Few things are more important to get right in the nutrition of plants than understanding what and how to use NPK fertilizers. NPK stands for nitrogen, phosphorus, and potassium. On most bags of commercially available fertilizers, NPK is the three numbers, separated by hyphens (#-#-#), that appear prominently on the front of the packaging and always in that particular order.
Although it is a gross simplification, the functions of NPK, can be summarized with the rhyme: up, down, all around. Nitrogen helps with all growth above ground, phosphorus helps with root development, and potassium promotes overall plant health. To be more accurate though, each performs many functions to support or catalyze a plant’s myriad biological processes.
Nitrogen (N)
Of all of the nutrients that plants need, that they do not get from the air, sun, or rain, nitrogen is the one they need in abundance. Oddly enough, the air around us (and of course, plants) is about 78 percent nitrogen, but plants cannot use the N from the air, they can only use it when it is in the soil.
Nitrogen helps plants develop and maintain their nice, verdant appearance. It is a primary component of chlorophyll, which gives plants their green color. It also contributes to rapid plant growth all over. Nitrogen is needed for all vegetative growth beyond leaves as well, like stems, shoots, and branches. It is critical for photosynthesis, helping the plant to create its own food source, keeping it strong and healthy.
Nitrogen is very mobile and volatile. It moves through soil quickly and is prone to leaching. This means that any amount not used by a plant is typically lost. It is why most fertilizers containing nitrogen include substantial amounts of it as it is not only needed in high quantities, but much is also lost and never even makes it to the plant, at least when it is applied in a water soluble form (more on that later). This means that excess nitrogen that is not used by the plant often ends up in waterways when applied to field-grown plants. For this reason, growers should not apply fertilizers containing more nitrogen than what the plants need at any given time or apply only slow-release forms of nitrogen.
Plants lacking nitrogen will let you know it. They grow more slowly, produce smaller leaves, and less top growth. Older, bottom leaves will start to turn yellow, and the rest of the plant will be a lighter green than it should be. Leaves can drop off prematurely.
Nitrogen is found in a lot of natural fertilizer sources in addition to commercially-produced forms. It is found in nutrients sourced from the sea, such as crab meal and fish meal. When sourced from land, it comes in the form of cottonseed meal, feather meal, or blood meal. It is also sourced from many types of manures and guano. Ammonium, nitrate, and urea are also forms of nitrogen that are available. For field production, many types of cover crops will take nitrogen from the air and move it into the soil for future use.
In addition to being a vital component of healthy plant growth, nitrogen is critical for decomposition. Compost piles would lie stagnant and leaf litter would not decompose on the forest floor if not for nitrogen. Most of the world’s flora and biomes would flounder and collapse if not for nitrogen. Too much nitrogen can be a problem for plants too, though it is far less common than not enough. Plants can grow too much, and not properly enter their flowering or fruiting stages, especially on plants that are not well-rooted or established with too much N.
Phosphorus (P)
Phosphorus is needed for root development and growth. As such, it is usually present in higher quantities in those fertilizers developed and marketed to support seed starting or transplanting. It, however, is responsible for or helps a wide range of plant functions well beyond supporting roots. Plant reproductive activities and seed development are supported by sufficient amounts of phosphorus. Flowers and fruit development are also aided by phosphorus as is a plant’s energy storage and transfer capabilities.
Slow growth, stunted, scorched, or disfigured leaves, and even defoliation in extreme cases can indicate a phosphorus deficiency. Another telling sign is discoloration of lower, older leaves. They can turn a very dark green or turn a reddish-purple when they are not getting enough phosphorus. Stems can show this same color variation as well. Sometimes the entire plant can appear yellow, though that likely indicates other deficiencies as well.
A less obvious sign is the absence of fruit. Plants that appear healthy and lush, but are not producing fruit at the expected times, may also be presenting symptoms of phosphorus deficiencies. A lack of blooms can also be an indicator.
Deficiencies in phosphorus are not unknown, but less common than some other nutrients because phosphorus is not as mobile as other elements. It is more prone to building up in the soil than nitrogen, which will leach out. This does pose a challenge in applying phosphate fertilizers because they take longer to become effective than some other element. When deficiencies are spotted and diagnosed correctly, there are several options for remediation. Phosphorus can also be present in sufficient amounts, but unavailable to the plant when pH is unbalanced.
Many of the same fertilizers that contain natural sources of N will have sufficient levels of P in them. These include fishmeal and bat or seabird guano as well as bonemeal. Other organic or natural sources include compost, alfalfa meal and rock phosphate. Non-organic sources are usually found as diammonium phosphate, superphosphate, or even triple superphosphate.
Potassium (K)
Potassium, sometimes referred to as “potash,” is a critical element in supporting plant immunity by promoting the function of several biological processes, such as chlorophyll formation. Either by itself or in conjunction with one or more other nutrients, potassium helps with photosynthesis, plant respiration, absorption, gas exchange, and protein production. It also helps with tissue growth, root, and plant development, as well as fruit color and flavor. Sufficient potassium amounts help plants during stressful periods due to weather extremes, or pest/pathogen pressures.
Like phosphorus, potassium deficiencies are not as common as other elements, but can still happen. When plants are experiencing potassium deficiency they often experience wilting on older leaves. Other symptoms may be scorching at the leaf tips with chlorosis between leaf veins (interveinal). Small, stunted, or underdeveloped fruit may appear as well.
Guano and kelp meal are good natural sources of potassium. Greensand, palm bunch ash, and sul-po-mag are others. More “traditional” sources are potassium sulfate, potassium nitrate, potassium chloride, and potassium phosphate.
