| Arable
Managing soil variability
Earlier last year one field in Bedfordshire was intensively sampled and mapped
as part of AHDB’s GREATsoils programme and an in-depth case study will be
published this summer. Here we provide a brief overview of the techniques
used and the options available to growers
oil variability is one of the main
factors determining differences
in crop growth within and
between fields. Variations in soil
texture, moisture holding
capacity, organic matter
content, nutrient availability, drainage,
compaction and soil depth affect consistency of
crop size and quality. As such, increasing
numbers of growers are looking to map soil
variability as the first step towards trying to
understand and manage crop growth and yield.
There are two main approaches to precision
soil sampling – grid and zone sampling. Both
sampling methods are valid options and both
have advantages and disadvantages.
Grid versus zone sampling – what’s the
difference?
Grid sampling uses a regular sampling
strategy to collect a number of samples per ha.
The location of each sampling point is GPS
logged and this information is used to create a
contoured map of soil pH and nutrients using a
method of data analysis to estimate values
between the measured points.
Zone sampling uses existing knowledge of
within field soil variability to direct where
samples are taken. Soil zones can be based on
measured soil and/or crop variability, for
example using soil electrical conductivity maps,
soil brightness maps, yield maps or any other
source of information on within-field variability.
Once the zones are defined, each is sampled
separately. The soil pH and nutrient maps
produced will reflect the boundaries between
the soil zones.
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12 | Farming Monthly | March 2018
Zone sampling focuses on managing areas
by soil type. It uses patterns and boundaries
evident from looking at soil surveys or yield
maps to form the basis of management zones.
However, grid sampling may identify ‘hot spots’
of soil fertility or pH (often related to field
management history) that cannot be detected
using zone sampling.
Economic benefits
Whichever soil sampling approach is used,
the number of soil samples taken will have an
important effect on the level of detail in the soil
pH or nutrient maps produced. The limiting
factor is usually cost – the more samples taken,
the more accurate the soil map is likely to be,
however the cost of additional samples may not
always be justified. The most common
commercially used sampling intensity is one
sample per hectare. Increasing the sampling
intensity from one to two samples per hectare
will typically increase the cost by a minimum of
£6/ha to reflect the additional laboratory
analysis (excluding any additional labour cost).
Soil mapping case study
Soil samples were collected for a case study
at Avenue field, F.B. Parrish & Son, in
Bedfordshire. For grid sampling, either one or
two composite samples were collected per
hectare. Each composite sample consisted of
16 subsamples taken in a spiral within a 3m
radius of a central point.
Analysis of these samples was used to ‘map’
soil pH and nutrient variability in Avenue field.
The single ‘whole field’ sample provided a
good measure of the mean field value for pH
and P Index. The more intensive grid soil
sampling revealed within-field variation in soil
pH and soil P Index.
“The more samples taken the more detailed
the soil map should be but the additional cost
may not be justified. You should decide upon
your sampling strategy based on the level of
detail required to make effective crop
management decisions” said, Lizzie Sagoo,
Principal Soil Scientist at ADAS.
It should also be noted that soil acidity can
be patchy and, irrespective of soil sampling for
nutrient status, regular spot testing of pH with a
soil indicator test across the field is often useful.
The complete case study will be published
this summer. Further information on the soil
precision project and soil management is
available from www.ahdb.org.uk/greatsoils
www.farmingmonthly.co.uk