Getting nitrogen fertilising right can dramatically improve citrus returns. Our latest trial results indicate that autumn applications are best for tree health and fruit quality.

As citrus growers, we need to make several decisions before applying fertilisers to our trees (as managers of many hectares of citrus on our research orchards we count ourselves as honorary growers!)

We need to measure which nutrients are needed, decide which commercial fertilisers are most suitable, how much of each to apply and when to apply it. Getting these decisions right will affect orchard profitability : tree health will be improved, fruit quality and yield increased, and wasteful fertiliser applications avoided.

In this article, we discuss the research we have carried out during the past three years on nitrogen fertilising of citrus.

Although nitrogen is only one of the elements affecting tree and fruit growth, its misuse can have huge effects and we considered it the most important starting point for our studies. We present some interesting data from trials on satsuma mandarins and oranges which can assist in making nitrogen fertilising as efficient as possible.

Nutrient Monitoring

Fortunately, we can detect nutrient deficiencies (and excesses) long before we can see them, by carrying out regular leaf analyses. How does leaf analysis work? A sample of leaves is picked in late February and sent to a laboratory where the level of each nutrient is determined. These results are then compared to a set of standards which have been determined over many years and confirmed in many countries as being ideal for tree performance. Each important element is described as being deficient, adequate or excessive, and a fertiliser programme to correct the imbalances can then be planned.

More than any other element, nitrogen levels seem to vary between different citrus cultivars. Vigorous cultivars such as tangelos and lemons quite often show deficiencies, while others such as satsuma mandarins commonly show toxic excesses. So it is very important to treat each cultivar separately when taking leaf samples and deciding on nitrogen rates.

Leaf analysis is an essential step in your citrus fertiliser programme. The cost of regular leaf analysis each year is easily offset by the benefits of improved fruit yields and quality. Soil analysis is also important, especially in deciding which fertiliser types can be used, and whether liming is needed.

Over many years of regular leaf and soil testing, a picture of the nutrient status of an orchard can be built up. This allows you to anticipate problems before they start affecting your crop. We recommend leaf analysis every year, and soil analysis at least every second year.

Well managed fertilising

1. As a newly planted tree, when all efforts are concerned with maximizing leafy growth and reducing floral growth. Flowering and fruitset at this stage of tree development drain the reserves needed to form a strong canopy.
2. As a young reproductive tree, when high levels of vegetative growth are still required for the tree to fill its allocated canopy space, but at the same time fruit production is necessary for economic reasons.
3. As a mature tree, when the tree has filled its allocated space, and management is directed at suppressing excessive vegetative growth and increasing fruit production.

Nitrogen deficiency results in lower fruit yields, but what are the adverse effects of too much nitrogen? Although trees rich in nitrogen have a wonderful dark green leaf colour, a common symptom is that fruit colour can be delayed and poor. This is especially so if large N applications have resulted in a potassium deficiency. Fruit yields are usually high in terms of numbers per tree, but fruit size is almost always smaller. Fruit appear rougher, and have thicker rinds. These in turn reduce the percentage of juice in the fruit. In some cultivars, sugar levels in the juice are lower.

Young Trees

On young trees, we should ensure that none of the flushes are lacking in nitrogen. Small, split applications are best up till the fourth year of growth (Table 1). From then on the trees should be switched into bearing mode, to emphasise yield and fruit quality. Leaf analysis should begin in the third February after planting.

It is very important when applying hand dressings not to place soluble fertilisers such as urea and CAN against the trunks of citrus trees. Also avoid large lumps or piles of fertiliser, which will burn the roots immediately beneath. Root damage from nitrogen fertilisers has been identified as one of the causes of `sudden death' of citrus trees on trifoliata rootstock.

Mature Trees

As a rule of thumb, no citrus orchard, regardless of tree size, spacing or age, requires more than 180 kg of nitrogen per hectare per annum. Many cultivars, and especially satsuma mandarins, will need far less than this, and again leaf analysis must be used to decide on quantities needed. It is also important to know the quantity of pure nitrogen contained in a commercial fertiliser (Table 2), as most recommendations refer to 'kilograms of N per hectare'.

Time of Application

It was clear that the trees were able to accumulate reserves of nitrogen in the autumn and early winter, mainly in the bark, wood and roots, and that these reserves were very important for early spring tree and fruit growth (Mooney et al. 1991; Mooney & Richardson 1992).

Soil temperatures during August/September are lower than during April/May (Figure 1), so uptake of nitrogen fertiliser is more efficient in the autumn. It is important though to wait until autumn before applying nitrogen, because a dose in summer will simply stimulate unnecessary extra growth on the autumn growth flush.

