Soil Testing for Phosphorus and Potassium

Routine Soil Testing goals

• Rapid• Affordable• Predictive• Reproducible• Widely applicable• Track changes in fertility• Develop nutrient

management plan

Soil testing starts with collecting a

good sample

Soil testing is not useful without meaningful samples

Soil Testing basics

Taking a Good Soil Sample

• Decide on samplingequipment, soil depth, numberof samples, and location

• Have a clean plastic pail formixing the individual cores

• A light coat on the interiorof the sampling probe of aspray lubricant (such as WD 40)can help with removal of the sample

• A field map or GPS unit to record where the samples came from

• Clearly labeled soil bags or boxes for sending to the laboratory

Taking a Good Soil Sample

• Divide the field or managementarea into areas depending ontopography, soils, managementhistory. A soil map will be helpfulfor this

• Take 15 to 20 individual soil coresand mix well into one compositesample to be analyzed by thelaboratory

• Clearly label the sample container and completely fill out the information sheet from the soil testing lab, so proper recommendations can be made for the specific field area and the crop to be grown

Where to Avoid Sampling

• Field borders, especially if closeto a gravel road with crushedlimestone

• Where there have been brushpiles, straw or haystacks, manurepiles, lime piles, etc. in the field

• Trouble spots, such as due toerosion or salinity, unlesssampled separately

• Old fertilizer bands in row crops• Injection knife tracks• Old fence rows, roads, or buildings• Animal excretion or congregating spots

Sampling Depth

• Plow/ Disc/ Chisel tillage:– Most frequently sampled to depth of 6 in., but may be – 7, 8, or 12 in. in some areas (depending on cropping system)

• Ridge-tillage– Sampled to a depth of 6 in., taken 6 in. from the row

• No-tillage or minimum tillage:– Sampled to a depth of 4 in., but may be 3 or 6 in.– (sometimes the surface 1 or 2 in. is sampled for soil pH)

• Established pasture and turf:– Sampled to depth of 3 or 4 in. Remove heavy thatch before

sampling

General recommendations are based on previous crop, tillage system, and fertilization practices. Follow the recommendations of your soil testing laboratory:

8 in.

16 in.

24 in.

32 in.

Forest

Residual Nitrate

Pre-Sidedress Nitrate Test

Turf

Plowed

No-till(2 samples)

Ridge-till

6 inches6 in

ches

General Recommendations for Depth of Sampling

Depth and Location of Cores Impact Variability

440 580330

440

160

330440

37 in.37 in.

0

10

Dep

th in

.

1606

8 in.160 – 580 ppm K

4 in.330 – 580 ppm K

Robbins and Voss, 1991 (IA)

Sampling in Ridge-Till Systemswith Residual Fertilizer Bands • Sample to 6 in. depth;

6 in. from the row• Avoid high P and K zones that

may have been band-appliednear the row

6 in.

6 in

.

Sampling Soils with Banded Fertilizer• In soils with residual fertilizer bands…

– The general recommendation is to double the number of cores in a composite sample sent to the lab to get a representative analysis

• If the location of the P fertilizer band is known: – 30-in. row spacing: Sample once in-the-band for every 20

between-the-band samples (1:20 ratio) – 12-in. row spacing: Sample once in the band for every eight

between-the-band samples (1:8 ratio)• If the location of the P fertilizer band is unknown:

– If <20 subsamples (cores) are taken, paired sampling in the field consisting of:

• 1. A completely random set of samples and• 2. A second set of samples, collected at half the fertilizer band spacing,

perpendicular to the rowThe greatest deviation from the "true" P soil test occurs when inadequate

sampling includes rather than excludes the band

Soil Sampling Orchards

• Leaf sampling is usually more accurate than soil analysis to monitor nutrient status of perennial crops, but soil testing still provides useful information

• Before planting, obtain a soil map and take samples according to soil type and field characteristics

Soil Sampling Orchards

Irrigated Orchards:• Sample in area wetted by irrigation • Take soil cores under the drip-line for

sprinkler or basin irrigation• With drip or micro-sprinkler irrigation, take

cores 1/2 to 2/3 of the way out from emitter towards wetted edge

Non-irrigated Orchards:• Sample in active rooting area• Take multiple cores around drip-line for a

composite soil sample• One-foot depth is generally adequate

0

20

40

60

80

100

0 2 4 6 8 10Number of sample cores

Per

cent

of v

alue

s fa

lling

into

the

mea

n ra

nge

• One core is not adequate to represent field variability!• The suggested number of cores depends on the degree of field variability• Taking 5 to 8 cores may be adequate, but 15-20 cores may be required to

get a representative sample

How Many Cores are Needed?

