Cropping frequency and N fertilizer effects on soil water distribution from spring to fall in the semiarid Canadian prairies

doi:10.4236/as.2011.23031

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Vol.2, No.3, 220-237 (2011)

doi:10.4236/as.2011.23031

Agricultural Scienc es

Cropping frequency and N fertilizer effects on soil water

distribution from spring to fall in the semiarid Canadian

prairies

R. de Jong1*, C. A. Campbell1, R. P. Zentner2, P. Basnyat2, B. Grant1, R. Desjardins1

1Eastern Cereal and Oilseed Research Centre, Central Experimental Farm, Agriculture and Agri-Food Canada, Ottawa, Canada;

*Corresponding Author: dejongr@agr.gc.ca

2Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current, Canada.

Received 26 May 2011; revised 24 June 2011; accepted 25 July 2011.

ABSTRACT

In the semiarid Canadian prairies, water is the

most limiting and nitrogen (N) the second most

limiting factor influencing spring wheat (Triti-

cum aestivum L.) production. The efficiency of

water- and nitro gen use need s to be asses sed in

order to maintain this production system. The

effects of cropping frequency and N fertilization

on trends in soil water distribution and water

use were quantified for an 18-yr (1967-1984)

field experiment conducted on a medium tex-

tured Orthic Brown Chernozem (Aridic Haplo-

boroll) in southwestern Saskatchewan, Canada.

Soil water contents were measured eight times

each year and plant samples were taken at five

phenological growth stages. The treatments

studied were continuous wheat (Cont W),

summer fallow - wheat, F-(W) and summer fal-

low - wheat - wheat, F-W-(W) each receiving

recommended rates of N and phosphorus (P)

fertilizer, and (F)-W-W and (Cont W) each re-

ceiving only P fertilizer, with the examined rota-

tion phase shown in parentheses. Soil water

conserved under fallow during the summer

months averaged 25 mm in the root zone, and

was related to the initial water content of the soil,

the amount of precipitation received, its distri-

bution over time, and potential evapotranspira-

tion. Under a wheat crop grown on fallow, soil

water contents between spring and the five-leaf

stage remained relatively constant at about 250

mm, but those under a stubble crop, with 40 mm

lower spring soil water reserves, increased

slightly until about the three-leaf stage. During

the period of expansive crop growth (from the

five-leaf to the soft dough stage) soil water was

rapidly lost from all cropped phases at rates of

1.87 mm·day–1 for F-(W) ( N+P), 1.23 mm· day –1 for

Cont W (N+P) and 1.17 m m·day –1 for Con t W (+P).

The initial loss was from the 0 - 0.3 m depth, but

during the la tter half of the gr o wing season from

deeper depths, although rarely from the 0.9 - 1.2

m depth. In very dry years (e.g., 1973, with 87

mm precipitation between spring and fall)

summer fallow treatments lost water. In wet

years with poor precipitation distribution (e.g.,

1970, with 287 mm precipitation between spring

and fall but 142 mm of this in one week between

the three- and five-leaf stages) even cropped

treatments showed evidence of leaching. The

above-ground biomass water use efficiency for

Cont W was 19.2 and 16.7 kg·ha–1·mm–1, respec-

tively, for crops receiving (N+P) and P fertilizer

only. Grain yield water use efficiency (8.91

kg·ha–1·mm–1) was not significantly influenced

by cropping frequency or N fertilizer. The 18

years of detailed measurements of plant and

soil parameters under various crop manage-

ment systems provide an invaluable source of

information for developing and testing simula-

tion models.

Keywords: Fallow Frequency; Water Use; Plant

Biomass; Spring Wheat; Soil Water

1. INTRODUCTION

In the semiarid region of the northern Great Plains

(i.e., the Brown Chernozemic soil zone), water is the

main factor influencing wheat (Triticum aestivum L.)

production [1,2]. Potential evapotranspiration (PET)

always exceeds precipitation (PPT) during the growing

season, and consequently farmers in this area often prac-

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

221221

tice summer fallowing (F) to conserve water for the next

crop. This practice also serves to control weeds and to

increase mineral nitrogen (N) in the soil, thereby helping

to alleviate the second greatest restriction to production

in this region (i.e., soil N deficiency) [1]. Although the

area devoted to summer fallow on the Canadian prairies

has been decreasing steadily in recent years, the rate of

decrease is lowest in the Brown soil zone (1.3% yr–1) [3];

thus, summer fallowing is still an important management

tool in this region.

