Will citrus fruit get sweeter by adding potassium or lime to the soil?
Although the notion of adding potassium and/or lime to sweeten citrus fruit has been around for a long time, it has not been substantiated by scientific research. Fruit from trees deficient in potassium are small and thin-skinned but are juicy and have a good acid and sugar content. Adding potassium to deficient trees will increase the size of the fruit. This has also been noted in other fruit trees, such as apples and peaches. An excess of potassium makes fruit large with coarse rinds, thick skins and poor eating quality.
A classic chart taken from the University of California’s Citrus Industry Volume II, shows that as the percent of potassium in the leaf rises, there is a slight decrease in the sugar content of the fruit’s juice. However, the vitamin C content increases.
If soils are truly potassium deficient, by all means apply potassium. However, sweeter fruit will not be a result of adding potassium.
In regards to lime (calcium carbonate), an old study (1958) in South Africa, on calcium deficient orange trees, noted that the fruit was acidic and low in sugars. The trees were stunted, chlorotic (yellow), sparsely foliated and exhibited profuse blooming and excessive young fruit drop. If the soil is truly calcium deficient, adding lime may increase the sugar content of the fruit. A soil analysis can determine whether the soil is deficient in calcium, as well as other nutrients.