant of the measured component. The term component is used on purpose because it could be a molecule that is measured and not a single element; for example, phosphorus (P) is measured as phosphate pentoxide (P2O5), and potassium (K) is measured as potassium oxide (K2O). This means that the percent weight is not just for the element looked at but includes the additional elements: in this case, oxygen (O). Nitrogen (N), on the other hand, is given as only the N, but the Guaranteed Analysis section will state where the N is derived from and will state this as a percentage of the nitrogen component as derived, since different forms of N behave differently and possess different properties. So, while the percentages are correct on the label, not everything is that straightforward and must be calculated to arrive back at the actual amount being applied. When two- or three-part nutrients are used, for example in some liquid fertilizers, add the similar element numbers together in order to arrive at the correct concentration.

Example:

Let’s determine the actual concentrations of nutritive elements as taken from a North American label where percentages stated are Mass/ Mass. A 50 pound pail of a liquid-based fertilizer has N-P-K values given as 10-20-10 (a suspected 1:2:1 ratio). In the Guaranteed Analysis section we have the following additional information:

Total Nitrogen (N)…………………….. 10%
10% Nitrate Nitrogen
Available Phosphate (P2O5)……….20%
Soluble Potash (K2O).…….…………..10%

This means that 10% of 50 pounds, or 5 pounds, is elemental N since it is listed as N, not a compound; 20% of 50 pounds, or 10 pounds, is P2O5; and 10% of 50 pounds, or 5 pounds, is K2O. These are the Commercial Percentages of the fertilizer package. For the elemental percentages, a conversion is required since both K and P weights include oxides. In this example, the percentage of actual P in the oxide form P2O5 is 44% and the percent K is 83%, so the actual weight of elemental P is 4.4 pounds (10 x 0.44) and K is 4.15 pounds (5 x 0.83). So the corrected numbers read 10% – 8.8% – 8.3%

So the actual ratio in the fertilizer of single elements in this example is 1.0:0.88:0.83 N-P-K, not the 1:2:1 the label indicates. All other elements given, whether they are actually taken up as a complex like sulfates or in elemental form, are expressed on the label as the elemental version, like nitrogen. In different measures of Mass/Volume, the numbers would be different and are also based on specific gravities. An example would be a root/flower additive fertilizer where the North American Mass/Mass convention would show a 0-10-11 NPK value. This might have Mass/Volume percentages expressed as 0-13-14, which would be dependent on the material it is derived from. The ratio is what is truly important: how much of each element is provided. Using higher or lower numbers is relevant to the amount that is applied as long as the ratio is close. Each species or, sometimes, variety of plant has a ratio specific to its needs even though many plants have identical needs and are sometimes grouped according to these needs. So given three different fertilizers labeled 0-10-11, 0-20-22, and 0-30-33, the ratio stays close and only the amount applied needs to be adjusted based on the needs of the crop. This is because, in the end, the root zone needs to have a certain amount available for the plant across the amount of time the plant needs to take it up, and many variables can and will affect this as a nutrient moves from the bottle or bag to the utilization sites in the plant.

LIMITING AGENTS

Limiting values are the speed limits of growth and development in a plant or any other life form. This is true whether it’s carbon dioxide (CO2) in the air, water in the soil, or a single element: any of these factors that are limited in availability will determine the potential for the plant’s development. These are known as Limiting Agents. Perfect ratios and amounts of fertilizer can be applied to a plant, but if available carbon (C) is limited by a lack of CO2 in the air, the plant will not be able to utilize all the applied nutrients, nor can structural elements and other processes be built or occur, and the plant fails: the limiter in this case is C.

In any system, the goal is to ensure that adequate levels of all the input components are maintained across time and adjusted when needed. This is because a plant requires different levels of some elements at different times or stages in its development. Most nutritive elements, as mentioned earlier, should be kept close to the needed levels because they tend to accumulate in the tissues of the plant where they can become toxic to the plant or to the animal that consumes it. The ratio in the root zone closely matches overall plant tissue composition; it is the overall concentration that gets growers in trouble with salt burns. Also, other factors can greatly influence nutrient availability to the plant such as pH or substrate composition and nutrient formulation. It does no good to apply the correct ratio of NPK if the pH is out of bounds since these nutrients will be made less or more available to the plant and will express this difference in tissue composition.

There are many ways or forms that can be used to engineer a fertilizer. For instance, nitrogen can be applied as ammonium nitrate, potassium nitrate, calcium nitrate, urea, etc., but each is different and each brings other components to the table. Phosphorus can be applied as superphosphate, triple superphosphate, monopotassium phosphate, ammonium phosphate, or bone meal, to name a few. Each of these must be “activated,” broken down, or form-shifted in the root zone in order to be taken up in one of the three forms of phosphate accepted by a plant. The pH of the environment will affect the form of the phosphate’s availability and will limit the ultimate availability of the desired monovalent form H₂PO₄^- at normal pH ranges between 5.2 and 7.2 by converting the phosphates into the unusable form H₃PO₄ or the less desired divalent form HPO₄^2-.

