Site-Specific Farming — Number 4
Site-Specific Farming and the Environment
SF-1176 (4) , June 1999
Dr. Dave Franzen, NDSU Extension Soil Specialist
Site-specific farming is based on the idea that the right inputs can be applied at the
right place at the right time. Fields are variable in nutrients and pests. Application of
a single rate of inputs may result, especially with nutrients, in over-application of
nutrients in part of the field and under-application in another. Over-application is a
concern because these areas tend to be over-supplied more than other areas for many years,
resulting in the possibility of leaching losses to groundwater or runoff losses to surface
water. Under-fertilization may also be a concern because it can result in a reduction of
crop residues returned to the soil which may lead to increased erosion and other
undesirable soil properties.
Reducing soil N levels
If plant nutrient levels are identified site-specifically, it is possible to reduce
fertilizer application in areas that are high and increase levels in areas that are low.
Using landscape-based zone sampling at two North Dakota site-specific study sites, there
was a 6 pounds of N per acre reduction in soil nitrate levels after a variable-rate
application of N was applied, compared to using a uniform composite sample approach.
However, at most sites and most years, the total N used was redistributed throughout other
areas of the field, resulting in similar total N used. The result was that at most sites,
the level of residual N gradually decreased and levels of N were less variable following
site-specific application of N fertilizer. These sites were in a wheat/barley/sunflower
rotation, a spring wheat/winter wheat/sunflower rotation, and a corn/soybean/wheat
rotation.
Sometimes traditional soil testing alone, even if conducted in zones or grids, is not
enough to identify fertility patterns. This was the case in an area of the Red River
Valley where sugarbeets, wheat and potatoes were grown in rotation for a number of years.
Sugarbeet quality was reduced, which is usually the result of high soil N levels. However,
grid soil sampling after sugarbeets and sampling to 4 feet deep before sugarbeets failed
to show any excessive N levels.
A study conducted in this area showed that large amounts, sometimes up to 400 lb/acre,
of N were being taken up by the sugarbeet, with most of that amount ending up in the
leaves. Soil N levels were very low following harvest (Figure 1) except in a few areas
where stands were reduced due to excessive water. Using sugarbeet top N to reduce the N
recommendation the following year for wheat by up to 100 lb/acre, yields in areas with
high sugarbeet top N were not different in yield than those that were fertilized with a
full rate of fertilizer N. The total N saved on the field was about 1,600 pounds.
Figure 1. Total N in sugarbeet tops compared to soil N
levels to 6 feet. (Reitmeier, Franzen and Giles) (11KB color image)
Soil sampling following potatoes revealed modest levels of N to the 4-foot soil depth,
but another 100-200 lb/acre of N were found 4 to 6 feet deep in the soil. By using this
deep N as a credit toward the following sugarbeet crop and applying a credit based on the
N content of the potato tops, no fertilizer N was recommended for the following year of
beets, reducing N inputs on this field by about 4,000 pounds. Despite the zero N rate
applied to this field, over 22 tons per acre of beets with over 17% sugar were produced.
Soil N levels in the field and total N levels in the soil and in the tops were reduced
compared to the previous year.
Remote imagery from satellites and aerial photographs were used to direct site-specific
applications (Figure 2) to sugarbeets following potatoes. These tools reduced the sampling
required to gather soil N information used to direct soil sampling in the rotation.
Satellite imagery and aerial photographs have also been used to direct a previous crop N
credit for wheat following sugarbeets, even where soil and sugarbeet top N is not as
excessive as the St. Thomas area study. Currently, NDSU recommends a system of previous
crop N credits based on general sugarbeet top color. There is zero N credit for yellow
beet tops, whereas as much as 60-70 lb N/acre are subtracted from N recommendations if
sugarbeet tops are green. This tool provides a fertilizer cost savings to producers as
well as an environmental benefit through decreased inputs that would otherwise be wasted
or moved off-site.
Figure 2. SPOT image of a St. Thomas sugarbeet field,
September, with red and dark blue areas related to high N levels, and lighter areas
related to lower N levels. By directing sampling to these areas, sample numbers can be
reduced. (10KB color image)
Site-specific tools can be environmentally beneficial because of the ability to limit
the buildup of nutrients in areas of a field. Fertilizer nutrients can be applied more
efficiently and used for the crop instead of lost into the surrounding environment.
The key to achieving environmental benefits through site-specific farming is to
identify levels of nutrients accurately and inexpensively in the field, and then applying
a variable-rate fertilizer application based on this knowledge. For N, this can be
achieved through zone sampling using topography, electrical conductivity, aerial
photography and satellite imagery as a basis for establishing nutrient management zones,
then using soil sampling to establish nutrient levels. For crops like sugarbeet that
accumulate N in the leaves, some plant analysis may be necessary to credit the following
crops. Crops that root deeply may require deeper soil testing than for other crops.
SF-1176 (4) , June 1999
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