By Thomas M. Dykstra
This article will appear in the September 2019 issue of Acres U.S.A.
For many decades now, the magic number of 12, in regards to leaf Brix, has been tossed around as the number to achieve if desiring to prevent insect pests from attacking your crop plants. Although not entirely accurate, this is a good point from which to begin a discussion.
This is not an article discussing Brix refractometers, how to use them, how to read them, or even how to argue the merits of digital versus analog refractometers. I will assume that the reader is familiar enough with their use. But for those who are hoping to garner some additional talking points above and beyond the magical value of 12 Brix, then I believe this short article can help.
Although refractometers are commonly used in the wine and citrus industries for testing grape and orange fruit sap, the notion of taking Brix readings from leaves finds more restricted uses among agricultural consultants and farmers in the know. Sometimes this is referred to as leaf Brix in order to differentiate it from the more common usage of testing sap from fruit. The Brix charts circulating around the internet include both leaf and fruit Brix, so I need to be clear that my discussion will focus on leaf Brix measurements.
First of all, one must understand at some level that insects do not attack healthy plants. Many people know this instinctively, but few have been told this explicitly. It is for this reason that knowing your leaf Brix levels is crucial to knowing your crop, whatever you may be growing. High Brix (14 and above) means not just that insects will not attack a given plant but that they will not even be attracted to the plant. In short, pest insects will pass over a high Brix field.
The converse is also true. Insects are very attracted to low Brix plants (6 and below). Unfortunately, if one uses insecticides to keep insects off plants, then it takes longer to realize this truth due to the insect indicators being repelled or killed. If you leave the insects alone, they will indicate to you the relative health of your plant. Now a logical, and seemingly heretical, conclusion to be drawn from this is that insecticides are totally unnecessary for protecting high-Brix plants. Financially speaking, excessive inputs reduce profit. Eliminating insecticides will increase a farmer’s profit — this should be your goal.
But if the only important Brix value to know is “12,” then what exactly is the purpose of having the other numbers? What types of information can be gleaned from Brix values of fourteen, or nine, or even five? For those who want to know something about their crop — immediately, right there on the spot — without having to send a sample away to some far-off laboratory, then you should be Brix-testing on your farm.
The vast majority of leaf Brix measurements will fall between 0 and 20. Therefore, I will restrict my analysis to those numbers (see chart). The leaf Brix chart I have constructed is broken down into both general categories as well as more specific levels. The general categories include (1) those plants between 0 and 2 Brix, (2) those between 3 and 7 Brix, (3) those between 8 and 11 Brix, and finally (4) those between 12 and 20 Brix.
Generally speaking, if a full-grown plant falls between 0 and 2 Brix, it already has one root in the grave. Plants with Brix readings that low are removed from planet Earth with alarming efficiency. Insects will move in quickly to consume these plants and disease will run rampant in these plants since they essentially have no immune system. These plants are unable to take care of themselves in a natural environment. If grown in an artificial environment, then they must be “spoon-fed” in order to survive.
For those who have ever played golf (not miniature golf), you have most likely walked on plants in this Brix range. Turfgrass cut that short, especially on the greens, has very little ability to effectively photosynthesize. Also, the prodigious amounts of pesticides sprayed on this same turfgrass through the sprinkler systems will indirectly prevent substantial root growth, making it difficult for the plant to store nutrients. If not continuously fed and watered, turf below 2 Brix will turn brown in a matter of days. Diverse insect groups will be attracted to it and will assist in the very natural demise. Spraying insecticides helps to hide the insect presence, and daily watering with synthetic fertilizers will be just enough to keep the turfgrass alive for one more day. These plants are being spoon-fed; their existence depends on it.
The next general category is substantially different. Those plants with leaf Brix readings between 3 and 7 have a fighting chance at survival. Many of our crop plants are between 3 and 7 Brix and are in considerably better condition than plants below 2 Brix. These plants require neither a daily dose of pesticide nor a daily dose of fertilizer. A farmer may only spray weekly or even biweekly, depending on the leaf Brix values, in order keep the crop alive. Most of the agricultural plants I have tested — perhaps 75 percent of them — fall within this range.
To be sure, these plants are struggling. They have enough inherent ability to get by and can even provide for their own basic needs, such as the production and storage of sugars. But they will not thrive. Size, health and yield will all be compromised.
Once most plants reach 6 Brix, there is a significant jump in the production of secondary plant metabolites. Secondary plant metabolistes are the phytochemicals that help contribute to a plant’s odor, color and taste. In addition, some secondary plant metabolites provide natural plant defenses against pests. These 6-Brix plants are finally able to devote their energy reserves into producing new proteins and diverse molecules. At 5 Brix and below, plants produce tasteless leaves, exhibit dull coloration and boast fruit with minimal odor signatures. For example, if you hold a tomato in the supermarket or out in the field and cannot detect any odor emanating from it, then that is a preliminary sign that it may be below 6 Brix.
Once a plant reaches a leaf Brix of 8, the secondary plant metabolites have really started to kick in and natural resistance begins. In my experience, Homopterous insects, such as aphids and scales, lose interest in the plant that obtains a value of 8 Brix. In fact, it is relatively common for me to spot these insects on plants below 6 Brix. When a plant reaches 8 Brix, the aphids lose interest, but other insects can and will move in to feed on the plant.
