Fertilizer Numbers: How To Make Sense Of The Numbers On Fertilizer Bags

So you’re trying to work out which fertilizer is best for your garden, but all you see is a bunch of numbers that don’t make sense. So just what do the numbers mean? You’re starting to feel like you need to have a chemistry degree just to find the right plant food for our lovely plants. Welcome to the intriguing world of numbers on fertilizer bags!

We’re going to take a look at the numbers on fertilizer bags, specifically the N-P-K values. Let’s dive right in!

Fertilizer labels have three main numbers that are usually bolded. These three numbers tell you what the main macronutrient makeup of the fertilizer is – that is, the primary nutrients that plants require being nitrogen(N), phosphorus(P), and potassium(K). The first number is the amount of nitrogen (N). The second number is the amount of phosphate (P2O5). The third number is the amount of potash (K2O) it contains. Thus the label you read is known as the fertilizer grade and it is a national standard. It is colloquially known as NPK. The range of NPK fertilizer options can come in liquid or granular form. It can also come as organic fertilizer formulations.

Nitrogen (N) – The Greening Nutrient

Nitrogen is the nutrient that plants need most for the growth of their leaves and stems. It is also responsible for making your grass green. Nitrogen is important for all plants, but it is especially important if you have lots of leaves or blooms. For example, nitrogen will be very beneficial to your tomatoes, but not so much to your roses. Nitrogen deficiencies are common in heavily fruiting plants because they need a lot of nitrogen to produce fruit. The result is yellow leaves with dry edges and a small growth rate. Plants that are deficient in nitrogen may stop growing altogether until they get more nitrogen. Most fertilizers contain some amount of nitrogen, and some contain just nitrogen (such as manure). Nitrogen is available in both organic and synthetic forms.

Phosphorus (P) – The Root Builder Nutrient

Phosphorus helps with root growth and the development of plant roots in new plants and seedlings as well as flower production in older plants. It also helps with blooming and fruiting because it controls the translocation of food in a plant.

It’s important to choose the right fertilizer for your garden because too much phosphorus can be detrimental as well. Phosphorus is highly soluble, meaning it moves about easily and is taken up quickly by plants. Because of this, it can seep into water supplies and cause pollution. One of the most common problems with excessive phosphorus is that it encourages algal growth in lakes, ponds, and rivers. The algae can then crowd out other aquatic life, depriving fish and other animals of oxygen.

You should test your soil before applying any kind of fertilizer because it is the best way to work out how to proceed. Adding too much phosphorus to soils that are already high in phosphorus can result in water pollution.

Potassium (K) – For Healthy Plant Cells

It is a key component of plant cell walls and helps to regulate water metabolism and photosynthesis and plant hardiness. Potassium is also involved in several biochemical reactions, such as stomatal closure and protein synthesis.

Most soils contain enough potassium for plant growth, but it can be depleted by heavy cropping or leaching. Potassium deficiencies can cause stunted growth, yellowing of leaves (known as chlorosis), and fruit disorders. adding potassium to the soil can correct these problems.

The best way to add potassium to your garden is by using compost or well-rotted manure. You can also use commercial fertilizer that contains potassium, but be sure to follow the directions on the package carefully. Over-fertilizing with potassium can damage plants.

If you suspect that your plants have a potassium deficiency, a soil test will tell the story. Your local cooperative extension office can help you interpret the results of your soil test and recommend the best way to add potassium to your garden.

NPK Numbers

So a bag of fertilizer (or a bottle of liquid fertilizer) with an NPK of 10-10-10 contains 10 percent nitrogen, 10 percent phosphate and 10 percent potash.

These different grades of fertilizer are manufactured by mixing two or more sources of nutrient together to form a fertilizer blend, thus they are commonly called ‘mixed fertilizers’.

There are many different blends made depending on the type of plants it is for. This is because different types of plants require different percentages of the 3 main nutrients in NPK fertilizer.

It’s also possible to get fertilizers with only a single one of the nutrients. A couple of the common Nitrogen sources are ammonium nitrate (33.5-0-0), urea nitrogen (46-0-0), sodium nitrate (16-0-0) and liquid nitrogen (30-0-0). Notice how the last two numbers of the NPK are ‘0’. That is because it only contains Nitrogen as a single source.

Phosphorus is usually provided as 0-46-0 and potash (the common name for Pottasium) as 0-0-60 or 0-0-50.

Calculating Macro Nutrient Content By Fertilizer Numbers

It is important to note that when we talk NPK, we are talking about macro nutirients. The whole micronutrient plant use discussion is an entirely different, but important discussion. More on that another time.

