Warm sunny days: The accumulation of heat units is most important in determining the sugar content in citrus. Simply put, citrus fruits become sweeter the longer they remain on the tree up to a point. If the fruit seems ready to pick, but the taste is not sweet enough, then leaving it on the tree a little longer should sweeten it.
Location, location, location: In certain places, fruit from a given tree will simply not produce enough sugars. Of the hundreds of citrus varieties, not all are adapted to a specific area. For a given region, some varieties won’t produce good quality fruit!
The rootstock factor - There are numerous citrus rootstocks. They impart a variety of characteristics to the fruit. Some rootstocks will enable the tree to produce an abundance of fruit, but with poor quality, in this case, low sugar content. For commercial growers, choosing the right rootstock is an important factor when selecting new trees for an orchard. Homeowners on the other hand, have few choices in rootstocks when purchasing a tree.
Will citrus fruit get sweeter by adding potassium or lime to the soil?
The notion of adding potassium or lime to sweeten citrus fruit has
been around for a long time, but it has not been substantiated by scientific
research. Fruit from trees deficient in potassium are small and thin-skinned, yet they can be 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 juice in the fruit. The vitamin C, however, content increases.
If soils are truly potassium deficient, by all means apply potassium. This being said, 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. Contact a local university cooperative extension to inquire about where an analysis is available.