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Fertilizer nitrogen recommendations for silage cornin high-fertility environment based on pre-sidedresssoil nitrate testB. J. Zebarth a , J. W. Paul b , M. Younie c & S. Bittman da Potato Research Centre , Agriculture and Agri-Food Canada , P.O. Box 20280,Fredericton, NB, E3B 4Z7, Canadab Transform Compost Systems Ltd. , 34642 Mierau Street, Abbotsford, BC, V2S 4W8,Canadac BC Environment , 46360 Airport Road, Chilliwack, BC, V2P 1A6, Canadad Pacific Agri-Food Research Centre , Agriculture and Agri-Food Canada , P.O. Box 1000,Agassiz, BC, VOM 1A0, CanadaPublished online: 20 Aug 2006.

To cite this article: B. J. Zebarth , J. W. Paul , M. Younie & S. Bittman (2001) Fertilizer nitrogen recommendations forsilage corn in high-fertility environment based on pre-sidedress soil nitrate test, Communications in Soil Science and PlantAnalysis, 32:17-18, 2721-2739

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FERTILIZER NITROGENRECOMMENDATIONS FOR SILAGE CORN

IN HIGH-FERTILITY ENVIRONMENTBASED ON PRE-SIDEDRESS SOIL NITRATE

TEST

B. J. Zebarth,1,* J. W. Paul,2 M. Younie,3 andS. Bittman4

1Potato Research Centre, Agriculture and Agri-Food

Canada, P.O. Box 20280, Fredericton, NB,

Canada E3B 4Z72Transform Compost Systems Ltd., 34642 Mierau Street,

Abbotsford, BC, Canada V2S 4W83BC Environment, 46360 Airport Road, Chilliwack, BC,

Canada V2P 1A64Pacific Agri-Food Research Centre, Agriculture and

Agri-Food Canada, P.O. Box 1000, Agassiz, BC,

Canada V0M 1A0

ABSTRACT

The purpose of this study was to measure corn response to

nitrogen (N), to evaluate the feasibility of using the pre-sidedress

soil nitrate test (PSNT) for making fertilizer N recommendations

for silage corn, and to evaluate the environmental and economic

implications of adopting fertilizer N recommendations based on

the PSNT in south coastal British Columbia, Canada, a region of

2721

Copyright q 2001 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author.

COMMUN. SOIL SCI. PLANT ANAL., 32(17&18), 2721–2739 (2001)

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high soil N fertility. Eighty-seven trials were conducted from

1994–1996 with corn or forage grass as a preceding crop on fields

having a history of manure application. Non-manured trials

received no spring manure application whereas manured trials

received a spring application typical of that farm field. In 1994,

treatments included sidedress N rates of 0, 30, 60, 90, 120, and

150 kg N ha21 replicated twice. In 1995 and 1996, trials with less

than 30 mg NO3-N kg21 soil to 30 cm depth prior to sidedress

received the same treatments as in 1994, whereas trials with

concentrations above this value received two treatments, 0 and

150 kg N ha21, replicated four times. Trials received less than

50 kg N ha21 at planting. Yield response to sidedress N was

limited, with 71, 87, and 55% of trials with 90% or higher relative

yield (yield at 0 kg N ha21 divided by yield at 150 kg N ha21) in

1994, 1995 and 1996, respectively. The limited yield response was

attributed to high soil N mineralization, which averaged 166 and

146 kg N ha21 for non-manured trials in 1995 and 1996,

respectively. The critical PSNT value (above which no yield

response to sidedress N is expected) was predicted to be 19 and

23 mg NO3-N kg21 soil using the Cate-Nelson procedure and a

linear-plateau regression procedure, respectively. A critical PSNT

test value of 32 mg NO3-N kg21 soil was predicted using a linear-

plateau regression of the estimated N rate which would maximize

economic return (NMER) against PSNT test value. In addition, a

model designed to minimize the risk of yield loss was developed

with a critical PSNT test value of 30 mg NO3-N kg21 soil.

Average soil NO3 content at harvest was slightly higher for

sidedress N rates based on the minimum risk model (70 kg N ha21)

than at NMER (64 kg N ha21). Based on a survey of farm fields, the

potential to reduce fertilizer N application in south coastal BC was

estimated to be about 90 kg N ha21 yr21.

