New Zealand mostly grows 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.
Preferred soil conditions
Citrus plants are shallow-rooted and require a well-aerated, free-draining soil that is high in organic matter. The soil pH should be between pH 6 to 6.5 (slightly acid). A soil pH below 5.5 (acid) or above 8 (alkaline) reduces access to essential soil minerals and may cause nutritional deficiencies in citrus plants.
Citrus require adequate levels of soil moisture at all times of the year, as a guide irrigation will be required from about October to March.
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.
Gypsum can be applied at rates up to 3,000Kgs/ Ha in areas where soil problems are worst and at lower rates where they are less severe. Free-draining, stony soils are easily leached and are often nutrient deficient which may be a consideration when applying nutrients.
Calcium
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.
Interestingly, the fruits of most plants contain much less calcium than the other organs such as leaves and shoots, arising from certain aspects of a fruit’s developmental physiology. Recent research suggests that ‘creasing’ (a physiological disorder of citrus) is associated with low fruit calcium. Although creasing can still occur in a calcium-rich soil, maintaining good levels of soil calcium should be seen as a priority.
Used as a fertiliser, gypsum is a good source of both sulphur and soluble calcium and has the advantage of providing calcium without affecting soil pH. Calcium in gypsum is provided in a water-soluble form.
Application rates of gypsum will be based on soil type and a recent soil lab test result.