The efficiency of soil water storage from precipitation

in the prairies is generally low and varies greatly de-

pending on soil texture, type of cultural practice, the

amount of crop residues left standing to trap snow, and

the amount and distribution of precipitation received in

the non-crop-growing period [4,5]. We recently assessed

the effects of cropping frequency and N fertilization on

soil water conservation based on the results from a 40-yr

experiment conducted on a medium-textured Orthic Brown

Chernozem (Aridic Haploboroll) at Swift Current, Sas-

katchewan [6]. We examined three treatments: continu-

ous wheat (Cont W) and fallow-wheat (F-W) each re-

ceiving N and phosphorus (P) fertilizer, and Cont W

receiving only P fertilizer. The results showed that at

harvest, F-W and Cont W (N+P) had similar amounts of

water in the soil profile, but Cont W (+P) had more be-

cause of less growth and reduced water use. However by

the following spring, soil water recharge, being propor-

tional to the amount of crop residues produced, had

conserved an extra 64, 55 and 40 mm of water in treat-

ments F-W, Cont W (N+P) and Cont W (+P), respec-

tively [6].

Numerous studies (see review articles [7,8]) have

been conducted in which the relationships between crop

yield and water use on the prairies have been assessed.

The effects of fertilization rates [9-11], tillage practices

[12-14], cropping frequency [15,16] and soil- and weather-

conditions [17] on water use efficiency have been docu-

mented extensively. However, less is known about the

seasonal changes in crop water use and in water use effi-

ciency of the plant biomass.

Agro-ecosystem models are being increasingly used

for site specific analyses and the development of site

adapted agricultural production systems [18,19]. On a

regional and/or national scale these models are used for

the evaluation of current land use and potential remedia-

tion measures through scenario simulations [20-22].

However, testing and validation of the models should be

done using independently measured data from field ex-

periments such as those described in this manuscript.

During the first 18 years (1967-1984) of the Swift Cur-

rent long-term crop rotation experiment, detailed meas-

urements of soil water and plant biomass were made,

with eight samplings between spring and fall on selected

treatments. Although some assessments of these data

were made [23,24], seasonal changes in soil water con-

tent and its distribution within the profile were not ex-

amined.

The objectives of this paper were to determine the ef-

fect of cropping frequency and N fertilizer on 1) soil

water trends and its depth distribution from spring to fall,

and to assess how these patterns were influenced by

weather conditions, and 2) the efficiency of water use to

produce above-ground biomass and grain of spring

wheat. Furthermore, we want to alert agricultural system

modellers to the unique nature of these long term ex-

perimental results.

2. MATERIALS AND METHODS

The Swift Current crop rotation experiment was initi-

ated in 1967 on a flat (slope < 2%) Swinton loam [25],

an Orthic Brown Chernozem [26]. Swift Current (50˚17′

N, 107˚48′ W, elevation 883 m) is located in the driest

portion of the Canadian Prairies, with long cold winters

and short growing seasons [27]. The soils are frozen

from mid October until March/April. Rainfall is mar-

ginal for many agricultural activities (on average 197

mm is received during the growing season) and timing

of rainfall is as important as total amount. Over the 18-yr

study period, the annual average precipitation was 324

mm, of which 108 mm was in the form of snow. A sub-

stantial proportion of the latter can disappear as sublim-

tion, snow blowoff and, after melting, as surface runoff.

No runoff from rainfall was observed during the growing

season. The 18-yr mean PET (calculation method de-

scribed in Section 2.2 below) was 661 mm.

The rotation experiment consisted of 12 cropping sys-

tems, of which we discuss five of the special plots. Spe-

cial plots (0.04 ha each) were sampled for soil water and

above-ground plant dry matter eight times between

spring and fall of each year between 1967 and 1984 [28,

29]. The experiment has been described in numerous

publications [23,24,30,31], thus we only present infor-

mation required to assess the factors examined.

