EM 9014    Published October 2010
Reviewed:
November 2018

Collecting soil samples to a depth of 6 to 8 inches is standard protocol for western Oregon. When the sampling protocol was developed, this depth was typical of tillage for seedbed preparation. As well as preparing the seedbed, tillage controlled weeds and mixed nutrients and lime into the soil. Presently, tillage is not always used, so a single standard sampling depth may no longer apply (figure 1).

Phosphorus (P), potassium (K), calcium (Ca),
magnesium (Mg), zinc (Zn), and lime have limited
mobility in the soil. When applied to the soil surface,
these materials remain in the top 1 to 2 inches
unless incorporated by tillage. Nutrient accumulation
at the surface occurs frequently in no-till or
direct-seed cropping systems, pastures, and fields
in which perennial crops have been grown for 3 or
more years. When soil is sampled to a depth of 6 to
8 inches, the top 1 to 2 inches, which have higher
nutrient concentrations, are mixed with the lower
6 to 7 inches, which have lower nutrient concentrations.
This situation results in uneven distribution of
the surface-applied nutrients in the soil sample. In
addition, continued application of ammonium-based
nitrogen (N) fertilizers acidifies the soil surface.

To evaluate surface and below-surface conditions,
collect soil samples from multiple depths. This
sampling method helps identify areas where nutrient
applications are warranted and reveals areas of low
pH at the soil surface.

Information in this publication will help producers
and agronomists minimize crop losses and maximize
economic returns.

The following section describes scenarios in
which collecting soil samples from multiple depths
would be beneficial. The examples include situations
in which pH or nutrient stratification has occurred
in western Oregon crops.

Soil pH Stratification

Applying N fertilizer acidifies soil at the approximate
rate of 0.1 pH unit per year for every 100 lb/a
of ammonium-N applied. Without tillage, the top
1 to 2 inches of soil will acidify much faster than soil
below. Table 1 shows soil pH stratification in four
Willamette Valley fields.

Example 1: No-Till Wheat

Winter wheat tolerates moderately acidic soil
(pH 5.4). However, wheat yield and growth are
markedly reduced if the soil pH is 5.0 or lower (figures
2 and 3). During germination and early growth,
winter wheat can be susceptible to low pH at the soil
surface, a condition that results from applying topdressed
N fertilizer to perennial crop fields, such as
grass grown for seed, that are direct seeded to wheat.

In a no-till winter wheat system, monitor soil
pH by collecting an incremental, or stratified, soil
sample. To do this, insert a soil probe 6 to 8 inches,
and then separate the top 2 inches of soil from the
remaining depth (figure 4).

When soil pH in the top 2 inches is below 5.0,
apply lime and mix with tillage to make a seedbed
suitable for germination and early growth. In this situation,
top-dressed lime without tillage immediately
before planting will not adequately change the pH in
the surface soil. Lime is immobile; thus, it does not
have the ability to move into the soil before seed germination.
When soil pH is below 5.5 in the top 6 or
8 inches, apply and incorporate lime (figure 5).

Example 2: Peppermint

Top-dressed lime creates pH stratification because it changes soil pH only in the top 0.5 to 2 inches
(figure 6). Below this depth, soil pH changes due
to top-dressed lime are uncommon even a year
after application. This situation is challenging; topdressed
lime creates a higher soil pH at the surface,
but pH in the soil below can be too low to maintain
adequate crop growth. Raising pH below 2 inches
requires mixing lime into the surface with tillage.

Example 3: Orchards

Similar to perennial grass seed fields, fruit and nut
orchards are managed in the absence of tillage and
receive top-dressed nutrients and lime. Soil collected
from several apple and cherry orchards in the Hood
River Valley revealed pH stratification (figure 7).
In these orchards, soil pH was measured under the
tree drip line (strip pH) and under the grass between
tree rows.

In orchards 1 and 2, soil pH under the drip line
differed little with depth. In all orchards, soil pH
was lower under the drip line than under the grass.
The lower soil pH under the drip line was a result of
repeated surface N applications. In orchard 3, which
received a surface lime application, pH in the top
2 inches of soil under the drip line was higher than
that in the soil below.

These three examples are from common production
systems in western Oregon. However, pH stratification
can occur in many other cropping systems
in which crops are perennial or grown with limited
tillage (e.g., caneberries, blueberries, nurseries,
Christmas trees, and pastures). Information from a
stratified soil sample can help you implement management
changes before conditions occur that create
serious crop growth limitations.

Nutrients in Surface Soil

The previous examples show that top-dressed
lime or N changes pH in the soil surface when tillage
is absent. In a similar manner, surface application
of other nutrients, such as P and K, without tillage
increases the concentration of these nutrients at the
soil surface. If these nutrients are applied to the soil
surface without tillage, they will remain in the top
1 to 2 inches.

For example, soil test P in a Willamette Valley
clover–grass pasture was 4 ppm. After 6 years of
P application and adequate crop growth, soil test P
was still 4 ppm. Both soil samples were collected to
a depth of 8 inches, the historical norm. In contrast,
the soil test P of a sample collected from the top
3 inches after 6 years of P application was 19 ppm.
This example shows that fertilizer P stayed at the
soil surface and was available for plant use. The soil
samples collected to a depth of 8 inches mixed subsurface
soil having a low P soil test with surface soil
having a higher P soil test. Taking soil samples from
multiple depths showed that fertilizer P applications
did not change soil test P uniformly in the soil.

Nutrient accumulation at the surface can occur
in a short time with relatively low nutrient application
and removal rates. Surface application of 30 to
40 lb/a P2
O5
will result in stratification within 2 to
3 years.

