Agriculture and Agri-Food Canada, Fredericton, New Brunswick
Most of the N in soils is organic and must be converted to nitrate or ammonium (mineralization) before it is available to plants (see The Nitrogen Cycle section); nitrate is the predominant inorganic form in most soils. Nitrate can be high in early spring in some fields as a result of the carry-over from the previous growing season. The amount of carry-over varies with the amount of soil nitrate present in fall and the losses over the fall and winter. In some locations, such as coastal regions of the Pacific Northwest which are mild and wet, carry-over is minimal. In locations where winters are cold and dry, carry-over can be considerable. When the amounts of carry-over nitrate plus mineralized N is less than the crop requires, application of N as fertilizer or manure is required.
The amount of soil inorganic N that will be mineralized during the growing season cannot currently be predicted with a chemical analysis (see Determining Nutrients Available in Soils section), therefore, soil N testing is limited to measuring inorganic N. In south coastal British Columbia, where soil N fertility on dairy farms is high due to a history of manure use, net soil N mineralization is commonly greater than 100 kg N/ha, with some fields exceeding 200 kg N/ha. In locations where carry-over nitrate is leached below the root zone over winter, a pre-plant test has limited value for determining fertilizer requirements. Since corn is a long-season crop (see Parable of the Fast and Slow Growing Crops section), soil nitrate testing can be delayed to just prior to sidedress fertilizer time in order to detect as much soil nitrate as possible from mineralization and nitrification.
Two Soil N Tests: PPNT and PSNT
The two most common soil N tests used for determining nitrogen available for corn are the pre-plant soil nitrate test (PPNT) and the pre-sidedress soil nitrate test (PSNT). The PPNT is based on the soil nitrate concentration to 60-cm depth measured before planting (Fig.1). This test measures mainly the amount of soil nitrate carried over from the previous growing season plus early-season soil N mineralization. This test works best in dry or cold climates where carry-over of nitrate from the previous year contributes a major proportion of the total soil N supply. Disadvantages of the PPNT are that the sample is taken prior to most growing season mineralization and nitrification of soil organic matter and spring applied manure N. The PPNT might be improved in some fields by including a measure of soil ammonium concentration.
The PSNT is based on the soil nitrate concentration to 30-cm depth measured at the six-leaf stage, just prior to the period of rapid crop N uptake (Fig. 1). The PSNT works best when no more than 20-30 kg N/ha (18-27 lb/ac) is banded at planting. Based on studies in coastal British Columbia, soil nitrate measured at this time includes little carry-over from the previous growing season, about 50% of the soil N mineralization which will occur throughout the growing season, and most of the available N from spring-applied liquid dairy manure (1). Usually, most nitrification of soil and manure ammonium is complete by the time the PSNT sample is taken, so measurement of soil ammonium is not required. However, where corn was planted after plough-down of forage grass in early May (i.e. after first cut), more than half of the plant available N in the soil was still in ammonium form when the PSNT sample was taken.
Thanks to the delayed sampling, the PSNT is more sensitive to soil N ineralization and spring manure application than the PPNT, and, except where carry-over is substantial, should provide better estimates of fertilizer N requirement than the PPNT. In cases where carry-over N is significant, it may be helpful to use the PPNT to choose an appropriate spring manure application rate, and to use the PSNT to choose an appropriate sidedress fertilizer N rate.
How to use pre-sidedress soil nitrate tests (PSNT)
It should be part of a nitrogen management system:
- Manage manure according to local environmental guidelines; in some years, liquid manure alone may supply enough N for your crop.
- Do not broadcast N before planting. Corn requires little N early in the growing season.
- Apply a low rate of N (20-30 kg/ha or lb/ac) with the planter. Nitrogen applied by the planter is not measured by the PSNT.
- Use the PSNT to decide how much, if any, fertilizer N to apply at sidedress.
- Sample to 30-cm (1-ft) depth midway between corn rows to avoid fertilizer banded with the planter.
- Take at least 10 cores per field when the corn is at the 6-leaf stage or 15-30 cm (6 - 12") tall.
- Keep the sample cool or frozen until it reaches the lab — a picnic cooler is handy; warm samples will release nitrate and give a fertilizer recommendation which is too low.
- Have the sample analysed for nitrate-N concentration in ppm.
- Corn will likely not respond to sidedress N in fields with PSNT values greater than the critical PSNT value. Critical PSNT values vary somewhat with region so use local information. The most common critical PSNT values in North America are 20-25 ppm.
- For PSNT test values less than the critical PSNT value, use local information to predict the fertilizer N requirement at sidedress. Table 1 shows values for coastal Pacific Northwest (1).
- Field measurements have shown that where corn was planted following a late-spring plough-down of forage grass, more than half of the plant available nitrogen in the soil was still in ammonium form when the PSNT sample was taken. In this case, PSNT will over-predict N needs.
Future technology: plant chlorophyll N tests
There is growing interest in estimating plant-N status based on leaf colour for making fertilizer N recommendations (2). This approach can be used rapidly in the field and does not require laboratory analyses. Also, the plant itself provides an indication of its N status, in contrast to soil tests which focus on soil N supply and cannot readily account for variation in crop N demand. The primary disadvantage of this approach is that factors other than crop N status can influence measurements.
Perhaps the most studied field-test method measures ‘leaf greenness’ because of the direct relation with chlorophyll concentration and N status. A hand-held device called ‘SPAD-502’ (Minolta Corp.) measures chlorophyll concentration of a leaf by shining a small beam of light through the leaf. SPAD measurements are quick and repeatable but readings are very sensitive to other factors such as the position of the leaf where the reading is taken, which leaf is tested, growth stage of the plant, and to corn cultivar due to differences in leaf thickness.
Used at the six-leaf stage, the SPAD meter has been found to be useful for identifying corn fields likely to respond to a sidedress N application. Some studies have found the SPAD meter to be as reliable as the PSNT in making a ‘yes/no’ decision on fertilization. However, the SPAD meter has not proved reliable for determining how much sidedress N to apply. Some studies have employed reference plots that receive abundant N fertilizer before planting, and used the ratio of SPAD readings from the field and the reference plots, but the advantage of this approach is not well proven. The SPAD meter might best be used as a preliminary screening tool to decide which fields require a PSNT test sample to be taken.
Readings with the SPAD meter taken after the six-leaf stage have been used successfully for making fertilizer N recommendations in irrigated corn fields. For these fields, N fertilization is done when the ratio of the SPAD readings taken from the field and from the reference plots drops below a threshold value, commonly 90 to 95%.
New instruments which measure light reflected from the crop canopy are being developed for indicating corn N status. None of these instruments are in common use at this time. However, one instrument which appears to be promising is the ‘Hydro N Sensor’. This instrument uses four tractormounted sensors, to measure light reflected from the canopy surrounding the tractor, to measure crop N status in real time. With suitable calibration, it may be possible in future to use such an instrument to perform real-time variable-rate fertilizerN applications.
1. Zebarth, B.J., J.W. Paul, M. Younie and S. Bittman 2001. Fertilizer nitrogen recommendations for silage corn in high-fertility environment based on presidedress soil nitrate test. Commun. Soil Sci. Plant Anal. 32, 2721-2739.
2. Zebarth, B.J. J.W. Paul, M. Younie and S. Bittman 2002. Evaluation of leaf chlorophyll index for making fertilizer nitrogen recommendations for silage corn in a high fertility environment. Commun. Soil Sci. Plant Anal. 33, 665-684.