Nitrogen Report Card 2001

Do Agassiz Corn Kings use more nitrogen (manure and fertilizer)?

As in past years, the fall soil tests taken in 2001 say ‘no’. There was no link between corn ranking and soil nitrate levels in the fall. For example, one of the top-ranking corn fields in the 2001 competition had the least soil nitrate, a very admirable 77 kg nitrate-N/ha (69lb/ac). The highest soil test came from the corn field with the second lowest rating.

Samples taken from fields entered into the 2001 Corn King Competition at Agassiz show that, on average, levels of fall soil nitrate were up from the previous year. Amount of soil nitrate-N to a 60cm (2 ft) depth in the 11 fields sampled averaged 125kg/ha (110lb/ac). Last year the Corn King fields averaged just 93kg/ha (83lb/ac). Over the previous five years of sampling, average soil nitrate-N levels have bounced between 48-150kg/ha (43-134lb/ac).

The fall soil nitrate test has been used for several years in BC and the Pacific Northwest as a “report card” to evaluate nutrient management practices on corn land. The report card enables farmers to evaluate their nitrogen application rates. Since it is hard to predict in spring how much N should be applied to a corn field, the report card gives farmers feedback on their nitrogen application rates. To get an “A”, the soil should contain no more than 90kg/ha (80lb/ac) of nitrate-N in the top 60cm (2ft) in the autumn. Levels above 90kg/ha indicate that excessive nitrogen was applied. Excess soil nitrate represents an economic loss to the farmer, as the nitrate will leach away during the winter months. At Agassiz in 2001, 7 out of 11 fields had over 90kg/ha of nitrate-N.

How does the fall soil nitrate test work? Corn can take up a limited amount of nitrogen so if nitrogen is over-applied, the surplus stays in the soil. Corn grows and consumes most of its nutrients in June and July. During August and September, soils are warm so the organic matter actively mineralizes, releasing nitrates into the soil at a time when corn no longer takes up the nitrates. More nitrate is released when soils are wet. In 2001, high rainfall in August (124mm or 5.5in compared to 54mm or 2.1in average) combined with warm temperatures could explain the higher fall nitrate. Probably because of the August rainfall, more than half the nitrate in 2001 was found at the 30-60cm (12-24in) soil depth. The year-to-year variation in fall soil nitrate from the Corn King competition is related to different weather, soil and management practices.

O. Schmidt, BCMAFF, Abbotsford.

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