In certain circumstances, the presence of aphids can be an indication that only a part of the plant is below 8 Brix. Diseases characterized by physical “plugs” that prevent the flow of nutrients through phloem and xylem tissue are often manifested in trees by dead or dying branches. Insects will focus their feeding on these weakened branches and ignore nearby branches with seemingly healthy leaves and/or stems. It is for this reason that different parts of the same citrus tree can display different leaf Brix readings when Citrus Greening takes hold, and even more so during drought conditions, when plugging of the vascular tissue is prevalent.
When plants ascend the leaf Brix ladder and reach between 8 and 11 Brix, insects metaphorically “fall off” the plant because the plant has a “sword and shield” that protects itself from insect predators. As a general rule, and although exceptions occur, sucking insects will not tolerate 8 Brix or higher. Chewing insects that eat the roots or leaves directly, such as caterpillars, grasshoppers, and beetles, will start to lose interest in eating a plant once the plant reaches 10 or 11 Brix.
I have witnessed grasshoppers taking bites out of 12-Brix leaves and then flying off the plant. I have also witnessed immature caterpillars of the Fall Webworm stop eating the leaves of a pecan tree once the Brix is increased to 12. As a result, these caterpillars will form a dense clump and then slowly die of starvation within inches of healthy growing pecan leaves. Virtually no insects will attack a plant at 12 Brix; this is why this figure is tossed around so commonly among growers.
Now variability is a hallmark of Nature. Fluctuations between Brix readings can and do occur throughout a growing season. Even if maintaining Brix levels in a given crop, it is not unusual for the leaves to fluctuate 1-2 Brix from one week to the next. It is for this reason that the safest place for your plants to be is at 14 Brix or above. In this way, one may be relatively secure that natural fluctuations do not take your crop below 12 Brix where it may become differentially attractive to various insect pests.
Although Brix is a measurement of dissolved solids, for our purposes it is the measure of sugar in plant sap. Sugar is the main product of photosynthesis. The more a plant photosynthesizes, the more sugar is contained in its tissues, and the higher the leaf Brix readings. This sugar is produced in the leaves, and is not only stored in the leaves but eventually descends to the roots as well. Depending on environmental conditions and the health of the plant, approximately 20-70 percent of the sugar (photosynthate) is expelled into the soil from the roots. This expelled sugar feeds the microbes that will, in turn, break down minerals and supply them to the plant. Therefore, high Brix plants will support a thriving subculture of microbes in the soil.
But sugar has another role. It is hygroscopic, meaning that it absorbs water. It may accomplish this by absorbing liquid water, such as from a spill, or by absorbing water vapor from the atmosphere, which can occur under conditions of high humidity. Either way, the more sugar you have in the soil, the higher the soil’s water retention. Hence, drought resistance and high-Brix plants go hand-in-hand. Plants with a Brix of 4 might only contribute 25 percent of their photosynthate to the soil, but plants of 10 Brix may provide the soil with 40-50 percent of its photosynthate sugar and still have enough sugar to grow reasonably well. By the time a plant reaches 14 Brix, there is so much sugar being pumped into the ground from the crop that microbial counts can reach 20 million or higher in a teaspoon of soil. On top of the soil, these plants are not only drought resistant, but freeze tolerant as well, since highly concentrated sugar water will not freeze above 26 F. Freeze warnings from the National Weather Service then become largely inconsequential to a grower.
Insects have a simple digestive system and cannot digest the same foods that we do. Low-Brix plants are designed for the insect gut. They do not have the digestive enzymes to break down healthy proteins from high-Brix plants, only the broken or incomplete proteins from low-Brix plants. High-Brix plants are meant for vertebrate animals, most notably humans. When we eat healthy plants, we augment our long-term health. When we eat low Brix plants, our long-term health is compromised, although the effects may take years to manifest.
Therefore, we should be eating high-Brix food for our long-term health. If eating meat, then our animals should be feeding on high-Brix plants. If continuously fed low-Brix plants, our cows, our sheep, our goats and the like will display compromised health and will suffer from various diseases and spontaneous miscarriages. It is only common sense to say that eating this unhealthy meat will then, in turn, compromise our own health.
It is important to repeat that this article refers to leaf Brix only. There are other parts of the plant, such as the fruit and the roots, that often display different Brix readings when compared to the leaves. This is to be expected. But roots are hard to get to and measure. Additionally, testing roots is often destructive to an individual crop plant. Testing leaves is not only easier, but is more consistent.
There are uncomfortably large fluctuations in both root Brix and fruit Brix readings throughout the season that make both of them difficult to decipher without the help of a specialist. Unless someone is very familiar with the differential readings from differing plant tissues, a farmer should stick with leaf Brix first and foremost as the chief indicator. Comparatively speaking, leaf Brix measurements remain much more consistent; hence, they are the gold standard when determining the relative health of your crop.
Insects recognize Brix levels better than we do. Insects are indicators. They indicate to us what plants are unhealthy to eat. If a codling moth caterpillar is in my apple, I am going to toss the apple. It is not worth eating. The insect has told me this by virtue of its presence. On the other hand, if insects are not present and no insecticides are being sprayed on the crop, thus allowing insects to choose, then this would be an indication to me that the crop may be healthy for my family to eat.
For those who are actively testing their crops with a Brix refractometer or have an agricultural consultant who is accomplishing the same, much information can be gleaned within just a few minutes about the current state of your crop. To be sure, there is more information that can be provided with further off-farm testing, at considerably greater expense. But in terms of an on-farm testing procedure, using a Brix refractometer is an inexpensive first step, and a wonderful one at that.
Tom Dykstra is an agricultural consultant based in Gainesville, Florida.