Soil testing: what it does and where is it going?

Karen Daly

Teagasc

Soil Fertility Conference, 19th October 2016.

Lyrath Co. Kilkenny

Soil testing: what it does & where is it going?

• What are we testing?

• Why we test:

• Major nutrients in soils we have to keep an eye on: Phosphorus

• Soil type effects

• Bringing more soil information into to testing

• New methods in development at Teagasc

Soil: a complex medium

sand

silt

….

.

clay

water

air

Microbial communities

water

Soil-water interface

(Soil solution)

45

5

25

25

Composition of mineral soil (%)

Minerals

OM

water

air

What are we testing?

Athea Castleisland Doonbeg Kishkeam Rossmore

Sites/surfaces for nutrient &

trace element retention – Al,

Fe, Ca,

Nutrient or Trace

element X

Nutrients and trace elements in soils ‘removal’

X

Fixed X

Available

reserves

X

available

Nutrients and trace element depletion or ‘removal’

X

Fixed X

Available

reserves

X

available

Crop offtake

Nutrients and trace elements in soils

fixed Available

reserves Available

Crop offtake fertiliser

input

Phosphorus pools in soils

Fixed P

500-1500

mg/kg

Available

reserves

100 mg/kg

Available

P 10mg/kg

Crop offtake Fertiliser

P input

Interpreting soil test P

results: excessive Index 4

Fixed P Available

reserves

P

Available

P

Crop offtake Fertiliser

P input

Tap in on full – oversupply.

Interpreting soil test P

results - optimum

Fixed P Available

reserves

P

available

P

Crop offtake Fertiliser

P input

Index 3 = Good supply only when needed by crop

Interpreting soil test P results: low/deficient

Fixed P

Available

reserves Available

P

Crop offtake fertiliser

input

not enough nutrient in

reserve to supply to solution

build up rates need

to build up reserves

Phosphorus: think in

terms of supply

Fixed P

500-1500

mg/kg

Available

reserves

100 mg/kg

Available

P 10mg/kg

Crop offtake Fertiliser

P input

Build back up labile P reserves to keep supply going.

Phosphorus: think in

terms of supply

Fixed P

500-1500

mg/kg

Available

reserves

100 mg/kg

Available

P 10mg/kg

Crop offtake Fertiliser

P input

Build back up available P reserves to keep supply going.

Index 3 = good supply

Build up influenced by soil type

Fixed P High Al to P

ratio Available

P

Crop offtake fertiliser

input

Index 1, 2

High Al influence P supply

Al:P ratio is too high – P

unavailable (strongly bound).

At lower Al:P, P is

easily released,

Al

P ? P

P ? P

?

?

?

p High P fixing soils

7.3 mg/l

Phosphorus availability is influenced by soil properties

Current trends in soil P fertility

14% 14% 15% 22% 24%

30% 27% 27% 31%

26% 25% 26%

28% 30%

29% 28% 27%

30%

30% 29% 28%

25% 25%

23% 24% 24%

22%

30% 32% 30% 25%

21% 18% 21% 22% 17%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

20

07

20

08

20

09

20

10

20

11

20

12

20

13

20

14

20

15

Trend in Soil P Index

P Index 4

P Index 3

P Index 2

P Index 1

high

Optimum

deficient

low

Where?

Current methods:

Soil sampling typically every 2- 4 ha,

once in 5 years.

Limited to small number of elements:

P, K, Mg, LR.

Almost never for texture, organic

matter & other soil attributes

Phosphorus soil test and what they measure?

• There are many different tests out there.

• Amount of P extracted by any one test depends on the soil

pools that are responsive to that test

(e.g. Mehlich3>Olsen>Morgan-P)

• A soil P test must access the same pools that the plant can,

No soil test is universal

Soil properties of soils

mapped at landscape

scale Manage soil fertility at

field or paddock scale

Soil testing:

Good idea to know the soil as well as test it.

Is it acidic in nature?

Al?

Clay?

Important soil properties to consider: pH, Al, Ca, %OM

Emerging techniques:

New methods for soil testing capture more soil properties

No chemical extraction,

lower cost & rapid results.

One scan gives info about

soil pH, OM, Al, Fe, Ca,

texture etc.

NIR / MIR

• Sample interacts with

light source

• Get spectral

information on the

chemical makeup of

the sample.

Spectra produced when infrared is absorbed by a soil sample

gives the overall chemical profile of the soil

A H Jean Robertson, H Rachael Hill, Angela M Main. 2013. Soil spectroscopy workshop. FAO.

Rome.

IR applications - collect multi-parameter data from a single scan

Athea Castleisland Doonbeg Kishkeam Rossmore

• % OM at surface horizon

• % sand, silt, clay along profile

• extractable Al, Fe, Ca in each horizon

• pH, CEC, TN, TOC,

Multi-parameter: Soil attributes affecting soil fertility – not

captured by current methods

What does this mean for soil testing?

Spatial scales

Temporal scales

Multi-parameter data

Model/forecast

soil fertility

indicators

Thank you for your attention

Karen.Daly@teagasc.ie

Johnstown Castle

Wexford