Nutrient Runoff Testing

cannabis-nutrient-runoff-testing-m3

Drain to Waste Gardening

* 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.

tds-pens-oakton-bluelab-m1

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.

 

growth-stage-ppm-m5
shadow

Runoff Equations:

Current Runoff – New Input =
Unused Nutrient since last feeding

Previous Runoff – Unused Nutrient =
Assimilated Nutrient since last feeding

For Feed Water Repeat 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 approximately 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.

An important variable not included below that you need to consider is your particular soils average Cation Exchange Capacity or CEC. If you are pouring 700 ppm solution in the top and only getting 550 ppm in the runoff, this means your mediums CEC is grabbing about 20% of the minerals from your solution. Even if your runoff is > the input you will need to recognize and account for your soils average CEC to have an accurate observation of what is really happening in the root zone. Not understanding how CEC works in mediums like peat can make your runoff observations seem to not make sense, read more about a soils CEC here.

Example Feed Water Repeat

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
   

Example Continuous Feed

When growing in inert mediums like peat or coco that have no nutritional value it is suggested to use a continuous feed routine instead of the feed water repeat scenario. The only difference is to make sure you are giving your plants an amount of nutrient (tds ppm) that it can fully assimilate before the next application. For a plant in late vegetative growth this will be around 700 ppm tds.

Feed Day

Current Runoff 720 ppm
New Input – 700 ppm
Total Nutrient Unused 20 ppm
   
Previous Runoff 710 ppm
Total Nutrient Unused – 20 ppm
Total Nutrient Assimilated 690 ppm
   

Alternative Runoff Equations:

Previous Runoff + New Input =
Total Nutrient Input

Total Nutrient Input – Current Runoff =
Total Nutrient Assimilated Since Last Feeding

Current Runoff – New Input =
Total Nutrient Unused Since Last Feeding

Example Feed Water Repeat

Water Day

Previous Runoff 710 ppm
New Input + 0 ppm
Total Nutrient Input 710 ppm
   
Total Nutrient Input 710 ppm
Current Runoff – 350 ppm
Total Nutrient Assimilated
since last feeding
360 ppm
   
Current Runoff 350 ppm
New Input – 0 ppm
Total Nutrient Unused
since last feeding
350 ppm

Feed Day

Previous Runoff 350 ppm
New Input + 700 ppm
Total Nutrient Input 1050 ppm
   
Total Nutrient Input 1050 ppm
Current Runoff – 710 ppm
Total Nutrient Assimilated
since last feeding
340 ppm
   
Current Runoff 710 ppm
New Input – 700 ppm
Total Nutrient Unused
since last feeding
10 ppm
grow room, grow rooms, grow room setup, grow room lighting, grow room equipment, grow room lighting, grow room environment, grow room security, grow room light, grow room light ballasts, grow room fans, grow room air filters, grow room carbon filters, grow room gardens, grow room gardening, grow room garden, grow room checklist, grow room ventilation, grow room gardening basics, grow room pests, grow room diseases, grow room insects, grow room maintenance, grow room location, grow room electricity, grow room reflectors, grow room nutrients, grow room plants, grow room cannabis, grow room marijuana, grow room papaver somniferum, grow room poppies, grow room poppy, grow room potting soil, grow room hydroponics, grow room faqs, grow room questions, grow room forum, grow room help, grow room soil, grow room seeds, grow room seed storage, grow room cleanliness, grow room propagation, grow room flowering, grow room vegetative growth, grow room air, grow room humidity, grow room temperature, grow room schedule, grow room mylar, grow room panels, grow room plans, building grow rooms, grow room space, grow room containers, grow room timer, grow room Q&A, grow room problems