Yams (Dioscorea spp.) are an important tuber crop for most of PNG’s lowland farming communities. For some, it is a staple food, also playing significant cultural values. The traditional yam-based farming system requires fertile soils to supply the required nutrients in order to produce economic tuber yields.
The crop is cultivated firstly after the natural vegetation has been cleared or after a long fallow period.
As a result of increasing population pressure on land, the length of fallow period has been drastically reduced to a few years in recent times. Farmers in Yule Island in the Central Province have mentioned that their fallow periods have been reduced significantly to less than six years for those who have a lot of land, and much lesser for those with little agricultural land.
Yams, like many other tuber crops, demand significantly higher quantities of the major soil nutrients – nitrogen (N), phosphorus (P), and potassium (K), calcium (Ca) and magnesium (Mg).
The major causes of declining soil fertility on Yule Island, as identified and discussed, were:
- increasing rainfalls and floods,
- soil erosion,
- cutting down of trees,
- continuous cultivation on the same piece of land,
- burning of grass/ bush fires,
- lack of knowledge,
- cultural beliefs,
- population pressure,
- land shortage,
- laziness, and
- lack of soil improvement efforts.
Soil samples collected at two depths (10cm – 15cm and 15cm – 30cm) at model farmer gardens indicated very low levels of the macro-nutrients N, P and K, although Ca and
Mg seem to have adequate levels. The soils being weakly acidic to neutral and alkaline.
The interpretations indicated that the highly nutrient-demanding tuber staples such as yams and cassava cannot continue to sustainably produce high quantities to meet the food demands for the increasing population on Yule Island, unless efforts are made to address the declining trend in soil fertility.
Soil improvement efforts have been minimal or non-existent due to lack of knowledge or farmers being oblivious.
Under the project, several interventions were trialed, which included hedgerows using Gliriciddia sepium (which is available in abundance); legume crop inter-cropping; and crop-rotation using peanuts, snake beans, dwarf beans, cow pea, pigeon pea and Mucuna spp, to name a few.
Mulching for soil moisture conservation was also demonstrated. Yam plantings were done using compost materials of local weeds and leaves of Gliricidia sepium alongside their common practice of without any soil improvement practices.
The importance of soil nutrient replenishment was emphasized during the training. Further demonstrations will be conducted when the yams are harvested later in the year.