Measuring Brix — The Fuel Gauge of Plant Health and How to Read It Correctly

June 05, 2026 Steinkraft GmbH

Imagine if, with a single flick of the wrist, you could know in seconds whether your tomato is truly nutrient-rich. Whether your carrot has developed its full beta-carotene potential. Whether your soil is doing what you hope it is. Whether your foliar fertilization has worked—before you see it in the harvest.
Such an instrument exists. It costs between 20 and 350 Euros. It fits in any jacket pocket. And it is used worldwide in regenerative agriculture as one of the most important tools available.
It's called a refractometer. And what it measures is called the Brix value. Here in the picture, we measured the value of the red onion from our organic box.

What is the Brix value—really?

The Brix value—named after the German scientist Adolf Ferdinand Wenceslaus Brix—measures the density of plant sap, meaning how many dissolved solids it contains. Originally developed to measure sugar content in the sugar beet industry, it has taken on an entirely new significance in regenerative agriculture.
This is because sugar only accounts for about half of the dissolved solids in plant sap. The other half consists of minerals, amino acids, enzymes, hormones, and phytochemicals. Nutrient density in plant sap comprises about half assimilated sugar and half dissolved organic matter and mineral salts.
This means: A high Brix value doesn't just indicate sweet plant sap. It shows a plant that is fully and balancedly nourished—with sugar as an energy source, with minerals as building blocks, and with phytochemicals for protection.

What Brix has to do with pests—the most surprising connection

Here lies one of the most fascinating and practical connections in regenerative agriculture.
As early as 1969, Francis Chaboussou described how sick plants attract pests and diseases and enable their spread. Unbalancedly nourished plants cannot realize their full photosynthetic potential. This also leads to incomplete protein synthesis: instead of complex protein molecules, more short-chain amino acids are produced, which are particularly attractive to aphids and other sap-sucking insects.
US entomologist Dr. Tom Dykstra, over more than 20 years of research, found that the Brix of leaf sap not only correlates with the health, stress resistance, and nutrient density of the examined plant—it also explains any insect infestation.

Specifically, this means: A plant with a low Brix value is an invitation for pests. A plant with a high Brix value protects itself.
John Kempf—one of the influential figures in regenerative agriculture—developed the so-called Plant Health Pyramid based on this. Studies show that plants begin to thrive once they reach a certain level of health. From this point on, they develop natural resistance to diseases and pests.
This leaf sheen—the visible sign of a vital, well-nourished plant—is known to anyone who has ever seen a truly healthy plant. It is not a coincidence. It is a measurable result.

The Brix scale—what the numbers mean

The most well-known reference scale dates back to Dr. Carey Reams—an American agronomist and biochemist who has been studying the relationship between soil fertility and plant health for decades. His Brix tables are now used worldwide in regenerative agriculture.

0 to 6 °Brix—critical:
The plant performs hardly any photosynthesis and has virtually no functioning immune system. It is susceptible to all pathogens, diseases, and pests. Unfortunately, this is the range in which a lot of conventionally grown vegetables fall.
7 to 9 °Brix—sufficient:
The plant has relatively good chances of survival. Pathogenic molds, algae, and fungi can no longer grow on it. It increasingly produces secondary plant compounds. The taste begins to become noticeable.
10 to 13 °Brix—good:
Fruits and vegetables reach a higher quality level. The plant can simultaneously supply microorganisms in the soil and promote its own growth and immunity. In this range, true nutrient density and real taste begin.
14 °Brix and higher—excellent:
According to the Reams scale, nutrient-rich food of medicinal quality begins here. Insects can no longer tolerate the sap due to the high sugar content—natural pest control without chemicals. The plant optimally supplies soil life with root exudates and actively builds humus.

The Reams reference table for vegetables:

Carrots: poor 4 — average 6 — good 12 — excellent 18
Tomatoes: poor 4 — average 6 — good 8 — excellent 12
Onions: poor 4 — average 6 — good 8 — excellent 10
Broccoli: poor 6 — average 8 — good 10 — excellent 12
Lettuce: poor 4 — average 6 — good 8 — excellent 10
Cucumbers: poor 4 — average 6 — good 8 — excellent 12
Potatoes: poor 3 — average 5 — good 7 — excellent 8
Bell peppers: poor 4 — average 6 — good 8 — excellent 12
Cabbage: poor 6 — average 8 — good 10 — excellent 12
Pumpkin: poor 6 — average 8 — good 12 — excellent 14
Young corn: poor 6 — average 10 — good 18 — excellent 24

The Reams reference table for fruits:

Apples: poor 6 — average 10 — good 14 — excellent 18
Strawberries: poor 6 — average 10 — good 14 — excellent 16
Grapes: poor 8 — average 12 — good 16 — excellent 20
Pears: poor 6 — average 10 — good 12 — excellent 14<
Cherries: poor 6 — average 8 — good 14 — excellent 16
Oranges: poor 6 — average 10 — good 16 — excellent 20
Watermelon: poor 8 — average 12 — good 14 — excellent 16
Pineapple: poor 12 — average 14 — good 20 — excellent 22

General rule: The higher the Brix value, the higher the nutrient density.

