S. Bittman, C.G. Kowalenko,
Agriculture Canada, Agassiz BC
BCMAFF, Abbotsford, BC
Can farmers grow top-notch corn crops without contaminating their groundwater with nitrates? Results from samples taken during the recent Corn King Competition for the Agassiz Fall Fair show that several farmers know how.
Nitrate left in the soil in the fall is subject to leaching due to our rainy weather in autumn and winter. Most of the nitrate leaching actually takes place soon after the heavy rains begin in November. The lost nitrate is a financial loss to the producer and a threat to groundwater quality. Water containing more than 10 parts per million of nitrogen in the nitrate form is considered unhealthy to drink, particularly for babies.
The three top corn fields for the Agassiz Corn Competition (based mainly on visual assessment of yield and maturity) had relatively low residual soil nitrate levels (see Table). The corn in the two fields with highest levels of soil nitrate did not rate as high. This shows that applying excess nitrogen as fertilizer or in manure is not necessary for top corn production.
|Nitrogen as nitrate in kg/ha*|
A few notes to help you interpret the results in the table: A corn crop needs about 175-225 kg/ha (160-200 lb/acre) of nitrogen. Two fields have enough nitrogen left in the soil to grow another entire corn crop (but of course the nitrogen will be long gone from the soil before next spring).
A fall-seeded cover crop will take up no more than about 20-30 kg/ha (18-25 lb/acre) of nitrate-nitrogen, most of it from the upper 15 cm (6 inches) of soil. Eight of 10 fields had plenty of nitrate in the top layer to support a cover crop so no additional nitrogen as manure or fertilizer was required for this purpose. The nitrate below 15 cm (6 inches) will be lost regardless of the cover crop.
A relay crop (Italian ryegrass) seeded between the corn rows takes up 40-70 kg/ha (35-55 lb/acre) of nitrogen from the upper 30 cm (12 inches) of soil. Such a crop would have cleaned up the nitrate from the top 30 cm (12 inches) of soil in all but two of the fields in the competition. Nitrate left in the 30-60 cm (12-24 inches) layer would be lost even with a relay crop.
How much nitrate is a threat for contamination of groundwater? When a soil with no cover crop contains less than 100 kg/ha (90 lb/acre) of nitrogen in the nitrate form, water percolating through that soil is unlikely to contain more than the critical 10 parts per million of nitrate-nitrogen. As a rule of thumb, Washington State University suggests that fields have recieved too much nitorgen if they contain more than 90 kg/ha (80 lb/acre) of nitrate-nitrogen in the fall. Eight of the 10 corn fields in the Agassiz Corn Competition this year were well within these standards. Note that relatively more nitrogen can be applied to a field that will have a well-established cover crop or better still, a relay crop, that will be harvested the next spring than to a field that will be left bare over the winter.