* All ppm numbers on this page use the U.S. 500 scale.

The TDS pen is a valuable and probably the most used tool in indoor grows. If you don’t have one already get yourself a good quality TDS pen so that you can take accurate measurements of your nutrient solution and nutrient runoff on a regular basis.

If you are using a drain to waste gardening style, the nutrient runoff test should be a regular task. A drain to waste garden is simply a containerized top feed style that typically uses an inert gardening medium like perlite and peat or coco. You feed from the top and let 10-20% (for well rooted plants) of the nutrient solution run out of the bottom of the container and then wait until your peat is nearly dry before the next application. Often this type of setup follows a feed water repeat or a continuous feed fertilizing schedule. You should perform nutrient solution runoff tests to check the ppm tds (parts per million – total dissolved solids) coming out of the container to determine how much of the nutrients from the last feeding were taken up by the plant and more importantly how much was left over. This is an important test because you want to avoid accumulating unassimilated nutrients/salts in the growing medium.

For example, if you were to apply 600 ppm of nutrient solution on a feed water repeat routine and on the following water day your nutrient runoff test is 200 ppm, then that means your plant has utilized 400 ppm of the nutrient solution since the last feeding and has left 200 ppm in the growing medium that partially washed out with the recent water application (600 new – 200 left over = 400 assimilated). In this example provided your Nutrient Ratios are correct, you would probably want to slightly adjust your nutrient solutions ppm up by 200 ppm for a total of 800 ppm since there will be a nutrient shortage of 200 ppm before the next feed day because we now know the plants are consuming 400 ppm between each application.

Utilizing a continuous feed schedule is the recommended option by Pro-Mix for a peat & perlite containerized drain to waste system as it is for coco based grows. For best results when utilizing a peat & perlite setup you should use nutrients made specifically for hydroponic gardening where the nutrient types are water soluble and immediately available for uptake by the plant. Otherwise you will need to incorporate microorganisms like Mycorrhizae, Bacillus Bacteria, and Amino Acids in order to keep Calcium and Phosphorus unlocked in the growing medium and to convert Urea Nitrogen (found in non-hydroponic fertilizers) into Ammoniacal Nitrogen that the plants can use.

Whether you decide to use a feed water repeat or a continuous feed routine you should start with the following nutrient charts weekly application totals and then adjust accordingly as determined by your runoff test results. Also be aware that well amended peat with a higher Cation Exchange Capacity will hold on to some of the minerals in your nutrient solution during application leading to an output ppm that is slightly less than the input ppm, this should be considered in your runoff tests to fully understand what is happening in the root zone.

When using either a continuous feed or a feed water repeat routine try to have 10-20% of your solution (water or fertilizer) run out the bottom of the container for well rooted plants at each application to help flush the growing medium of any left over salt buildup and then dispose of the runoff. Young plants with minimal roots should be watered more sparingly to prevent overwatering so that the roots can search out the medium for moisture and not sit in a waterlogged medium which can become anaerobic.

You should run some medium runoff tests prior to planting to see exactly what you are starting off with and to make sure you didn’t over apply calcitic lime as that will toxify the soil and lock up your pH, read more about Amending Peat for Cannabis Cultivation.

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