Farmer's Weekly (South Africa)
Soil health: Rotation with cash crops benefits sugar cane yield
Decades of monocropping has compromised soil health on many South African sugar cane farms, negatively affecting their productivity. Dreyer Senekal, co-director of Senekal Familie Boerdery, explains to Lloyd Phillips how he is experimenting with strategic crop rotation to improve the soil of his sugar cane enterprise.
Sugar cane was first planted in South Africa in 1848, and for most of the time since then was produced in a monocropping system. It was only in more recent years that agriculturalists and farmers began to understand the importance of biodiversity both above and below the soil surface.
As one sugar cane farmer, Dreyer Senekal, observes drily, “We used to have the view that if you needed to rotate your old sugar cane crop, you just planted a new sugar cane crop straight after it. The biggest change we might have made back then was to plant a different cane variety to the one we’d ploughed out.”
Senekal is the full-time agricultural manager of the Senekal Familie Boerdery (SFB), a diversified mega farming business established in 1978 by his father, Charl Senekal, who remains actively involved in the operations.
SFB’s agricultural enterprises cover 4 500ha of irrigated lands in Mkuze, northern KwaZulu-Natal, with water piped from Jozini Dam. The primary enterprise is sugar cane, but SFB also produces citrus, macadamia and chillies. In addition, Senekal has a small commercial beef herd that he runs as a hobby.
“Our access to irrigation and our warmer climate allows us to harvest our sugar cane every 12 months. Depending on the sugar cane variety, we get eight to 10 harvests before we plough out and replant. On average, we replant 400ha to 600ha annually on a rotational basis across our sugar cane operation. Our main varieties are N49 and N57, and we’ll soon be harvesting trials of newer varieties to see how they do. All of these varieties are specifically bred for production under irrigation,” says Senekal.
He adds that, for most of the year, the Mkuze area experiences high number of daytime heat units, about 7,5 hours of sunshine per day, an average annual temperature of 22,3°C, and a virtual absence of frost, all of which make conditions ideal for vigorous sugar cane growth. However, irrigation is essential, given the average annual rainfall of only 550mm.
CLAY SOIL CHALLENGES
The two most common soil types in the area are Hutton and Shortlands. On the SFB farm, clay content averages a relatively high 35% to 40%, while in some parts of the farm this reaches as high as 70%. The main challenges with high
clay soils, especially in a sugar cane operation that uses heavy machinery, are surface capping and both surface and subsurface compaction.
A further challenge with SFB’s Hutton and Shortlands soils is that they tend towards an alkaline pH of 8 and above, as well as a build-up of exchangeable sodium in the deeper layers. This sodic environment exacerbates the dispersion of soil particles and the breakdown of soil aggregates, leading to compaction, run-off and erosion.
The sodium build-up became an even greater problem as drought conditions lowered Jozini Dam’s water level to 36% of capacity, thereby increasing the water’s concentration of natural sodium. However, the level of the dam is now at 56% and rising following recent rains, according to Senekal.
“To try to reduce soil sodicity, for the past three years we’ve been applying 2t/ha of gypsum to our sugar cane lands during replanting and after harvesting of ratoons. For fields particularly high in sodium, we fertilise only with ammonium sulphate as it helps lower soil pH and neutralise some of the sodium. We’re already seeing positive results from these actions.”
Senekal points out that in the years before SFB began its soil health improvement activities, yield on monocropped sugar cane lands declined to as low as 110t/ha. Yet the farm’s records show that yields of up to 170t/ha could be achieved on lands newly planted to sugar cane.
Stressed sugar cane is increasingly prone to competition from weeds such as couch grass ( Cynodon dactylon) and watergrasses
( Cyperus rotundus and C. esculentus), and to attacks from pests and diseases. The weeds can ‘steal’ as much as 100kg/ha of nitrogen from soil in only a few months.
Senekal and his management team realised that monocropping of sugar cane and their conventional management practices for this crop were taking out more from the soil than was being replaced. So, about five years ago, he began researching ways to sustainably rejuvenate the soil. A key finding was the importance of rotating the sugar cane with other crops such as cotton, which had long been grown in the nearby Makhathini Flats area.
“I first planted a trial of 18ha of irrigated cotton in sugar cane fields I had just ploughed out for replanting,” explains Senekal. “I chose a genetically modified Roundup Ready variety which, unlike sugar cane, is tolerant of the broad-spectrum herbicide glyphosate. This allowed me to make a big dent in the weed populations that grew between the cotton.