The treatments examined were summer fallow-wheat

F-(W), summer fallow-wheat-wheat-F-W-(W), and con-

tinuous wheat (Cont W) each receiving N and P fertilizer,

and (F)-W-W and (Cont W) each receiving only P fertil-

izer. The rotation phase shown in parentheses was the

special plot treatment. Fertilizer N and P were applied in

accordance with the soil NO3-N (0 - 0.6 m depth) and

soil P (0 - 0.15 m depth) levels in individual plots,

measured the previous fall (mid-October) [30]. Fertilizer

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

222

N, as ammonium nitrate, was applied by broadcasting it in

spring prior to seedbed preparation. We used N rates

recommended by the soil testing laboratory at the Uni-

versity of Saskatchewan [32]. Wheat grown on summer

fallow received about 9 kg·N·ha–1·yr–1 and wheat grown

on stubble received an average of 30 kg·N·ha–1·yr–1 during

the 18-yr period. Phosphorus fertilizer (monoammonium

phosphate) was applied with the seed, with the desig-

nated treatments receiving 9 to 10 kg·N·ha–1·yr–1 in ac-

cordance with the general recommendations for the area

and crop [33]. Nitrogen in the P source was accounted

for in the N rates. All phases of each rotation were pre-

sent each year. There were three replicates.

Full-sized farm equipment was used for all field op-

erations. Tillage on summer fallow was performed two

to five times with a heavy-duty cultivator and/or rod

weeder. Late fall application of 2,4-D ester to control

broadleaf weeds was customary and other herbicides

were applied to cropped areas as required [31]. No fall

tillage was performed on the plots. The field manage-

ment operations (i.e., seedbed preparation, in-crop her-

bicide application, seeding, harvesting, and tillage of

summer fallow areas) were generally similar to those

discussed previously [34].

Seeding dates ranged from May 3 to June 5 (average

May 17) and harvest dates ranged from August 16 to

September 23 (average September 1).

2.1. Plant and Soil Sampling

At the three-leaf, five-leaf, shot blade (also known as

flag leaf complete) and soft dough growth stages, which

on the Feekes scale [35] correspond approximately to

stages 1, 2, 10 and 10.5.3, respectively, and at harvest,

plant samples were taken from two plant rows, each 3 m

long and located 1 m from the edge of the plot (sampled

area = 1.115 m2). Plant samples were dried at 70˚C and

the mass of the above-ground parts determined. The soil

was sampled at the aforementioned growth stages, as

well as prior to spring seeding in early May, at plant

emergence, at harvest, and in the fall, just prior to freeze-

up. The fallow plots of rotation (F)-W-W (+P) were

sampled at the same time as the cropped plots and

therefore its sampling times will also be referred to as

spring, emergence, three-leaf, etc. Over the 18-yr period

most sampling stages occurred within a 2- to 4-wk range.

Soil samples were taken (three cores per plot were

bulked) from the 0 - 0.15, 0.15 - 0.3, 0.3 - 0.6, 0.6 - 0.9

and 0.9 - 1.2 m depths. These samples were analyzed for

gravimetric soil water content, which was converted to

volumetric units using measured bulk densities of 1.20,

1.22, 1.26, 1.49 and 1.67 Mg·m–3 for the five depths,

respectively [28]. The lower limits of available water, i.e.

the permanent wilting point, as determined from field

measurements [36] were 27, 28, 28, and 31 mm for the 0

- 0.3, 0.3 - 0.6, 0.6 - 0.9 and 0.9 - 1.2 m depths respec-

tively, for a total of 114 mm for the 1.2-m depth. Water

contents at –0.03 MPa (i.e., field capacity) were 88, 87,

97 and 109 mm for the same depth intervals, for a total

of 381 mm for the 1.2-m profile [37].

