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Research
on taro and sweet potato was conducted in Papua New Guinea. Sweet potato is the main staple
crop in most parts of the country. Field
experiments were conducted on-farm and on the university experimental
station.
The main aim was to increase crop
yields using organic and inorganic inputs (i.e. inorganic fertilisers,
poultry litter) as well as improved fallows in shifting cultivation
systems. A brief description is given below.
On-farm research
The
on farm research was conducted in Hobu, which is about 15 km North of Lae.
Three series of experiments were conducted: In the first experiment, plots
were planted with
Piper
aduncum,
Gliricidia sepium and Imperata
cylindrica which were slashed after one year whereafter sweet potato
was planted. Sweet potato yield was lowest after gliricidia fallow but no
yield differences were found after piper and imperata fallow. In the
second season, there was no significant difference in sweet potato yields.
The second experiment consisted of a factorial fertiliser trial with four
levels of N (0, 50, 100, 150 kg ha–1) and two levels of K (0,
50 kg ha–1). Nitrogen fertilisers increased yield in the
first season but depressed tuber yields in the second and third season.
Vine yield was significantly increased by N fertiliser in all three
seasons. Potassium fertiliser had no effect on marketable tuber yield but
increased non-marketable tuber yields. The third experiment consisted of a
comparison between N from inorganic fertiliser and poultry litter at four
rates (0, 50, 100, 150 kg ha–1). No difference was found
between the inorganic fertiliser and poultry litter and highest yields
were found at 100 kg N ha–1. In the second season no
significant response was observed. Although yield variation was
considerable this series of experiments has shown that sweet potato yield
can be significantly increased by inorganic or organic N applications.
Sweet potato yields after fallows were variable than after inorganic
nutrient inputs. Inputs through inorganic fertiliser or poultry litter may
strongly increase or decrease tuber yields.
On-station research
Besides the on-farm trial a series of experiments
were conducted at the university station to
investigate the effects of inorganic fertilisers and poultry litter on
sweet potato and taro. The experiments ran for several seasons and yield,
yield components and soil nutrient dynamics were investigated. Nutrient
use efficiency (see below) and modelling of the nutrient uptake and allocation to
various plant components (including roots - see below) were part of the research. Funding came from
ACIAR and the University of Technology in Lae. Although the research was
primarily on
nutrient and yield dynamics, additional research focused on the incidence
of nematodes and sweet
potato weevils and the effects on crop production. Also the seasonal
effects of rainfall was investigated and it was found that sweet potato
yields were higher in seasons with lower rainfall.
Nitrogen
use efficiency
The
effects of inorganic N fertilizers on upland taro (Colocasia esculenta) and sweet potato (Ipomoea batatas) were quantified with the aim to investigate
relationships between inherent soil fertility, N uptake, N application
rates and crop yield. Five levels of fertilizer N (0,100, 200, 300 and 400
kg ha-1) were given in split applications.
The
yield of marketable taro corms was not affected by N fertilizer but
non-marketable corm yield doubled at high N fertilizer rates. High N
applications yielded 8 to 11 Mg ha-1 more taro tops. Marketable
and non-marketable sweet potato yield was negatively affected by N
fertilizers. High N applications yielded 26 Mg ha-1 more vines
than the control treatment. Nitrogen fertilizer significantly reduced the
harvest index in both crops. When no fertilizer was applied, the total N
uptake of taro was 32.0 kg ha-1 of which 9.7 kg was taken up in
the marketable corms. At 400 kg N ha-1 the total N uptake was
67.5 kg ha-1 of which 23% was taken up by the marketable corms.
Uptake of N in the marketable sweet potato tubers was less than 11 kg ha-1
and for most treatments more N was taken up in the non-marketable tubers
than in the marketable yield. Up to 156 kg N ha-1 was taken up
with the sweet potato vines.
Three-quadrant diagram linking N
application, N uptake and marketable yield (left diagram) and total fresh
biomass yield (right diagram); I. Yield against uptake (Nitrogen Use
Efficiency), II. Yield against N rate (Fertilizer Use Efficiency), IV. N
uptake against N application rate (Fertilizer Recovery). From Hartemink et
al. (2000).
Despite
the negative effect of N on sweet potato yield, sweet potato had a higher
N use efficiency than taro due to a higher above-ground biomass
production. The N fertilizer recovery was 25% for the sweet potato but
only 9% for the taro indicating considerable N losses, likely caused by
leaching.
