Tools of the Trade — Using Refractometers & Penetrometers

tools-of-the-tradeby Gary Digiuseppe

Forage producers can measure the percentage of sucrose and other soluble content of their grasses with the use of a refractometer, although the accuracy of the reading can be dependent on the cost of the instrument.

Martin Capewell, owner of Agriculture Solutions LLC in Strong, Maine, says an analog refractometer costs around $50-$200, while a digital model can run anywhere from $190- $10,000. A “simple, hand held, decent” digital refractometer, Capewell says, costs about $350 and will last a long time provided it’s properly cleaned and maintained and protected from extreme temperatures.

All refractometers work by passing light through sap or juice extracted from a crop, and then measuring the angle of the light as refracted by a prism. The readout, a percentage of soluble content, is called Brix, named for one of the originators of the method, Adolf Brix. The Brix reading is determined by where the line produced by the light refraction crosses the scale on the device. Technically, Brix is the percentage by weight of sucrose in a solution, but the refractometer can’t differentiate between sucrose and other dissolved solids.

The scale is calibrated relative to a temperature of 20°C, and Capewell says refractometers are either temperature compensated or non-temperature compensated. “The great thing about one with automatic temperature compensation is that it gives you the correct Brix reading, and you don’t have to do that formula yourself,” he says.

An analog refractometer has to be held up to a light source to be read; digital models use an internal light source. “When you click the ‘measure’ button, within about 3 seconds you can get an exact reading to one decimal place,” said Capewell. “It’s much more accurate, quicker and easier to use.”

In order to extract a sample for measurement by the refractometer, the farmer crushes several blades of grass in a vice grip or garlic press that should have a filter so the crusher will not push the solid material through the holes.

Cheryl Pike of Pike Agri-Lab Supplies in Jay, Maine, says there are devices specially designed to squeeze juices out of plants or fruit for sampling purposes. If a garlic press is used, it should be sturdy and made from stainless steel. “Most of the cheaper garlic presses won’t stand up to it, especially if you’re talking about pasture and trying to get the juice from grasses.”

Pike says most recommendations for an acceptable Brix reading are at 12 or higher. “The higher you have that number, it’s going to be a higher quality grass,” she said. “There have been studies showing the feed quality values matching the Brix reading. They are positively correlated — as one goes up, the other goes up.” Some studies indicate higher sucrose levels in pasture grass increase a ruminant animal’s conversion rate of feed to milk.

Temperature is just one of four variables that affect the reading. Concentration of dissolved solids, the atomic weight of the substances in the liquid and the number of covalent bonds, which are higher when components like amino acids and proteins are present also play a part. However, the farmer just needs to know if the meter is temperature compensated and to adjust for temperature if it’s not.

“If you have a very watery substance, it would read lower than something that has a lot of solids dissolved in the liquid,” said Pike.

In short, says Pike, “The more nutrition you have in your juice, the higher the reading will be … the more complex, and the more desirable things that are in our vegetables, fruits and grasses are the things that make the reading go higher.”

To ensure an accurate readout, Pike cautions that some inexpensive instruments they tested from overseas suppliers were not very accurate. “You get what you pay for.”


Another popular tool for farmers is the penetrometer, which measures resistance in soils. In addition to providing farmers with an idea of the tilth and oxygen content of soil, Capewell says it can also be used to determine the depth of the hardpan layer, so the producer can tell how much room a plant’s roots will have to grow.

He says in order to operate the penetrometer, push it into the soil at a constant rate while keeping an eye on the dial that shows how much resistance it is facing. “It might go up to 200 pounds per square inch (psi),” he said. “You’ll know that you’ve got oxygenation as you’re pushing it in, and you’ll certainly hear it go ‘thud’ as you’re hitting a harder layer. You can continue to push it past that point to see how hard it is to push through that, too, but when you hear it go ‘thud’ you can mark on the penetrometer where it met the harder layer, and you can see how deep your topsoil is and how good the tilth is.”

Penetrometers start at $250 with digital models being more expensive. They’re made of stainless steel and should last a number of years, but the tips may need replacing if they’re used very frequently. Capewell explains, “If you’re pushing into something all the time, you may hit rocks, and it may get blunted over time” which would affect the measurement. Luckily, the tips are inexpensive to replace.

Pike adds that plants do not perform well on ground where 300 psi or higher is required to push through the soil. The penetrometer produced by her company has marks along the length of the probe every 3 inches so the operator can record the depth at which the 200 psi, 300 psi mark, or hardpan is reached.

Finding the hardpan would be useful in a situation where the ground has been plowed. She says when that layer has been detected, “People are using different treatments, not necessarily just the plowing, that are supposed to either lighten up the soil or alleviate hardpans.” The penetrometer can be used before and after to gauge the efficacy of those treatments.

This article appears in the August 2014 issue of Acres U.S.A.

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