INTRODUCTION

South coastal British Columbia (BC), Canada, is an agricultural region

characterized by high animal densities and intensive field and horticultural crop

production. The region has high N inputs as manure and fertilizer relative to crop

N removal (1). As a result, there is growing concern over the contribution from

agricultural production to groundwater contamination by nitrate.

Silage corn (Zea mays L.) is a major field crop in the region, accounting for

about 20–25% of the land used for dairy production, and is grown in rotation

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with forage grass, primarily orchardgrass (Dactylis glomerata L.). Most corn

fields receive heavy annual applications of manure. Consequently, large

quantities of N are supplied to the corn crop by manure and by mineralization of

soil organic N (2). However, the quantity of N supplied to a corn crop from

manure and mineralization varies substantially among fields and years, making

the fertilizer N requirement of corn difficult to predict. No established system for

making fertilizer N recommendations for corn exists in this region.

The pre-sidedress soil nitrate test (PSNT) was developed in Vermont to

predict the inorganic fertilizer N requirement of corn (3,4). The test is based on a

soil sample taken when the corn is at approximately the six-leaf stage. This

timing is chosen to be just prior to the period of rapid crop N uptake, yet after

significant mineralization of soil organic N has occurred (4). The PSNT has

proved successful in a number of regions in the United States for identifying corn

fields for which a yield response to additional N at sidedress is not likely to occur

[e.g., Meisinger et al. (5) and Klausner et al. (6)]. In some cases, the accuracy of

N recommendations using the PSNT was improved by separating sites into

different crop yield potentials (7,8). The PSNT is generally more effective in

identifying the need for N fertilizer at sidedress than for predicting the quantity of

fertilizer N required (6).

Previously, growing season soil N dynamics were used to identify the

PSNT as the most promising approach for a soil N test for silage corn in south

coastal BC (9). Most of the additional inorganic N associated with spring manure

application was already present, and nitrification of manure ammonium had

occurred, at the time when PSNT sampling is done. In addition, up to about 60%

of the total apparent net mineralization which occurs during the growing season

may have occurred by the time the PSNT sample is taken (unpublished data by

authors).

The purpose of this study was to measure corn response to N, to evaluate

the feasibility of using the PSNT for making fertilizer N recommendations for

silage corn, and to evaluate the environmental and economic implications of

adopting fertilizer N recommendations based on the PSNT in south coastal BC.

Soil and climatic conditions and crop production practices in this region are

similar to those in coastal areas of the Pacific Northwest of the United States, and

are characterized by high soil N fertility.

MATERIALS AND METHODS

Trials were conducted at 17, 16, and 12 field sites in 1994, 1995, and 1996,

respectively. The sites were chosen to reflect the range of soil types on which

silage corn is grown, and included both corn and forage grass as preceding crops

(Table 1). Soil sand, clay (pipette method), and organic matter (loss-on-ignition

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Table 1. Crop and Soil Response Parameters for Trials in Three Years by Preceding Crop and Spring Manure History

Soil Nitrate at Harvestd

Year

Preceding

Crop

Spring

Manure N

PSNT Test

(mg N kg21

Relative

Yield a (%)

Crop N

Accumulationb

(kg N ha21)

Apparent net

Soil N

Mineralizationc

(kg N ha21) 0 kg N ha21 150 kg N ha21

1994 Corn No 11 22 (9–42) 91 (80–104) 147 (108–182) NA 49 (11–184) 80 (23–169)

Yes 11 32 (11–54) 97 (72–119) 194 (125–220) NA 81 (24–156) 164 (85–272)

Grass No 6 43 (22–72) 97 (86–104) 219 (182–255) NA 131 (39–364) 212 (94–501)

Yes 6 51 (22–70) 98 (91–105) 201 (134–252) NA 161 (69–278) 257 (93–520)

1995 Corn No 13 27 (14–49) 97 (82–110) 171 (136–209) 144 (55–217) 56 (11–137) 125 (23–289)

Yes 12 43 (16–76) 103 (93–129) 201 (150–268) NA 147 (31–378) 241 (111–493)

Grass No 3 36 (29–46) 99 (91–104) 169 (121–205) 249 (126–333) 126 (16–213) 190 (92–266)

Yes 3 62 (57–67) 99 (94–106) 190 (175–208) NA 177 (112–248) 308 (191–417)

1996 Corn No 8 13 (7–18) 87 (82–96) 140 (96–207) 137 (66–219) 48 (6–103) 48 (22–89)

Yes 7 27 (7–56) 95 (80–105) 170 (100–230) NA 85 (10–218) 91 (24–200)