An example using kilograms:

If you want to get fancy like the farmers do, you can calculate the pounds or kilos of nitrogen in a bag of 10-10-10 fertilizer by multiplying the weight of the bag by 0.10. You can do the same process for calculating the amounts of phosphate and potash. So as an example, a 25kg bag of 10-10-10 contains a total of 7.5 kgs of nutrients: 2.5 kgs nitrogen, 2.5 kgs phosphate and 2.5 kgs of potash. The remaining 17.5 kg weight is just filler, usually sand or limestone.

Another example using pounds:

To calculate the pounds of nitrogen in a 50-lb. bag of 8-0-24 fertilizer, you would multiply 50 by .08, which equals 4. Calculating the pounds of phosphate is very easy because the ‘0’ tells us there is none. To work out the pounds of potash simply multiply 50 by .24, giving you 12.

So a 50-pound bag of 8-0-24 fertilizer has a total of 16 lbs of nutrients with 4 lbs nitrogen, absolutely 0 lbs of phosphate, and 12 lbs potash. The filler amount is this case is 34 lbs.

Importance Of A Soil Test To Determine NPK Formula To Apply:

A soil test is important to determine the NPK formula to apply because it can help identify which nutrients are needed by plants and in what quantities. It can also help determine whether a soil is too acidic or too alkaline, which can impact plant growth. Additionally, a soil test can help diagnose nutrient deficiencies in plants and correct them before they cause significant damage to your crop. Overall, if you want to get the most out of your plants and maximize their yield potential, a monthly soil test is the way to go.

Selecting A Fertilizer Grade: Its All in The n-p-k Ratio

If you want to really drill down into the detail on what fertilizer is best for your garden, you can have your soil tested. The results of such a test usually comes back with a recommend fertilizer grade you should use. However, another approach is just to get one of the standard home gardener fertilizer packs at a garden or hardware store which usually labels what it is best used for. In general, a lawn requires higher amounts of nitrogen than typical vegetable crops, although leafy greens also require a significant amount of nitrogen as well. When you need to choose a plant fertilizer, there are a lot of options. Some of the typical grades you’ll find labeled for lawns and gardens include:

  • 6-6-18: his fertilizer is a good choice for lawns, gardens, flowers, vegetables, and trees. 
  • 5-10-5
  • 5-10-10
  • 10-10-10: This fertilizer can be used on fruit trees, shrubs and evergreens. It contains equal parts of nitrogen, phosphorus and potassium (potash). 
  • 8-0-24
  • 20-20-20

Organic Vs Synthetic NPK Fertilizer: What Does Blood Meal Do For Plants?

As you can see above, Blood meal contains 13%-14% nitrogen. Where Sodium nitrate has 16%. Blood meal acts as a slower release fertilizer, and one more suited for younger plants. In contrast, Sodium Nitrate is faster acting and is best appropriate for later stage growths.

Blood meal is also an organic source of nitrogen macronutrients which are beneficial to the gardening or farming process. Blood meal provides a good breakdown compared to the other high N sources we mentioned earlier. However its important not to use too much blood meal as it will burn your plants as easily as any chemical fertilizer can!

Where Can I Get Blood Meal?

Blood meal is useful in gardens or farms of almost any size, however, some people prefer not to use blood products. You can substitute dry blood products for a variety of other organic materials, all of which are useful in different ways.

Dry Blood Products Substitutes

In some situations, bone meal is a good alternative to synthetic NPK fertilizer as it provides slow-releasing nitrogen. It is a good option if you prefer to avoid blood-based products.

Spreading Fertilizer

It is very important to spread fertilizer evenly, otherwise, plant growth will be uneven. Uneven fertilizer distribution can lead to stunted growth in some areas, while other areas may experience excessive growth. This can cause the overall appearance of your plants to be unbalanced and unsightly. Additionally, unevenly fertilized plants are more susceptible to stress and disease. This can be especially noticeable with lawn fertilizer spread unevenly.

You may have noticed lawns that look somewhat stripped. This can occur when fertilizers are spread unevenly.

To make sure that you get even coverage when spreading fertilizer, it is best to use a spreader.

The two most popular types of fertilizer spreaders are cyclone spreaders and drop spreaders. Cyclone spreaders are generally better at spreading evenly, therefore giving you the best results. When doing the job, make sure you overlap your pattern by proceeding in a cross-hatch manner. Apply half the fertilizer in one direction and the rest in the opposite direction. Also, make sure you break up any fertilizer clumps before proceeding to prevent clogging. Clumps tend to form if any moisture gets into the stored fertilizer so make sure you store it in a dry place.

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