The phosphates will bind other available elements as well as to substrate particles and become unavailable to the plant even though they’re still showing in the system. So fertilizers must be designed not only to provide the right ratios of elements in the right amounts, but also for a dynamic environment of temperature and pH fluctuations and across different substrates.

RATIOS

Ratios are the true indicator of the correctness of the fertility program. When designing a fertility program, it is critical to know all the sources of nutritive elements available to a plant, and what those ratios and concentrations are. By knowing these, the rest of the question is a math question.

If the grower is using a medium that has a starting fertility ratio of NPK 0-1-0.5, and the plant shows a total tissue ratio of 4-2-3, then they will have to add a ratio of 4-1-2.5 to get the correct fertilizer addition needed. However, it must be remembered that these are at perfect values of pH, temperature, and across the growth cycle, and the values are seldom perfect.

Plants seldom take up nutrients equally and will influence the root zone to give up more of what it needs. Plants also change their needs slightly during development whereas tissue analysis is a slice of time, so tissue taken at the end of the crop cycle will only show the cumulative value of these stages and not reflect how a plant takes these nutrients up over time. Juvenile plants take up a different ratio than flowering plants do; when a plant anticipates seed, it will begin to accumulate phosphates.

Where the nutritive elements are all correct except for one of them which is low, then the low element will be the limiter: where this is a minor (micro) element used only in a few processes, say sodium, then the effect, while present, is minimal. In the case that a major element is the limiter, say phosphorus, then the effect can be dramatic because those compounds made from phosphorus will not be complete, and those processes dependent on P will not occur such as nutrient uptake, transport, and conversions. By applying sufficient concentrations of these elements in the correct ratio, along with the proper environment, the plant never sees a limiting agent and growth will proceed at the maximum genetic possibility.

It is important to apply sufficient concentrations of these elements, but caution must be observed in not applying too much: and here both high concentrations and incorrect ratios can play a hand. Just because one nutrient is limited does not mean the plant will avoid taking up all the other needed elements. These unused elements usually find their way to the vacuole of the cell and there they remain: vacuoles not only provide water storage and structure support, they also serve as garbage dumps. Heavy metals like copper, boron, molybdenum, and manganese cause issues in animals that consume them: plants will also accumulate non-nutritive elements such as lead and uranium if present in the available or free form in the root zone. Where not enough ATP is available to totally convert nitrates to usable N, then nitrites can accumulate. Excess ammonium shunted to the vacuole converts to nitrates and nitrosamines, a cancer-causing agent. Keeping these ratios close, while avoiding limiting values, is the ultimate goal of a fertility program, and the best way to keep consumers consuming.

PLANT NEEDS

So what does a plant need in the way of phosphorus, how do we provide this, and what can we expect over time and development? The best way to know a plant’s needs is to know what makes up the plant and the ratio of these elements to each other. Once this is known, and once what exists in the substrate is known, it is fairly easy to apply the balance by using several known fertilizing materials. However, it is equally important to know these values at the different growth stages of the plant and adjust at each stage.

The other way is to use a product that was designed, based on the plant itself, from research done correctly by the company that produces the fertilizer (a complete fertilizer), based on the substrate involved. Care must be taken by the grower to get all the variables correct, such as pH and temperature, or at least to give the company what it asks for. It is equally important to use the substrate it was designed for, as these will cause those ratios discussed earlier to change. The grower must be sure to follow the guidelines of the company closely, taking care not to substitute products as most will provide different levels of the components or in a different format.

Equally important, from the balance point of view, is to provide the ratio that the plant wants when it wants it. A plant’s need for phosphorus goes up during the earlier stages of flowering, then falls back to completion; but still the need has been escalating across the plant’s development all along. This is known as the Phosphorus Utilization Curve, appearing as a bell curve on a graph. The only additional phosphorus a grower needs to apply is the amount the plant requires that the main fertilizer does not contain, and that will vary over time: the ratio game again. The plant will change the environment around the root surface to influence the activation of phosphates to bring them into the plant. The total need for phosphorus in the root zone will ultimately be based on not only the need of the plant, but also the level of activity the environment will have on ultimate phosphate availability.

SPECIAL CONSIDERATIONS

The phosphorus pathway in the plant is wide and all encompassing. Phosphorus starts out in the seeds at high levels to ensure the plant has enough to initiate all the metabolic processes it will require, as well as the growth processes. ATP is used to build structure, chemical compounds, and uptake the other elements needed for these processes. More phosphorus is found in root tissues because much is needed to move nutrients into the plant and into the transport pathways. It comes into and is turned into ATP for use locally or transported to all the other cells to be transformed into ATP or used as ATP (assimilated). Once it is the form of ATP or one of the other energy components, it is released for the energy and then is free to be used in the formation of other phosphorylated compounds. It can also be converted back to ATP. More phosphates are also found in the flowers themselves because of the decrease in produced ATP locally, and because the plant is accumulating phosphorus for the seeds and other energy-draining requirements of the flower tissues, such as pollen.

