SWEET	POTATO     PRODUCTION       GUIDE    FOR   THE DRY	PAPUAN     LOWLANDS

By Graham King, Horticulturist,  Laloki Plant Research and
Quarantine  Station, D.P.I.

INTRODUCTION

In the lowlands sweet potato is the second most important
staple after bananas.   It is a crop that has great commercial potential in Central Province. The crop is suited both to small scale village production, and to large scale commercial production which uses fertil-

VARIETIES

Five varieties  have been se- lected from the Laloki col- lection as being superior in yield and disease resistance. These varieties  are available at Laloki in small quantities.

Variety        Source

izer, agrichemicals  and
irrigation.

The climate in Central Province is good for sweet potato.
Sweet potato plants need high temperatures  and sunshine. Although water is needed in large quantities  for the young

Laloki 2
Unu I
NG7571
Murua I
Murua 2

PLANTING

Laloki, C.P.
Cape Rodney, C.P.
IITA., Niger ia
Kerema, Gulf
Kerema, Gulf

plant, dry weather  is good
during tuber formation.

SOIL TYPE AND DRAINAGE

The field must be well drained. Loam or sandy loam is best for sweet potato cultivation.   On heavy clays, stems and leaves grow well but the yield is low with badly shaped tubers.

LAND PREPARATION

The field should be ploughed  to bUry grass and weeds.  It
should then be harrowed one or two times to make the soil fairly fine.  The ground  should t~en be ridged.  This helps
g~ve high yields and it makes
harvesting  easier.  Ridges are
Usually 1-1.5 m apart.

Sweet potatoes are grown by
planting	cuttings from the existing	 or previous  season's crop_	Usually only the stem
tips (about 30   cm) are used. The tips are more vigorous  and there is a better chance that they will grow well.

The cuttings  are planted in a single	row on the ridge 30   cm apart.	 Plant only one cutting at each position.

TIME OF PLANTING

If irrigation  is available  it is possible  to grow sweet potato all through the year.

The information in this article first appeared as   D.P.I. Laloki Information Bulletin No.2.

59

e time when yields will most obably  be poor is during the

In large scale production, chemical weed control is essen-

t season.  Most varieties

tial.  Three chemicals  which

tablished  under irrigation
r harvest in the wet season

are easy to obtain in Port
Moreshy and give good weed

not give high yields.  This especially  so on poorly

control in sweet potato are:

ained  soils.

Eptam:  Controls grasses,  nut

grass and mimosa.

wever, Unu 1, Murua 1 and

Rate: 2.5-7.0 litres/ha

rua 2 do give high yields

Crop stage: pre-emergence

der wet conditions.   Laloki 2

Weed stage: pre-emergence

d NG7571 are better suited to

y conditions  in the last 2

Incorporate  thoroughly  into the

nths of growth.

upper 10-15 cm of the soil with-

n no irrigation  is available

in 10 minutes of application.
Apply before ridging.

in the	wet season	for	
in the	dry.		

eet potato should only be

nted

Lasso:  Controls  annual grasses

rvest

D CONTROL

trol	of weeds in the first weeks	after planting  is
y important.   Yields can be atly reduced  if the crop
not kept weed free.  There two methods of weed control

and some annual broad leaf weeds.

Rate:    2.4 litres/ha
Crop	stage: immediately  after planting
Weed stage: pre-emergence

Must	be irrigated  immediately after		application.

Gramoxone:   This chemical w{ll

hand weeding and chemical

kilt-many grasses  and broad-

trol.

d weeding

d	weeding uses a lot of our	and so is expenslve.
ee hand weedings are usually essary,  at 2, 4 and 8 weeks
m planting.   As the plants

leaf weeds on contact.   If other herbicides  do not give
good control or instead of hand- weeding,  a directed  spray of Gramoxone  will kill weeds grow- ing between the rows of sweet potato.   It will also damage sweet potato plants if the
spray hits them.

w, it becomes easier to

age them with hoes.  Yields

Rate:  50 ml Gramoxone  plus 30

be reduced  if the vines
disturbed  too often.

ml Agral or other wetter in a
15 1 knapsack.
Crop stage: post planting

er 8 weeks the vines should

Weed stage: post emergence

er all bare ground and weeds
not usually a problem.

mical control

use	chemicals  you must have right	equipment.   If only

PEST	CONTROL Insects

psacks  are available  then

For details of the insect pests

t-emergent  sprays only can

of sweet potato, and their con-

used effectively.   A boom-

trol.  See the Entomology

ay is necessary  to apply

Bulletins  on pp. 90-98 of this

-emergent  sprays.

issue.

