Water, EC and Nutrient Solution

The concentration of the nutrient solution has an enormous effect on plant development and growth. Measuring the overall concentration or strength of a ‘balanced' solution is essential. Focus on nutrient balance and concentration in the solution to head off deficiencies before they cause big problems.

The concentration of the nutrient solution has an enormous effect on plant development and growth. Measuring the overall concentration or strength of a ‘balanced’ solution is essential. Focus on nutrient balance and concentration in the solution to head off deficiencies before they cause big problems.

Fertilizers (dissolved ionic salts) conduct electrical current when in solution. The ions in an ionic compound are held together by ionic bonds. These ions “cation” (+ positive) and “anion” (– negative) have positive and negative charges that attract one another and bond. Nutrient (salts) concentrations are measured by their ability to conduct electricity through a solution. A dissolved salts meter measures the overall concentration or strength of a nutrient solution. For example, pure distilled water has no resistance and conducts virtually no electrical current. When nutrients (dissolved ionic salts) are added to pure distilled water it conducts electricity. A greater concentration of nutrients in solution conducts more electricity.

Irrigation water is often full of ionic salts that affect the uptake of nutrients.

Several scales are currently used to measure how much electricity is conducted by nutrients including: Electrical Conductivity (EC), Conductivity Factor (CF), Parts per Million (ppm), Total Dissolved Solids (TDS), and Dissolved Solids (DS). Most US gardeners use ppm to measure overall fertilizer concentration. European, Australian, and New Zealand gardeners use EC, however they still use CF in parts of Australia and New Zealand.

Each variety of cannabis has an ideal EC range for optimum growth. Some varieties are incredibly heavy feeders while others are easy to over-fertilize. Check with seed and clone vendors for details. A high EC causes ‘water stress’ causing plant cells to lose water. The water moves via osmotic pressure into the more concentrated solution surrounding roots. Wilting foliage is the first sign of an EC that is too high. When a mild EC overdose occurs plants compensate and foliage growth becomes tough or hard, with brittleness to it. Foliage is often darker green and plants are shorter with smaller leaves.

Brittle leaves are the product of a high EC.

Many commercial cannabis gardeners give their flowering crops progressively higher EC concentration. Flower buds plump up and put on weight but this practice tends to make flower buds develop a very harsh taste when smoked or vaporized due to the excess salts left in plant tissues. The residual ash is also very dark and abundant.

EC is also affected by water uptake. On hot days when more water is taken up from the solution, nutrients concentrate and EC climbs. A low EC also causes more water uptake and foliage soon becomes weak and soft, often lighter green too. However, lowering EC during hot spells is essential to avoid problems. Measure EC daily and adjust in relation to growing conditions accordingly.

An imbalanced EC caused the potassium burn.

To check the EC of the nutrient solution, collect samples from the reservoir, growing medium and runoff. Save time and effort; collect EC and pH samples simultaneously. Collect samples with a syringe or turkey baster used for cooking at least 5 cm deep into the rockwool or growing medium. Collect separate samples of runoff and from the reservoir. Place each sample in a clean jar – washed and triple rinsed with double distilled water. Use a calibrated EC meter to measure each of the samples and record measurement on a piece of paper.

Foto collecting nutrient solution samples with a large syringe

a. Nutrient Reservoir
b. Substrate
c. Runoff

Measure pH and EC (ppm) at the same time every day.

Under normal conditions, the EC in the growing media and runoff should be a little higher than the nutrient solution in the reservoir. If the EC of the solution drawn from the growing medium is substantially higher than the one from the reservoir, there is a fertilizer salt buildup in the substrate. Correct the imbalance by leaching substrate thoroughly with diluted nutrient solution, and replace with new solution. Regularly check the EC of your water, slab, and runoff.

EC Guidelines

Growth Stage EC Range

Seedling 0.8-1.3
Clone 0.5-1.3
Vegetative 1.3-1.7
Flowering 1.2-2

Note: These guidelines are recommendations only. Different varieties of cannabis require higher or lower EC values than listed above.

Let a minimum of 20 percent of the nutrient solution drain from growing media after each irrigation cycle to help maintain EC stability. The runoff carries away any excess fertilizer salt buildup in the growing medium. If the EC level of a solution is too high, increase the amount of runoff so that 30 percent of the solution drains out the bottom of containers. To raise the EC, add more fertilizer to the solution, or change the nutrient solution.

This sick leaf is a product of an imbalanced EC. The native input water had a ppm reading of 680!

Many factors can alter the EC balance of a solution including irrigation, evaporation and nutrient uptake by roots. For example, if the substrate is under-watered or allowed to dry completely, the EC reading will rise. In fact, the EC may increase to two- or three-times as high as the input solution when too little water is applied to rockwool. This increase in slab EC causes some nutrients to build up faster than others. When the EC doubles, the amount of sodium can increase as much as four- to six-fold under the right conditions! There should not be any sodium present in your garden unless it is in the water supply, and it should not be in excess of 50 ppm.

Nutrient solution concentration levels are also affected by nutrient absorption by roots and by water evaporation. The solution weakens as plants use nutrients, but water also evaporates from the solution, which increases the nutrient concentration. Counteract the concentration of fertilizer salts by regularly adding plain water to the nutrient solution to replace that used by plants.

A constant readout pH meter makes keeping track of the nutrient solution much easier.

Solution Mixing and Maintenance

If possible get a water analysis before it is mixed with hydroponic nutrients. A water analysis will indicate the dissolved ionic salts already in solution. For example, hard water contains elevated levels of calcium and magnesium. Both elements should be added sparingly to nutrient solutions. Soft water has very few impurities (ionic salts) that cause pH to fluctuate, requiring chemical buffers, usually calcium and calcium compounds, to be added to solution. If no water analysis is available from your local water district, a simple EC reading will measure the overall concentration of the dissolved solids (ionic salts) in native water. If growing hydroponically and EC is 0.3 or higher, treat water with reverse osmosis before adding nutrients.

Use an accurate easy-to-read container to measure out nutrient dosage.

Plants use so much water in relation to nutrients that nutrient solutions need to be replenished regularly. Casually “topping off” the reservoir with pH-balanced water daily will keep the solution relatively balanced for a week maybe two. Use an electronic EC pen to monitor the level of dissolved solids in the solution. Occasionally you will need to add more fertilizer concentrate to maintain the EC level in the reservoir during “topping off.” Keep the reservoir full at all times. The smaller the reservoir, the more rapid the depletion and the more critical it is to keep it full. Employing an automatic filling function for smaller reservoirs will help ensure a balanced nutrient solution.

A reverse osmosis filter will remove ionic salts (dissolved solids) from native input water.

A few gardeners top off the nutrient solution with 500-600 ppm-strength nutrient solution every two to three days. If topping off with nutrient solution keep the EC within safe limits. Avoid problems by draining it and adding fresh solution regularly.

Most gardeners leach the entire system with weak nutrient solution for an hour or more between changing the reservoir. A mild fertilizer solution avoids an absence of nutrients for any amount of time. But, the EC will still drop to the levels the medium is leached with which removes all the excess, resets the ratio, and ensures the plant has nutrients at all times.

Check EC of reservoir, growing medium, and runoff nutrient solution at the same time every day. Check solution temperature to ensure adequate dissolved oxygen is available to plants.