Crop Inputs In view of our findings to date, the following
suggestions are made to assist with interpretation of the soil N test for N recommendations for corn: 1. If forage legumes are burned down or plowed down in the spring,
or if manure is applied before planting in the spring, the PSNT may be of some value. The preplant N test should
not be used in these cases. It should be noted that the above suggestions
are tentative and need to be confirmed by further research.
Comments on the Soil N Test for Corn
By Dr. Eric Beauchamp and Dr. Bev Kay, Department of
Land Science, University of Guelph
The soil N test for corn was made available in OMAFRA Publication 296 (Field Crop Recommendations)
in about 1992. Although it is not heavily used by farmers, the test is one of the most objective ways to estimate
the N requirement of a corn crop. The soil N test result can be affected by previous crop residues (e.g., forage
legumes), manure application and weather conditions. These factors should be taken into account when the test result
is used as a guideline.
Most
of the comments made here are based on results from field trials recently at the
Elora Research Station and other research stations in Ontario. A focus of these
trials was to develop a new soil N test, but the ability of the currently used
soil N test was also studied with particular reference to the management of previous
legume crop residue N. The following are some points arising out of this research.
First, it was observed that corn yield response to N was generally dependent on soil organic matter content (or
organic C content) in the 0-30 cm depth. Also, NO3 (nitrate) accumulation during the months of May and June was
likewise dependent on soil organic matter content It was observed that, if the organic C was greater than 2.0-2.5
per cent (approximately 3.5 - 4.3% organic matter, since soil organic matter is about 58% carbon, 0-30 cm) and
with a clay content of about 15 per cent, there was little likelihood of a response to N fertilizer. As the organic
C decreased below this level, we would expect an increased response to N fertilizer. The accumulation of NO3 during
these months was linearly related to organic C content.
Second, it was observed that NO3 accumulation rate during April, May and June
was related to soil temperatures. Soil temperature units (STU) were calculated
by averaging the daily minimum and maximum soil temperatures at a given depth.
A temperature less than 0oC was considered to be 0oC. The accumulatio
n of STUs
with temperatures measured at 5 cm depth from April 1 to June 30 for 4 years at
the Elora Research Station is shown in Fig. 1. It is evident that substantial
differences in STU accumulation occurred between years. The data in Fig. 2 show
the relationship between soil NO3 content and STU during May and early June (2
sites in each of ‘97, ‘98, ‘99; 1 site in ‘00). The data show that the greater
the accumulated STU, the greater the soil NO3 level. There were differences between
years. In particular, the June 19, 2000 sample indicated a decrease in NO3 from
the May 15, 2000 sample. This decrease is attributed to relatively high rainfall
during May and early June this past spring. It is thus evident that both soil
temperatures and rainfall in the early growing season affect NO3 accumulation
and need to be taken into account in using the soil N test for predicting N requirements.
Third, the soil N test accurately reflected the N available following chemical burndown of forage legumes in the
fall. Previous research by Vyn et al. showed that fall plowdown of red clover produced similar results. On the
other hand, the NO3 accumulation pattern with burndown or plowdown of forage legumes in the spring was different.
Nitrate content (0-30 cm) in early May was actually lower than with the ‘no legume’ treatment but was considerably
higher later in early to mid June. The early growth of alfalfa or red clover resulted in NO3 uptake which probably
reduced soil NO3 content before plowdown or burndown. Rapid N release from these residues occurred thereafter in
late May and June.
Fourth, it was observed that the accumulation of NO3 during the April-June period increased at an increasing rate
if rainfall was not unduly great. Thus the date of sampling, especially for the presidedress soil N test (PSNT),
needs to be taken into account. For example, the measured NO3 level in the last week of May will be considerably
different than that in the second week of June, yet this difference is not taken into account with soil N test
interpretations in Publication 296.
2. The preplant soil N test or PSNT may be appropriately used if forage legumes are herbicide-burned or plowed
down in the fall.
3. The soil N test table in OMAFRA Publication 296 is appropriate if average weather conditions prevail during
the April to June period.
4. If unduly wet conditions (for example, 2000) prevail for a significant period during May or early June, soil
NO3 levels are likely to be lowered by PSNT sampling time. Nevertheless, a low PSNT would indicate a greater fertilizer
N requirement to compensate for NO3 losses.
5. It would generally be appropriate to use the recommended N rates according to the soil N test (PSNT) for samples
taken in the first week of June. If samples are taken in the last week of May, the recommended N rates should be
decreased by about 10 per cent. For samples taken in second week of June, the recommended N rates should be increased
by 20-30 per cent.
1