Blog by Christine Bukania
For many of us, deserts represent dry wastelands where almost nothing can take root, let alone grow and flourish. But for the last three years, Dr. Hari Krishna has been growing sweetpotato in the desert in Abu Dhabi, with considerable success. In 2013, Dr Krishna went to Abu Dhabi on a short-term consultancy assignment for the Abu Dhabi Farmers’ Services Center (ADFSC) and stayed for four years.
In 2014 while at ADFSC, Dr. Krishna, a development agronomist and postharvest technologist with over 35 years’ experience wrote to the International Potato Center (CIP) to request sweetpotato cultivars that can potentially grow in drier environments. As fate would have it, the breeding programme in Mozambique under the leadership of Dr. Maria Andrade had spent many years developing drought tolerant orange-fleshed sweetpotato (OFSP) varieties. He received 11 drought tolerant varieties from CIP and set up trials to see how far into marginal lands he could push this resilient crop. Most of these varieties were developed and released by two of the 2016 World Food Prize Laureates, Drs. Maria Andrade and Robert Mwanga.
“I remember very well when we got this little package of two kilograms ofcuttings. Now we have several greenhouses with multiplication plots and we are harvesting over 25 tonnes of roots from extension farms,” Dr. Krishna says.
Out of the 11 cultivars, ten did well and after further trials in a controlled research environment, six were found to be good for commercial production. Cuttings were mass propagated and were planted in extension farms in three locations. The entire production process was also monitored carefully. As a first quality check, there is a central nursery system, which ensures the quality of the cultivars going out. Any rooted cuttings that are found to be defective or diseased are destroyed. Only disease free vigorous cuttings are released for production.
The growth process
Cuttings are pre-rooted in a polystyrene tray with potting mix and are ready for transplanting after one week. Each tray holds 220 cuttings. “The plug transplant is the critical component in the successful establishment of sweetpotato in desert farming,” explains Dr. Krishna. This is because the nutrient rich potting mix holding adequate moisture acts as a buffer and helps rooted cuttings to rapidly acclimatize and grow into the sandy soil amended using cow manure and some basal fertilisers like superphosphates. Irrigation needs to be carefully controlled during establishment of transplants. In the 2016-17 growing season, the project expanded to 16 farms around the country growing the most promising cultivars Irene, Erica, Melinda, Sumaia, Kabode and Vita. Preliminary results suggest that all these cultivars performed well with yields ranging from 10 to 25 tonnes per hectare. There is however limitation when irrigation water salinity increases. Yield losses of nearly 50% can be expected in higher salinities.
For desert agriculture, irrigation water is tapped from aquifers through bore wells and drip irrigation is mostly used to grow crops. Currently, there are more than 20,000 farms using drip irrigation for cultivation. Fertigation and chemigation are common practices. Fertigation is the injection of soluble fertilizers into the irrigation system for plant nutrition while chemigation is the injection of chemicals such as pesticides to control pests and diseases.
In addition, row covers are used to protect the young sweetpotato crop from bird and insect attacks, and from intense radiation to reduce heat stress on young plants during the crop establishment phase.
The main sweetpotato growing period is from October to December with harvesting in January. A second crop can be established in December and harvested in April. This allows farmers to double crop in one season. This helps to increase farm income. Sweetpotato roots grown in the desert are ready for harvest between 120 and 140 days after transplanting. Manual harvesting is carried out by gently prying out the sandy soils around the roots. “This means digging mostly by hand with a help of a ‘pharaoh’s stick’ as I call it. This is just a stick with a blunt edge to ease the sand away from the root before pulling it out. The technique has been used in Egypt since antiquity. It prevents root damage from improper harvesting techniques,” Dr. Krishna elaborates. Because most sweetpotato crops are only about 1,000 square metres, harvesting can be completed in two working days.
In a desert environment, postharvest handling of the roots are critical for quality and storage potential. Soon after harvest, the roots are trimmed and washed in four consecutive baths full of water to remove sand particles. This gets the roots clean, and also hydrates them. Humidity plays an important role in postharvest handling of sweetpotato. In the interior parts of the desert, humidity can be as low as 30% drying out roots, which quickly shrivel and lose their appearance. Because humidity never goes above 60%, manual hydration is critical to help the roots hold their moisture and reduce water loss. To market prepare the roots, they need to be trimmed, washed, sorted and graded into small, medium and large roots.
Some findings from the desert trials
Dr. Krishna’s desert trials have yielded interesting findings about the resilience of sweetpotato varieties that were developed for
drought and disease tolerance in tropical Africa. “Irene is our best performing cultivar. Yield ranges between 15 and 25 tonnes per hectare from about 130 days of growing depending on the location and quality of irrigation water in which it is grown,” he says.
The sweetpotato varieties have remained true to their genetic makeup. For example, Kabode, a variety developed in Uganda, is irregular and knobbly, while Irene is smooth shaped. Both of them retained these characteristics in the sandy soils in which they are grown.
Storage is one of the major challenges affecting the sweetpotato value chain. Dr. Krishna’s trials found that washed and cured roots stored between 12 and 15 °C, had reduced incidence of rotting and sprouting even after seven months of storage. Although there were some rots, these could be minimised with increased aeration during storage. In a cool, dark, humid and well ventilated environment, roots may be kept for a shorter duration between one and two months.
For farmers in marginal arid and semi-arid regions, these drought resistant varieties could potentially provide a source of food and nutrition. The vines and leaves can also be fed to livestock.
Dr. Mwanga, notes sweetpotato and other tropical crops will give good yields as long as they receive some little water and are grown in soils that are not too poor in fertility. This applies even in desert conditions where fertiliser inputs are minimal. Dr. Andrade agrees. She recalls growing sweetpotato in a coastal area in Cape Verde, which was sandy and dry using fertigation and drip irrigation. “I harvested a sweetpotato root weighing 13 kg. There is no problem with sweetpotato as long as you have water and some fertiliser. If you have plenty of solar radiation sweetpotato would love it.”