2.2. Weather Data

Daily maximum and minimum air temperatures and

precipitation were measured at a meteorological site

located 1 km west of the experimental site. Potential

evapotranspiration was estimated from a regression

equation relating latent evaporation (i.e., evaporation

from Bellani plate atmometers) to meteorological infor-

mation [38]. Although equations using a number of

weather elements were developed, the most common one,

using daily maximum and minimum air temperature data,

was used:

LE = 0.928 Tmax + 0.933 Trange + 0.0486 Qo – 87.03

where, LE is latent evaporation from a Bellani plate sur-

face (cm3·day–1), Tmax is daily maximum temperature

(˚F), Trange is the difference between daily maximum

and daily minimum temperature (˚F), and Qo is the daily

amount of solar radiation at the top of the atmosphere

(cal·cm–2). The latter can be calculated for a given lati-

tude and day of the year. LE is multiplied by a factor of

0.096 to obtain PET in mm·day–1 [39]. The empirical

Baier-Robertson equation, which has been calibrated and

validated for Canadian conditions, was chosen over the

more physically based Penman-Monteith formulation,

because net radiation, windspeed and humidity data were

not continuously available during the 18-yr study period.

2.3. Data Anal ysis

Grain yields and above-ground plant biomass at the

various stages of growth were related by regression

analysis to water use (WU) and relative water use, WU/

PET. Water use was defined as: (spring soil water - soil

water at a later stage) + precipitation received during

that period.

3. RESULTS

3.1. Soil Water under Summer Fallow

3.1.1. Eighteen-yr Mean Soil Water Contents

Producers use summer fallow to conserve extra water

for the next crop. The main portion of the 20-mo fallow

eriod in which water is conserved in the semiarid prai- p

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

223223

Ta b le 1 . 18-yr (1967-1984) mean soil water (0 - 1.2 m depth), above-ground dry matter, and precipitation (PPT) received between

sampling times in five crop rotation phases at Swift Current, Saskatchewana.

Rotation Phase Sampled

(F)-W-W(+P) F-(W) (N+P) F-W-(W) (N+P) Cont W (N+P) Cont W (+P)

Average

calendar

day

Stage

sampled Soil water

(mm)

Soil water

(mm)

Dry matter

(kg·ha–1)

Soil water

(mm)

Dry matter

(kg·ha–1)

Soil water

(mm)

Dry matter

(kg·ha–1)

Soil water

(mm)

Dry matter

(kg·ha–1) Period Period PPT

(mm)

123 Spring

(Sp) 213 251 - 211 - 210 - 203 -

Sp to

Em 29.2

151 Emergence

(Em) 217 248 - 218 - 211 - 210 -

Em to

3L 17.5

162 Three-leaf

(3L) 222 252 90 217 87 214 87 211 78

3L to

5L 29.8

177 Five-leaf

(5L) 225 246 503 214 422 211 424 213 365

5L to

SB 28.1

188 Shot blade

(SB) 224 215 1792 192 1343 190 1428 188 1176

SB to

Do 52.1

224 Soft dough

(Do) 230 158 5130 157 3808 152 3788 158 3102

Do to

Ha 11.1

238 Harvest

(Ha) 233 154 4957 150 3706 150 3463 154 2877

Ha to

Fa 40.9

290 Fall (Fa) 238 161 - 160 - 160 - 162 -

Sp to

Fa 208.7

aSoil water and dry matter are for rotation phases shown in parentheses.

rie is the 5.5-mo period from May to mid-October [5].

Thus we observed an 18-yr average gain in soil water in

the 0 - 1.2 m depth of 25 mm (i.e., 0.15 mm·day–1) dur-

ing this period (Table 1). On average, the 0 - 0.3 m

depth starts in spring close to 70% of field capacity with

a little over 60 mm of water stored from the first eight

months of the fallow period (Figure 1). Between spring

and fall, the water content in this segment stayed fairly

constant as gains from precipitation were balanced by

evaporation and drainage losses. Water in the 0.3 - 0.6 m

depth at the spring sampling time was slightly over 50

mm and the amount gradually increased to as much as

that in the 0 - 0.3 m depth by the shot blade stage (Cal-

endar day 188), and thereafter remained constant at

slightly over 60 mm (like in the 0 - 0.3 m depth) until

fall. Water in the 0.6 - 0.9 m and 0.9 - 1.2 m depths was

about the same (45 mm) at the spring sampling. The

water contents in these two depths increased gradually

until the end of August, when it reached about 55 mm,

i.e., slightly above 50% of field capacity.

3.1.2. Soil W ater Con tents under Summer Fallow

during Selected Years

As might be expected, the amount of water stored and

the pattern of gain over time were quite variable, de-

pending primarily on the rainfall frequency, the amount

years (1970 and 1982) (Figure 2).

