Soil management practices

Surveys in the Highlands by the National Agricultural Research Institute and Australian Centre for International Agricultural Research have indicated that pests, diseases and unsustainable cultivation practices have caused up to 60 per cent decline in sweet potato yields, in recent years, Yapo Jeffery writes
Wild Mexican sunflower is a good source of soil nutrients.

SWEET potato is an important food and cash crop in the Highlands of Papua New Guinea.
It plays an important role in food and income security for rural farming communities.
However, surveys in the Highlands by the National Agricultural Research Institute (Nari) and Australian Centre for International Agricultural Research (ACIAR) have indicated that pests, diseases and unsustainable cultivation practices have caused up to 60 per cent decline in sweet potato yields, in recent years.
To mitigate these threats, Nari has been conducting studies on the use of pathogen-tested (PT) planting materials and improved soil fertility management options.
This work has been undertaken since 2017 with the support of the ACIAR at the Institute’s Highlands Regional Centre at Aiyura, Eastern Highlands.
One vital part of this work is the use of PT planting materials.
PT planting materials are sourced from initial stocks of sweet potato plantlets that have been grown under sterilised conditions in a tissue culture laboratory.
These pest and disease free or resilient stocks are then established in special insect or vector proof nurseries.
Sweet potato cuttings, or slips, are generated from these safe nurseries for distribution to farmers.
This helps to minimise the use of infected materials sourced from old gardens.
As part of the studies, trails compared yields of PT cultivars grown using organic soil fertility management methods against outputs of non-PT cultivars grown using traditional farming methods.
The trails used PT cuttings or plantlets of the local Wahgi besta cultivar and non-PT plantlets were collected from old gardens.

Dried chicken manure ready for use as organic fertiliser.

Organic soil fertility management options used included coffee pulp, chicken manure, compost and soybean, peanut, natural and improved fallow rotations.
Trial results indicated that higher yields were being obtained from integrating PT planting materials with organic soil fertility management options.
These accounted for around 35 tons of harvests compared to just 18 tonnes per hectare produced from non-PT cultivars, grown without soil fertiliser options.
This represented a yield improvement of about 51 per cent.
Furthermore, it was also found that PT planting materials needed very high amounts of soil nutrients to produce better yield qualities.
With the use of organic soil fertility management options, the tubers harvested were found to offer more commercial and nutritional value as they were comparably bigger, smoother and of better eating qualities.

Nari and an ACIAR staff harvesting PT Waghi besta sweet potato at Aiyura organic soil fertility management trials in Eastern Highlands.

However, it is important for farmers to maintain organic soil management methods when planting PT sweetpotato cultivars as their high intake of soil nutrients may cause fertility to levels to decline, over time.
Despite that, organic soil fertility management options are environmentally friendly and usually have no harmful effects on the surrounding plants and wildlife.
Another interesting aspect was the comparison between performances of the different organic fertiliser options.
It was found that applications of chicken manure, coffee pulp, improved fallow and natural fallow rotations gave better results.
These options were more effective as they had provided higher amounts of nitrogen and potassium concentration in the form of organic matter.
However, care should be exercised so that right amounts of organic fertilisers such as chicken manure and coffee pulp are applied to encourage healthy growths and yields of sweet potato.
Many of these options can be accessed and used with very minimal costs as they are locally available, especially in the Highlands.
Farmers can obtain chicken manure and coffee pulp from their own poultry or coffee farms to use after having them dried well and stored for use.
They can also do improved fallow by selecting and growing wild legume and nutrient accumulating plants such as fish poison bean (Tephrosia vogelii); smooth senna (Senna septemtrionasis) wild daka (Piper aduncum); and, Mexican sunflower (Tithonia diversifolia).
Crop rotation practices involving legumes crops such as soybean and peanut are also good options that farmers can consider.
Furthermore, leaves and stems of crops from improved fallow and crop rotation can make very rich compost because store high amounts of nutrients such as nitrogen, phosphorus and potassium from the soil.
Generally, all of these methods help to improve and sustain soil fertility by fixing soil nutrients; increasing microbial activities; improving soil structure, and retaining soil moisture.
Nari is doing such studies with various PT sweet potato cultivars in selected places in the Highlands for commercial production.
One of these sites is the Asaro Valley in Eastern Highlands.
It was observed that many farmers here have keenly adopted the organic soil management practices to grow and supply quality sweet potato tubers.
This has enhanced their demand in urban markets in cities such as Port Moresby and Lae.
We hope farmers from other parts of the Highlands and the country would be interested to adopt the innovative PT and organic soil management methods.
This will help to improve sweet potato production to sustain income and food security for the growing population in the country.

  • Yapo Jeffery is a junior scientist in soil and water management based at Nari’s Aiyura Research Centre in Eastern Highlands

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