Fertility and Nutrients
Benchmark soil sampling: steps to success
Fine-tune fertilizer applications using benchmark soil sampling.
November 29, 2007 By Donna Fleury
Soil sampling and analysis are commonly used tools for managing nutrients in
crop production. For years, random soil sampling was the typical method of sampling,
with between 15 and 25 soil samples collected randomly across the field. Other
techniques, such as grid sampling were used for high value crops. However, as
prices of fertilizers rise, crop prices fluctuate and margins shrink, farmers
are looking for ways to maximize nutrient inputs and management.
"Benchmark sampling is becoming more popular and, as fertilizer prices
rise, it's a good tool for fine-tuning your nutrient management program,"
explains Dr. Ross McKenzie, research scientist – agronomy with Alberta
Agriculture Food and Rural Development (AAFRD) in Lethbridge. "The downside
of random sampling is if you have a variable field with a combination of high
and low areas, the nutrient levels will also vary and the soil test results
could be misleading. Benchmark sampling at the same location year after year,
on the other hand, makes it easier to compare soil test results from one year
to the next."
Benchmark sampling requires the selection of a representative area in the field
that is fairly uniform, flat and productive. The area selected, which is usually
between one-half to one acre, is marked using GPS co-ordinates and soil samples
will always be taken from that site in the future. In some situations, two or
more benchmark sites may be established to address very different management
areas of the field. A fine-tuned fertility management strategy can be achieved
even without variable rate technology.
"This type of sampling speeds up the whole process because you only have
to go to one area of the field and sample, rather than driving over the whole
field," says McKenzie. About 15 samples are selected from the benchmark
area to ensure you have a good representative mix of the soils. For example,
if you happen to sample in a spot where nitrogen or phosphorus was banded in
the previous year, taking too few samples could compromise the results. "Benchmark
sampling gives you a better sense of changes from year-to-year at that site,
making it easier to compare results."
How deep should you go?
In terms of soil sampling depth, different laboratories and companies may recommend
different sampling depths, with zero to 12 inches fairly common and sometimes
12 to 24 inches as well. However, McKenzie prefers to see soil sampling done
in three increments, including zero to six inches, six to 12 inches and 12 to
24 inches. "Taking samples from these increments gives producers the best
picture of what is in their field for nutrient planning," says McKenzie.
"By taking a sample in the zero to six inch layer where the topsoil or
A horizon is located, and the six to 12 inch mineral layer below, you get a
better idea of what is in each of those two levels." Both phosphorus (P)
and potassium (K) tend to be in that zero to six inch depth, and a more accurate
measure will result by taking a sample in that topsoil layer. For nitrogen (N)
and sulphur (S), particularly in wetter years, there tends to be more S at deeper
levels, especially in the Brown and Dark Brown soil zones. Therefore, sampling
at 12 to 24 inches gives a better analysis of N and S at that level, as well
as measure electro-conductivity for salt levels.
"Another reason to consider taking samples at various increments is when
you make a conversion from parts per million (ppm) to pounds per acre, the ppm
in a zero to six inch and six to 12 inch samples are multiplied by a factor
of two, while the ppm in the 12 to 24 inch sample is multiplied by four. Therefore,
if there is an error and it's multiplied by a factor of four, that just compounds
Although McKenzie recommends sampling at three increments, he notes that often
industry dealers prefer to sample at zero to 12 inches. If they are using automatic
soil samplers, these machines are designed to only take samples at one or two
depths, not three. Sampling at zero to 12 inches is better than nothing, but
it is far better to go to depth sampling. Sampling the 12 to 24 inches can make
a huge difference in the amount of N and S fertilizer being applied. With P
and K, it is most important to know what is in the top six inches. Therefore,
testing for P and K at the zero to six inch depths on most fields is recommended,
except for manured fields.
"For annual crops, in my opinion, you should be sampling at least representative
fields every year to allow you to fine-tune fertilizer recommendations,"
says McKenzie. Although P and K tend to not change much from year-to-year, N
and S can. "For annual crop production, especially with the cost of N fertilizer
between $0.40 and $0.50 per pound, sampling for N every year to fine-tune how
much you should put on would be a benefit." With S, you do not want to
run short but on the other hand, if it is available at depth, you can save money
by not over-applying, he says.
"For permanent crops like alfalfa, timothy or grass, you may not need
to soil sample every year, especially if you have good records and know what
is happening in the field." With crops like timothy, in most years there
will not be much N left in the spring, so regular annual rates will be applied.
For alfalfa, once a field is established and if proper soil testing has been
done before establishment, there should not be a need to sample every year.
Soil testing should be done late in the fall, and not until the soil temperature
drops down to between five and seven degrees C in the daytime. Taking soil tests
too early means running the risk of continued microbial activity releasing N,
making it difficult to estimate how much N may actually be there. Soil moisture
levels are also an important consideration for soil testing. Typically, the
months of August and September are the driest months, with the crops having
used up much of the N in the soil. The drier soil conditions provide for minimal
microbial activity to break down organic matter and release N into the soil.
"However, in 2005, with all of the rain over the summer and fall, there
will be continued microbial activity releasing more N," says McKenzie.
Along with the likelihood of more N and other nutrients becoming more available,
the rain will have moved the nutrients further downward, meaning there will
likely be more nutrients at depth. "In an odd ball year like 2005, soil
testing at the end of the fall is important to be able to properly plan for
next year. Fall testing will help you make informed decisions and ensure you
are putting on the right amount of fertilizer, rather than too little or too
much for the next crop."
Another tool available for fine-tuning fertilizer applications is an AAFRD
software program called AFFIRM (Alberta Farm Fertilizer Information and Recommendation
Manager). A new revised version was released in 2005. The program requires soil
test results and allows farmers to really fine-tune fertilizer requirements
and economics. McKenzie says the program can really change the economic picture
of how much fertilizer should be applied.
"More and more farmers are moving to benchmarking soil sampling, because
it is another step in fine-tuning fertilizer and nutrient management,"
says McKenzie. "When N fertilizer was $0.20 per pound and wheat was $5
per bushel, farmers maybe didn't have to get quite so serious about it, but
when N fertilizer prices are projected to get to $0.50 per pound by next spring
and grain prices expected to be lower, then farmers really have to start sharpening