As various aspects of precision agriculture are implemented
in Nebraska, some of the most frequent questions asked by producers,
fertilizer dealers and crop consultants relate to soil sampling.
Should I soil sample this field on a grid? What grid spacing
should I use? How often should I sample? Can I use a yield map to
tell where to soil sample? All of these are good questions,
but often we do not have definitive answers. Site-specific management
research conducted in recent years in Nebraska, however, provides
some direction on how to implement a soil sampling program for precision
agriculture.
Basic Sampling Principles
Historically, the objectives of soil sampling have been to determine
the average nutrient status of a field and to provide some measure
of nutrient variability in a field. Soil sampling for precision
agriculture has these same objectives with some modifications. Instead
of a field, producers are interested in areas within fields. They
also are interested in relating trends in soil fertilizer levels
to other field properties that are predictable or easily measured.
Knowledge of factors influencing soil nutrient levels including
soil type, topography, cropping history, manure application, fertilizer
application and leveling for irrigation will help the producer determine
the most effective sampling approach.
The basic principles of soil sampling still apply to precision
sampling. An adequate number of samples should be collected to accurately
characterize nutrient levels. The samples should be collected to
the proper depth for non-mobile and mobile nutrients. Samples should
be handled and stored to minimize contamination and degradation.
Grid Sampling
When variable rate fertilizer application was first practiced 8-10
years ago, application maps were most often derived from grid soil
samples collected at average densities of one sample for every three
to four acres. In research studies conducted in Nebraska, fields
have been grid sampled at much higher densities (up to 42 samples
per acre) to approximate the true spatial variability of a number
of soil nutrient levels. Sampling at high densities allows the evaluation
of lower sampling densities on nutrient maps. In some cases, fewer
samples can result in inaccurate maps. Figure 1 shows how
a tenfold range in sampling density at a research site in Lincoln
County resulted in significantly different patterns. In this case,
the coarser sampling grid missed a systematic pattern in soil nitrate,
probably related to livestock fencing. The average recommended nitrogen
rate for the field at the higher grid density was 148 lb N/acre.
The average recommended nitrogen rate was 162 lb N/acre at the lower
grid density; 45 percent of the field received a different nitrogen
recommendation with the coarser grid. The coarse grid was denser
than most commercial grid sampling practiced by fertilizer dealers
and crop consultants.
In other situations, accurate maps can be generated at much lower
sampling densities. At a site in Buffalo County, a grid density
of 14 samples per acre was compared to a density of one sample per
3.7 acres. The coarse grid is similar to that used commercially.
In this case, the nitrogen rate maps were not greatly different
–17.6 percent of the field received a different nitrogen recommendation
with the coarser grid, and the average nitrogen rate was the same
for both grids – 158 lb N/acre.
The optimum grid density depends on the site, and to some extent
what nutrient is being assessed – soil organic matter, nitrate,
phosphorus, zinc, etc. It helps to know the spatial variability
of the field in order to know the optimum grid density – which,
after all, is the reason for grid sampling. This also raises the
basic question of why we would choose to grid soil sample. Is there
a better way to obtain the desired information?
Directed Sampling
Directed soil sampling is in many ways simply an extension of how
soil samples were often collected in the past. For example, if a
field contains significant areas of more than one soil series, the
University of Nebraska recommendation was to collect samples from
each soil series. Also, if parts of the field had different preceding
crops, different fertilization histories, eroded areas, or an old
farmstead location, these areas were to be sampled separately. In
these situations, the producer is using his knowledge of spatial
factors to direct where samples are taken to determine if they have
different fertilizer needs. The new tools of yield maps, aerial
photographs and remotely sensed images simply provide more information
about variability in the field and where soil sampling can help
interpret variability.
Figure 2, three sources of spatial information are provided
for a field in Clay County: the soil survey (2a), a bare
soil photo (2b), and a yield map (2c). In this case,
the soil survey provides little spatial information – the study
area is located on one soil series (Crete silt loam). The aerial
photo shows areas that vary in soil color. In Nebraska, much of
the variation in the color of bare soil is related to soil organic
matter content. The yield map shows an area of higher yield that
is consistent with darker soil on the aerial photo. Soil samples
from the field indicate that areas which are darkest on the aerial
photo, and have the highest yield, are highest in soil organic matter
(3.1 percent) and that soil organic matter is lowest in the lighter,
lower yield areas (1.9 percent). This information can be used in
making recommendations for variable rate fertilizer or herbicide
applications.
Cooperative Extension EC 154
