Nutrient Runoff Testing

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Drain to Waste Gardening

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 soil or an inert gardening medium like perlite and peat or coco. You feed from the top and let a little nutrient solution run out of the bottom of the container and then wait until the medium is completely dry before repeating the process. Often this type of setup follows a feed water feed water fertilizing schedule in order to prevent toxic salt buildup in the growing medium. The grower will perform a nutrient solution runoff test 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. If the input nutrient solutions ppm is consistent at every feeding one can determine if they need to adjust their nutrients ppm up or down.

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For example, if you were to apply 300 ppm of nutrient solution at each feeding and your nutrient runoff test is 400 ppm, then that means that your plant has only utilized 200 ppm of the nutrient solution since the last feeding and has left 100 ppm in the growing medium that washed out with the latest feeding (300 new + 100 left over = 400ppm). If the nutrient solution isn’t adjusted down to compensate for this, salt buildup in the growing medium is a possibility unless you implement the feed water feed water routine and even then you can have problems if your input is too high. But you can do better for the plant by adjusting the nutrient solution ppm down, in this example case to 200 ppm. Then your plants will be able to utilize nearly 100% of what was given to them which will allow you to use a feed feed feed feed routine instead provided that you are using nutrients more in line with hydroponic gardening. The following equations will help you calculate the important variables:

 

Current Runoff – New Input =
Unused Nutrient since last feeding

Previous Runoff – Unused Nutrient =
Assimilated Nutrient since last feeding

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In both examples described below, you will want to get as close as possible to your target Current Runoff ppm so that you can avoid nutrient deficiencies or toxic salt buildup in the growing medium.

For ‘feed water feed water’ gardens you will want the Current Runoff to be approximately equal to the Previous Runoff ÷ 2 on the Water Day and then on the Feed Day the Current Runoff should be approximately equal to the Previous Runoff x 2. The goal here is to give the plant an amount of nutrient on the feed day that it can assimilate 50% of by the water day. If you are giving your plants the correct amount and Nutrient Ratios at each feeding you will be in good shape. If your Nutrient Ratios aren’t correct, like not giving enough Potassium or Magnesium, you may have trouble with unassimilated nutrients building up in the growing medium because the elements in fertilizer have both antagonistic as well as synergistic effects on other elements availability. Check out the Mulders Chart to see this nutrient relationship. You must also consider the size of the plants because this is not a static formula and will increase over time as the plants get larger.

Example Feed Water Feed Water

Water Day

Current Runoff 350 ppm
New Input – 0 ppm
Total Nutrient Unused 350 ppm
   
Previous Runoff 710 ppm
Total Nutrient Unused – 350 ppm
Total Nutrient Assimilated 360 ppm
   

Feed Day

Current Runoff 710 ppm
New Input – 700 ppm
Total Nutrient Unused 10 ppm
   
Previous Runoff 350 ppm
Total Nutrient Unused – 10 ppm
Total Nutrient Assimilated 340 ppm
   

For ‘feed feed feed feed’ gardens you will want the Current Runoff to be slightly greater than the New Input, ideally by around 10-50 ppm otherwise you really don’t know the active root zone situation. If the Current Runoff = New Input then the plant was either at 0 ppm Neutral and ready for fertilization or the plant is nutrient deficient by an unknown amount. Once you know your plant is optimally healthy the goal would be to give a near exact amount of nutrient which can be fully assimilated between each feeding, this is known as paying the maintenance cost. This can be a little tricky to figure out but after some trial and error you should be able to work out how much of each nutrient your plants want, check out the Cannabis Nutrient Ratios page to get a general understanding of this for a full size plant.

Example Feed Feed Feed Feed

Feed Day

Current Runoff 360 ppm
New Input – 350 ppm
Total Nutrient Unused 10 ppm
   
Previous Runoff 350 ppm
Total Nutrient Unused – 10 ppm
Total Nutrient Assimilated 340 ppm
   

Next Feed Day

Current Runoff 360 ppm
New Input – 350 ppm
Total Nutrient Unused 10 ppm
   
Previous Runoff 360 ppm
Total Nutrient Unused – 10 ppm
Total Nutrient Assimilated 350 ppm
   
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