On the front of fertilizer bags, you’ll find three bold numbers, called N-P-K numbers. These represent the three primary nutrients in fertilizer: nitrogen (N), phosphorous (P), and potassium (K). These exist to help you quickly compare products and use the right ratios for your growing Crop condition.

Don’t mistake these labels for chemical compounds. Their molecular formulas are actually N for nitrogen, P2O5 for phosphate, and K2O for potash (which contains K for potassium).
So to read these numbers, take for example a fertilizer mixture labeled as 0-20-20. This means the mixture contains 0% nitrogen, 20% phosphorous, and 20% potash.

The remaining 60% is called “filler,” which is usually sand, granular limestone, or a blend of other substrate materials.

These fertilizer numbers are the fertilizer grade, which quickly explains to you the ratios of the three primary nutrients mixed into the blend. Various grades are available because different types of plants thrive better with a different balance of nutrients

𝐂𝐚𝐥𝐜𝐮𝐥𝐚𝐭𝐢𝐧𝐠 𝐍𝐮𝐭𝐫𝐢𝐞𝐧𝐭 𝐂𝐨𝐧𝐭𝐞𝐧𝐭 𝐅𝐫𝐨𝐦 𝐅𝐞𝐫𝐭𝐢𝐥𝐢𝐳𝐞𝐫 𝐍𝐮𝐦𝐛𝐞𝐫𝐬

We now know that the 3 numbers on fertilizer bags are percentages. For example, if the first number is 10, then that means that 10% of the total 25-pound fertilizer bag is made up of nitrogen.
If you recall from your math classes, you can divide a percentage number by 100 (by moving the decimal place to spots to the left) to represent it as a decimal instead. So 10% is the same as 0.10, just written differently so it can be used in math equations.

This allows us to take the total weight of the bag and multiply that by the percentage in decimal format to find out exactly how much of a primary nutrient is in the fertilizer mixture.
In our example, we can take 25 pounds and multiply it by 0.10 to find that our fertilizer bag contains 2.5 pounds of nitrogen.

Let’s run through a full example with a 50-pound bag of 6-6-18 fertilizer (remember, the order of the numbers is N-P-K):

Nitrogen: Divide 6% by 100 to get 0.06 and multiply that by 50 pounds, which equals 3 pounds.
Phosphorous: The number is also 6%, which ultimately equals the same 3 pounds.
Potash: Divide 18% by 100 to get 0.18 and multiply that by 50 pounds, which equals 9 pounds.
This means a 50-pound bag of 6-6-18 fertilizer contains a total of 15 pounds of nutrients. These are 3 pounds of nitrogen, 3 pounds of phosphorous, and 9 pounds of potash. The remaining 35 pounds is made up of filler material.

𝐖𝐡𝐲 𝐓𝐡𝐞𝐬𝐞 3 𝐅𝐞𝐫𝐭𝐢𝐥𝐢𝐳𝐞𝐫 𝐍𝐮𝐦𝐛𝐞𝐫𝐬 & 𝐍𝐨𝐭 𝐎𝐭𝐡𝐞𝐫𝐬?

Though there are 17 nutrients that are critical to supporting all plant life, nitrogen, phosphorous, and potassium are the three primary macronutrients that they need more than all others.
There are others, like iron, that are also essential to plant life but are only needed in much smaller amounts. Flowers, like azaleas, need more iron than your lawn, for instance.
This is why when you look at specialty fertilizers, they will mention the plant-specific nutrient on the front and in the ingredients list.
Because plants need these macronutrients in larger quantities, they can drain their soil of these nutrients over time, sometimes depleting them entirely. Heavy and constant rain causes nitrogen to flow out of the soil (called leaching). Replenishing the soil with these three nutrients matters more than any others, which is why these three are the numbers on fertilizer bags.

𝐇𝐨𝐰 𝐃𝐨 𝐓𝐡𝐞𝐬𝐞 3 𝐍-𝐏-𝐊 𝐍𝐮𝐦𝐛𝐞𝐫𝐬 𝐁𝐞𝐧𝐞𝐟𝐢𝐭 𝐏𝐥𝐚𝐧𝐭𝐬?
We know each of these 3 macronutrients is critical to plants, but how exactly? Nitrogen (N) provides the ability to grow at a normal rate with plentiful and rich green leaves.
Of course, this helps them have more chlorophyll and helps with photosynthesis (absorbing light and converting it to energy).

A low amount of nitrogen causes plants to slow their growth and lose their green color, resulting in yellow-green leaves (called chlorosis). Not only is it unsightly but your garden crops will produce a lesser and weaker abundance of fruit and vegetables.
Too much nitrogen causes plants to focus on their leaves with less emphasis on the flower’s bloom or the food product.

Phosphorous (P) helps the plants use all other nutrients efficiently, but also guides the plants through a strong development of their roots and blooms.
The right amount of phosphorous means you’ll have beautiful flowers, healthy fruits, and potent seeds. Too much or too little will lead to a lopsided development.

Potassium (K) aids in general growth, including root and top growth. It not only boosts growth in the right amounts but regulates overgrowth where a plant can over-extend itself.
It also strengthens the cell walls, allowing for stronger stems and wind and weight resistance. It also helps with disease and pest resistance as well as from cold weather or drought conditions.

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