60

Rats become a major problem in the dry season.  Rat traps can be placed around the crop or a suitable rat poison can be put in bamboo tubes and placed
around the crop at 2-3 m inter-
vals. Diseases
The major diseases of sweet potato in Papua New Guinea are scab (Elsinoe batatas)   and
'little leaf', a virus-like
organism.   To control both diseases use clean planting material and destroy infected
plants.

For a fuller description  of scab,	 see the Plant Pathology Note	on pp. 99-100 of this issue.		For details of sweet potato	little leaf, see the Plant	 Pathology  Note in HARVEST Volume	 7 (1): pp. 50-51.

FERTILIZER

As sweet potato uses nutrients very efficiently  compared with other crops, fairly good yields can be obtained without fer- tilizers.

Sweet potato is a crop which is tolerant of acid soils.  The crop will still grow well when the pH of the soil is 5.0 or even less.

Nitrogen and potassium  in to- gether give the greatest  in- crease in yields.  Approxim- ately twice as much potassium as nitrogen  should be applied. The best results are obtained when the fertilizer  is applied in furrows on the ridge at planting,  or broadcast  (scat- tered over the whole area) before ridging.

~:     Apply 50 kg Nlha plus 50 kg K/ha at planting,  then 50 kg K/ha 2 months after planting.

IRRIGATION

About 12 mm of irrigation should be given every 3 days until the crop is growing vigorously.   After that, irrigate the crop every 2 weeks.  Up to 50 rom of water should be given each time. Most sprinkler irrigation systems are designed to give
10~12 rom per hour.

HARVESTING

The proper time for harvesting is when the growth of leaves and vines has stopped and the storage	of nutrients  in the
tubers is at a maximum.   In the lowlands this is usually 4-5 months after planting.   It is better to harvest after a spell of dry weather.   You should not harvest immediately  after rain because tubers then contain
too much water and rot very
quickly.  A chain digger can be used but the vines have to be cut away before digging.

Harvesting	a   crop of sweet potato  by hand at Laloki

61

TORAGE

weet	potato tubers can be tored		for short periods - up
o I month after harvest.  You hould	only store tubers which how no sign of sweet potato

Institutions  (e.g. Police, Army, Hospitals,  Hostels)

Advantages	- willing to offer high prices.
- require  large quantities regularly.

eevil attack.

Disadvantages  - may not allow

contracts  for supply.

ry to harvest tubers for

- growers have to shop around

torage on a fine day, not when

looking for a buyer.

t is rainy.  When digging,

ake care not to damage the

Traditional  markets

ubers.
Advantages  - very high prices

or the first two or three days

Disadvantages  - takes a long

fter harvesting  store the

time to sell.

ubers in a warm spot.  It

- have to pay someone to sell

hould be about 30oC, but not

the produce.

n direct sunlight.  If pos-

- only small quantities  can

ible, cover with a sheet of

be sold.

lastic during these two or

hree'days.   This will keep the

Central Food Marketing

umidity  (the amount of water

n the air) high.  High humid-

Advantages  - will almost always

ty, plus a temperature  of 30-

accept produce.

50C will cure the tubers  (that
s, heal the damage from

- may sellon  commission
which means a higher price.

igg ing) .

Disadvantages  - unless sold on

ter	this, store the tubers in cool		place away from rain.

RKETING

eet potato can be sold at any e of three plac~s.

commission  prices are low.

It is estimated  that the weekly demand for sweet potato in Port Moresby  is 30-35 tonnes at least.  With suitable land,
good varieties  and the proper
cultural practices  many farmers

production	of sweet	potato	to	
meet that	demand.			

would be able to increase their

62