Franzen and Berglund , 1997

“More” Cores Improves Precision and Accuracy

Soil test P category upper limit, ppm

Freq

uenc

y (5

0 to

tal)

0 2

1720

73 1

05

10152025303540

5 cores per sample

0 0

8

34

8

0 005

10152025303540

10 20 30 40 50 60 70

20 cores per sample

True average

Soil Sampling Equipment

Sampling Tools:– Shovel:– Use clean tools– Sample from the proper depth and location– Place samples in clean bucket for mixing

Soil Sampling EquipmentSampling Tools:

– Shovel:– Use clean tools– Sample from the proper depth and location– Place samples in clean bucket for mixing

Push probe:– One-inch diameter tube is most common– Convenient to use in soils without stones– Easy to clean tube and sample to a

consistent depth

Soil Sampling EquipmentSampling Tools:

– Shovel:– Use clean tools– Sample from the proper depth and location– Place samples in clean bucket for mixing

Push probe:– One-inch diameter tube is most common– Convenient to use in soils without stones– Easy to clean tube and sample to a

consistent depth

Auger:– More convenient in rocky, wet, and hard soils– Easier to sample to deeper depths– A variety of tips and designs are available for

different soil textures

Equipment-Mounted Sampling Equipment

Tractor-mounted Truck-mounted ATV-mounted

Sample Handlingand Shipping• Once the individual cores have been collected

in a bucket, break the lumps, remove stones, and mix well

• Mix the soil completely and fill the sample box or bag to the “full mark” (usually one to two cups of soil)

• Avoid taking wet soil samples, but allow to air dry if the samples are too wet for shipping

• Carefully label each sample container and make careful description on a field sketch or field notes of where the samples were taken

• Accurately complete the field information sheet requested by the lab in order to get the most accurate recommendations possible

Practical Sampling Equipment Considerations• Probes and shovels do not work well in rocky soils• Bucket augers may work best in sandy-textured soils• Special tools are sometimes used in sampling turf• Use a tool that permits sampling to a consistent,

accurate depth

Time and Frequency of Sample Collection• Seasonal variability does exist

– But more for soil pH thanfor P or K

• If possible, sample at the same time of year to reduce variability

• If not possible to sample at the same time, the soil analysis will still be useful for making nutrient decisions and tracking trends

What season of the year?

Corn Grain Yield

bu/A

Topsoil variation

Low soil P and K

End-row compaction

Soil pH

Historically managed less intensively

Treeline and end-row compaction

Greater weed pressure

Nearly flat, ponding

Natural and Man-Made Variability Impacts Soil Productivity

Map Courtesy of Kitchen, USDA - ARS

Soil P Concentrations1-2 mg P/kg

3-4

5-6

7-8

9-10

11-18

When to Take Directed Soil Samples?Suspected Field Variability

Indirect Indicators:– Topography– Aerial photos– Soils map– Yield map– Soil EC

Direct Indicators:– Cropping history– Fertilizer history– Manure history– Old homesteads– Old feedlots

Map Courtesy of R. Koenig, Washington State University

Sampling by Soil Type May Be Best Choice for Some Fields• In highly variable landscapes,

sampling by soil type (zone) is superior to a random sampling scheme

• Sampling by soil type and landscape position is frequently the best way to get accurate information on the fertility status of a field

Zone or Grid Sampling?

• Zone sampling (Stratified sampling)– Uses farmer knowledge of field variation– Excellent if location of variation known– Use if there are regular or repeating patterns– Good for large sampling areas (> 5 acres)

• Grid sampling– Will help locate unknown sources of variation– Easy to manage fertilizer with field maps – Can increase knowledge of the field

Grid Sampling: Different Approaches• Composite (or cell) system

– More robust for large grid size– Requires more effort– Less variable grid

• Grid point system– Assumes sampled areas can predict

unsampled areas– Difficult on narrow fields– Superior if you can afford a small grid size

point

xxxxx xx

xxx

xxxxxxx

xxxxxxxx

xxxxxxx

xxxxxxxx

xxxxx xx

xxx

xxxxx

xxxxx

composite gridx

x

x x x

x

xx

xxx

x

x

xxxxx

xx

xx

x xx

x

xx

xx

x

x

x

xx

xxx

x

x

x

xxx

x x

x

xx

xxx

x x

x xx

x

Grid or Zone-Based Soil Sampling• Easiest with the assistance of a

GPS monitoring system to record sampling sites

• Cores are collected and composited surrounding the pre-determined sampling site

• Results from the soil analysis are processed with GIS data-handling software to make field maps

More Intensive Sampling Results in Better Data…but More Expense Too

- - - - - Soil P concentrations - - - - -

Set2Set1 Set3

5 24 43 62 81 100lb/A

sampled on 2.5-acre grid sampled on 0.15-acre grid

Map Courtesy of Kitchen, USDA - ARS

Field

compo

site

Stra

tified

compo

site

Grid

Examples of Sampling Strategies

• Field Composite– Sampling representing the mean

concentration of the field• Stratified/Zone Composite

– Separate samples based on known or expected field variability

• Grid– Samples taken based on pre-

determined pattern and spacing

Choosing a Soil Sampling Strategy• First consider the sources and degree of field variability

(both natural and man-made) • Whole field (random) sampling most appropriate when:

