Agriculture Reference
In-Depth Information
representing 31% of dealers (Akridge and Whipker, 1999), is the use of the standard
soil survey at the 1:20,000 scale. This practice reduces the number of soil samples.
But the soil survey was never intended for site-specific use and will generally result in
a poor characterization of soil nutrient status because of soil type variability within
map units and past field management. At present, a preferred method is directed or
smart sampling based on management zones (Franzen et al., 2000). However, this
method requires basic information such as soil characteristics, landscape param-
eters, yield maps, crop management historical and present information, and aerial
imagery. Hence, sample management for soil and crops is very important for a rea-
sonable fertilizer plan. The soil nutrients and crop productivity are closely related
with time and zone variability. Wollenhaupt (1997) found that time influenced 67%
of crop productivity variation, zone influenced 10% of crop productivity variation,
and the average amount of fertilizer influenced 10% of crop productivity variation.
Considering the variability of soil nutrients, there are some restrictive factors for
soil sampling, including depth of sampling, soil layer effect, soil mixed sampling,
and optimal sampling time. Different sampling methods can be used according to
the management purpose. The first method is random sampling. You can divide the
field into different grids, and randomly select some sample for each grid; the aim is
to obtain a certain amount of sample for the whole field. You can choose the same
number of samples in one grid, or you can chose different samples in different grids.
The random sampling method is not suitable for the acquisition of maps. The second
method is nested sampling. You can first divide the field into different grids, and
then divide the grid into smaller grids until you get the final unit of soil sample.
This method can reduce the cost of sampling, but it cannot supply a uniform sample
distribution in the whole field. Using the nesting sampling, you can still obtain a
reasonably accurate of the soil zone variation. The third method is rules grid sam-
pling. In this method, you divide the field into equal grids, and you can also divide
the field into different triangles, rectangles, and hexagons according to soil nutri-
tion zone variation. The fourth method is systematic stratified sampling, which can
solve the problem of periodical distribution of soil nutrition in zone variation. This
method is an ideal method for positioning operation systems. Another method is
aided-sampling, which needs the aided information of soil characteristics, landscape
parameters, yield maps, crop management historical and present information, and
aerial imagery.
Using the above sampling methods, we can obtain the nutrition map of soil and
crops. The soil map is usually applied to predict the product potential of crops. The
soil map can be used to analyze the zone variation of soil texture and organic mat-
ters, and it also can be used to analyze the effective utilization of soil nutrition, time
variation, and zone variation of soil water content. The crop nutrition distribution
map can be used to find the main factors of productivity variation, since the crop
map is related to the farmland farming system, soil nutrition, soil water content,
crop disease, pest and weeds distribution, former management methods, climate, etc.
The main principles for sampling are the five main aspects: (1) coefficient of
variation of soil nutrition; (2) aided information of field; (3) the amount of samples
for management; (4) feasibility of sampling methods; and (5) time, labor, and cost
for sampling.