In 1968, 177 mm precipitation that was well distrib-

uted throughout the season was received between spring

and fall (Figure 2(a)). The PET during this period was

calculated to be 579 mm, and as a result the soil profile

only gained 25 mm of water, or 14.1% of the precipita-

tion. The early increases of about 28 mm of water in the

0.3 - 0.9 m depth between spring and emergence was

received, and also on the temperature and wind which

control evaporation in the southern prairies. We present

examples of trends in water storage under fallow [(F)-W-

W (+P)], in two dry years (1968 and 1973) and two wet

peculiar because there was little precipitation received in

this period (15 mm). We suspect that this anomaly might

be due to spatial variability because the coefficient of

variation of the sampled spring soil water content was

high, i.e., 39%. From emergence to fall, the soil water

contents of all depths remained fairly constant, except

between harvest and fall when the 0 - 0.6 m depth re-

flected the substantial precipitation (91 mm) received

during this period.

With only 87 mm precipitation and PET totalling 626

mm between spring and fall, 1973 represented a very dry

year (Figure 2(b)). The 0 - 0.3 and 0.3 - 0.6 m depths

gradually lost water throughout the spring to harvest

period, while the water content in the 0.6 - 0.9 m depth

remained fairly constant. On the other hand, the 0.9 - 1.2

m depth gained 12 mm water from spring to the five-leaf

stage, and then remained relatively constant. Thus there

was evidence of water moving from upper to lower

depths prior to the five-leaf stage. Overall, 21 mm of

water was lost from the soil profile under summer fallow

conditions due to evaporation exceeding the well dis-

tributed, but small occurrences of precipitation events in

this dry year.

During the very wet years of 1970 and 1982, when

summer fallow started in the spring with a modest

amount of water in the profile (220 mm and 170 mm,

respectively), large amounts of precipitation received in

ay and/or June, before evaporative demands were very M

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

224

Figure 1. 18-yr mean trend in soil water content under summer fallow [(F)-W-W (+P)] as a function of time of sam-

pling, precipitation received during the sampling periods, and depth (successive sampling times are spring, emergence,

three-leaf, five-leaf, shot blade, soft dough, harvest and fall).

high, resulted in marked gains in soil water, i.e., 53 mm,

or 18.5% of the precipitation, in 1970 and 105 mm, or

31.6% of the precipitation in 1982 (Figures 2(c)-(d)).

About half (142 mm) of the total precipitation received

in 1970 fell in a one week period between the three- and

five- leaf stage, causing wetting of all depths to 1.2 m

and possible deeper. After the five-leaf stage small rain-

fall events were balanced by evaporation and thus, bar-

ring the anomalous water contents in the 0.9 - 1.2 m

depth at the soft dough stage and the one in the 0 - 0.3 m

depth at harvest, the soil water contents of all depths

remained constant till fall. In 1982, when the precipita-

tion was better distributed throughout the season than in

1970, soil water contents in the upper two depths

reached almost field capacity at the five-leaf stage and

stayed constant and high till harvest before increasing

slightly till fall. The steady gains in water in the 0.6 - 1.2

m depth were 84% of those in the 0 - 0.6 m depth.

Water draining beyond the 1.2 m depth during wet

years has also been demonstrated in several soil water

simulation studies [37,40,41]. Thus, the amount of water

conserved during the summer months under summer

fallow depends on the complex combination of amount

and time-distribution of precipitation received, the initial

soil water content in spring, and the evaporative de-

mands during the season.

3.2. Soil Water under a Crop

3.2.1. Eighteen-yr Mean Soil Water Contents for

0 - 1.2 m Dept h

Soil water conditions under cropped systems in the

semiarid prairies reflect the net balance between pre-

cipitation received and water lost via evaporation and

transpiration. Drainage through the root zone should be

rare except when very wet conditions occur in the early

part of the growing season, before the crop is well estab-

lished. The 18-yr mean amount of soil water in 0 - 1.2 m

depth under wheat being grown on fallow [F-(W) (N+P)]

was approximately 250 mm from spring to the five-leaf

stage; thereafter, soil water decreased rapidly to 158 mm

at the soft dough stage (i.e., at a rate of 1.87 mm·day–1)

(Table 1). Between harvest and fall there was an average

gain of about 7 mm of water in response to an average

precipitation of 41 mm received in this 52-day period.