Potassium is similar to P. It remains where it is
placed when applied at agronomic rates. Figure 8
illustrates two important concepts. First, soil test
K in the surface inch of soil can increase rapidly.
After only 3 years of surface K fertilizer applications
of 80 lb/a and straw residue burning in a perennial ryegrass field, soil test K in the top 1 inch and top
6 inches was 180 and 78 ppm, respectively. Second,
soil test K stratification can affect fertilizer recommendations.
The Oregon State University Extension
fertilizer guide for perennial ryegrass (Perennial Ryegrass Grown for Seed (Western Oregon), FG 46-E;
Hart et al. 2005) recommends applying K fertilizer
when soil test K is below 150 ppm. On the basis of
the 6-inch sample, K fertilizer application is recommended.
However, the K value from the top inch of
soil indicates that no fertilizer is needed. This example
illustrates the importance of stratified sampling.

Plants obtain nutrients from much of the root
zone, including the soil surface. Nutrients in the
top 2 inches of soil supplement plant nutrient needs
from the entire rooting zone. Low soil test values
from a sample collected from a 6- to 8-inch depth
will result in a recommendation for fertilizer application
even when sufficient nutrients such as P and
K are available in the surface soil. On established
fields, collect soil samples from (1) the top 2 inches
of soil and (2) the 2- to 8-inch layer of soil. Compare
soil test values from each depth to evaluate changes
in nutrient concentration over time.

Tillage and Nutrient Stratification

Nutrients accumulate rapidly in the soil surface
as a result of repeated fertilizer applications without
tillage. Surface nutrient accumulations can be
measured after a short time and from continual low
application rates.

Conversely, tillage mixes P, K, Ca, Mg, and lime,
which have limited mobility in soil. Table 2 shows
that conventionally tilled fields did not exhibit much
nutrient or pH stratification and that nutrient stratification
did not occur in fields with intermittent
tillage. Minimal or infrequent tillage eliminates differences
in soil test values between the top 2 inches
and the 0- to 6-inch depth. Therefore, when a field is
tilled at least once during a 3-year period, take soil
samples at the conventional depth of 6 to 8 inches.

Stratified Soil Sampling: Putting the Idea into Practice

Do not sample soil only from the surface; you
need to know the nutrient and pH status of more
than the surface 2 inches. To take a stratified
soil sample, insert a soil probe 6 to 8 inches, and
then separate the top 2 inches of soil from the
remaining depth.

For no-till or direct-seed cropping systems, pastures,
and fields where perennial crops have been
grown for 3 or more years and have received annual
surface fertilizer, wait 2 to 3 years after establishment
to begin stratified sampling. Use the following key to
determine if collecting a stratified soil sample would
be useful.

1. What is the purpose of sampling?

• Agronomic or to determine nutrient or fertilizer application rate

→ Go to Question 2.

• Monitoring or regulatory to follow nutrient
movement or accumulation

→ Refer to Pacific Northwest Extension
publication PNW 570-E, Monitoring
Soil Nutrients Using a Management Unit Approach
(Staben et al. 2003).

2. Is the field tilled at least once every 3 years?

• Yes → use a conventional (6 to 8 inches) sampling depth.

• No → Go to Question 3.

Has fertilizer been top-dressed for more than 2 or 3 years?

•No → Use a conventional sampling depth.
The crop will not respond to nutrient
application even if the nutrient concentration
differs between the surface
(0 to 2 inches) and subsurface
(4 to 6 inches) samples.

• Yes → Stratified sampling may provide a benefit because

- the field has not been tilled for 3 or more years, which is sufficient time for stratification to develop;

- when the crop grows rapidly and needs nutrients, roots are present in moist soil near the soil surface; and

- the higher nutrient content in surface soil compared with subsurface soil means that a conventional sample will underestimate nutrient availability.

Conclusions

The idea of collecting shallow soil samples is not
new. Comparison of soil tests results from before
establishment with results from various depths continues
to be important in western Oregon cropping
systems. Measuring the surface soil pH can help
you reduce potential stand losses, and obtaining
similar information for soil surface nutrients such as
P, K, Ca, Mg, and Zn will help you make informed
nutrient management decisions and maximize
economic returns.

References

Choate, J.B. 2004. “Phosphorus Availability in BiosolidsAmended
Soils.” Master’s thesis, Oregon State University,
Corvallis.

Hart, J.M., M.E. Mellbye, D.A. Horneck, G.A. Gingrich, W.C.
Young III, and T. Silberstein. 2005. Perennial Ryegrass
Grown for Seed (Western Oregon)
. FG 46-E. Corvallis,
OR: Oregon State University Extension Service.

Horneck, D.A. 1995. “Nutrient Management and Cycling in
Grass Seed Crops.” PhD diss., Oregon State University,
Corvallis.

Staben, M.L., J.W. Ellsworth, D.M. Sullivan, D. Horneck,
B.D. Brown, and R.G. Stevens. 2003. Monitoring Soil
Nutrients Using a Management Unit Approach
. PNW
570-E. Corvallis, OR: Pacific Northwest Extension
Publications.

For More Information

Hart, J., G. Pirelli, L. Cannon, and S. Fransen. 1996. Pastures
(Western Oregon and Western Washington
). FG 63. Corvallis,
OR: Oregon State University Extension Service.

Jackson, T.L., E.H. Gardner, and T.A. Doerge. 1983. Peppermint
(Western Oregon—West of Cascades
). FG 15. Corvallis,
OR: Oregon State University Extension Service.

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