Root
biomass and nutrient uptake
There
is a paucity of information on biomass production and nutrient uptake of
roots in tubercrops. We investigated nutrient uptake of taro roots in
relation to corm yield and above ground biomass. Fertilized (100-50-100 kg
NPK ha-1) and unfertilized plants (n=4
each) were harvested at 126 DAP (mid-season) and 231 DAP (harvest).
Rooting depth at both sampling times was < 0.2m and an unit soil area
equivalent to the planting distance (0.5*0.8m) was removed from the field
whereafter roots were washed on a 0.5 mm sieve.
Root
biomass at 126 DAP was 0.26 Mg ha-1 (15% of total biomass) in
the unfertilized plots and 0.52 Mg ha-1 (13% of total) in the
fertilized plots but at harvest root biomass was similar (0.50 Mg ha-1).
Root nutrient concentration at 126 DAP was similar in both plots but N, Ca
and S significantly declined in the unfertilized plots at harvest whereas
B significantly increased with 18 mg kg-1. In the fertilized
plot P, K , Mg, Mn and Cu had decreased at harvest whereas Zn had
significantly increased. Nutrients in the root biomass as a fraction of
the total nutrient uptake were similar at 126 DAP for both treatments. At
harvest, however, the fraction of nutrients in the root biomass was
considerable lower in the fertilized plots. Striking was the high uptake
of Mg by taro roots in the unfertilized plots at 126 DAP (38% of total)
and at 231 DAP (36%).
The
study has shown that the amount of nutrients taken up by roots of
fertilized and unfertilized taro was similar at harvest but that a much
larger proportion of plant nutrients is allocated to the roots under
unfertilized conditions.
Last
update: 10th January 2007
Publications
Hartemink, A.E. 2003 Integrated nutrient management research with sweet
potato in Papua New Guinea. Outlook on Agriculture 32: 173-182
Hartemink, A.E. 2003
Sweet potato yield and nutrient dynamics after
short-term fallows in the humid lowlands of Papua New Guinea. Netherlands
Journal of Agricultural Science 50: 297-319
Hartemink, A.E., S. Poloma & J.N. O’Sullivan & 2001 Integrated nutrient
management research with sweet potato at Hobu. In: Food Security in Papua
New Guinea. R.M Bourke, M.G.
Allen & J.G. Salisbury
(Eds). ACIAR
Proceedings no. 99, Canberra pp. 698-711
Powell, K.S., A.E. Hartemink, J.F.
Eganae, C. Walo & S.
Poloma 2001 Sweet
potato weevil (Cylas formicarius) incidence in the humid lowlands of PNG.
In: Food Security in Papua New Guinea. R.M Bourke, M.G.
Allen & J.G.
Salisbury (Eds). ACIAR Proceedings no.
99, Canberra pp. 736-745
Hartemink, A.E., S.
Poloma, M. Maino, K.S. Powell, J.
Eganae & J.N. O’Sullivan
2000 Yield decline of sweet potato in the humid lowlands of Papua New
Guinea. Agriculture, Ecosystems and Environment 79: 259-269
Hartemink, A.E., M.
Johnston, J.N. O’Sullivan. S.
Poloma 2000 Nitrogen Use
Efficiency of taro and sweet potato in the humid lowlands of Papua New
Guinea. Agriculture, Ecosystems and Environment 79: 271-280
Hartemink, A.E. & R.M.
Bourke 2000 Nutrient deficiencies of agricultural
crops in Papua New Guinea. Outlook on Agriculture 29: 97-108
Louman, B & A.E.
Hartemink 1998 Sweet potato production in hedgerow
intercropping systems in the lowlands of Papua New Guinea. Papua New Guinea
Journal of Agriculture, Forestry & Fisheries 41: 91-98.
Offprint
Hartemink, A.E. & M.
Johnston 1998 Root biomass and nutrient uptake of
taro in the lowlands of Papua New Guinea. Tropical Agriculture 75: 1-5.
Offprint
Sayok, A. & A.E.
Hartemink 1998 Erosion and soil fertility changes under
leucaena intercropped with sweet potato in the lowlands of Papua New
Guinea. Papua New Guinea Journal of Agriculture, Forestry & fisheries 41:
85-90.
Offprint
Hartemink, A.E., M.
Johnston, P. John, W.
Julius & A. Kerru 1997
Biomass
production and nutrient uptake of taro roots. Papua New Guinea Journal of
Agriculture, Forestry & Fisheries 40: 6-12.
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