The 18-yr mean soil water content in the 0 - 1.2 m

depth at spring sampling under well-fertilized stubble

rop wheat systems [e.g. F-W-(W) (N+P) and Cont W c

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

225225

Figure 2. Examples of soil water contents under summer fallow [(F)-W-W (+P)] as a function of time of sampling, daily

precipitation, and depth for four selected years (values in brackets are precipitation totals between spring and fall).

(N+P)] was 210 mm (Table 1). Generally, these drier

systems gained a few mm of water by the three-leaf

stage, as gains from precipitation exceeded losses due to

low evapotranspiration rates related to minimal dry mat-

ter production and generally cool spring conditions. Here

too, as was the case for wheat grown on fallow [F-(W)

(N+P)], soil water in the profile decreased rapidly be-

tween the five-leaf and soft dough stage to about 155

mm in 1.2 m depth (at about 1.23 mm·day–1). Between

harvest and fall these well fertilized, stubble cropped

systems then gained on average 10 mm of water from

the 41 mm of precipitation received in this period.

As discussed by [6], Cont W (+P), because it had less

standing stubble than Cont W (N+P), trapped less snow

over winter; thus in spring, on average, it started with

about 7 mm less water in the profile (Table 1). Like

well-fertilized systems of wheat grown on stubble, Cont

W (+P) gained a small amount of water (10 mm) be-

tween the spring sampling and the five-leaf stage, then,

like the other cropped systems, it lost water rapidly at a

rate of 1.17 mm·day–1 until the soft dough stage, before

regaining about 8 mm between harvest and fall. Note

that the rates of decrease in soil water during the period

of rapid growth (i.e. the five-leaf to soft dough stage)

were proportional to the rate of above-ground dry matter

production (Table 1). This suggests that transpiration

was mainly responsible for the water loss in that period.

3.2.2. Eighteen-yr Mean Soil Water Contents at

Individ ual Depths

The 18-yr mean soil water content in the 0 - 0.3 m and

0.3 - 0.6 m depths at spring sampling under wheat being

grown on summer fallow [F-(W) (N+P)] was about 64

mm (Figure 3(a)). Soil water remained almost constant

till the three-leaf stage (five-leaf stage for the 0.3 - 0.6 m

depth), then decreased sharply to about 35 mm at the

soft dough stage as evapotranspiration markedly ex-

ceeded precipitation. The soil water contents then re-

mained constant for a short time until harvest. Between

harvest and fall (with no transpiration) the water content

of the 0 - 0.3 m depth was recharged with precipitation

to reach about 46 mm of water by the fall sampling. The

water content of the second depth remained constant till

fall, as excess precipitation between harvest and fall was

insufficient to wet the soil beyond the 0.3 m depth. Both

the 0.6 - 0.9 m and 0.9 - 1.2 m depths had about 59 mm

of soil water at the spring sampling, and water levels in

these two depths remained almost constant till the five-

leaf stage before decreasing slightly in both depths till

the shot blade stage. Thereafter, soil water in both of

these segments decreased sharply, though faster in the

.6 - 0.9 m depth, till harvest, and then remained con- 0

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

226

Figure 3. 18-yr mean trend in soil water content under spring wheat as a function of time of sampling, precipitation received

during the sampling periods, and depth (successive sampling times are spring, emergence, three-leaf, five-leaf, shot blade, soft

dough, harvest and fall).

stant till fall when the 0.6 - 0.9 m depth had about 38

mm and the 0.9 - 1.2 m depth had about 46 mm of water.

Thus, in the F-(W) (N+P) system most of the water

was used between the five-leaf and soft dough stage, and

mainly from the top 0.6 m of soil. This was related to the

period of greatest growth (Ta b l e 2 ) and evapotranspira-

tion. Water was used more slowly from the 0.6 - 0.9 m

depth and even more slowly from the 0.9 - 1.2 m depth.

After harvest there was generally only sufficient extra

water to recharge the 0 - 0.3 m depth by the fall.