Grass No 4 22 (9–36) 90 (64–102) 149 (78–208) 168 (65–226) 66 (5–104) 75 (26–119)

Yes 3 23 (13–29) 93 (79–102) 132 (88–189) NA 54 (7–91) 105 (47–167)

a Dry matter yield for 0 kg N ha– 1 rate divided by yield for 150 kg N ha21 rate.b Crop N accumulation for the 0 kg N ha21 rate.c Increase in soil plus crop N between planting and harvest, calculated for the 0 kg N ha21 rate for non-manured trials only. Data not

available for 1994.d Soil nitrate content to 90 cm depth at harvest.

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at 4508C) contents ranged from 20–740, 80–590, and 28–224 g kg21,

respectively. All sites had a history of manure application, mostly as liquid

dairy manure with some sites receiving solid poultry, solid horse, or liquid swine

manure. Detailed information on the site management and soil properties is

presented elsewhere (10).

A total of 87 trials were conducted, with two trials conducted at most

sites; one following the normal spring manure management for that farm and

one with no manure applied in spring. Autumn applied manure has no effect

on corn growth or N uptake in the following growing season in south coastal

BC because of intensive leaching during the autumn and winter (2). Starter N

applied with the planter averaged 27 kg N ha21, and was never more than

50 kg N ha21. No pre-plant broadcast of N was applied to any trial. The

experimental unit was a plot 12 m� 3 m (4 corn rows) in size established at the

time of corn emergence.

In 1994, each trial consisted of six treatments in a randomized complete

block design with two replications. Treatments included N rates of 0, 30, 60, 90,

120 and 150 kg N ha21 as NH4NO3 applied at sidedress when the corn was at

approximately the six-leaf stage. Fertilizer N treatments were not modified based

on starter N rates. The fertilizer was applied as a band approximately 15 cm from

each side of the corn row and 5 cm deep using a precision fertilizer sidedresser,

modified from a commercial small plot seed drill and with tines mounted on the

frame to perform tillage for weed control.

In 1995 and 1996, soil samples were collected to 30 cm depth from each

trial approximately one week prior to the time of sidedress and analyzed for soil

NO3 concentration. Trials with concentrations less than 30 mg NO3-N kg21 soil

received the same treatments as in 1994 replicated four times, whereas trials with

concentrations of 30 mg NO3-N kg21 soil or more received two treatments, 0 and

150 kg N ha21, replicated four times.

Soil samples were collected from each site in early spring prior to manure

application in 1995 and 1996 for depth increments of 0–15, 15–30, 30–60 and

60–90 cm. Each sample was a composite of 10 cores for the 0–15 and 15–30 cm

depths, and 5 cores for the 30–60 and 60–90 cm depths, using a 2.54-cm

diameter soil probe. Soil samples were collected at the six-leaf stage by replicate

in 1995 and 1996, and from each half of each replicate in 1994 (in order to get

four samples from the two replicates), and were collected at harvest from each

plot, for the same soil depths. All soil samples were kept frozen until analyzed. A

15-g subsample of moist soil was oven dried at 1058C to determine gravimetric

soil water content. A 20-g subsample of moist soil was extracted using 100 mL of

2 M KCl and 1-h shaking time (11). Concentrations of NO3-N and NH4-N in the

extract were determined spectrophotometrically by flow injection analysis (2).

Soil NO3-N and NH4-N concentrations were converted to units of kg N ha21

using soil bulk density determined using the soil core method.

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Corn was hand harvested from a 1.5 m (2 corn rows)� 6 m harvest area in

each plot and total silage yield determined. Ten representative corn plants from

each plot were chopped, and a 700 g subsample dried at 608C for determination of

dry matter content and ground to pass a 2-mm sieve. Total N concentration was

determined by a dry ash method using a Leco FP-428 N Determinator (Leco

Corporation, St. Joseph, MI) or by a standard Kjeldahl digestion, followed by

determination of NH4 concentration in the digest using an AutoAnalyzer. The

two methods were cross-calibrated to ensure consistency of results. Statistical

analyses were performed using the GLM procedure of SAS (12).

Apparent net soil N mineralization (ANM) was estimated from the

0 kg N ha21 treatment for non-manured trials in 1995 and 1996 as:

ANM ¼ crop N uptake 1 soil N at harvest 2 soil N in early spring

2 starter N ð1Þ

where soil N is NO3-N plus NH4-N measured to 90 cm depth.