“The cotton also grows really strongly in our climate, and I can sell my harvest to recoup its production costs and possibly even make a profit.”
Sugar cane’s root system is a combination of highly branched superficial roots, buttress roots, and vertical roots (rope roots). Cotton has a taproot system that drills down into the soil to help break up subsoil compaction.
BENEFITS OF ROTATION
The first harvest of the follow-on sugar cane crop planted after the cotton yielded an average 18t/ha to 20t/ha more than was projected. SFB’s agriculture management team was so impressed that that they resolved to start growing cotton as a rotation on most of the sugar cane fields that were being ploughed out.
The cotton crops are typically planted in October at 53 000 seeds/ha at an inter-row
spacing of 91cm, and harvested five months later in March. This plant population and row spacing allow for sufficient airflow under the plant canopy to minimise the development of fungal diseases. Fertilising mixtures and rates are calculated based on the recommendations from soil and leaf analyses. SFB’s cotton yields currently range from 3t/ha to 5t/ha.
A year or two after he began experimenting with cotton as a rotation crop, Senekal’s ongoing research suggested that he should follow it immediately with a crop of sugar beans. Bean plants are leguminous, so their symbiotic rhizobia bacteria fix atmospheric nitrogen into the soil that can be used by the following crops, thereby reducing applied inorganic nitrogen requirements. Sugar bean plants’ fibrous root system also helps loosen and aerate the soil.
“We start planting our sugar beans two days after the cotton has been harvested, because we want to take advantage of the March and April heat units to boost the beans’ germination and early growth,” he explains. “We use a relatively short-season variety that’s ready for harvesting in August and September. The beans are planted at a rate of 75kg/ha of seed and at a 76cm inter-row spacing. After that, we replant these fields to sugar cane.”
A bonus of SFB’s sugar bean harvest is that it is ready during the window when the peak of South Africa’s local sugar bean supply has passed and before the country starts importing this commodity to achieve consistent national supply. So, the prices during this short period are typically above average national prices during the rest of the year.
In 2020, SFB achieved around R21 000/t for its beans, which yielded at least 2,5t/ha. Any beans rejected during on-farm sorting are given to the beef cattle as high-protein feed or sold to buyers at a reduced price.
The fibrous plant residue from the harvested cotton crops is slashed and then disced into the soil before the beans are planted. The softer plant residue from the sugar beans is worked into the soil as preparations for planting the follow-on sugar cane crop get under way.
BENEFITTING IN CASH AND KIND
According to Senekal, the cotton and sugar bean crops collectively generate a substantial cash income during the 12 months between when an old sugar cane field is ploughed out and a new cane crop is planted. In addition, the field gains a number of benefits: improved soil structure with a greater biological component, nutrient cycling with increased nutrient availability to plants, more organic carbon, better moisture penetration and retention, improved aeration, greater tolerance to pests
MONOCROPPING SUGAR CANE MINES THE SOIL, TAKING AWAY MORE THAN IT REPLACES
and diseases, and reduced erodibility and competition from weeds. All of this leads to higher sugar cane yields and sucrose levels.
LEY CROP TRIAL
Senekal’s efforts to improve soil health through crop rotation include an experiment with a ley crop of sunn hemp ( Crotalaria juncea) in his high bulk density, high-clay soils (above 55% clay content). A ley crop (grown in the rest period between commercial crops) is typically a non-incomegenerating crop that serves to improve soil health and break the pest and disease cycles of primary crops. Sunn hemp produces far more above-ground biomass than cotton and sugar beans do, and this is mechanically worked into the soil to decrease its bulk density (the weight of the soil in a given volume) and improve its aggregation and thus its infiltration capacity, permeability, aeration and the effective rooting depth for primary crops.
“It has been three years since we first began rotating sugar cane with cotton and sugar beans, and we’re still seeing the benefits in the annual yields and quality of the cane. We’re still learning about how long these benefits will last, but we’ll continue our strategy, and we look forward to when all our sugar cane fields have been rotated in seven to 10 years’ time. Once we’ve identified how long the benefits last and we’ve determined our costs versus our savings and returns, we may even discover that it’s more profitable to reduce the number of years between ploughing out and replanting a sugar cane field,” says Senekal.