Visual evaluation of deficiency and toxicity symptoms are useful to make positive diagnosis of citrus malnutrition. Incipient nutritional imbalances, which result in reduced fruit quality and lowered yields, often cause no obvious visual symptoms. Leaf analysis gives the best early indications of nutrient imbalance, allowing the orchardist to rectify the situation before it becomes a major problem. It is important to remember that economic losses occur long before these types of visual symptoms become apparent. The leaf is metabolically very active, functioning as the site of photosynthesis, which determines the primary processes occurring within the plant.
Leaves are also a major site of carbohydrate and mineral storage.
Consequently, the mineral status of the leaves not only influences the efficacy of photosynthesis but also reflects the nutrient status of the plant and therefore its fruitfulness. Although other organs within the plant may act in a similar manner, the leaf is the most readily available source of tissue for analysis. The most suitable sampling time for New Zealand grown citrus is in late February to early March. Nutrient levels, in particular nitrogen, phosphorus, potassium, calcium and magnesium, have also been found to differ depending upon leaf position on a shoot. For example nitrogen, phosphorus, and potassium contents are lower and calcium and magnesium levels are higher on fruiting branches.