ReStory narrator: Andrew Ardington
After more than 100 years of tillage, the sugarcane fields of Cranburn Farm’s soil organic carbon is at healthy levels. Ever since the ground was first turned the soil carbon levels of KwaZulu-Natal’s sugar cane fields have plummeted. Despite the fact that sugarcane is a perennial crop and the fields don’t have to be ploughed every year, the extensive tilling work done every ten or so years when then crop is replanted results in considerable carbon oxidation into the atmosphere. Other management practices that contribute to further carbon oxidation from the soils include burning of the trash (dead leaves) before a field is cut and the application of synthetic fertilisers.
For the first couple of decades, until after WWII, Cranburn’s fields were worked with oxen, but thereafter tractors entered the fray and significantly increased the tillage. Before synthetic fertilisers became common fertility was maintained by not burning the trash (which remained as a mulch) and by growing cover crops of velvet beans or cowpeas. The cover crops were harrowed and ploughed into the fields before the sugar was planted.
From the 1960s fertilisers replaced the green manure from the cover crops and across the industry the burning of the trash became the norm. Burning was popular because it made harvesting and land preparation before planting easier and cheaper. Of course the burning immediately put large quantities of carbon into the atmosphere and also meant that until the crop had grown back and closed the canopy there was no mulch on the soil. The soil was exposed to the sun, wind and rain, soils eroded and more soil carbon was lost. Cranburn Farm changed to synthetic fertilisers, but did not change to burning its trash unless a field had borer.
In the 1970s Cranburn Farm introduced herded cattle into sugar fields, following the cane cutters and eating the tops (the green leaves not sent to the mill) as their winter feed. The cattle processed the tops and returned them to the fields in their manure and urine, returning carbon and stimulating the soil microbiology. With the advent of portable electric fences for controlling cattle movements, this incorporation of green manure intensified. Cranburn’s harvesting of unburnt cane was costly as cutters took longer to cut a field but it had many other benefits. The surface mulch and cow dung and urine was now adding more organic matter back to the soil and due to the cover it provided chemical control of weeds was reduced by 30%.
In 1996 the old practice of fallowing with green manure crops was resumed. A full year of fallow was followed with multispecies summer (e.g. sun hemp, sorghum, cowpeas) and winter (e.g. oats, rye, vetch) cover crops. Instead of being incorporated into the soil with machinery, these crops were repeatedly grazed by cattle. Aside from adding organic matter to the fields, the heavy grazing and trampling ensured that before replanting to sugar no mechanical management of the surface litter was needed. Despite the longer fallow periods out of sugar cane and that fertiliser was applied at 30% lower than the recommended rate, Cranburn’s sugar cane yields compared favourably with peer group farms.
Finally, in 2000 the local soil tests were improved to include a reading for carbon. Carbon which we know to be a crucial component of soil health had long been neglected in a world dominated by chemical tests for nitrogen, phosphorus and potassium. In the region, after nearly 100 years of tillage, soil organic matter had dropped significantly, in some poor soils to below 1%. The test results from Cranburn Farm varied between 3% in weak soils, up to 7% in the good soils. Years of mulching, cattle dung and cover crops were making a difference.