Steps to success with tree crop nutrition

Correct nutrition management is essential for achieving high yields of quality macadamias and avocados and most growers work with a local agronomist to help them manage and refine their fertiliser programs.

Correct nutrition management is essential for achieving high yields of quality macadamias and avocados and most growers work with a local agronomist to help them manage and refine their fertiliser programs. 

These specialists in plant nutrition work with growers to assess the available nutrient levels in the soil and trees and keep an eye on other constraints and limitations which may affect growth. 

To provide a high standard of advice, your agronomist may have attended professional training to become a Fertcare^® accredited adviser. This is an industry initiative designed to ensure growers are receiving soil management and fertiliser advice based on good practice and accepted science. 

I have written these ‘steps to success’ to help growers identify areas for continuous improvement in nutrition in their orchards. 

A quality fertiliser is a key ingredient, and ^®Cal-Gran is ideal for those tree crops, but it is only part of the story.

Use regular testing

Annual soil and leaf testing will provide valuable information about the reserves of nutrients available in the soil and currently being used by the trees.

Leaf tissue testing is a particularly efficient and accurate guide to nutrient uptake.

In avocado crops, collect 80 recently expanded, mature and healthy leaves from non-fruiting terminals of the recent summer flush. In macadamia crops, take 80 mature leaves (six or seven months old) from the second whorl of the current season’s growth, from non-flushing terminals.

The recommended time for soil and leaf tissue testing is around April-May in avocados and September-November in macadamias, or just before the peak spring flush.

Soil samples are generally taken to a depth of 15 cm, but every few years, it is worth also investigating the subsoil area to a depth of 90 cm.

Tree roots may be using nutrients that have moved into this area, or you may find subsoil constraints are inhibiting root activity. When checking for subsoil constraints, segment the samples in 15 cm increments. Six to eight cores are enough for one sample when deep testing in tree crops.

Where deep soil tests are taken, mark the sites so the same areas can be sampled again in two to five years. This is the best way to monitor trends or changes over time.

Use a NATA accredited and ASPAC certified laboratory for reliable results from any soil or plant tissue testing program. Nutrient Advantage® Laboratory Services offers a high quality, accredited soil, plant and water testing service for growers across Australia.

Regular water testing is a good idea in irrigated orchards, as water quality can change and nutrients or salts in the water may significantly affect your trees. 

Consider fertiliser types

There are two main methods of fertiliser application in tree crops – fertigation and broadcasting granules.

Fertigation is the easy way for growers to split fertiliser applications and apply nutrients in regular small doses. However, fertigation can limit the area of soil which is treated with fertiliser to a narrow band around the irrigation line, which may not be adequate to feed the roots of the entire tree.

Depending on the fall of the land and irrigation equipment set-up, fertigation may also result in uneven nutrient application. Fertiliser will not be uniformly applied unless the irrigation water is uniformly distributed.

Broadcasting granules under the trees is better for undulating blocks, supplying the entire orchard with targeted nutrients.

When broadcasting granules, bring the fertiliser band no closer to the trunk than one third of the canopy radius. Extend the band of fertiliser beyond the canopy line into the inter-row by this same distance.

Apply fertilisers regularly

Fertilisers should be applied in small, frequent applications. This is the best way to ensure good uptake and use by the crop.

In macadamias, for example, the total nitrogen requirement should be split into five or more applications between April and October.

In avocados, the general recommendation is to apply 40% of the fertiliser requirement after fruit drop, another 40% four to six weeks later and the remainder when the flush is maturing, around April.

Review your blend

Cal-Gran fertiliser blends are ideal for supporting avocado and macadamia trees with nitrogen and a range of other nutrients.

Unlike urea blends, Cal-Gran fertiliser blends contain nitrogen in the ammonium and nitrate forms and are not prone to volatilisation losses (a gaseous loss of nitrogen to the atmosphere).

That’s why Cal-Gran is so well suited to surface fertiliser applications in tree crops. 

It is a long-term low volatilisation risk management option that can help ensure more of the nitrogen applied is available to the crop.

Growers can broadcast Cal-Gran fertilisers in a fortnightly or monthly routine and be confident that the nutrients will be moved into the soil with the next rainfall or irrigation and start to work.  

Cal-Gran contains nitrate and ammonium nitrogen, as well as sulphate sulphur and calcium. 

There are many Cal-Gran blends that are suited to the nutrition needs of avocado and macadamia crops. Custom fertiliser blends can also be created by your agronomist based on soil and plant tissue test results and crop history.

Don’t forget trace elements

Review and improve

* Contains sulphate of potash as the potassium source

One surprisingly good fit for many macadamia growers is Cal-Gran Aftergraze. Although originally designed to boost pasture growth, it suits macadamias with its high rate of nitrogen, 3% phosphorus and 7.5% potassium. 

One word of caution with these blends – it is best to avoid applying potassium chloride in avocados (as well as lychee, longan and mangosteen crops) because they are highly sensitive to salts. Agronomists can design Cal-Gran fertiliser blends using a sulphate of potash source. Sulphate of potash is also best for crops where there are high chloride levels in the soil or irrigation water. 

To maintain optimum product quality, growers are encouraged to order Cal-Gran and Cal-Gran blends as close as possible to the intended time of use. That’s because these products are hydroscopic (they will absorb moisture), potentially reducing product quality and making application difficult.

Last, but not least, be aware that if switching from urea (46% nitrogen) to Cal-Gran (23.9% nitrogen), the total fertiliser rate will need to be increased to apply the same total rate of nitrogen to the trees.

Don’t forget trace elements

Although micro-nutrients such as zinc and boron are required in much smaller amounts, they are still essential for good plant nutrition and the production of quality fruit.

Cal-Gran blends can include zinc or boron to ensure adequate nutrition for your crop. 

If magnesium is required, Nitrophoska Special may be used as it contains some magnesium and more can be blended with the fertiliser as needed. Cal-Gran is not suitable for blending with magnesium. 

Review and improve

Take time to sit down with your adviser each year to review the success of your fertiliser program and look for opportunities to improve. Consider rainfall received and crop yield variations too.

Avocado and macadamia growers can achieve significant yield responses through correcting nutrient deficiencies and addressing any hidden hunger that the trees may have, so it is time well spent.

For more information or to discuss nutrition in avocados and macadamias further, feel free to call me on 0428 111 471 or rob.dwyer@incitecpivot.com.au.