Simple quantitative and qualitative field test for diagnosis of small holder farm problems

Ray R. Weil

University of Maryland

Talk to the

Farmer

• Trust but verify

Visually assess

Field variability

• Crop growth

• Soil colors

• Topography

Test Soil

• pH / EC

• Nutrients

• Physical tests

Test Plants

• Petiole/stem sap

• Foliar symptoms

• Cut open stem

• Dig up roots

Talk to the farmer

• What’s the problem?

• What was done?

• What was applied?

• Trust but verify

In-field variability

Can we make the whole field as good as the “good spots?

What makes the “good spots” so good?

Maize field in Tigray

Terrace catches water in Ethiopia

pH 5.4, Labile C 660 mg/kg

pH 4.8, Labile C 400 mg/kg

Furrow-irrigated maize in Malawi

One source of variability.

Farmer shows me how he incorporates manure: Trust but verify

Test Soil

• pH / EC

• Nutrients

• Physical tests

• Signs of life

Collecting surface soil composite sample using farmer tools.

Soil Chemical Properties of surface soil (composite sample)

Chemical Properties of soil profile (mini-pits)

Physical soil properties

Biological soil properties

My current field tool box of soil and plant testing materials ~ $1600.

And “on the ground”

NO3 meter: $350

EC meter: $20

pH meter: $35

$0.50 bottle drinking water

0 50 100 150

Sum (K + + NO 3-), mg/ kg soil

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

EC

of

1:1

so

il:w

ate

r su

sp

en

sio

n,

dS

/m

Ustult-falTropept- ter

SOIL

Range : 0.05 to 1.0 dS/m

EC relates to fertility

Often indicative of general fertility level in humid region soils.

EC, pH and NO3 meters also used for water that may be used for irrigation

In a smallholder’s farmyard

1: 1 soil: water slurry Soil pHH2O

Soil electrical conductivity

Add calcium chloride to EC to > 2.5 Soil pHCaCl2

Delta pH (pH H2O - pHCaCl2

CaCl2 extractable nitrate-N (ion specific meter)

CaCl2 exchangeable K+ ions (ion specific meter)

CaCl2 extractable Sulfate-S (turbidity with BaCl2)

CaCl2 extractable phosphate-P (colorimeter)

Nitrate-N

pHw in the field using bottled drinking water is comparable to standard lab pHw.

Nitrate-N

Soil NO3-N in field using general purpose Horiba NO3 meter is comparable to standard lab NO3-N extraction only for samples with > 15 mg /L.

Out of the rain in my “no-star hotel”

$15 balance

good to 0.01g

$50 colorimeter

programmed for

PO4 to 0.01 ppm.

P reagent

packets $0.26

per sample

$0.18 glass

“cuvette” &

Rx vessel

Tropept- ter Ustult-fal

SOIL

4.0

4.5

5.0

5.5

6.0

pH

w-

or

pH

Ca

Cl2

pHCaCl2

pHw

Tropept- ter Ustult-fal

SOIL

0

10

20

30

40

50

60

70

N0

3- -N

o

r K

+ ,

mg

/kg

so

ilK+NO3_N

For:•Soil pHCaCl2

•Delta pH (pH H2O - pHCaCl2)•Salt extractable nitrate, sulfate and potassium

Clear sample after settling

Muddy sample after stirring

0 10 20 30 40 50 60 70

K+ using muddy suspension, mg/L

0

10

20

30

40

50

60

70

K+ u

sin

g c

lea

r so

lutio

n,

mg

/L

Ustult-falTropept- ter

SOIL

10 20 30 40 50 60 70

NO3-N using muddy suspension, mg/L

10

20

30

40

50

60

70

NO

3-N

usin

g c

lea

r so

lutio

n,

mg

/L

Ustult-falTropept- ter

SOIL

Clear vs. muddy samples for K and NO3 meters

NO3 K

1: 10 soil: Mehlich3 extraction solution

Mehlich3 extractable K+

Mehlich3 extractable PO42-

Mehlich3 extractable SO4-S

Tests for P and S use light beam and so require filtering.

2.5 g dry soil in 20 mL 0.02 M KMnO4 (method of Weil, 2003)

Labile carbon (indicative of SOM, degradation)

Earthworms and channels

Termites, ants and nests

Dung beetles and other fauna

Plant roots, nodules, mycorrhizae

Fungal mycelium /fruiting bodies.

Bio-interpretation of nitrate and labile C tests

Flat-tip pH meter on moistened soil

Direct-reading EC probe inserted into moistened soil

• Presence of free carbonates (10% HCl drops)

• Reduced iron (α-α diperidil)

• Mn oxides (H2O2 drops)

• Physical observations (e.g. water, density, colors, nodules)

Example of soil profile limitations:Guambeh brothers, Nimba County, Liberia

Guambeh brothers farm in Nimba, Liberia

“Good” area

“Poor” area

Test Plants

• Petiole/stem sap

• Foliar symptoms

• Cut open stem

• Dig up roots

Farmer used urea and DAP

Ion specific meters for K+

and NO3-N

Colorimeter for PO4

Good quality garlic press ($18.00)

0.54 reading = 12 ppm S in vial

= 240 ppm S in sap

1.99 reading = 48 ppm S in vial

= 960 ppm S in sap

S = 960 ppmN = 530K = 6680P = 9 ppm

S = 240 ppmN = 53K = 6500P = 21 ppm

Farmer used urea and DAP

Future farmers in Ethiopian finger millet field