Three approaches were used to evaluate the relationship between crop

yield response and the PSNT test value (soil NO3-N concentration to 30 cm

depth at approximately the corn six-leaf stage, sampled midway between corn

rows to avoid N fertilizer banded at planting). The first was the Cate-Nelson

procedure (13), adapted to computer spreadsheet form (14). This procedure can

choose a critical value for separating measured response to some parameter,

such as a soil test, into two categories. For the PSNT, the critical value is the

PSNT test value above which no additional yield response to sidedress N is

expected to occur. The calculations were done using relative yield (yield for the

0 kg N ha21 rate divided by yield for the 150 kg N ha21 rate) and PSNT test

value. The optimum sidedress N rate for PSNT test values greater than or equal

to the critical value is zero. This procedure cannot be used to predict the

optimum sidedress fertilizer N rate for any PSNT test value below the critical

value. A relative yield of 90% was chosen to separate trials which were

responsive or non-responsive to sidedress N.

Second, a linear-plateau regression model was fit to the relative yield and

PSNT test values using the NLIN procedure of SAS (12). The critical value is

defined by the intersection point of the linear and plateau portions of the response

curve. Interpretation of the critical value is similar to that for the Cate-Nelson

procedure.

Third, the rate of sidedress N estimated to maximize economic return

(NMER) was plotted against the PSNT test value. A regression model of the form

Y ¼ a 1 bX 1 cX2; where Y is dry matter yield and X is the fertilizer N rate at

sidedress, was fit to the corn dry matter yield treatment means for all trials which

had a PSNT test value of less than 40 mg NO3-N kg21 soil, and in which six rates

of sidedress N were applied. The expected shape of the response curve (i.e., b > 0;

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c, 0) was obtained for 37 of the 54 trials to which curves were fit. Where the

expected shape of the response curve was not obtained, NMER was set to zero. For

each trial with the expected shape of the response curve, NMER was estimated

using the method of Zebarth et al. (15) using a cost:price ratio (CPR) of 17.1,

assuming values for fertilizer N and silage corn of CDN$0.89 kg21 N and

CDN$0.052 kg21 silage (dry weight), respectively, realistic values for south

coastal BC at the time of the study. The NMER for each trial was also calculated

for CPR values of 12 and 22. For three trials with CPR ¼ 12 where NMER was

greater than 150 kg ha21, the highest sidedress N rate used, NMER was set to

150 kg ha21. A linear-plateau regression model was then fit to the NMER and

PSNT test value data, and the intersection point between the linear and plateau

portions of the curve was interpreted as defining the critical PSNT value.

Growing season (April to September) air temperatures at Agassiz, BC

were 1.2, 1.4, and 0.28C above the long term (1961–1990) average of 14.98C,

and growing season rainfall was 82, 84 and 120% of the long term average of

516 mm, in 1994, 1995, and 1996, respectively (16). Similar climatic trends

were present elsewhere in the region. Growing conditions were generally good

for corn in 1994 and 1995 although the warm dry spring in 1994 may have

resulted in moisture stress in some fields. Very wet spring conditions and cool

May temperatures in 1996 resulted in late corn planting and delayed early

growth.

RESULTS AND DISCUSSION

Crop Response

Corn dry matter yield at the 150 kg N ha21 rate averaged 16.4 and

17.9 t ha21 in 1994 and 1995, respectively, compared to 14.1 t ha21 in 1996 (data

not presented). Corn yield was generally not sensitive to preceding crop or spring

manure management. A wide range in dry matter yield was obtained among sites,

however. For example in 1994, dry matter yield for the 150 kg N ha21 rate ranged

from 10.6 to 25.0 t ha21.

Yield response to sidedress N was limited, with corn relative yield (yield

for 0 kg N ha21 rate divided by yield for 150 kg N ha21 rate) of 90% or greater for

71, 87, and 55% of the trials in 1994, 1995, and 1996, respectively (Table 1).

Nitrogen accumulation in the above-ground portion of the crop for the

0 kg N ha21 rate from non-manured trials was high at 170, 170, and

143 kg N ha21 for 1994, 1995, and 1996, respectively, despite the low starter N

applied. These results suggest that soil N fertility levels were high, and that crop

response to sidedress N application was small, and therefore that much of the

variation in dry matter yield among sites was not due to N fertility.

