Sweetness in Citrus

  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.

Citrus pruning

Lemon trees grow vigorously, especially the Lisbon variety. If not pruned on a regular basis they will develop a very dense canopy, allowing little light to penetrate into the interior of the tree. Under this condition, the tree will produce few interior blooms and subsequently a small amount of fruit.  In addition, pesticide sprays become ineffective because of the inability of the spray to penetrate the dense foliage.  Left unpruned, a lemon trees will also become quite tall, making picking difficult. 

Other citrus trees like orange and grapefruit are not as vigorous in their growth, and therefore do not need as much pruning as lemons. These trees can be left unpruned without serious consequences. If desired, a large tree can be pruned by simply removing a few of the interior branches to open the tree up to more light. Note: studies have shown that yields are reduced in proportion to the severity of pruning. To rejuvenate an older tree, pruning may range from moderate thinning of the canopy to heavy pruning. This pruning will stimulate a new flush of growth; however, the recovery will be temporary unless the cause of the trees decline has been corrected.

When to prune

 The timing for pruning is not critical.  After the last piece of fruit is removed for the season, is generally a good time.  Citrus like Valencia oranges and some mandarins tend to alternate bear. That is, they produce heavily one year followed by light production the next.  In this case, it would be best to prune during the ‘off’ or light year. In addition, if you have the time, it is prudent to remove the dead twigs from a tree. These harbor disease spores which, during a rain, will splash and infect nearby fruit causing the fruit to blemish. 

Lychee

Inconsistent fruit production in lychee.

 When it comes to producing fruit, lychee trees, as well as longans, are very temperamental.  In order for the tree to bloom profusely, there are certain requirements that need to be met:  a combination of a dry period, usually about 4-6 weeks, along with low temperatures. The cool temperatures then need to be followed by warm weather for good fruit development. Among the many varieties of lychee, kaimana requires less cool temperatures and is therefore a good variety for warm areas.

 It is apparent that in order for lychee trees to produce well, they need a very specific climate. When this is not met, inconsistent fruit production will occur.