By John T. Lee
Soil testing trends: fall 2005.
|Figure 1. Soil nitrate variability between
unplanted fields in Manitoba (percentage of fields testing in each soil
test range). 477 unplanted fields tested in fall 2005.
Soil testing wrapped up in many areas of Manitoba in November. Excessive May
and June rainfall in southern and eastern Manitoba prevented planting of thousands
of acres. Many of these fields were not tilled for the first time until late
in July. Weed growth was excessive in many fields while weeds were controlled
with herbicides on others. Water stood in many fields for an extended period
of time, which resulted in nitrogen losses. Excessive water can result in nitrogen
loss due to leaching on well-drained soil types. Excessive water on fine textured
poorly drained soil types will result in nitrogen losses due to denitrification.
The nitrogen level in the zero to 24 inch soil profile varied a lot from field-to-field
this fall because of the different circumstances each field experienced during
the growing season (weed growth and control, tillage, ponded water, fall applied
N). The results summarized the nitrogen test results from 477 fields, which
were not planted in Manitoba this past spring. Figure 1 shows that the amount
of N remaining in the zero to 24 inch soil profile varies a lot from field-to-field.
With nitrogen prices what they are now, it is important to test each of these
fields to know what amount is needed for next year's crop. Guessing at the rate
of N to apply could result in spending many dollars more than needed or could
result in not applying enough N to reach profitable yields in 2006.
The general trend for soil nitrogen level following wheat production in Manitoba
is higher than a year ago. The wheat crop was a little disappointing in some
areas, which resulted in more nitrogen being left in the soil than previous
years. Figure 2 shows that the amount of N left in the soil profile varies quite
a bit from field-to-field. The amount of N left in the soil profile is a result
of many things including: rate of N applied, crop yield achieved, amount of
N lost to leaching or denitrification, amount of N mineralized from organic
matter and so on.
The reason nitrogen soil testing is necessary each fall is that we cannot calculate
these factors in advance. Nobody knows if a season will be warm or cool, wet
or dry or whatever. Soil nitrate testing in the fall gives us a starting point.
Once we know how much nitrogen is left in the soil, we can make a plan for next
year to apply only what is needed. -30-
*John T. Lee is a soil scientist
for AgVise Laboratories at Northwood, North Dakota.
* * *
AgVise Laboratories posted its Canadian
soil test summary for all nutrients on its web site in early December. Visit: