RefractometerProduct Guide

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The refractometer is a very useful instrument for the monitoring of plant health. The tool is used to read the Brix, which is an indication of the total soluble solids, minerals, vitamins and sugars in the sap of the leaves and fruit or seeds of the plant. Within a given species of plant, the crop with the higher refractive index Brix and crop maturity will have a higher sugar, mineral, and protein content, as well as a greater specific gravity of density and lower water content. This adds up to a sweeter-tasting, more mineral nutritious food with lower nitrate and water content, and better storage characteristics. Harvested fruits and vegetables with high Brix have shown better resistance to post harvest disease and lower water loss during storage. Crops with higher sugar content will also have a lower freezing point and therefore be less prone to frost damage. A high Brix will produce more alcohol from fermented sugars and be more resistant to insects.

Plants with Brix readings of 3 - 5 (higher than average) have improved resistance to most plant sap sucking insects. This phenomenon has been verified by Organic Ag Advisors for the following insects: aphids, spider mites, leafhoppers, whiteflies, leafminers, cucumber beetles, squash bugs, lygus bugs, Colorado Potato Beetles and others. Chewing insects are not as dramatically affected by high Brix as are sucking insects. In addition, Organic Ag Advisors has identified a correlation between high Brix readings and a reduction in Botrytis in grapes, as well as brown rot in stone fruits. Further research is being conducted to verify this phenomenon with other fungi and bacteria.

How to Take & Read a Sample

There can be marked differences in Brix readings between varieties as well as between species. We’ve seen readings taken from the sap of mature leaves range from a low of 1 to a high of 26. Some varieties have consistently low Brix readings no matter how they are grown. These tend to be varieties that have the most problems with disease and insects. In sampling your growing area, it is important to compare readings for the healthy plants, the weaker plants and the super vigorous plants. Also compare samples from areas where there is a pest or disease outbreak with areas where there is no problem. Within the same plant, Brix readings will vary greatly, depending on whether the sample is taken from young leaves, growing points, mature leaves, petioles, immature and mature fruit, roots, etc. The optimum Brix reading for each area of the plant is unknown.

To standardize the process and establish a reliable baseline of information for accurate interpretation, Organic Ag Advisors has chosen to focus on newly mature leaves or petioles, because they yield sap most easily and are less prone to the wide swing in Brix, which is seen in new leaves and growing points. In addition, while mature leaves respond to foliar stimulation more slowly than younger leaves, they maintain the Brix increase over a longer period, thus giving a better indication of the effectiveness of a foliar spray or other nutrient adjustment. No matter which portion of the plant is chosen, it is vital to consistently sample the same portion of the plant. It is also important to sample consistently from the same side of the plant, as the sun

affects photosynthesis and samples taken from the shaded or north side of the plant may vary from portions of the plant that receive full sun.

A minimum of 20 plants is required for sampling. The same plants should be sampled on each sample date. The plants chosen should be of the same physiological age. The time of day is critical to the sampling procedure. Plants tend to be most active at sugar accumulation/manufacture during the late morning and early afternoon. Many growers sample between 10 AM and Noon. Do not sample at night or in the evening. Do not sample when the temperature is above 100°F. Realize that the Brix is going to be lower in early morning than at noon. Always sample at the same time of day.

The weather will have an impact on Brix accumulation. Prolonged warm, sunny weather is stimulating to the plant photosynthesis process. Conversely, periods of cool, cloudy or foggy weather will lower plant sap Brix. Be sure to note your weather and weather patterns with your sampling to determine if there was a weather-related Brix change. Drought, extreme heat, mechanical damage, compaction, excess rain or irrigation all can effect Brix readings. For most plants, the sap Brix in the newly matured leaves should continue to climb until the fruit of that crop begins to mature (verasion). At this point, it is typical for the Brix reading in the leaf to stall or drop as the sugars and other nutrients are translocated to the ripening fruit or seed. To follow this process, compare the accumulating sugars in the maturing portion of the plant against the Brix in the mature leaves. Increasing leaf Brix readings during the maturing phase will increase the Brix of the harvested crop.

