Speaking at a recent Fertiliser Association of Southern Africa symposium, Dr Gerhard Nortje, a fertiliser consultant and senior lecturer in environmental and soil science at the University of South Africa, said that the fertilising norms for macadamia tree cultivation set by international growers are not defined narrowly enough for South African growing conditions, and that in order to rectify this, cultivar trials specific to South Africa’s conditions needed to be conducted.
Through his years of experience, however, Gerhard has identified certain fertiliser needs for macadamia trees that can be applied in local orchards.
Macadamia trees are not heavy feeders, and their fine lateral roots are efficient in absorbing phosphorus and calcium, even where there are low levels of these nutrients in the soil.
However, according to Gerhard, the first thing producers must realise is that the phenological cycle is the primary determining factor for macadamia orchard management and fertilisation, and not the calendar.
Macadamia flower initiation begins in May, with August and September the main months for flowering and root growth. Nut set occurs in September and October, and from November to January nut growth occurs, as well as oil accumulation.
(Nut growth increases linearly up to mid-December, thereafter nuts convert starch into oils. The oil accumulation period is very important, as nut quality improves with increased oil content).
Between March and May, the mature nuts drop from the trees. However, nut drop varies from cultivar to cultivar, with ethylene spray used to initiate nut drop in the Beaumont cultivar, Gerhard says.
“The aim of a fertilisation programme should be to keep the tree and the root system healthy, and to replace, at least, the amount of nutrients removed each year by the crop,” Gerhard says.
A yield of 3,5t of Nut-in-Shell (NIS)/ha, which is the average viable crop/ha, would absorb about 63kg of nitrogen (N), 3,5kg of phosphorus (P), 70kg of potassium (K), 17,5kg of sulphur (S), 35kg of calcium (Ca) and 5,25kg of magnesium (Mg), per hectare per annum.
“Macadamias need low P and Ca levels, but high iron [Fe] levels. Iron deficiencies will result in sparser leaf cover and possible tree dieback.
“It also needs well drained soil, and NPK levels should be replenished as trees require,” Gerhard says.
Gerhard says farmers should sample and analyse the soil every two years, while leaf samples should be conducted on marked trees every year. This, he says, is sufficient to monitor soil fertility over time.
“A history of soil fertility and nutrient status in orchards should be built over time. Leaf samples must be taken between October and November each year by sampling the fourth leaf behind the growing point of an actively growing shoot, or a shoot that has recently completed a growth flush, on which the terminal bud is dormant. Only leaves from healthy plants must be sampled,” Gerhard says.
Fertilisation traditionally begins in July, when N and K are applied. Usually only 25% N is applied, but the farmer may also choose to apply 25% Ca. Then in September, 40% N is applied, and 50% Ca.
Immediately after planting, non-bearing trees need to be given time to adjust to their new environment, overcome any transplanting shock, and for the roots to begin growing out of the potting medium into the surrounding soil.
It is thus important not to apply any fertiliser to trees for the first six to eight weeks after planting.
The analysis of data is more meaningful when recommendations are based on historical trends, rather than on the results recorded after only one year.
“Look back as far as possible and build records for specific blocks and orchards,” Gerhard advises.
Any nutrition management programme must take into account a visual appraisal of the trees, as well as information such as yield data, historical soil analyses, historical leaf analyses and data of any previous fertiliser programmes, Gerhard says.
In Australia, macadamias adapted to highly weathered soils, with high levels of Fe available for plant absorption in high rainfall areas. Macadamia trees are therefore highly sensitive to Fe deficiencies in soil.
These acidic Australian soils also have very low P levels available for plant absorption. As a result, macadamia trees have developed root systems that absorb P exceptionally well.
As such, the plant absorbs high amounts of P from soils with adequate to high P levels.
Gerhard says that plant and soil analyses taken from various macadamia orchards with serious Fe deficiencies have shown that these orchards have extremely high P levels.
He has thus concluded that Fe deficiencies are induced by high levels of P. However, this claim is disputed by some in the industry.
Since 2011, Gerhard has identified Fe deficiencies as the reason for tree dieback in some macadamia orchards. Using the Bray-1 soil test, a test used to analyse P levels in soil, it was discovered that over 50mg.kg-1 of P had a negative impact on crop performance.
In the orchards that Gerhard tested, results showed that P levels were as high as >200mg.kg-1, with the P levels of some soils reaching up to 300mg.kg-1 or 400mg.kg-1.
Some researchers, however, believe that the problem may lie with too high Zinc levels in the soil, instead of P levels. This is something that needs further investigation, he says.
Many farmers attempt to fertilise soil by spreading chicken manure in their orchards. This can, however, lead to too high levels of P. Moreover, many macadamia orchards in South Africa are planted on traditional tobacco fields.
These soils often have inherited high levels of P, and so Fe deficiencies may be inevitable even before planting begins. Thus, soil analysis is critical, Gerhard says.
Research conducted by researchers in Hawaii indicates that Fe deficiencies in macadamias are also evident in cases where soil pH is too high.
Soil pH analysed in water should not be more than between 5,5 and 6, while soil pH analysed in KCl should not be more than between 4,5 and 5.
The effect of high pH and high P levels on Fe levels is disastrous for macadamia trees, Gerhard explains. Soil pH and P levels should be analysed and corrected during land preparation and before planting.
Thorough incorporation of soil ameliorants such as lime, gypsum, K and P, and the aeration of the soil, facilitated by deep ripping, will provide an optimal environment for root development, Gerhard says.
Macadamias need lots of organic matter, so the incorporation of as much compost as possible, up to 10t/ha prior to planting, is an added benefit, especially on sandy soils.
Email Dr Gerhard Nortje at firstname.lastname@example.org.