Understanding How to Choose the Proper Fertilizer for Optimal Plant Growth

“NPK? Is that a ’90s singing group?” Full disclosure, I thought it was. Don’t judge, it was during the era of BBD (Bell Biv Devoe), SWV (Sisters With Voices) , and TLC (T-Boz Left-Eye Chilli) — so when I heard NPK mentioned for the first time, I thought I was missing out on new music. This misunderstanding on my part was cleared up once I asked, “Tower Records have the CD?” I got schooled that day, and here is the takeaway about understanding the letters and numbers on plant fertilizer labels.

Image by Hemerocallis

The Letters

Nitrogen (N), Phosphorus (P), and Potassium (K) are the three key nutrients that plants require to thrive.

Nitrogen encourages foliage growth, among other benefits. Fertilizers high in nitrogen are often used for grass, or for other plants where green foliage growth is more important than flowering.

Phosphorus contributes to healthy root growth, assists with disease resistance, and aids in the production of fruits, vegetables, and setting flower buds.

Potassium assists with the growth of strong stems, root development, and increases the ability of plants to resist diseases.

The Numbers

Fertilizers are clearly labeled with a three-part number on the front of the package. The number will be separated by dashes, reading something like this: “5-10-5”. This measurement refers to a percentage by the weight of the three major nutrients in the fertilizer product.

First Number

The first number gives the percentage of Nitrogen. A “5-10-5” fertilizer would contain 5 percent nitrogen by weight. So for every pound of fertilizer applied there is really only .05 pounds of nitrogen. In a 10-pound bag of “5-10-5” fertilizer, then, there is 0.5 pound of nitrogen.

Second Number

The middle number gives the percentage of Phosphorous. A “5-10-5” fertilizer contains 10 percent phosphorous by weight or .1 pounds of phosphorous per pound of product. A 10-pound bag, then, contains 1 lb. of phosphorus.

Third Number

The final number gives the percentage of Potassium. A “5-10-5” fertilizer contains 5 percent potassium by weight or .05 pounds of potassium per pound of product. In a 10-pound bag, there is .5 lb. of potassium.

Choosing a Product

Do not get overwhelmed with the vast selection of products on the market. I generally choose products labeled: “Certified Organic” or “Demeter Certified Biodynamic.” This is a personal choice. Use the product that you feel will best benefit your garden.

Fertilizers have a nutrient release rate of:

  • Immediate (0 to 1 month)
  • Short term (1 to 4 months)
  • Long term (4+ months)

Some fertilizer products are used:

  • for direct application to your soil
  • to be added to your water for watering in
  • as a foliar spray to get nutrients directly to the leaves

Here is a handy guide PDF compiled by the folks at Peaceful Valley Farms on their extensive range of fertilizers. We use this guide to help us understand and purchase fertilizer products.

Conducting a Soil Test

It is a good idea to test your soil, especially if you are starting new on a plot. Taking the time and expense to do a soil test once will save you from headaches in the long run. A soil test allows you to know what you are working with, and what you need to address in regards to NPK and other nutrients.

Photo by Kim Mendoza

“Don’t guess — test! Applying fertilizers and soil amendments without the guidance of a professional soil test is like driving downtown with your eyes shut. Applying too much of one nutrient can tie up others and wreak havoc on your soil biology. “

– Fedcoseeds.com

There are many companies that do soil tests by mail. They send you a handy packet to collect a sample in, then you simply return that sample to them. Within a couple of weeks, you’ll get your results along with suggestions on the products needed to get your soil ready for growing! Start by finding your local Cooperative Extension to locate a soil testing lab in your area. You can also use this service.

When it comes to fertilizers, the first thing is: DO NOT GET OVERWHELMED! There are many resources available to help you in your gardening efforts, and to help you work towards successfully following the Five Steps To Farming With Your Fork. Understanding the overall needs of your plants and soil enables you to enjoy gardening while Farming With Your Fork!

Do you have a product you highly recommend? Please share with us.


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