The higher the Brix reading, the stickier the sap will be and the harder it will be to get the sap out of the leaves. “Sap extractors” work better than a garlic press. However, for some plants (especially stone fruits) a hand-held juice squeezer will be inadequate. We have found that a wheat grass juicer (available through health food stores) will extract the juice from almost anything.

We have found a strong correlation between Brix readings and Nitrate Nitrogen sap tests. We have observed plants with a relatively high Brix reading which also have pest pressure. The use of the Nitrate meter has revealed that in virtually all instances, these high Brix reading plants also had high to very excessive Nitrate levels. Apparently, when plants have excessive nitrates, usually caused from over application of Nitrogen rich fertilizers relative to the actual needs of the crop, the Brix resistance phenomena is overwhelmed by the excess Nitrates. A plant high in Nitrates will have an excess supply of free amino acids (excess amino acids not yet synthesized into complete protein by the plant). Insects prefer free amino acids to proteins and feed on plants to supply themselves with these excess amino acids, which they use to manufacture proteins in their bodies. In general, complete proteins (with all 18 amino acids in balance) are not consumed by insects. It is desirable to balance any excess Nitrates in the plant sap with the additional minerals needed by plants, to stimulate the plant to manufacture the other amino acids necessary for protein synthesis. We suggest the use of a Cardy Nitrate meter to improve the information collection and understanding. There are numerous methods used by

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growers to increase the Brix of the plants. Apply all nutrients which are low or defi cient in the soil or plant (determined via the use of a plant tissue and/or soil analysis). This can be accomplished using soil applied amendments and fertilizers (generally cost effective, but slow acting), via fertigation (generally more expensive, but has a much quicker effect on the plant) or foliar feeding (the quickest, and often the most cost effective approach). Foliar feeding with the proper material(s) can increase the Brix readings in 24 - 48 hours, fertigation in 3 - 7 days and soil applied fertilizers and amendments react in 2 - 4 weeks. A properly constructed and timed foliar application can bump up the Brix 1 - 3 degrees in 24 hours, and the plant will often “perk up” and insect resistance will improve.

As a rule of thumb, foliar feeding only has a limited duration of effect on the plant, from 7 - 45 days depending on material and crop. Longer term fertilization via soil or water application is also necessary to sustain the Brix, unless multiple foliars are applied. Many growers apply 3 - 7 foliar sprays to a crop, usually on a weekly basis, some more frequently. Fertigation and foliar feeding with calcium, phosphorus, potassium and trace minerals (such as found in kelp products) almost always have a positive effect on Brix readings.

Variety Poor ExcellentApples 6 18Avocados 4 10Asparagus 2 18Bananas 8 14Cantaloupe 8 16Cabbage 6 12Cassaba 8 14Carrots 4 18Cherries 6 16Caulifl ower 4 10Coconut 8 14Celery 4 12Corn Stalks 4 20Corn, Sweet 6 24Cow Peas 4 12Grapes 8 24Grapefruit 6 18Green Beans 4 10Honeydew 8 14Endive 4 10Escarole 4 10Lemons 4 12Limes 4 12Kohlrabi 6 12Lettuce 4 10Mangos 4 14Oranges 6 20Onions 4 10Papayas 6 22Peaches 6 18Pears 6 14Peanuts 4 10Parsley 4 10Pineapple 12 22Peas, English 8 14Peas, Field 4 12Peppers, Bell 4 12Peppers, Hot 4 10Potatoes, Irish 3 7Potatoes, Red 3 7Potatoes, Sweet 6 14Raisins 60 80Raspberries 6 14Rutabagas 4 12Strawberries 6 16Squash 6 14Tomatoes 4 12Turnips 4 10Watermelon 6 16Alfalfa 4 22Grains 6 18Sorghum 6 30

Refractive Index of Crop Juices Calibrated in %sucrose or Brix:(For Reference, pure water has reading of “0”)

The chart to the right is provided as a general information service and is not intended to act as a recommendation for this process or any other fertilization and/or pest control technique. These suggested guidelines are implemented strictly at the user’s risk. No warranties or guarantees are expressed or implied. Peaceful Valley Farm Supply and Organic Ag Advisors do not accept any responsibility for errors or omissions.

Factors which can negatively effect raise Brix levels include excess irrigation, excess nitrogen fertilization, excess salts such as sodium or chlorides, cold weather, prolonged clouds or fog.