The dividing line secret

With analog refractometers, there's a quality characteristic that goes far beyond the pure numerical value, which most users don't know.
The dissolved solids that aren't sugar make the refractive boundary in the refractometer blurry. The blurrier the refractive boundary, the higher the plant's nutrient uptake.
Specifically, this means: If you look into the refractometer and the boundary line between light and dark is sharp and clear—then the sap mainly contains sugar and water, but few minerals. If the line is fuzzy and blurred—then the sap is rich in minerals, amino acids, and secondary plant compounds.
A high, fuzzy line is the best sign a refractometer can give you. That's the moment you know: this plant is truly well-nourished.
That's why many experienced users in regenerative agriculture recommend an optical refractometer over a digital one—because only the optical one shows you this dividing line. The digital one only provides a number.

The devices—which refractometer for whom?

Level 1—Entry for the garden:
Analog handheld refractometers from brands like KERN & SOHN or inexpensive ATC models. Measuring range 0 to 32 °Brix, automatic temperature compensation (ATC) is important so you don't have to convert. You look through the eyepiece and read the line. When buying, look for a measuring range of at least 0 to 18 °Brix, better up to 32 °Brix.
Advantage: Inexpensive, robust, shows the dividing line.
Disadvantage: Subjective reading, no data storage.

Level 2—Professional field device:
ATAGO PAL-1—the worldwide classic for professional use. Result in 3 seconds, IP65 waterproof, measuring range 0 to 53 °Brix. Easy calibration with distilled water.
Anton Paar SmartRef—the most modern device on the market, from Graz. Result in under 2 seconds, IP66, Bluetooth connection to a free app for iOS and Android. The app stores all measurement data, visualizes it, and exports it. Perfect if you want to compare multiple crops over a season.

Practical recommendation: Combine two devices—an inexpensive optical one for assessing the dividing line and a digital Anton Paar or ATAGO for precise, comparable numbers throughout the season.

Step by step: How to measure correctly

Calibration before each measurement series: Place a few drops of distilled water on the measuring surface. The displayed value must be exactly 0 °Brix. If not, correct it with the calibration screw on the analog device or via the menu on the digital one.

The right time of day—the most important detail: The Brix value fluctuates significantly throughout the day. It increases with the sun and reaches its peak between 11 AM and 2 PM. For comparable measurements, always measure at the same time of day. Ideally between 11 AM and 2 PM. If it's cloudy, wait 5 minutes for sunshine.
Choose the right leaf: Select medium-sized, fully grown leaves—not very young ones, not very old ones. Leaves that are in full sun. For trees, the 4th or 5th leaf from the current year's shoot. 4 to 6 samples per plant for a representative picture.

Extracting the sap: Lightly squeeze the leaf or press it out with a small plant press. A modified garlic press works well. For hard vegetables like carrots, lightly crush a piece on a clean surface. Only 1 to 2 drops of sap are needed. Measure the sap immediately—it oxidizes quickly.

Measure and read: Place the sap on the measuring surface, close the lid on optical devices. On the optical device, simultaneously observe the sharpness of the dividing line.

Cleaning after each measurement: Rinse the measuring surface with distilled water and gently pat dry with a clean cloth—never rub, as this will scratch the optics.

When and how often to measure?

For farmers and larger operations: Create a simple measurement log—date, crop, variety, time of day, weather, Brix value, sharpness of the dividing line. After each foliar fertilization or soil treatment, measure after 24, 72, and 144 hours—this way you can see exactly if and how strongly your measure has worked. Even an increase of 2 °Brix after foliar fertilization is a clear positive signal.
Also compare the Brix values of your crops with neighboring weeds. The Brix value of the weeds should always be significantly lower than that of the cultivated plant.
For hobby gardeners: Once a week during the main growing season is sufficient for orientation. After each treatment with Grünkraft Calcium, measure again after 2 to 3 days—the increase is the most direct confirmation that the treatment has worked.

What the Brix value also tells you

Detect boron deficiency: The Brix value should drop in the evening as the plant transports sugar to the roots. If the value does not drop in the evening—there might be a boron deficiency. Boron is the "door opener" for sugar transport from the leaves to the roots.
Detect water stress: In extreme drought, a high Brix value can be misleading—the plant concentrates all substances in the sap due to water loss. In such cases, also check the electrical conductivity of the leaf sap.
Assess frost protection: Plants with high Brix values have naturally higher frost protection. The high sugar content in the sap acts like a natural antifreeze.
Determine harvest time: A stable high Brix value over several consecutive days indicates optimal ripeness in fruit.
Indirectly measure humus quality: In cloudy weather, the Brix value drops quickly in fields with little humus. In fields with a lot of humus, it remains more stable. The Brix value in bad weather is an indirect humus indicator.

What the Brix value has to do with Grünkraft Calcium

STEINKRAFT Grünkraft Calcium with Zeolite for plants to spray and water 650g increases Brix value and nutrient density

The Brix value is the most direct measurement tool for the effect of Grünkraft Calcium. Calcium is the key to all minerals. It opens the stomata for CO₂ uptake, strengthens cell walls for nutrient storage, and activates the pathway for all other minerals into the plant cell.
A treatment with Grünkraft Calcium that works—shows itself in the Brix value within 2 to 3 days. The value increases. The dividing line becomes blurrier. The plant literally glows.

A beautiful quote (no idea from whom anymore)

"We pay doctors to make us healthy—but we should rather pay our farmers to keep us healthy."

The refractometer is the tool that makes this connection visible. It connects the farmer who produces with the person who eats. It makes nutrient density measurable. And it shows that healthy food is not a luxury—but the result of healthy soils and informed decisions.
Are you still guessing, or are you already Brixing?