Enhancing soil productivity in the H’lands


THE traditional farming system of shifting cultivation to enhance soil fertility has marginal impact in managing crop production in the Highlands.
Natural fallows to replenish adequate supplies of essential crop nutrients are determined by the length of the fallow period.
A study by Nari in 2005, funded by the Australian Centre for International Agricultural Research (ACIAR), revealed that fallow periods in much of the Highlands region have drastically reduced from several decades to less than 12 months.
The absence of a break between cropping cycles is also becoming common. As a result, soil productivity has diminished over time.
Unsustainable soil management practices such as slash and burn, continuous mono cropping, maximum tilling, inappropriate fertiliser application and improper crop rotation, just to name a few, have further deteriorated soil conditions.
All in all, land use intensification to meet the growing demand for food and income is unsustainable. High population densities eventuating from a rapidly growing population is the underlying cause.
Synthetic fertiliser use is not viable for the majority of PNG rural farmers. Hence, acceptable land management technologies for better soil health to improve crop yields are needed.
Where fallow periods are inefficient in revitalising soil productivity, improved fallows can become important.
FAO (the UN’s Food and Agriculture Organisation) describes improved fallows as land resting from cultivation but the vegetation comprises planted and managed species of leguminous trees, shrubs and herbaceous cover crops that rapidly replenish soil fertility in one or at most two growing seasons.
Hence, soil fertility can be restored in a shorter time, compared to the same interval under natural fallow.
Tilling of the slashed vegetation further releases essential plant nutrients during decomposition.
Even with the vast diversity of leguminous species, the Agri Cultures Network elaborates that a good fallow specie should have quick growth to close the canopy quickly and suppress weeds and minimise erosion; high biomass production its litter breaks down without N-barrier (closer C/N ratio); and N2 fixing deep rooting for pumping leached-out nutrients or nutrients from decomposing rocks.
The species should also efficiently absorb nutrients, not accidentally spread into cultivated areas by flying seeds and so on, be easy to clear and supply products that make its use attractive (fruits, stakes, folk medicines, feed etc).
The Highlands is fortunate to have many kinds of leguminous species that are able to empower the use of improved fallows in farming systems.
These include leguminous species such as: Tephrosia vogelli, Crotalaria spectabilis, Flemengia macrophylla, Senna septemtriolasis, Crotalaria quinquefolia, Crotalaria incana and Desmodium species.
Incorporation of non-leguminous, nutrient accumulator plant species such as Tithonia diversifolia and Piper aduncum can also be rewarding. This is due to their high nutrient content of phosphorus and potassium, respectively.
Attempting to mitigate the impacts of soil infertility in the Highlands for sweet potato production, Nari through its Highlands Regional Centre in Aiyura, has conducted studies of low input soil fertility management options. Being a cadet research scientist, I led field trials from 2013 to 2015, studying impact of these systems on sweet potato yield.
A mixed improved fallow-sweet potato rotation system was an area of interest. Common legume species in the highlands were broadcasted and left to compete and establish among the natural fallow vegetation before cutting and ploughing the top biomass for sweet potato cultivation.
This approach was to avoid the extra efforts that the farmers may experience in managing a fallow, which may discourage uptake of the innovation.
Alternative systems studied included natural fallow-sweet potato rotation and legume-crop rotation systems involving peanut and soybean.
Application of organic supplements, or fertilisers, such as; coffee pulp, chicken manure and improved composts were also experimented.
Marketable tubers after mixed improved fallow was 16.15t/ha; this was almost a t/ha less subsequent to natural fallow in the first season. Its result was one of the three (3) least performing treatment apart from peanut and soybean.
However, marketable tuber yield was 17.12t/ha after the managed fallow treatment in season two. Natural fallow gave 15.41t/ha. These were the results from six months fallow.
This is an encouraging indication that improved fallows being able to improve and maintain yields over successive seasons. On the contrary, despite a 2t/ha increase in yield, natural fallows may have the shortcoming of unable to replenish adequate soil nutrients in a similar time frame.
Despite the conclusion of the cadet project, Nari and ACIAR have realised the importance of such a study. Post-cadet research activities have been expanded in Aiyura under the ACIAR SMCN 2012/105 project – Sustaining soil fertility in support of sweet potato cropping systems.
– Nari

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