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Soil Inorganic Nitrogen

Soil NO3 content was low in early spring, with an average of 53 and

23 kg N ha21 to 90 cm depth for 1995 and 1996, respectively (data not presented).

Soil NO3 was not measured in early spring of 1994. These results are generally

consistent with previous studies which concluded that much of the nitrate

remaining in the soil after harvest is leached out of the root zone over the autumn

and winter in south coastal BC (2,17). However, the higher average soil NO3

content in 1995 was due to high soil NO3 from 30–90 cm depth at three sites

where incomplete leaching of nitrate appears to have occurred. In comparison,

soil NO3 content to 90 cm depth at sidedress averaged 139 (range 65–329), 143

(range 61–232) and 107 (range 47–209) kg N ha21 for non-manured trials, and

183 (range 104–303), 220 (range 98–393), and 171 (range 66–411) kg N ha21

for manured trials, in 1994, 1995 and 1996, respectively. Soil NH4 content was

relatively low, ,50 kg N ha21, for most trials in early spring and at sidedress. The

exception to this was one site in 1994 with organic subsoils which had soil NH4

contents as high as 800 kg N ha21. Therefore, the limited yield response to

sidedress N, and high crop N uptake with no sidedress N for non-manured trials,

was attributed primarily to high soil N mineralization.

Apparent net mineralization, as calculated for non-manured trials using

Eq. 1, averaged 166 (range 55–333) and 146 (range 65–226) kg N ha21 in 1995

and 1996, respectively (Table 1). Highest apparent net mineralization values

were commonly associated with a preceding forage grass crop. There was no

significant relationship between apparent net mineralization and soil sand, clay,

and organic matter contents as determined using multiple linear regression (data

not presented).

The estimates of apparent net mineralization in this study are similar to or

higher than the 96 to 158 kg N ha21 reported previously for silage corn fields in

south coastal BC (2). The high soil N mineralization was likely due primarily to

the history of manure use at these sites, and in some cases to the preceding forage

grass crop. Manure management history is an important factor influencing soil N

mineralization (2,18), which is consistent with the lack of a significant

relationship between apparent net mineralization and soil sand, clay, and organic

matter contents at these sites.

Pre-sidedress Soil Nitrate Test

PSNT test values ranged from 7 to 76 mg NO3-N kg21 soil (Table 1). As

expected, there was a general trend toward higher PSNT test values following

forage grass than following corn, and for manured trials as compared to non-

manured trials.

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The critical PSNT value was estimated at 19 mg NO3-N kg21 soil using the

Cate-Nelson procedure (Fig. 1a). In comparison, the critical PSNT value

estimated using the linear-plateau regression model was 23 mg NO3-N kg21 soil

(Fig. 1b). The effect on the critical values of separation of trials into those with

corn or forage grass as a preceding crop, into those with or without spring manure

application, and into those with high or low yield potential, was investigated for

both approaches. Generally one of two results was obtained for each case: either

Figure 1. Critical PSNT values as estimated by the Cate–Nelson procedure (A) and by a

linear-plateau regression procedure (B) for 87 trials 1994–1996.

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critical values remained similar to those obtained with the complete data set, or

the resulting estimate was erratic based on an insufficient data set.

In previous North American studies, critical PSNT values were estimated

to range from 13 to 25 mg NO3-N kg21 soil using the Cate-Nelson procedure

(6,7,14,19–24), and from 20 to 27 mg NO3-N kg21 soil based on a regression

model approach (5–8,25). Bundy and Andraski (8) estimated a lower critical

value for medium yield potential (19 mg NO3-N kg21 soil) compared to high

yield potential (21 mg NO3-N kg21 soil) sites, or a critical value of 26 mg

NO3-N kg21 soil for all sites combined, using a linear-plateau regression

procedure.

For CPR ¼ 17:1; NMER ranged from 0 to 108 kg N ha21. A linear-plateau

regression model of NMER against PSNT test value predicted a critical value of

32 mg NO3-N kg21 soil, with a maximum predicted sidedress N rate of

62 kg N ha21 for a PSNT test value of zero (Fig. 2). The regression model under-

Figure 2. Best fit and minimum risk of yield loss models for the relationship between the

estimated sidedress N rate to provide maximum economic return (NMER) and PSNT test

value for cost:price ratio of 17.1 for 54 trials 1994–1996.

