Growing Media

How to test and read a soilless growing mix

A soilless mix report looks like a soil test report. It has the same pH, the same EC, and the same nutrient lines in ppm, but almost nothing about it transfers directly. Three different extraction methods give three different numbers for the identical pot, the ppm is measured by volume instead of mass, and peat, coco and bark each carry their own chemistry before a single drop of fertilizer touches them. This covers testing a container mix on its own terms, not as a stand-in for a field soil report.

Updated September 2026 9 min read

Which method you're reading

Three water-extraction methods test a soilless mix: one you send out, two you can run yourself. They are not interchangeable readings of the same thing, they are three different procedures that produce three different numbers for the identical pot, so the first thing to know about any reading is which one produced it.

SME (Saturated Media Extract). The standard most commercial and university labs run. Distilled water is worked into a sample until the surface glistens, a true saturated paste, and the extract is drawn off after it equilibrates. Needs lab equipment and a bit of skill; not something you do on a bench in the greenhouse.
1:2 dilution. Air-dried media and water combined at a fixed 1:2 volume ratio, shaken, filtered. Simple enough to run yourself with a pH/EC pen, which is its main appeal.
PourThru. No sampling or lab step at all: irrigate a representative container to saturation, wait roughly 30 to 60 minutes for it to drain, and catch the leachate. The only method that reads the mix as the plant's roots sit in it, and the only one you can run daily without sending anything anywhere.

Each adds a different amount of water relative to what the mix holds, so the resulting solution is diluted by a different amount, and every reading downstream, EC included, comes out different. The same pot, tested all three ways on the same day, can read as three different nutritional stories: one widely cited comparison has an EC of 2.6 mS/cm come back "too high" under 1:2, "normal" under SME, and "low" under PourThru.

Match the table to the test

Whatever interpretation table you're reading a number against, confirm it was built for the same method that produced the number. A "normal" range copied from an SME sheet applied to a PourThru reading isn't approximately right, it's reading against the wrong scale entirely.

Pulling a representative sample

A field soil sample can sit for weeks without changing much, but a soilless mix in a container cannot: its EC and pH shift within hours of a feed, and shift again as the plant draws water and leaves salts behind between feeds. Sampling here is as much about timing as it is about where you dig.

Same mix, same crop, same age. Composite several containers of the identical mix, crop and planting date, not a spread of different pots at different points in their cycle. Mixing those together blurs a real signal into noise.
Sample the root zone. Pull from several points around each container, an inch or two below the surface and down into the middle of the root ball, avoiding the very top crust and the drainage layer at the bottom, both of which read differently from where roots are.
Fix the timing relative to the last feed. Whatever point in the fertigation cycle you sample at, test at that same point every time. A reading taken right after fertilizing and one taken just before the next feed can look like two different growing programs when it's the same one, sampled at two different moments.
PourThru has its own protocol. Irrigate to the point of light leaching, wait about 30 to 60 minutes, then collect what drains. Too little wait time under-extracts; too long lets the sample sit and drift.

pH and EC: the two that move fastest

pH in a soilless mix runs lower than the field-soil target most growers already know: about 5.4 to 6.4 for most greenhouse and nursery crops, with SME specifically calibrated closer to 5.5 to 6.0. Below that band, iron and manganese can climb toward toxicity in sensitive species; above it, iron and manganese lock up the same way they do in a high-pH field soil.

EC is not a nutrient reading, it's the total dissolved salt concentration, every ion in solution added together, fertilizer and any residual salts alike. A high EC says the root zone is salty; it says nothing on its own about which nutrient, if any, is short or in excess. Target ranges depend on both the method and the crop stage, since a young seedling can be injured by an EC an established finishing crop handles without notice:

1:2 dilution, normal 0.8 to 1.3 mS/cm The most dilute of the three, so the lowest numbers.

SME, normal 2.0 to 3.5 mS/cm Established plants; seedlings and young transplants run lower, roughly 0.7 to 1.0.

PourThru, normal 2.6 to 4.6 mS/cm The least dilute of the three, so the highest numbers, for the same actual salt level.

"Normal" ranges by method, established plants. Notice the same underlying salt level reads as three different numbers depending only on which test drew it out.

The table above is for interpreting a reading you already have. A second, related number, how strong to mix the fertilizer solution you feed in the first place, is a different question with its own target, and the two aren't interchangeable: a feed mixed to the right strength and a substrate that has been accumulating salts between waterings can show very different EC.

EC to ppm Converter Converts between EC scales and ppm, and gives target feed strength by crop, stage, and growing medium, including soilless mix, for the solution you deliver, not for a test reading.

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Reading the nutrient lines

A soilless report tests the same handful of nutrients as a field soil report, nitrate-N, phosphorus, potassium, calcium and magnesium, sometimes the micronutrients too. But on an SME sheet the numbers arrive in a different unit entirely, and land in a different range because of it:

Nitrate-N 60 to 175 ppm The nutrient most likely to swing week to week with the fertigation program.

Phosphorus (P) 5 to 15 ppm A tenth of a typical field soil's sufficiency floor. See the callout below before assuming this mix is short on P.

Potassium (K) 75 to 200 ppm Close in scale to a field soil reading, which makes it the easy one to mistakenly compare directly.

Calcium (Ca) 75 to 250 ppm A fraction of a field soil's range; container crops carry far less total calcium reserve to draw on.

