Getting to the root of declining pecan yields

The yields of mature pecan nut orchards are failing to reach optimal levels, and concerned producers have been searching high and low to find answers, investigating everything from foliar applications to soil management. The latest indications are that the solution seems to lie in the trees’ roots.

Getting to the root of declining pecan yields
Cover crops offer many advantages for orchard farmers; in particular, they aid root health. However, the drier climate of pecan production areas can make cultivating cover crops a challenge.
Photo: Lindi Botha

“Yields are down in the pecan industry, and we have many more questions than we have answers,” says Hardus du Toit, senior technical officer at the South African Pecan Nut Producers’ Association.

“In the Vaalharts area, where most of the industry is concentrated, we should be getting at least 2,5t/ha. Instead, we’re averaging 2t/ha, taking alternate bearing years into account.”

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Exacerbating the effect of unrealised yields is the ever-increasing cost-price squeeze. The price paid for pecans has decreased over the past few years, after spiking a few prior to that.

“The sums are a little tighter than they were a few years ago,” laments Du Toit.

“With input costs having risen sharply, farming sustainably requires increasing our yields. Instead, we’re seeing a big compounding effect of lower yields and higher input costs that has hit the entire industry at once.”

In their search for answers, Du Toit and others held discussions with irrigation experts from Netafim, who suggested they investigate soil compaction, as this influences water absorption, which in turn affects root growth and nutrient uptake. Du Toit and the Netafim team then spent several days measuring compaction across the region using a penetrometer.

“The results were astounding,” says Du Toit. “We realised just how inadequate our soil preparation had been, and the negative effect our current soil management was having on the trees.”

He explains that while pecan trees can be rather forgiving in their early years, problems start to show once they have reached their teens. In addition, many pecan farmers in this area are first-generation tree farmers, having come from cash crops.

“It’s a big mind  shift to make. If you can’t afford to perform a deep rip on the cash crop fields this season, you can postpone it to next season without any major consequences. But with pecan trees, if you don’t do everything exactly right on the first try, you’ll sit with problems for the lifetime of that orchard.

“As the trees are growing older, we’re seeing problems worsening. Sunlight management is an issue, as is soil compaction.”

Constricted roots
Netafim agronomist Jovan Erasmus explains that optimising production involves many aspects, but a strong, deep root system is the key to a productive, high-yielding tree that consistently produces high-quality nuts.

“Overall, long-term production potential is adversely affected when you have compacted soil, as it complicates nutrition and irrigation. This means increased expenses and makes life difficult.”

He explains the extent of the challenges that pecan farmers face by describing the typical soil conditions where most of South Africa’s pecan production takes place.

“This is aeolian soil that compacts easily, severely affecting root growth. It compacts to high densities that prevent good, strong root development. The typical breakdown of soil in Hartswater, for example, is 88% sand, of which 80% is fine sand. The remainder is usually silt and clay. The uniformity in size and distribution means that the soil is packed very tightly and can reach a high bulk density. This impedes root elongation, and can be measured using a penetrometer.

“Penetrometer measurements show that the resistance at which roots struggle to grow is 1,2 megapascal (MPa). The critical point at which root growth is seriously impeded is 2MPa.

When compaction was measured in Vaalharts and Hartswater, readings far exceeding 2MPa below 30cm of soil were common. This means the trees have only about 30cm of soil in which to grow. If most of the roots are above 30cm and there’s no mulch cover, the surface temperatures get so hot that the roots can burn.”

Erasmus adds that if the soil is too compacted, the roots will expend energy struggling to grow, leaving less energy available for the growth of leaves and nuts. In short, the easier it is for the roots to grow, the more energy there is for a crop.

For optimal crop production, roots should be able to grow easily up to 1m deep.

“If the soil is overly compacted, trees are less able to absorb nutrients, especially immobile elements such as phosphorus and potassium, and mobile elements such as calcium, nitrate, iron, magnesium and zinc.

“Transpiration will also be lower, leading to reduced yield. And as the movement of water in the soil is adversely affected, the plant has less ability to store water and nutrients.”

Because optimal irrigation becomes problematic, farmers end up either under-irrigating or over-irrigating. The latter leads to leaching and saturation of the compaction layer, and maintaining optimal nutritional balances within the root zone then becomes difficult.

Free the soil
When the Netafim team set out to find a solution to the lower yields, taking soil compaction and nutrient distribution into account, they investigated soil preparation practices on several farms. The results were disconcerting.

While some farmers had not ripped the soil before planting their pecan trees, preferring to make the planting holes with an auger, others had ripped and cross-ripped to a depth of 60cm and then also used an auger to make the planting holes.