– the existing fertility is high and/or variability is low• Field-zone sampling (by soil type or landscape position)

may be most appropriate when: – location of variation is known– sampling areas are large– resources are limited

• Grid sampling may be most appropriate when the location of variation is unknown and future management can address the spatial variability

What is the Right Soil Extractant?• Chemical solutions are added to soil samples that

mimic root and soil processes- estimating both current and future nutrient availability

• The nutrient extracting solution should simulate the natural processes found in different types of soils

• Some extractants and methods are better suited for particular soils and the lab results must be calibrated with local field research

Soil Surface

Phosphate

Extractant Provides an“Index of Availability”

Nutrient Extraction Process: Theory• Measure what is currently available and predict

what will soon become available to the plant• Not a prediction of the total quantity of nutrients

in the soil

Selecting a Soil P Extractant

• The extracting solution should remove plant-available P from the soil through at least one of these reactions:

1. Dissolving action of acid

2. Anion replacement to enhance P desorption

3. Complex the cations that bind P

4. Hydrolysis of cations that bind P

Potassium

Ammonium AcetateModified MorganSodium AcetateMehlich 1 or Mehlich 3

Phosphorus

Bray 1Mehlich 1Mehlich 3Modified KelownaModified MorganSodium-Bicarbonate (Olsen)

Extraction Analysis

Selecting a Soil Extractant• Select a soil extracting solution that has been previously calibrated

for the soils in a specific region. Commonly used extracts include:

Neg

ativ

ely

Cha

rged

Mem

bran

e

K+

Mg2+

Ca2+

SO42-

HPO42-

H+

HCO3-

HCO3-

HCO3-

HCO3-

HCO3-

H+

H+

H+

H+

Posi

tivel

y C

harg

edM

embr

ane

NO3-

Ion Exchange Membranes and Resins• Membranes are designed

to simulate a plant root by attracting anions (on cation resin) or cations (on anion resin)

• Exchange membranes estimate nutrient availability without soil disturbance

• Sequential measurement can provide an estimate of the nutrient availability rate

Choose a Well-Established Soil Testing Labthat Uses Appropriate Techniques and Participates in a Quality-Assurance Program

Steps to Sampling Success

• Good Field Sampling is the First Step• Accurate Chemical Analysis is the Second Step• Data Interpretation is the Third Step

– Analytical accuracy is essential… but of little value in the field without relating these lab numbers to actual crop response

– Are the fertilizer response predictions accurate for your soil types, crops, and management practices?

Recommendations

Analytical results

Two Essential Parts of a Soil Test Report

0

20

40

60

80

100

0 20 40 60 80 100

Soil Test Index of Nutrient Availability

Perc

ent o

f Max

imum

Fertilizer Requirement

Crop Response

Very

Hig

h R

espo

nse

Prob

abili

ty

Med

ium

Res

pons

e P

roba

bilit

y

Littl

e or

No

Res

pons

e Pr

obab

ility

No

Res

pons

e Ex

pect

ed

Examples of Relationships between:• Soil Test Values• Crop Response • P and K Fertilizer Recommendations

Sampling pastures and Fields Receiving manure• Accurate assessment of nutrients

in fields receiving animal waste is important for nutrient management planning

• Highly variable fertility levels across the field make it difficult to collect an accurate soil sample

• Careful soil sampling allows better decisions to be made and efficient use of essential plant nutrients

Sampling manure-Amended Soils• Poor estimates of soil nutrient

status makes it difficult to have an accurate nutrient management plan:

– Poor agronomic results – Unwanted environmental impacts

• Non-uniform manure application makes it difficult to get a field “average” of nutrient content

• A large number of cores is necessary to represent both high soil test areas and low soil test areas

Sampling Pastures

• Avoid sampling in areas that arenot representative of the area – consider that animal activities area huge source of variation such asaround feeders, water, shade trees

• Avoid sampling near fresh manurepiles or recent urine spots sincethey may not be representative of the field

• Use a random zig-zag pattern to collect 15 to 20 individual cores for each field (less than 20 acres)

• Remove plant and manure debris, break the cores, and thoroughly mix the samples before submitting for analysis

Sampling pastures: Guidelines• Divide fields into

smaller management zones (usually less than 20 acres)

• Avoid sampling adjacent to roads, fence lines and congregation spots

• Take at least 15 – 20 cores at random points along a zig-zag pattern

Summary• Before sampling, decide on the

purpose of soil testing and how the information will be used

• Choose an appropriate sampling strategy for your individual situation

• Take appropriate number of cores, using appropriate equipment to get accurate results

• Thoroughly mix the cores and send samples to a well-respected laboratory that uses appropriate analytical techniques for your situation

• Review the results and recommendations to verify that they fit with your field experience

International Plant Nutrition Institute (IPNI)

655 Engineering Drive, Suite 110Norcross, GA 30092-2837Phone: 770-447-0335; Fax: 770-448-0439Website: www.ipni.net

Reference: 06128