ISSUE 13 July 27, 2006
SOIL SAMPLING AFTER HARVEST
There are three sources of N for crops. The obvious source is fertilizer and supplements, such as manure. Another source is the soil, which includes in my mind the previous crop credits we give for crops following legumes, sugarbeets and perhaps another one or two I am experimenting with presently. The soil source is always estimated through N calibration experiments, and averaged over sites and years, at least to date. The other source is the inventory of available nitrate through soil testing. A grower should inventory this N and take it into account through soil testing each year prior to growing a crop that requires supplemental N.
In previous years, I have encouraged people to follow the combine, usually in August and early September to sample for soil nitrate. This year, due to the very dry conditions, in some areas it might be prudent to hold off for rain, or at least later in the fall. In extremely dry areas, soils will be very hard and the physical problem of soil sampling may prevent good cores from being extracted. Secondly, in these areas, mineralization has been very low due to the heat and dry soil conditions. When it does finally rain, I expect a flush of soil microbial activity. I would rather the sample was taken following this flush. In areas that are dry, but not under a true drought, such as the northeastern counties in the state, I see no problem with following the combine with a soil sampler.
In the event that it remains dry, soil samples should still be obtained this fall. Accomplishing this task may take some innovation on behalf of the sampler in hard soil, such as using an auger on a hefty drill, plugged into a generator. There will no doubt be higher than normal N remaining in the soil, which will be economically important to growers considering the high price of N fertilizer.
There will also be crops that were not harvested that should be treated next spring as a green manure. Estimate the dry matter, assume the material has an N content of about 4%, and consider that about 1/3-1/2 of the N will be available under some kind of tillage, and about 20% available under no-till.
Dr. Dave Franzen
NDSU Extension Soil Specialist