Fruit Quality Trials

Figure 1: Soil temperatures at 10cm depth throughout typical seasons in NZ citrus districts	Figure 2: Harward Late leaf nitrogen levels

Figure 2 shows the Harward Late leaf levels of nitrogen for the duration of the trial. Note that the levels are already high in spring for trees which had applications in the previous autumn. Levels in trees which had spring applications remained high during fruit sizing in December/January.

During both seasons, fruit quality of both cultivars was as good if not slightly better on trees where the nitrogen was applied in autumn rather than in spring (Table 3).

Because most citrus districts experience moderate to high rainfall in autumn, it makes economical sense to apply nitrogen fertilisers in two doses (mid April and mid May) to avoid leaching losses.

Types of Nitrogen Fertiliser

Nitrogen is most commonly applied as a dressing of solid mineral fertiliser. However, foliar sprays and fertigation are other ways of getting nitrogen into a citrus tree. Organic fertilisers such as blood and bone, manures and composts are also potential sources of nitrogen. Table 2 summarises the nutrient content of these different types.

Urea is the cheapest form of nitrogen, and has become the standard compound for N application. However, calcium ammonium nitrate (CAN), which should not be confused with calcium nitrate, is becoming more popular as a solid dressing. Although it is more expensive per kilogram of N than urea, it has several advantages over urea. CAN has equal portions of nitrate and ammonium, is less acidifying than urea, and is less likely to be lost through leaching or volatilisation. It also contains a small amount of calcium, which is commonly deficient in many of our citrus orchards.

If calcium levels are low, and if the soil pH does not allow for further liming, then calcium nitrate is a good option for supplying nitrogen and calcium.

Compound mixes such as Nitrophoska Blue can be used to supply nitrogen, but only where potassium and phosphate are also required. This is where leaf analysis becomes vital - many local citrus orchards are now showing potassium excesses, presumably because of a long history of applying mixed fertilisers rich in potassium.

This imbalance has unfortunately resulted in deficiencies of calcium and magnesium, and in poorer fruit quality (excessive potassium results in high citric acid levels.) Mixed fertilisers which contain potassium are not recommended on satsuma mandarins, which appear to have a very low K requirement.

Organic nitrogen sources are slow-release, and because of their low N content, need to be applied in bulk (3 - 4 tonnes per hectare on mature trees). This fact, combined with the price and lack of easy availability of many organic materials, has not favoured their use. However, they are not readily leached and, with the exception of fresh animal manures, are safe to apply in relatively large amounts.

Poultry manures should be analysed for salt content (sodium and chloride) where it has been used as a feed additive. For many biosustainable orchard regimes, they are the only acceptable sources of nitrogen.

Foliar options for nitrogen are urea, potassium nitrate and magnesium nitrate. Again, urea is the most readily available and cheapest form, but care must be taken that only low biuret urea (LBU) is used. Biuret is a byproduct of the urea manufacturing process, and is very phytotoxic to citrus and many other plants. The biuret content should be lower than 0.4 %. LBU is applied at 1% (10 kg per 1000 litres water), and is particularly useful for `quick fixing' nitrogen levels in trees which are deficient.

When soil temperatures are abnormally low, as they have been during the past two seasons, then foliar applications may be the only effective way to get nitrogen into the tree. LBU can be combined with foliar trace element sprays, and there is now good evidence from Californian research that a mid-winter spray of LBU will increase flower and fruit numbers in the following season.

Forms of nitrogen which can be fertigated include urea, ammonium nitrate, calcium nitrate, potassium nitrate and mono- and di-ammonium phosphate. All these compounds are best injected towards the end of the irrigation cycle to avoid leaching losses. Individual fertigation doses should not exceed 500 grams total fertiliser per tree. If more nitrogen is needed, then split it into two or three doses, preferably with an irrigation between each dose.

Summary

• Nitrogen must be maintained in balance with other elements in a citrus tree. Leaf analysis is critical for getting the balance right.
• Young trees need small, split applications of N to promote each growth flush and develop strong canopies.
• Bearing trees (5 years and older) need autumn applications only, to build up reserves for spring and promote fruit quality in the next season.

Acknowledgements

This research was financially supported by voluntary citrus grower levies administered by the Citrus Sector of NZ Fruitgrowers Federation. We thank Ian & Pearl Wilkinson and Roger & Rae Davies for use of their orchards.

References

Mooney P.A., Richardson A. & Harty A. (1991) Citrus nitrogen nutrition - A fundamental approach. Proc. Citrus Res. Seminar, NZ. June: 69-88

Mooney P.A. & Richardson A.C. (1992) Seasonal trends in the uptake and distribution of 15N in satsuma mandarins. Proc. Int. Soc. Citriculture. In press.

Source: The Orchardist, May 1993