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estimated NMER for almost all trials in which NMER was greater than zero,

because of the large number of trials for which NMER was equal to zero. There

was no consistent pattern with respect to preceding crop, soil texture, or soil

organic matter content for trials in which NMER was equal to zero.

The best-fit relationship between the estimate of NMER and PSNT test

value has been reported in previous studies (6,8,21). The critical value

estimated by this approach was similar to or higher than obtained using the

Cate-Nelson or linear-plateau regression model procedures (8,21). In each

study, the best-fit relationship underestimated the optimum N rate by

50 kg N ha21 or more for some trials. As a result of the relatively poor

relationship between PSNT test value and NMER, the PSNT is generally more

effective in identifying the need for N fertilizer at sidedress than for predicting

the quantity of fertilizer N required (6). In this case, the PSNT can be used to

make a yes/no decision for sidedress N application, and traditional methods,

for example crop yield potential and N credit systems, can be used to predict

the rate of fertilizer N to apply.

It is important to producers in south coastal BC to obtain optimal crop yield

because of high land values. Producer acceptance of the PSNT will be therefore

be limited if there is a risk that the optimum fertilizer N rate will be significantly

underestimated. Such a risk is present for the best fit model between NMER and

the PSNT test value (Fig. 2). The PSNT could be used to make a yes/no decision

for sidedress N application, however, no other approach is available for use with

the PSNT to predict the fertilizer N rate to apply if the PSNT value is below the

critical value. Therefore, a model designed to minimize the risk of yield loss

(Fig. 2) was developed between NMER and PSNT test value such that most

extreme NMER values were encompassed by the model. Though not developed

mathematically, it is a model which is readily accepted by growers. It is

recognized that this approach will tend to overestimate the actual fertilizer N

required to achieve optimum yield. It may be feasible to reduce fertilizer N

recommendations somewhat as grower acceptance of the PSNT increases.

The points which lie along the ascending portion of the minimum risk

model (Fig. 2) are from all three project years, include trials with both corn and

forage grass as a preceding crop, include manured and non-manured trials, and

represent sites with a range of soil textures and organic matter contents.

Consequently, these recommendations were assumed to apply to all silage corn

fields, except possibly the small number with organic soils. The minimum risk of

yield loss model was generally suitable over a wide range of CPR values (Fig. 3).

Fertilizer N recommendations based on the PSNT were developed from the

minimum risk of yield loss model using increments of 25 kg N ha21 (where PSNT

has units of mg NO3-N kg21 soil): 0 kg N ha21 for PSNT>30; 25 kg N ha21

for 30$ PSNT > 26; 50 kg N ha21 for 26$ PSNT > 21; 75 kg N ha21 for

21$ PSNT > 18; 100 kg N ha21 for 18$ PSNT > 14; and 100 kg N ha21 for

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14$ PSNT. On average, fertilizer N recommendations based on the minimum

risk of yield loss model are 42 kg N ha21 higher than NMER.

The reliability of the PSNT is commonly evaluated by determining the

number of trials which fall into each of four quadrants based on the PSNT critical

value, and based on the relative yield (90% relative yield in this study) chosen to

distinguish between sites which are responsive or non-responsive to sidedress N

(Fig. 4). In Quadrant I, the PSNT correctly predicts no yield response to sidedress

Figure 3. Minimum risk of yield loss model based on a cost:price ratio (CPR) of 17.1,

plotted against the relationship between estimated sidedress N rate to provide maximum

economic return (NMER) as a function of PSNT test value for CPR ¼ 12 or 22.

Figure 4. Distribution of individual trials falling into the four quadrants (see text for

definition and interpretation of quadrants) for trials in 1994 to 1996 for critical PSNT test

values of 19, 23, and 30 mg NO3-N kg21 soil based on the Cate–Nelson procedure, linear-

regression procedure, and minimum risk of yield loss model, respectively.

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N, and in Quadrant III the PSNT correctly predicts a yield response to sidedress

N. The success rate for the PSNT is commonly defined as the proportion of trials

falling into Quadrants I and III. In Quadrant II, the PSNT predicts that a yield

response to sidedress N will occur, whereas no yield response was measured. In

this case, the PSNT prediction is incorrect, however, optimum crop yield is still

obtained. In Quadrant IV, the PSNT predicts no yield response to sidedress N,

whereas a yield response was measured. This case is of most concern in south

coastal BC because optimum yield is not obtained.

