non-exchangeable potassium in southern african soils: a
TRANSCRIPT
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Non-exchangeable potassium in southern African soils: a neglected
reserve from a crop nutritional perspective
Neil Miles1 & Mart Farina2
1South African Sugarcane Research InstituteUniversity of KwaZulu-Natal
2Consultant, Howick, South Africa
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The cost squeeze in agriculture
Years
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The cost squeeze in agriculture
Years
Improved efficiencyIn the use of inputs
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Estimated current annual expenditure on NPK fertilisers in the South African sugar
industry
N P K TotalR millions
448 212 489 1 149
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Soil testing for potassium• Most soils have relatively large amounts of total K,
but small amounts of plant-available K.
• Soils may contain minerals (typically micas & illites) that release K slowly and/or ‘fix’ it.
• Routine soil testing involves extraction of exchangeable / soluble K with salt solutions.
• Slowly-available (‘reserve’) K may be estimated using boiling HNO3 or sodium tetraphenylboronextraction.
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Potassium pools in soils
K in clay structures
‘Fixed’ K in micas &
illites
Exchangeable KK in soilsolution
‘Soil-test’ K
Slowly available‘Nitric K’
Exchange
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The issue…….• In soils containing micaceous clays, slow
release of K is a major reservoir of K for crop growth.
• Although long known, this phenomenon not accommodated in routine soil testing.
• Potential for appreciable savings in K ?
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This presentation
• Data from maize and sugarcane field trials
• Boiling nitric-acid extractable K (‘nitric K’) in 429 topsoil samples from five Southern African countries
• Prediction of nitric K using routinely-measured soil properties and MIR
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Field trial results…
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Long-term maize trial at Bergville
• Avalon soil, 36% clay• Nitric K = 2.08 cmolc/kg• No yield response over 25 years (all stover
and grain removed at harvests).
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Bergville Maize Trial
0
100
200
300
400
500
600
700
0
20
40
60
80
100
120
140
160
180
200
0 5 10 15 20 25 30
K re
mov
al (k
g K
/ha)
K s
oil t
est (
mg
K/L
)
Years
K soil testK removal
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Sugarcane response to potassium on a Glenrosa soil in Mpumalanga
• Initial soil K = 150 ppm
• Nitric K = 2.6 cmolc/kg
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Potassium response of sugarcane on a Glenrosa soil in Mpumalanga
No response to K over three
ratoons (estimated
annual removal = 250 kg K/ha)
12
14
16
18
20
22
24
26
28
0 75 150 225
ERC
(t/h
a)
K (kg/ha)
LSD0.05
Sucrose yields in third ratoon
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Long-term sugarcane trial at Mt Edgecombe (BT1 Trial)
• Arcadia (vertic) soil
• Nitric K = 1.52 cmolc/kg
• No response to NPK for 18 years
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Sugarcane trial on the Umfolozi flood-plain
• Dundee (alluvial) soil (42% clay)
• Soil test K: 110 mg/L
• Nitric K = 3.84 cmolc/kg
• No response to K for 2 seasons
Top-yielding farmer: no K for 5 years
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So….. no responses to applied K in
field trials with nitric K’s of 1.5 to 3.8
Overseas field trial findings…?
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Criteria suggested for modification of K recommendations in Australia
• Haysom (1971) field trial data interpretation:
Nitric K (cmolc/kg) Category Recommended K
(%)<0.8 Low 100
0.8 – 1.5 Medium 821.5 – 2.5 High 45
>2.5 Very high 0
• Schroeder et al. (2006): Reduce K recoms by ±20% if nitric K >0.70
Never implemented..?!
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Nitric K levels in Southern African sugarcane topsoils
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Median nitric K levels in Southern African soils
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0N
itric
K (c
mol
c/kg)
100
4030
92
30
72
38
27
SA field trials
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Nitric K: which soils have higher reserves?
• Many soils in drier irrigated areas
• Alluvial soils (e.g. Umfolozi flood-plain)
• Structured soils in rainfed areas
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The problem of soil heterogeneity
• Frequent high soil heterogeneity implies marked variations in K reserves over small distances.
• Not possible to extrapolate results over large areas.
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Frequency distribution of nitric K in sugarcane fields on the North Coast of SA
0
2
4
6
8
10
12
14
16
18
20
22Fr
eque
ncy
Nitric K (cmolc/kg)
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Nitric K in profiles surrounding SASRI
HuttonFernwood
Arcadia
Glenrosa
Shortlands
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HuttonFernwood
Arcadia
Glenrosa
Shortlands 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Hutton Fernwood Glenrosa Arcadia Shortlands
Nitr
ic K
(cm
olc/k
g)
156 kg/ha K appliedper ratoon (R1900/ha)
Nitric K in profiles surrounding SASRI
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Imperative: the routine prediction of nitric K in order to modify K recoms
• Multiple regression based on routinely-measured soil properties (R2 = 40.4)
• Offer nitric K as an optional supplementary analysis at additional cost?
• Use MIR to routinely estimate nitric K on all samples?
How .… nitric K determination not suited to use in routine soil-testing lab
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Mid-infrared reflectance prediction of nitric K reserves in
soil of the Southern African sugar industriesPr
edic
ted
Measured
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Mid-infrared reflectance prediction of nitric K reserves in
soil of the Southern African sugar industries
Pred
icte
d
Measured
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Bad news for the fertiliser industry???
OPPORTUNITIES
Improved efficiencies → enhanced sustainability.
More effectively address other crop nutrient problems…N, S, micros.
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Conclusions• Field trial results point to the unnecessary
use of K on many soils in Southern Africa
• Exploiting slowly-available K reserves → significant cost savings, without compromising production.
• MIR → possible solution to the problem of routine measurement of K reserves