Laboratory leaf sap analysis has given us remarkable insights into plant nutrition. We have learned a great deal about nutrient interactions and plant nutrient absorption of different products and in different environments, and have become champions of sap analysis.
Occasionally I am asked how we might use the Horiba meters to measure sap contained nutrients in the field, rather than submitting them to a lab.
I don’t consider Horiba meters to be a viable option if we really want to manage plant nutrition properly. Here are a few reasons why:
- We need to know the levels of many more than four or five nutrients. Knowing the levels of only nitrate, potassium, calcium, sodium, pH, EC, and Brix doesn’t begin to approach the thoroughness of data needed to make informed decisions about nutrient management. For example, manganese influences potassium absorption, and boron influences calcium absorption, to a significant degree. Trying to manage the macronutrients without knowing the levels of the trace minerals promises to be an exercise in frustration and mismanagement.
- Nitrate is only one of many possible forms of nitrogen contained within a plant. In a healthy plant with proper protein synthesis, upwards of 80% of the nitrogen will be in the form of enzymes – complete proteins, which don’t register at all on a nitrate meter. It is possible to have a crop with abundant levels of total nitrogen and record a non-detect nitrate on a Horiba nitrate meter. With lab-based sap analysis, nitrate, ammonium, and total nitrogen are all measured separately. Our goal is to have abundant total N, with nondetectable levels of ammonium and nitrate. A goal that we achieve quite regularly.
- In many cases, (not always) in-field analysis with the Horiba meters is conducted on the sap contained within the petiole, rather than in the leaf. Yet, we know the petiole is a nutrient and water transport pipeline, and nutrient levels in the petiole sap can fluctuate by as much as 30-40% at different periods during each 24-hour photocycle. With what other analytical methods would we accept a possible 30+% error margin? None, of course. We can reduce this margin of error by collecting samples at the same period of each day, but this doesn’t resolve the challenge that the nutrients contained within the petiole don’t always reflect what is present in the leaf.
In – lab analysis of leaf sap overcomes all of these challenges. This is why we only use the sap analysis from Crop Health Labs.