3 Key Concepts for Understanding Fertilizer

Providing sufficient nutrients to the plants in your commercial greenhouse is one important part of your successful business. With inadequate nitrogen, phosphorus, potassium, and other amendments, plants wilt, grow leggy, underproduce flowers, and die. Proper feeding helps you grow vibrant, healthy plants! In this article, we’ll go over some of the common terminologies you’ll need to understand as you research and plan a fertilizer program for your greenhouse. 

PPM

PPM stands for Parts Per Million, and it’s the typical unit used to tell growers how much nitrogen to feed a plant. For example, a common recommendation for many crops is a constant feed in the irrigation water of 150-250 ppm N. This indicates the concentration of fertilizer solution to water.

A typical balanced fertilizer like 20-20-20 NPK contains 20% nitrogen. How much soluble fertilizer to use depends on the injector ratio and desired ppm. To provide 200 ppm N using 20-20-20 fertilizer and a 1:100 injector, you’ll need to use about 13.33 ounces of fertilizer per gallon of water. Since PPM is a universal unit, you can easily determine the needed amount of fertilizer using a basic equation.

This equation will determine ounces of soluble fertilizer needed per gallon:

Desired Concentration x Dilution Factor (injector ratio)  

Percentage of N in fertilizer x 75

For the example above: 

(200 ppm 100)(20 75) =13.33 oz/gallon

Or, for a 1:200 injector:

(200 ppm 200)(2075) = 26.66 oz/gallon

TDS

TDS stands for Total Dissolved Solids, which is another term for the ppm500 scale. A digital TDS meter measures the total dissolved salt and mineral content using ppm units. EC meters are interchangeable with TDS meters. TDS measurements are typically expressed as ppm or milligrams per liter. EC measurements are usually represented as millisiemens per centimeter (mS/cm). Most digital meters will switch modes to give you EC in mS/cm or TDS in mg/l or ppm. 

Use the TDS/EC meter to take an initial reading of your clear water before you add nutrients and also to test the EC of the solution before you feed it to your plants. 

EC

Electrical Conductivity is a measure of how well a solution conducts electricity, which indicates the salt content of the solution. More salt leads to better conductivity, while less salt leads to lower conductivity. EC doesn’t account for specific nutrients but is a broad indicator of soluble salt content. Check out our article on the PourThru EC method for more information on how to test the levels in your greenhouse soil and optimize your fertilizer program. 

To convert an EC reading to PPM, multiply based on the ppm scale you’re using. Standard ppm scales include ppm500, ppm650, and ppm700. An EC reading of 2.4 mS/cm converted to the ppm500 scale would equal 1200ppm. It’s very important to pay attention to which scale a given source is using. Most meters come with a chart to quickly convert an EC reading to whichever ppm scale you’re using. Once you do the math, you can know the range you want to stay in and what that range is for EC and ppm. 

As always, we’d love to answer your questions, so you have the information you need to successfully grow vibrant, viable crops!