Citrus has its origins in south-east Asia where its fruit formed part of the human diet over 6000 years ago. Through migration and trade its cultivation had spread to the Mediterranean by the time of the Roman Empire 2000 years ago. Nowadays, citrus is the highest-value fruit crop in terms of international trade. Brazil, China, USA and Mexico together produce about 60 million tonnes – about half of the world’s total. The major citrus types grown are oranges (55%), tangerines (23%) and lemons/limes (11%).
Citrus was first planted In New Zealand by missionaries in Northland in the early 1800s. By the mid 1800s there were commercial orchards in Northland, Auckland and Bay of Plenty. Nowadays, we have around 1,700 ha of citrus located mainly in Northland, Bay of Plenty and Gisborne regions. Production (~20,000 tonnes, ~$40m) has risen relatively slowly and most fruit is consumed locally. We grow mostly oranges, mandarins and lemons with tangelos and grapefruit grown in smaller amounts.
Citrus thrives in consistently warm (subtropical) temperatures with high humidity and plenty of sunshine. They do not cope well with frost. The NZ climate is predominantly temperate and some consider it marginal for citrus.
Like all plants, roughly half of a citrus tree lives underground – the roots. Therefore, a tree’s general health is best when soil conditions are just right. A well-managed soil increases the yield and quality of the fruit and also helps to reduce the incidence and severity of many pests and diseases.
Citrus is shallow-rooted and requires a well-aerated, free-draining, soil that is high in organic matter. The soil should be between pH 6 to 6.5 (slightly acid) – a pH value below 5.5 (acid) or above 8 (alkaline) reduces access to essential soil minerals and causes problems.
Poor drainage and associated anaerobic soil conditions are not well tolerated by citrus. The use of gypsum to remedy poor drainage in heavy or compacted soils is recommended. To gain best advantage, gypsum should be distributed at a rate that depends on the severity of the problem – a high rate (up to 4,000 kg/ha) in areas where soil problems are worst and lower rates where they are less severe. Adequate levels of soil moisture are required at all times of the year and this requires irrigation when evapotranspiration exceeds precipitation – usually from about October to March.
Gross symptoms of calcium-deficiency (e.g. chlorophyll fading on leaf margins) are uncommon in New Zealand citrus but good supplies of soil calcium are required for fruit set and for enhancing disease resistance. To maintain a good balance of soil minerals, commercial citrus production requires a proper fertiliser programme based on regular leaf analyses.
Gypsum is a good source of both calcium and sulphur and has the advantage of providing calcium without effecting soil pH. Another advantage is that this calcium is provided in a water-soluble form. Free-draining, stony soils are easily leached and so are often nutrient deficient. Used as a fertiliser, gypsum at 1,200 kg/ha is a good source of both sulphur and soluble calcium.
Interestingly, the fruits of most plants contain much less calcium than the other organs such as leaves and shoots. This peculiarity arises because of certain aspects of a fruit’s developmental physiology. As a result, the fruit of many species (apples, kiwifruit, tomatoes etc) suffer specific disorders associated with their low calcium status. Recent research suggests that ‘creasing’ (a physiological disorder of citrus) is also associated with low fruit calcium. Although creasing can still occur in a calcium-rich soil, its incidence and severity is usually much reduced. In the light of this, maintaining good levels of soil calcium should be seen as a priority.