The success rate was 79, 75, and 67%, and the number of trials which fell

into Quadrant IV was 10, 7, and 5%, based on the critical PSNT value predicted

using the Cate-Nelson procedure, the linear-plateau regression procedure, and the

minimum risk of yield loss model, respectively. Therefore, the lower the

predicted critical PSNT value, the higher the success rate, but the greater the risk

of yield loss for trials with PSNT test values greater than the critical value. In

comparison, the success rate for the PSNT in previous studies ranged from 68 to

88% (6,14,20,21,24) and the proportion of trials which fell into Quadrant IV

ranged from 0 to 7% (6,20,21,24).

Environmental Implications of PSNT Adoption

Soil NO3 contents at harvest in this study were quite variable, with

generally higher values for manured trials than non-manured trials, for trials with

a preceding grass crop than with a preceding corn crop, for the 150 kg N ha21 rate

than for the 0 kg N ha21 rate, and in 1994 and 1995 as compared to 1996 (Table

1). Soil NO3 to 90 cm depth at harvest where no sidedress N was applied varied

widely from 5 to 378 kg N ha21 and from 22 to 520 kg N ha21 for the 0 and

150 kg N ha21 rates at sidedress, respectively.

Soil NO3 at harvest is an estimate of the risk of nitrate leaching in south

coastal BC, and 100 kg N ha21 has been suggested as the maximum acceptable

soil NO3 content in the root zone at harvest from a nitrate leaching perspective

(26). The percentage of trials with soil NO3 to 90 cm depth at harvest exceeding

100 kg N ha21 when no sidedress N was applied was 23 and 50% for non-

manured and manured trials, respectively. The high soil N content at harvest was

attributed to high soil N mineralization. The risk of nitrate leaching in these fields

is expected to be enhanced where forage grass was the preceding crop, where

there is a history of high rates of manure application, on soils with high to very

high soil OM, especially organic soils, and in years with climatic conditions

favorable for mineralization.

When all trials were considered, the PSNT test value explained 57% of the

variation in soil nitrate content to 90 cm depth at harvest where no sidedress N

was applied (Fig. 5). Based on this regression, average soil NO3 at harvest of 50,

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63, and 86 kg N ha21 would be expected at PSNT test values of 19, 23, and 30 mg

NO3-N kg21 soil, the critical PSNT values predicted using the Cate-Nelson

procedure, the linear-plateau regression procedure, and the minimum risk of yield

loss model, respectively. For the 54 trials for which NMER was calculated,

average soil NO3 content to 90 cm depth at harvest was 64 kg N ha21 at NMER,

compared to 70 kg N ha21 at the fertilizer N rate based on the minimum risk of

yield loss model. Overall, the results suggest that use of the fertilizer

recommendations based on the minimum risk of yield loss model should result in

a similar or slightly higher risk of nitrate leaching as compared to NMER.

Excessive soil NO3 at harvest was primarily due to a greater supply of N from soil

N mineralization, mineralization of forage residues from the previous growing

season, and spring manure application than was required by the crop, rather than

from excessive fertilizer N application.

Economic Implications of PSNT Adoption

The economic implications of adopting the PSNT have been evaluated

previously (27–29). It is difficult to perform a similar economic evaluation of

adoption of the PSNT in south coastal BC because no established system for

making fertilizer N recommendations for silage corn exists. The lack of an

Figure 5. Regression of soil NO3 content to 90 cm depth at harvest for the treatment

receiving no sidedress N against PSNT test value.

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established system may be due in part to the nature of N cycling in the region,

where high but variable contributions of N to the crop from soil N mineralization

and manure application are common.

Soil samples were taken from a total of 37, 74, and 111 silage corn fields

in south coastal BC in 1994, 1995, and 1996, respectively to measure the

PSNT test value (Fig. 6). A total of 70, 81, and 41% of the fields exceeded the

critical PSNT value of 30 mg NO3-N kg21 soil based on the minimum risk of

yield loss model in 1994, 1995, and 1996, respectively, suggesting that no

sidedress N was required to obtain optimum crop yield. For the fields with a

PSNT test value below the critical value, an average of 9, 7, 4, 7, and 5% of

fields fell into the ,14, 14–18, 18–21, 21–26, and 26–30 mg NO3-N kg21

soil test categories when averaged across the three years of the study (Fig. 6).

