Co-Authored by Ian McDonald
Background: In 2013 a range of corn fields were sampled to determine what impact the weather has had on the level of residual soil nitrate. This year April was cool and relatively wet while the first half of May came in warm and dry followed by conditions since mid-May that have been cooler and wetter than average. What impact has the weather had on residual N supply and therefore on N application rates?
To provide some insight on this question soil samples were taken from 95 fields on June 3,4 and 5 from across the province and included a range of previous crops and soil types.
Results: On the medium textured soils (loam in the name) that had not received fertilizer nitrogen or manure, and where the previous crop was not red clover or alfalfa, soil nitrate levels were slightly below the long term trend. The average from these soils this year was 10.7 PPM nitrate N, compared to an historical average of 11.0 PPM and compared to 12.2 PPM in 2012 (warm spring) and 9.5 PPM in 2011 (cool spring). These levels correspond to approximate sidedress N recommendations of 95 (12.2 PPM), 105 (11.0 PPM) and 118 (9.5 PPM) lbs of N/acre.
This year the lighter textured soils seem to be showing less residual nitrate (9.0 PPM) compared to either the clays (11.4) or the loams (10.7). In some cases this may be explained by the fact the sand fields frequently correlated with areas of heavier April rains.
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The other finding that seems to stand out in this year’s survey is that fields with fall manure or red clover or alfalfa are not showing residual soil nitrates levels that are as high as normally observed. Fields with alfalfa or red clover as the previous crop had readings that averaged 14.8 PPM nitrate, and fall manured fields averaged 13.8 PPM. These call for applications of sidedress N of 70-80 lbs of N/acre. Speculation is that a cool April may have reduced N mineralization from these organic sources in fall applied manure. Mineralization will continue to occur but growers are advised to trim N credits by one-third; i.e. red clover credit at 49 lbs N/acre as opposed to 73 lbs N/acre, or to run PSNT samples on their own fields to verify.
1) In general terms, on the medium and heavy textured soils there appears to be no reason to depart from average or slightly above average sidedress N rates in 2013. Our survey suggests that growers on sandy soils might be justified in slightly higher than average sidedress N rates.
2) Producers with fall manure, red clover or alfalfa should be careful to apply normal or slightly above normal N rates. There is no evidence in our survey that spring-applied manures (principally inorganic N supply) are below average in their potential N supply to the corn crop.
3) Heavy May rains that caused saturated conditions and standing water in fields are not fully represented in this survey and growers should be aware of the potential N loss from denitrification in these fields.
4) We remind side-dressers that the numbers presented here are averages and that there is considerable variability across sites. Taking your own pre-sidedress N test in order to fine-tune application rates is strongly recommended (See Table 2).
Soil samples were collected from a range of fields across southern Ontario. OMAF & MRA would like to acknowledge the contributions to this survey by the many farm co-operators and industry stakeholders who participated in this effort. Appreciation is also extended to SGS AgriFood Laboratories for rapid soil sample analysis.
Click the above image to enlarge.
To convert soil test results from kg/ha to ppm for a 30-cm (12-in.) sample, divide kg/ha by 4. For example, if the nitrate-nitrogen concentration of a sample taken from the top 30 cm (12 in.) of soil is 32 kg/ha, the nitrate nitrogen is 32 kg/ha ÷ 4 = 8 ppm.