The farmers who had used augers only were no better off than the others, however, with all experiencing compaction of over 2MPa from 30cm downwards.

Erasmus says the reason for this lay in the continual wheeled traffic in the orchards, which was effectively compacting the soil with every trip between the rows.

“When it comes to compaction, the biggest issue by far is wheeled traffic. This is why we often see that the penetrometer shows compaction up to 60cm, but then below that the soil is looser.”

An alarming fact is that the very first set of wheels to roll over a newly tilled land comprising sandy soil recompacted the soil to 80% of its original density, almost entirely negating the point of tilling and loosening the soil.

The Netafim team suggests that pecan farmers follow current trends in orchard farming and introduce ridges and cover crops as avocado and macadamia producers have done.

“We’ve seen that cover crops and mulch have a hugely positive impact on soil structure and reducing compaction in orchards in the Lowveld. Ridges are also proving to be far more beneficial than initially thought, as they not only provide a large layer of top soil, but traffic is also forced to stick to the middle of the inter-row and away from the area directly under the trees where the roots are,” explains Erasmus.

The first step in solving the pecan dilemma is therefore controlling traffic in the orchard.

“Distinguish between active root zones and wheeled traffic zones, and use every means to keep the traffic off the root zone. Ridges are preferable, as they not only prevent wheeled traffic in the root zone, but roots are far more developed when trees are planted on ridges. This has been shown in research.

“It also appears that trees grow much faster in the early years and start producing earlier when planted on ridges. This is because water and nutrient management is easier, so the trees get what they need through properly developed root systems and well-managed soil.”

These healthy citrus trees show the value of planting on ridges and using drip irrigation: they have well-developed root structures that easily grow to a depth of 1m.

An additional layer of organic matter can go a long way to preventing natural recompaction, making planting cover crops an attractive practice. A thick layer of mulch provides nutrients to the roots and also helps to keep soil surface temperatures low, preventing root burn.

In managing the cover crops, a side mulcher should be used to place the green matter under the tree trunk, without driving over the root zone.

Cover crops in the pecan-growing areas are more of a challenge than in the lush Lowveld, however, as they cannot depend on the latter’s high rainfall to sustain them through summer. Crops that have lower water requirements should therefore be chosen.

Correct irrigation scheduling
The wetter the soil, the greater the compaction, so it’s important not to over-irrigate. Low-flow drip irrigation is therefore a better option than flood and sprinkler irrigation.

Michael Esmeraldo, regional agronomy manager at Netafim in Mbombela, urges farmers to dig soil profile pits to determine where roots are concentrated, and to manage irrigation accordingly for the best results.

“If you know where the roots are, you know where to place your irrigation drippers and how to schedule irrigation to ensure you wet all the roots with every irrigation cycle. The more roots that come into contact with the fertigation, the better the potential for a good yield. It’s also important to manage oxygen in the root zone, so over-irrigation should be avoided,” he explains.

Optimal irrigation entails dividing the root zone in two, namely the active roots and the buffer zone, and managing each differently. The active roots need to receive adequate water with each irrigation cycle to ensure that the soil stays wet for longer and dries out slowly.

“If the roots are up to 1m below the surface, the irrigation cycle should wet the soil to about 70cm, because this is where 70% of the roots are. Then, once a week, carry out a longer cycle to wet the soil all the way to the bottom.”

Esmeraldo cautions that the exact timing of irrigation cycles should be calculated based on readings from a soil moisture probe. In sandy soils, these probes should be placed 10cm from the dripper and in the inter-row space, in line with the dripper. This will give the best estimate of how well the water is penetrating the soil.

“If it takes three hours to completely wet the root zone to a 1m depth, then you know you cannot irrigate for longer than that or you’ll leach all of your nutrients. So it’s very important to determine the cycle length.

“In the profile pit, you’ll also be able to see whether the water is infiltrating the soil properly or if there’s run-off in the wrong direction. If there are many roots under the
drippers, you’ll know the water is adequate. If there aren’t any, either the irrigation cycle is too long and the roots are not getting enough oxygen and therefore not growing, or there’s too much run-off, and the roots are not getting a chance to absorb the water.”

Esmeraldo says that when low-flow drip irrigation technology was introduced, there was scepticism over whether the small drippers would be able to wet the entire ridge.

“But profile pits quickly put this argument to rest, as they clearly showed it could be done, even with 1ℓ/hour drippers.”

He adds that is not uncommon to find pecan trees in excess of 100 years old. Taking care to manage the soil properly and so avoid compaction can lead to decades of good yields.

Email Hardus du Toit at [email protected], Jovan Erasmus at [email protected], or Michael Esmeraldo at [email protected].