The weighted average sidedress N that would be recommended was 21, 17,

and 41 kg N ha21 in 1994, 1995 and 1996, respectively. If it is assumed that all

fields would receive an average of 25 kg N ha21 as starter, then corn fields

would, on average, receive 46, 42, and 66 kg N ha21 in 1994, 1995 and 1996,

respectively.

Information on the total quantity of fertilizer N applied to a corn fields in

south coastal BC is not available. Nitrogen budgets for the Lower Fraser Valley

estimated an average of 140 kg N ha21 inorganic fertilizer N for silage corn (1). If

this value is correct, and assuming the three years of the project are typical with

respect to PSNT test values, then the average potential reduction in fertilizer N is

about 90 kg N ha21, or about $80 ha21 not including any reduction in fertilizer

application costs.

Figure 6. Relative frequency of PSNT soil test values in farm fields in south coastal BC

in three years.

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While these estimates are not directly comparable to those obtained in other

studies, the potential economic benefit estimated from this study is generally

higher than reported elsewhere. Musser et al. (29) estimated that use of the PSNT

in Pennsylvania would increase economic return to producers by from $9 to

$34 ha21, and reduce fertilizer N use by 23 to 67 kg N ha21. Babcock and

Blackmer (27) estimated use of the PSNT in Iowa could result in an average

reduction in fertilizer N use of 38%, and an increase in economic return to the

producer of up to $54 ha21. For Pennsylvania, Roth et al. (28) estimated that use

of the PSNT would result in reduced fertilizer N application for 52% of fields,

with an average reduction of 25 kg ha21 across all corn fields.

The greater potential economic benefit associated with the use of the PSNT

in south coastal BC is likely due primarily to the very high level of soil N fertility,

and the lack of an established system for making fertilizer N recommendations,

allowing substantial reductions in fertilizer N application to be made while

maintaining optimum yield. Such potential reductions are large despite the choice

of a relatively high critical PSNT value.

CONCLUSIONS

Corn yield response to N fertilization in south coastal BC was limited due

to high soil N fertility. The high fertility was due in part to high soil N

mineralization, resulting from favourable climatic conditions and a history of

annual manure application on most corn fields. High and variable soil N supply to

the corn crop was provided by soil N mineralization, spring manure application

for manured trials, and in some fields from plough-down of a preceding forage

grass crop.

Critical PSNT test values of 19 and 23 mg NO3-N kg21 soil were estimated

using the Cate-Nelson and linear-plateau regression approaches, respectively.

These critical values, and the associated success rates for the PSNT, were similar

to those reported in previous studies. Producer acceptance of these critical values

is limited because of the risk of sub-optimal yield, because this approach does not

allow prediction of fertilizer N recommendations below the critical PSNT value,

and because no alternative system for making fertilizer N recommendations

below the critical value exists for this region.

For CPR ¼ 17:1; NMER ranged from 0 to 108 kg N ha21. A linear-plateau

regression model for the relationship between NMER and PSNT test value

indicated a higher critical PSNT test value of 32 mg NO3-N kg21 soil, however,

this model underestimated NMER by 50 kg N ha21 or more in some trials. As a

result, a minimum risk of yield loss model, which encompassed most extreme

NMER values was developed which had a critical PSNT value of 30 mg

NO3-N kg21 soil and a maximum fertilizer N recommendation of 125 kg N ha21.

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Adoption of fertilizer N recommendations based on the minimum risk of

yield loss model should result in a similar or slightly higher risk of nitrate

leaching as compared to fertilization at NMER. Excessive soil NO3 at harvest was

primarily due to a greater supply of N from soil N mineralization, mineralization

of forage residues from the previous growing season, and spring manure

application than was required by the crop, rather than from excessive fertilizer N

application.

Adoption of fertilizer N recommendations based on the PSNT was

estimated to reduce fertilizer N application to silage corn in south coastal BC by

90 kg N ha21, or about $80 ha21, not including any reduction in application

costs.

ACKNOWLEDGMENTS

The assistance and cooperation of all the growers who allowed us to work

on their fields is gratefully acknowledged. Funding for the project was provided

by Agriculture and Agri-Food Canada, the Canada-British Columbia Green Plan

for Agriculture, Coast Agri Ltd., and the Fraser River Action Plan of

Environment Canada. The assistance of M. Betts, BCMAFF in conducting trials

on Vancouver Island is gratefully acknowledged. Laboratory and field

assistance was provided by B. Harding, G. Telford, L. Birston, D. Chapple,

and C. Watson.

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