Magnesium (Mg) 40 to 75 ppm Watch alongside calcium: a mix fed hard water or dolomitic lime can run high on both without anyone adding either on purpose.

SME "normal" ranges for containerized greenhouse crops. 1:2 and PourThru run to their own, more dilute scales, same as the EC table above.

This is not a soil test, don't compare the numbers

A field soil report reads phosphorus in mg per kilogram of soil, a mass basis. A soilless report reads it in mg per liter of extract, a volume basis, because container growing media is bought, mixed and irrigated by volume, not weighed out by the pound. The two units share a name, ppm, and nothing else: 15 ppm P on a soil report and 15 ppm P on an SME report are not the same amount of phosphorus relative to the root zone, and a mix's own sufficiency floor sits far below a field soil's for exactly this reason. If you're working from the field soil version of this guide, its ppm numbers don't transfer here.

What "available" means here

A ppm reading, however you produced it, is what's currently dissolved in the extract, and that is not the same thing as everything the mix will eventually give up. How much of a gap there is, and which direction it runs, depends on what the mix is made of.

Peat and bark carry a meaningful share of their nitrogen and sulfur locked in organic matter that hasn't broken down yet. That pool releases slowly, driven by soil microbes and therefore by temperature and moisture, not by anything a single extraction captures. A fresh, peat-heavy mix can test low on soluble N or S in week one and still have real reserve arriving over the following weeks as that organic fraction mineralizes, the soilless equivalent of the slowly-released organic nutrient pool a field soil holds in its own organic matter.

Coco coir runs the opposite problem. Its exchange sites arrive from the factory already loaded with potassium and sodium, up to roughly 40% and 15% of its capacity, with little calcium or magnesium on board at all. Feed a nutrient solution straight into unbuffered coir and the coir strips the calcium and magnesium out of it, holding them in exchange for dumping its own potassium and sodium back into solution, right where the roots are.

Buffer coco before you plant, not after

Soak unbuffered coir in a calcium-and-magnesium (cal-mag) solution for 8 to 24 hours before use. That pre-loads the exchange sites with calcium and magnesium so your feed solution isn't spent displacing sodium and potassium the first time it touches the mix. Skipping this step is the single most common reason a coco-grown plant shows a calcium or magnesium deficiency despite a fertilizer program that supplies plenty of both.

Soilless Mix Calculator Build a mix by parts, peat, coco, perlite, vermiculite, bark and compost, and scale it to any batch size.

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Keeping it in range

A container's whole nutrient supply passes through it with every watering, so drift is normal and expected, not a sign anything is wrong on its own. Two habits keep it inside the range rather than chasing it after the fact.

Leach on purpose. Irrigate with roughly 10 to 20% more solution than the container holds, so some drains out the bottom every time. That excess is what carries away salts left behind as the plant takes up water faster than it takes up fertilizer, which is most of the time. Skip it for more than a few feeds and EC climbs on its own, independent of how much you're feeding.
Test on a schedule, not just when something looks wrong. PourThru's whole advantage is that it costs nothing but a saucer and a meter, so it's the one method suited to a weekly check rather than something you send out and wait on. Catching a pH or EC drift a week early is the difference between a small correction and a rootball that needs flushing.

What to take away

SME, 1:2 dilution and PourThru give three different numbers for the same pot. Match the interpretation table to the method that produced the reading, always.
EC measures total dissolved salt, not any one nutrient. High or low on its own says nothing about which nutrient is short or in excess.
Soilless ppm is mg per liter (volume); field soil ppm is mg per kilogram (mass). The two are not comparable, and a soilless mix's own sufficiency ranges run far lower for exactly this reason.
Peat and bark hold back organic N and S that release slowly over weeks. A fresh mix testing low isn't necessarily short, it may just not have mineralized yet.
Buffer coco coir with cal-mag before planting, or its exchange sites will trade your calcium and magnesium for the sodium and potassium it arrived with.

Sources

General guidance, drawn from university extension services and one peer-reviewed journal article. Target ranges shift by crop, growth stage and the specific mix, so an interpretive chart matched to your specific method and crop, whether that comes with a meter, a self-test kit, or a lab, is the more precise word on any one of these.

University of Massachusetts Extension, "Methods of Greenhouse Media Testing and How They Differ" ↗. The SME, 1:2 and PourThru comparison, including the EC table and the 2.6 mS/cm example.
University of Massachusetts Extension (Soil and Plant Nutrient Testing Laboratory), "Commercial Floriculture Soilless Media Report" ↗. Target pH and nutrient ppm ranges under SME.
University of Georgia Extension, "EC and pH: What Is It and Why Does It Matter?" ↗. What EC measures, and target ranges by crop stage.
USDA Natural Resources Conservation Service, "Soil Quality Indicators: Bulk Density" ↗. Typical mineral soil bulk density, for comparison against peat's.
Silber, A. et al., "Nutrient Chemistry in Coconut-Mineral Mixed Substrates for Soilless Culture," Advances in Horticultural Science ↗. Coco coir's exchange chemistry and its native potassium and sodium load.
New Mexico State University Extension, "Sulfur and New Mexico Agriculture" ↗. The organic and adsorbed sulfur pools an extraction can miss, and how the monocalcium phosphate method accounts for the adsorbed fraction on mineral soils.

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