I often get asked about the use of a refractomer and Brix readings as a tool to evaluate plant health. It can be a useful tool in the hands of a dedicated frequent user. However, the tool provides highly variable readings, and requires an observant and experienced user to produce a reliable indicator of crop health. I described the reasons we don’t use a refractometer as a management tool in this blog post.
The best method I am aware of to accurately use a refractometer as a management tool was described by Arden Andersen in his book Science in Agriculture:
Of course, you can achieve similar outcomes much more accurately and easily and without the extensive time requirements when you use sap analysis.
Construct two or more plastic or wire rings that encircle a 5 or 10 square-foot area. The area of a circle is 3.1417 times the radius squared. The circumference of a circle is 2 times 3.1417 times the radius. A ring encircling 5 square feet would have a diameter of 2.523 feet (30.28 inches) and a circumference of 7.93 feet (95.15 inches). A ring encircling 10 square feet would have a diameter or 3.568 feet (42.8 inches) and a circumference of 11.21 feet (134.52 inches). Because there are 43,560 square feet in an acre, 10 square feet equal 1/4356 of an acre. Using a 10 square foot ring makes calculations easier, but this size of ring is more difficult to carry around unless you make it so that it can be disassembled.
Take these rings to the field and drop them 3 to 5 feet apart. One ring will be used as a control or check area; the others will be designated as test areas. Check the refractometer reading within each ring and record these readings. Be consistent with the methodology and the parts of the plants used for these readings so each test area is sampled identically.
In a spray bottle, mix the exact fertilizer and water ratio that your sprayer is calibrated to apply. For example, if you would apply 2 quarts per acre of 6-12-6 in 20 gallons of water, you would mix 0.8 ounces of 6-12-6 in 1 quart of water. Twenty gallons equal 80 quarts, so a 1-quart mix for your spray bottle would be 1/80 of your per-acre mix. This is calculated by multiplying 2 quarts by 32 fluid ounces per quart to get 64 ounces. Divide 64 by 80 to get 0.8 ounces of 6-12-6 for 1 quart of water.
Next, you need to determine the number of squirts from the spray bottle to apply to the 10 square-foot test area in order to equal a spray rate of 20 gallons per acre. Because 10 square feet equal 1/4356 of an acre that would get 20 gallons of spray, you would apply 1/4356 of 20 gallons or approximately 0.6 ounces, which is about 2.4 tablespoons. This is calculated by multiplying 20 gallons by 128 fluid ounces per gallon (2,560 ounces), and dividing this number by 4,356. Because there are about 4 tablespoons per ounce, 0.6 ounces times 4 tablespoons per ounce equals 2.4 tablespoons.
Take a measuring cup and count the number of squirts from your 1-quart spray bottle to equal 0.6 ounces or 2.4 tablespoons. This is the number of squirts you will apply to your 10-square-foot test area to equal the amount that would be applied if you sprayed the field with your sprayer at 20 gallons per acre.
After misting this spray mix on the test area, wait 30 to 60 minutes and recheck the refractometer reading of the crop in the test area, as well as the control. If the brix reading in the test area increased by at least two full points net, the spray is desirable and would benefit the field. You can then spray the entire field with the confidence that the spray is beneficial. You can mix several different sprays and check each one. If the refractometer reading remains unchanged or drops, the spray is undesirable for that particular day for that particular crop. It is not necessarily a reflection on the product quality, but rather the plant’s need at that time. The only exception to this guideline is when there might be a delayed reaction where no change in refractometer reading is observed for several hours. In this case, leave the rings in the field for 24 hours and recheck the brix reading of both the control and test areas. If an increase in the brix reading of the test area is observed, this spray would then be sprayed on the entire field. The delayed response might occur where a specific prescription has been formulated for a particular field and the spray test is a verification of its appropriateness. The delay could be a result of weather, temperature, water, and so on.
Keep in mind that, before and during a storm, the refractometer readings of the growing crop will be lower, as they will after several days of cloudy weather. The lower the nutrient reserve, the more these refractometer values will be lowered by such circumstances. Imperative to maintaining adequate crop brix readings is the continuous maintenance of the soil conductivity reading. It must remain above a net 200 ergs, or you will not be able to hold the brix above 12 when you do get it to that level.
It is recommended that you purchase an automatic temperature compensated refractometer so that your readings will be more accurate and you will not have to calibrate the refractometer every time the temperature changes.
To reiterate, be consistent in the location of the plant where the refractometer reading is taken. The poorer the plant’s health (correlated to lower refractometer readings), the more the brix readings will vary throughout the plant and throughout the day and season. Always compare the refractometer values in the test area with those in the control area.
You desire a net increase, meaning an increase over the change observed in the control. If the control area had no change and your test area increased by two points, you have a net increase of two. But if the control area increased by one (which can happen as the sun rises in the morning) and your test area increased by two, you have a net increase of one.
The question often arises: How can commodities with high refractometer readings have insect and disease problems? Common examples include sweet com at 24 brix having corn ear worms, and grapes at 18 brix having white flies, mites, and leaf hoppers. The answer is that the refractometer reading taken at the weakest part of the plant in question must be considered. The aforementioned problems indicate that the refractometer readings were taken of selected plant parts (e.g., ear, plant) only, and not of all the parts of the plant. The weakest link determines the outcome of insect and disease infestation. An ear of com at 24 brix with corn ear worms inevitably will have leaf or stalk refractometer readings below 12. Grapes at 18 brix with insect infestation inevitably will have cane or leaf refractometer readings below 12 brix.
With apples, the opposite seems to occur. An apple with apple scab fungus will itself have a low refractometer reading (below 12); however, the leaves on the branch supporting the sick apple will have very high refractometer values (above 12 or even in the upper 20s). In any event, there is a mineral imbalance/deficiency in the crop.
Modern hybridization has produced plants that create such previously mentioned imbalances to satisfy cosmetic desires without considering mineral balance or the plant’s natural ability to satisfy cosmetic desires if it is just provided with the necessary mineral to do so. Hybridization has resulted in plants that will accumulate sugars in given parts, yet not be able to metabolize, transfer, or convert them to plant parts.