Several insightful pioneering agronomists have recommended the use of a refractometer and Brix readings as a useful management tool to evaluate overall crop quality and the effectiveness of product applications. Carey Reams popularized the idea in the ’70s and Dan Skow, Arden Andersen, and others have further developed and shared this idea. 

It can be a useful, even powerful, qualitative tool as long as we understand the long list of caveats, and how to avoid being misled. 

The foundational idea is that the refractive index of plant sap correlates to the content of dissolved solids, including sugars, and can be used as an overall assessment of plant health. When plants reach a certain threshold, they can become resistant to almost all insects and diseases. In principle, this has been demonstrated to be accurate and correct many times, on many farms. Putting it into practice is tricky though. 

It is tricky because of Brix levels exceptionally high variability over time, weather, location on the plant, water availability, and more. It is also important (and challenging) to be consistent in extracting sap, and using the same amount of pressure to get a consistent sample each time.

Brix levels fluctuate through each 24-hour photocycle, usually peaking mid to late day because of accumulated photosynthates. In healthy plants with the proper mineral balance for good photosynthate transport, Brix levels often drop 30% or more in the leaves from evening until morning, as sugars are moved to the sugars sinks and used or stored.

Brix levels fluctuate based on weather. Plants can anticipate storms, sometimes by as much as several days, and move all the sugars possible into the roots so they can rapidly recover in case of storm damage. Brix readings should drop quite a bit in advance of a storm. 

Brix levels fluctuate based on water availability. Dehydrated crops will have a higher Brix reading because the dissolved solids are more concentrated, but the crop certainly isn’t healthy. 

Brix levels fluctuate at different locations within the plant. There are often big differences between old leaves and new leaves, or spur leaves and new shoot growth, or on the fruit leaf or ear leaf. It is very common for the fruit and the leaves most closely associated with the fruit to be the lowest Brix. This is true because the fruit often has the highest nutritional requirement, and is the last location for nutritional integrity to be achieved. For this reason, we can have disease and insect resistance leaves and susceptible fruit on the same plant. 

Some crops have also been bred to have artificially inflated Brix reading on the fruit in the absence of nutritional integrity, while the remainder of the plant is still very low. Sweet corn is the classical example, there are others.

Each of these described fluctuations can be significant and can produce as much as a 60%-70% swing in Brix. The less healthy a plant is, the more dramatic the fluctuations.

The location and time with the lowest Brix level determine the degree of insect or disease resistance for the whole crop. 

If you wish to use Brix levels effectively as a management tool, it will require committing the time to collect regular samples, at different locations on the plant, within different fields, in different weather conditions, at the exact same time of day, at least several times per week. Because of the inherent variability, effective management is a result of managing the trend, not each individual measurement. 

Many growers don’t have the bandwidth to develop the degree of familiarity needed with Brix readings to use it as an effective tool. This is where sap analysis becomes a useful tool, because it requires less time, less familiarity, and because it can identify immediately which nutrients should be addressed.

To be clear, I am a fan of Brix readings, and developing familiarity with what it can tell us about a crop. However, we need to be clear-eyed about its limitations, and what is required for it to be used effectively. I know of only a handful of commercial-scale growers that use it to the degree necessary to get good results.

What aspects of Brix readings did I miss?