"The time has come," the walrus said "to think of many things" - one of the things this wise old sage would be telling you to think about this February is the nutritional status of your citrus orchard and having leaves analysed.

Leaf analysis is a useful guide to more efficient and economical fertilizer practices, and can be used to diagnose specific nutrient problems. In addition to the quantification of nutrient uptake, this technique provides a means of evaluating the actual nutritional status of a plant at a given time and comparing it with optimum nutrient levels. In our experience, nutritional imbalances in New Zealand citrus orchards are common (Sale, 1989; Mooney et.al., 1991). This suggests that there are a large number of citrus orchardists who are not paying sufficient attention to their plant nutrition. Surely the main aim of citrus orcharding is to maintain trees in good health, to sustain high yields of quality fruit.

A major problem appears to be that of incorrect nitrogen fertilization which has resulted in imbalanced nitrogen cycling within the trees (Figure 1). Of all the major elements, nitrogen has the most influence on leaf analysis interpretation (Reuther and Smith, 1954). For example when nitrogen levels are low, then phosphorus, potassium and sulphur levels are likely to be higher than in comparable trees with adequate nitrogen levels. Similarly, excessive levels of nitrogen are correlated with low levels of phosphorus, potassium, sulphur and the trace elements. Leaf analysis gives both the levels of the nutrients as well as the ratios of one element with another, therefore interactions between elements are more discernable, and hence more easily rectified.

Figure 1
Incorrect nitrogen cycling during the growing season may result in excessive vegetative growth and poor quality fruit.

Visual evaluation of deficiency and toxicity symptoms are useful to make positive diagnosis of citrus malnutrition. However, the expression of visual symptoms generally indicate a serious imbalance (Figure 2). 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.

Figure 2
It is important to remember that economic losses occur long before these types of visual symptoms become apparent.

Why Use Leaf Tissue?
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.

Which Leaves Should You Select?
The following physical factors govern the type of leaf which should be used to determine the nutritional status of an orchard: leaf age, its position on the tree, the type of shoot the leaf is on, the age of the tree and the rootstock/scion combination.

Leaf age: During the early stages of leaf development the nutritional status of the leaf is variable, therefore the general rule is to sample fully developed, current season's spring flush leaves. The most suitable sampling time for New Zealand grown citrus is in late February to early March.

Positional Effect: Shading, aspect and sample height have all been found to affect the nutrient content of leaves. Therefore to avoid positional effects, leaves should be taken from unshaded positions on the surface of the canopy, at the north, east, south and west points of the tree, at a uniform height above the ground (on large trees this should be standardized to about 1.5 m high). Nutrient levels, in particular nitrogen, phosphorus, potassium, calcium and magnesium, have also been found to differ depending upon leaf position on a shoot. This source of variability is minimised by selecting the first mature leaves from the end of the shoot (Marchal, 1984).

Type of shoot: The variability of leaf nutrient contents has been found to be greater in leaves on fruit-bearing shoots than non-fruiting shoots (Harding et al., 1962). For example nitrogen, phosphorus, and potassium contents are lower and calcium and magnesium levels are higher on fruiting branches. Lateral branching may also affect the nutrient content. Therefore leaves should be selected from non-fruiting, non-branched shoots. These shoots are also more likely to bear fruit in the following year so that their composition is of direct interest. They are also always present on young trees and biennial trees during an 'off-year'.

Rootstock and Scion Effects: Rootstock effects occur because each rootstock has a different root growth pattern, which influences nutrient uptake (eg. lemon trees grown on trifoliata stock have been found to be higher in potassium than those on rough lemon). Different scion species regulate uptake, transport and metabolism of nutrients to varying degrees (Pitman, 1975). Therefore every rootstock/scion combination must be sampled individually.

Tree Age: Age of the tree has been found to influence the nutrient content of the leaves. Therefore trees of similar ages within an orchard block should be sampled separately.

Sample size: As a general rule, 20 to 50 trees are sampled from an area not exceeding 2 ha. Trees should be selected randomly, using a zig zag or diagonal pattern, however, trees from the outer perimeter should be excluded. The sample should consist of 100 leaves which are healthy, free of all pests and diseases and physical damage, unless this is typical of the orchard. When harvesting leaves with visual malnutrition symptoms for analysis, where possible a second sample of healthy leaves from the same area should be collected for comparison.

To summarize, the sample should consist of leaves which are fully developed current year spring flush, from unshaded, non-fruiting wood, at the north, east, south and west points of the tree, and a uniform height above the ground. Every rootstock/scion combination and trees of different ages should be sampled separately. A sample of about 100 leaves should be selected from about 20 to 50 randomly selected trees.

Sample Handling
The leaves should be washed carefully in cold water to remove surface contamination, patted dry with tissue and then placed in a clean brown paper bag. The sample should then be sent off promptly to a laboratory for nutrient determination.

What To Do With The Results
Once the results of the leaf nutrient status are available, experienced orchardists can use the data to determine the rates and type of fertilizer which needs to be applied. For those growers who are not sure how to interpret the results, we suggest that the data be discussed with a technical advisor who has experience in the area of citrus nutrition.

The primary aim of plant analysis is so that annual fertilizer application may be optimized to produce higher yields of quality fruit. In the longer term the aim should be to use plant analysis to minimize the amount of chemical fertilizers applied to the orchard while maintaining optimal production (Martin-Prevel et al., 1984). To achieve this it is necessary to keep a record of annual management and fertilizer practices which serves as a reference when interpreting nutrient analyses and making future management decisions.

References
Harding RB, Ryan TM & Bradford GR. 1962. A comparison of macroelement composition of orange leaves from non fruiting and fruiting terminals. Proc. Soc. Am. J. Hort. Sci. 80: 255-258.

Marchal J. 1984. Citrus. In: Plant analysis as a guide to the nutrient requirements of temperate and tropical crops. [Martin-Prevel et.al. (eds)]. Lavoisier Publishing Inc. New York. pp 320-354

Martin-Prevel P, Gagnard J, Gautier, P, Benton Jones J & Holmes, MRJ. 1984. Plant analysis as a guide to the nutrient requirements of temperate and tropical crops. [Martin-Prevel et.al. (eds)]. Lavoisier Publishing Inc. New York.

Mooney PA, Richardson A & Harty AR. 1991 Citrus nitrogen nutrition - A fundamental approach. N.Z. Kerikeri Horticultural Research Station Citrus Research Seminar, June:69-88

Pitman MG. 1975. Ion transport in plants. Baker and Hall, Amsterdam.

Reuther W & Smith PF. 1954. Leaf Analysis of Citrus. In: Fruit Nutrition [Childers NF (ed.)], Rutgers University, New Jersey. pp 257-294.

Sale P. 1989. Survey highlights nutritional trouble spots. The Orchardist of N.Z. February: 14-15

Source:
The Orchardist, February 1992