Refractometers are traditionally used to test ripening fruits & vegetables, however, they can also be used in a relatively new method called "Plant Sap Analysis". Sap is squeezed from fresh plant tissue (leaves) & tested for Brix°, pH & EC.
The starting point for plant sap testing is Brix° (see sidebar). Brix° indicates the degree of complexing into sugars or proteins in the photosynthesis factory – the leaf. If Brix° is low, some element(s) are missing and other tests are used to identify problems.
The sap pH indicates the level of balance of nutrient uptake and helps to identify which elements may be out of balance.
EC (conductivity) indicates the level of simple ion uptake into the plant sap. Low Brix° & low EC (of sap) indicate that elements are not being made available to the plant. If sap EC is too high, it shows that elements or ions are not being “complexed” & ions such as NO3 may be at excessive levels.
Other tissue tests such as potassium & nitrate are also used to identify troubles in plants.