In some substrates, such as mineral soils, roughly 50% of the phosphates applied are rendered immobile and become permanently fixed in the medium. As a result, more has to be added to accommodate this capture, so while the amount added is higher, the amount realized is lower. Plant mediums that have active micro-life will also see a depletion occur of available phosphorus that is used by the micro-life since ATP serves all life forms in equal roles. The pH of the soil solution will affect available phosphorus as will temperature and overall concentrations of other elements such as potassium, a synergistic effect which is a ratio issue as well. The grower has to be aware of all these variables in designing a fertility program for their crop. Most nutrient lines are designed with the line in mind: in other words, the ratio, composition, source, and application rate of each component product adds to the final ratio of every nutrient that would be required by the plant.

THE MARKETING EFFECT

The noise level about phosphorus is just that: noise. Numbers on fertilizers are legitimate in most incidents, especially where regulated: these are not wrong. They do nothing but indicate the concentration of the constituent elements. The type or source of these elements can be a determining factor in final availability based on the overall system. Complete fertilizers are designed to provide the correct ratio of the elements required once the entire line is mixed according to instructions.

The problem with phosphorus is knowledge, and old laws that dictate how to measure and report the element. When viewed correctly, phosphorus should be in the correct range as adjusted for the root zone environment. Knowing how to read and accept both labels and reported findings, and interpret the data, is critical in determining the truth behind the advertizing and statements made about products and results.

WHAT TO LOOK FOR

What should the grower be looking for? First, a grower must decide if they are going to use an off-the-shelf version of a complete fertilizer or build their own. An off-the-shelf product must be designed for their plant/crop and the methods they will use to grow. Building a fertilizer takes some extensive knowledge of chemistry and horticulture; this is generally not the best method for growers of smaller commercial operations or hobbyists.

Phosphorus can be applied in many formulations depending on the base mineral it is derived from. Base minerals have other elements associated with them: some good, and some not-so-good. For example, monopotassium phosphate with an NPK analysis of 0-10-11 commercial and 0-4.4-9.13 elemental applies potassium (good) as well. Or sodium nitrate with an NPK of 15-0-0, elemental, but it also applies sodium as well (not so good).

The bigger the phosphorus number, the less will be used. Make sure that it is small enough to not make costly mistakes when applying smaller measured amounts. Bigger numbers may or may not decrease the unit cost of phosphorus as it is based on a different mineral which sometimes has costly other materials attached. Diammonium phosphate has an NPK of 21–53-0; the nitrate is expensive and the composition of the product is going to require some balancing with other components and care in application as it is very acid-forming. Using an off-the-shelf version will probably offset most of these issues and make for an easier process of application.

The grower should be aware of two issues: the first is nutrient contamination and the second is the fact that nutrient sources will vary in characteristics and availability. Some nutrient constituents become contaminated with other elements either through the mining or the manufacturing process. Contaminants such as lead or other heavy metals can accumulate in the plant to injure the plant or the consumer. Some nutrient constituents can have adverse effects on pH, be less soluble and therefore less available, or can be in a less than desired form. The ammonium ion, while an acceptable source of nitrogen, becomes less acceptable as the concentration increases to the point of becoming toxic. So, the grower should look for nutrients that are high quality, clean, and designed correctly. Find or request the heavy metal analysis for the nutrient line before using the products: this will tell how clean they are.

For complete fertilizers, the grower should be dealing with a quality company that has the grower’s success in mind: one that does the research, in a legitimate manner, and maintains high quality standards. This is especially true for complete fertilizers or fertilizer lines, and it goes further. The company should understand all the relationships that affect delivering nutrients to the plant and should never, never attempt to sell their products based on the shortcomings of their market’s (consumer’s) knowledge levels. A good company will educate the market and hold true to the science: a market-oriented company will sell the glitz and make the science fit its ends.

Bibliography

Brady, Nyle C., and Ray R. Wells. The Nature and Properties of Soils. 13th. Upper Saddle River, NJ: Prentice Hall, 2001.
Epstein, E. Mineral Nutrition of Plants: Principles and Perspectives. New York: Wiley, 1972.
Epstein, E. “Silicon.” Annu. Rev. Plant Physiol. Plant Mol. Biol. 50 (1999): 641-664.
Paul, E. A., and F. E. Clark. Soil Microbiology and Biochemistry. 2nd. San Diego: Academic Press, 1996.
Plant Research, B.V., interview by Geary Coogler. Conversations on Phosphorous Utilization Oosterhout, (October 27, 2009).
Schwarz, A., W. Wilcke, and W. Zech. “Heavy Metal Release from Soils in Batch pH (stat) Experiments.” Soil Sci. Soc. Am. J. 63 (1999): 290-296.
Taiz, L., and E. Zeiger. Plant Physiology. 3rd. Sunderland: Sinauer Associates, Inc., 2002.
Yamagata, M., and A. E. Noriharu. “Direct Acquisition of Organic Nitrogen by Crops.” JARQ 33, no. 1 (January 1999): 15-21.