NPK Ratio Calculator
Knowing what each of these three macronutrients, are, what they do, and how to know if your plant is showing signs of deficiency, does not tell you how much you need to apply either as a preventative or as remediation. There is a wealth of published and easily found information out there about what particular crop NPK needs are. Once you know how much of a particular nutrient are needed per acre or per plant, calculations still need to be made to translate the bag of fertilizer in-hand, and how much of it to use.
Fortunately, there is also a free NPK calculator available online at the Hydrotek website. This calculator is useful not only to help you decipher how much of a particular nutrient to apply or in what ration, but also to calculate amounts prescribed for large scale farming into amounts more suitable for indoor or outdoor applications, or small-scale cultivation.
- Convert the rates of fertilizer recommended to comport with any particular formulation that you may have.
- Calculate the amount of area to be treated or fertilized.
- Calculate fertilizer costs per square foot, or acre.
A typical bag or package of fertilizer will have NPK numbers on them such as 5-10-5, 10-6-4, 17-17-17, 21-7-7, just as a few examples, but they could be any number between zero and one hundred as long as the total sum of the three numbers does not exceed one hundred. These numbers represent the percentage (not weight) of each nutrient in that package. A fifty pound bag of 10-6-4, has 10 percent (5 lbs.) of nitrogen, 6 percent (3 lbs.) of phosphorus, and 4 percent (2 lbs.) of potassium. Filler materials makes up the balance in each package and could be sand, sawdust, peat moss, ground corn, limestone, and even dirt or compost.
Your research of your particular crop needs or preferably, the results of a soil analysis, will prescribe how much of each nutrient to give. Results are usually given in pounds per acre, but that is where one of the online calculators comes in handy to convert that number into a more practical number for most small-scale growers.
If you determine or are prescribed to use for example, three pounds of nitrogen per thousand square feet, one pound of phosphorus, and half a pound of potassium, then whatever formulation you have, should have a ratio of NPK of 3:1:1/2. This means that you could have a bag that is 18-6-3, or 9-3-1.5, or something comparable.
Additionally, if your recommendation is to apply a nitrogen containing fertilizer at a rate of one pound per thousand square feet of growing area, in the examples above, you would apply ten pounds of a 10-6-4 fertilizer or just under six pounds of a 17-17-17 fertilizer. It can be easy to get overwhelmed with all of the numbers, but just remember that the three numbers tell you the percentage of each nutrient in each bag, not the number of pounds (unless it is a 100 lb. bag) of each nutrient.
When to Use What NPK Fertilizer
The final piece of the puzzle beyond understanding the role of NPK, and how much of it to apply, is to understand more about the fertilizers themselves. The only sure way to know how much NPK, when to apply, and what type of fertilizer to use is through testing. Testing of soil or tissue can seem overwhelming, but it is actually quite easy and inexpensive.
Many growers waste hundreds to thousands of dollars on NPK fertilizers that they do not need because they do not do any testing. There are labs for use throughout each state and province that are run either by universities, governments, or private interests. In any case, soil testing, and many types of plant tissue testing are affordable.
Soil testing for field-grown crops should be done at an absolute minimum of once every 5 years but doing so every year will result in being able to address deficiencies in some cases before they become a problem. Greenhouse and hydroponic stock and media should be evaluated far more often, especially if using any kind of a “home-made” blend.
In addition to sending off samples to be analyzed, there are a plethora of available testing kits that have high degrees of accuracy that can be used. In either case, knowing what your current nutrient levels are, will give you far greater insight into the “facts on the ground” than taking your best guess. Results typically come with recommendations for fertilizers considering your particular setup and your intended usage of the crops.
NPK fertilizers can be found in slow-release or quick release formulations. With some exception, liquid fertilizers are quick release while granular ones are slower. Another way to look at this is that water soluble fertilizers are those that go right to work, once activated with water, but they are also prone to leaching and wash out.
Slow release fertilizers are activated by the biological activity in the soil and not by water. These fertilizers stay around much longer providing nutrients to the crop or plants over several weeks or months. The down-side to these besides not being immediately available is that they tend to be much lower amounts of available fertilizer than water-soluble types.
This can lead to the discussion on organic versus synthetic NPK fertilizers. Your analysis will not likely recommend one or the other, because your plants do not know the difference. If nitrogen is present for their use, it does not matter to the plant if it was sourced organically, sustainably, or otherwise. Organic fertilizers tend to be slow-release and granular. Crops and plants generally require more organic fertilizer than synthetic to produce desired results. Though organic fertilizers do not work as fast as synthetic, over time, they can result in more uniform growth than with continued dosing of synthetic fertilizers.
Organic sources of NPK have other benefits too in addition to being easier on the environment. They do not add excess salts or acid to the soil and encourage living soils that are full of growth-friendly microbes. They also do not burn plants if used in excess or in the case of accidental spillage. They also do not find their way downstream causing unwanted algae blooms in waterways.
As far as when to use NPK fertilizers, it depends if they are field-grown, or grown inside. Either way though, plants being grown from seed or just transplanted, will need access to enough phosphorus to develop healthy roots. When plants have entered into their vegetative stages, they will need more nitrogen fertilizers. This also goes for crops that are grown solely for their leaves such as lettuces or cabbages. These need an NPK fertilizer where “N” is the biggest number.
During the blooming phase, it is good to go back to a fertilizer that has a higher “P” number on it. Fertilizers high in potassium (K), should be used throughout the active growing season to encourage good defenses against various threats.
