First lab managers’ meeting of the South-East Asia Laboratory NETwork (SEALNET 2.0) - Quality improvement in Asian soil laboratories: towards standardization and harmonization of soil analyses and their interpretation, Bogor, Indonesia, 20 - 24 November 2017.

1. General principles of soil test interpretation

2. • Soil pH, EC
o Different soil/water ratios (1:5, 1:2.5, 1:1)
• Extractable P
o Different extractants, soil:extractant ratios,
extraction times (Bray-P1, Bray-P2, Olsen-P)
• Extractable K
o Different extractants (NH4OAc, NH4Cl)
• Organic C
o Different oxidation methods (Walkley-Black,
combustion)
Methods used to measure key soil properties in some SEALNET
labs

3. Will these different methods give different results?
• Soil pH
o Soil pH decreases with decreasing soil:water ratio
o High correlation between results from different
soil:water ratios
For example, in some Vietnamese soils,
pH (1:5 water) = 1.09 pH (1:1 water) – 0.01 (r=0.94)

4. Relationship between ECse and the equivalent EC1:5 determined from
soil clay content and SCAMP texture type.
Corresponding EC1:5 (dS/m) based on % clay content
ECse (dS/m) 10–20
(Sandy, S)
20–40
(Loamy, L)
40–60
(Clayey , C)
60–80
(Clayey,
C)
2.0 0.16 0.20 0.23 0.31
4.0 0.30 0.40 0.50 0.62
ECse = electrical conductivity of a saturation extract; SCAMP = Soil Constraints and Management Package
Will these different methods give different results?
• Soil EC
o Soil EC increases with decreasing soil:water ratio
o High correlation between results from different
soil:water ratios

5. Will these different methods give different results?
o Equivalent values for 1M NH4OAc and 1M NH4Cl
extractable K
• Exchangeable K

6. • Available P
Will these different methods give different results?
Olsen-P
Extractant: 0.5 M NaHCO3
Soil:extractant ratio: 1:20
Extraction time: 30 min
Separation: filtering; centrifuging
Analytical finish: colorimetric Murphy and Riley (1932)
Bray P1 and Bray P2
Extractant: Bray P1: 0.03 M ammonium fluoride + 0.025 M HCl
Bray P2: 0.03 M ammonium fluoride + 0.1 M HCl
Soil:extractant ratio: original 1:7; often 1:10
Extraction time: original: 60 sec; often 5 min
Separation: centrifuging; often filtering
Analytical finish: Murphy and Riley (1962); Watanabe and Olsen (1965)

7. • Olsen-P and Bray-P1 may be correlated for similar soils-
0
20
40
60
80
100
120
140
160
180
0 20 40 60 80 100 120 140 160 180
Bray 1-P (mg/kg)
Olsen-P(mg/kg)
but we cannot be confident that the relationship can be
applied to other soils!

8. Bray-P: acidic, complexes Al, dissolves Ca-P
Olsen-P: alkaline, hydrolyses org C, reduces
soil solution Ca activity
Bray-P and Olsen-P may extract P from different sources
in different soils
Sorbed P
(attached to -O- groups
on clay mineral surfaces)
Mineral P
(apatites, fertiliser residues)
P
P P
P
Solution P
P
precipitation/
dissolutionsorption/
desorption
dissolution
Organic P
mineralisation
?

9. Quite simply, a soil (P) test is of no use unless it is related
to plant growth!
Australian national dataset
for wheat grain yield response
to P
Colwell-P best related
to wheat grain yield

10. Determination and Interpretation of Soil Properties for Soil Fertility Assessment
• Different species
have different
nutrient
requirements
• Critical soil test
values may differ
by a significant
amount Ymax Maize = 8 - 10 t/ha
Ymax Peanut = 3 - 6.5 t/ha
Ymax Soybean = 3 t/ha
Bray-P and Olsen-P will have different critical values for
different crops
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0 10 20 30 40 50 60 70
Available P (mg/kg)
Grain/podyield(kg/ha)
Soybean Wheat Peanut B Maize B Peanut A Maize A
Colwell-P (mg/kg)

11. • Coefficients of variation: Olsen-P 13%; Bray-P1 27%
But what is meant by a ‘critical value’
• Better to talk about ‘critical ranges’!

12. • Organic C
oWalkley-Black organic C: partial oxidation
oTotal organic C: complete oxidation by combustion
Will these different methods give different results?
• Walkley-Black organic C = 75% to 90% total organic C,
depending on soil type and soil texture

13. 5 major soil carbon pools
<10%2. Soluble - fresh residues (labile)
<5%1. Living organisms and roots (labile)
10-50%3. Particulate organic C -decomposing (labile)
33-50%4. Humus (decadal)
1-30%5. Charcoal/Resistant (inert)
WalkleyBlackorganicC
5 major soil carbon pools
<10%2. Soluble - fresh residues (labile)
<5%1. Living organisms and roots (labile)
10-50%3. Particulate organic C -decomposing (labile)
33-50%4. Humus (decadal)
1-30%5. Charcoal/Resistant (inert)
WalkleyBlackorganicC
5 major soil carbon pools
<10%<10%2. Soluble - fresh residues (labile)
<5%<5%1. Living organisms and roots (labile)
10-50%10-50%3. Particulate organic C -decomposing (labile)
33-50%33-50%4. Humus (decadal)
1-30%5. Charcoal/Resistant (inert)
WalkleyBlackorganicC

14. 5 major soil carbon pools
<10%2. Soluble - fresh residues (labile)
<5%1. Living organisms and roots (labile)
10-50%3. Particulate organic C -decomposing (labile)
33-50%4. Humus (decadal)
1-30%5. Charcoal/Resistant (inert)
TotalorganicC

15. Summary
• Soil pH and soil EC determined at different soil:water
ratios can be compared using regression equations.
• Available P determined by different methods may not
be interconvertible.
• Exchangeable K values determined in 1M salt solutions
are equivalent.
• Walkley-Black organic C is a variable proportion of total
organic C.
• Guidelines exist for the interpretation of soil pH and soil
EC.
• Critical ranges for available soil P tests and exchangeable
K will vary with crop and perhaps soil type.