Soil water distribution with depth and changes from

spring to fall were similar for the stubble crop systems

F-W-(W) (N+P) and Cont W (N+P) (Figures 3(b)-(c));

therefore, we will only discuss the results for Cont W

(N+P). Soil water content and response in the 0 - 0.3 m

depth between spring and fall in this system were almost

identical to that of a summer fallow crop system F-(W)

(N+P) (Figure 3(a)). The stubble crop system, with

much less time for soil water to be recharged (8 mo vs

20 mo), had only about 51 mm of water in the 0.3 - 0.6

m depth in spring. There was sufficient precipitation and

maybe some drainage from the 0 - 0.3 m depth to

slightly increase the soil water content in the 0 - 0.6 m

depth to about 55 mm by the five-leaf stage. From the

five-leaf to the soft dough stage, soil water in both the 0

- 0.3 and 0.3 - 0.6 m depths decreased sharply and line-

arly at the same rate to about 35 mm, which is similar to

that for the F-(W) (N+P) system. Thereafter, as for F-(W)

(N+P), soil water in the 0 - 0.3 m depth increased, and

that of the 0.3 - 0.6 m depth remained constant, until fall.

Soil water in the 0.6 - 0.9 m and 0.9 - 1.2 m depths, like

in the 0.3 - 0.6 m depth, started the spring with much

less water than in F-(W) (N+P). Although this pattern of

water loss from these two lower depths mimicked those

for the respective depths in F-(W) (N+P), the lower rate

of water use by the lower-yielding stubble crop com-

pared to the fallow crop (Table 2), resulted in less water

being taken-up from these two depths under the stubble

crops. Consequently, the soil water contents in these

lower depths were generally similar in Cont W (N+P), F-

W-(W) (N+P) and F-(W) (N+P) by fall.

Soil water content in the 0 - 0.3 m depth under Cont

W receiving only P [(Cont W (+P)] (Figure 3(d)) was

similar in amounts and distribution with depth as Cont

W (N+P) from spring to fall. However, soil water in the

0.3 - 0.6 m depth under Cont W (+P) exceeded that in

the 0 - 0.3 m depth from spring to the five-leaf stage

constant at about 66 mm). This may be related to the (

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

227227

Tab le 2. Effect of cropping frequency and N fertilizer on above-ground dry matter accumulation at Swift Current, Saskatchewan

(1967-1984)a.

Dry matter (kg·ha–1)

Year Stage

F-(W) (N+P) F-W-(W) (N+P)Cont W (N+P)Cont W (+P)

PPTb (mm)

(Spring to harvest)

PETc (mm)

(Spring to harvest)

1967 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

161

606

1474

2984

3401

128

522

1329

2960

3834

111

566

1376

2489

3345

345

1032

2210

2687

54 450

1968 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

432

1072

2237

3044

101

432

917

1198

1243

121

502

1169

1306

1261

358

764

1346

1315

87 466

1969 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

101

355

911

2013

4379

104

348

897

2083

3992

108

318

917

2076

4302

101

237

794

2023

3694

135 464

1970 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

348

2060

5107

3410

311

1256

3633

3107

429

1742

4202

3310

452

1882

3473

2486

244 460

1971 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

419

3536

7059

5491

375

2267

3667

3405

476

2930

3490

3709

372

1762

3051

2976

124 522

1972 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

177

897

4082

3360

281

1075

4116

3730

214

884

3606

2337

224

961

3832

2379

155 536

1973 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

991

1799

3596

2888

532

1156

3064

2418

535

1178

3473

2588

465

991

2505

2251

64 503

1974 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

104

917

2019

4488

5435

104

717

1956

3231

3896

111

791

1979

3483

3778

114

724

1711

3366

3643

258 464

1975 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

111

1215

3128

5486

3489

101

853

2144

4357

3375

101

747

2164

4756

3870

687

1861

3985

3468

210 457

1976 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

134

515

3610

9417

8053

141

603

3074

6343

5563

148

556

3295

6772

5100

101

489

2368

5486

5030

223 479

1977 Three-leaf

Five-leaf

Shot blade

Soft dough

134

429

2063

8774

180

964

7090

221

1142

6480

207

1175

5419

227 551

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

228

Harvest

7719 5796 5337 4725

1978 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

261

680

3211

5602

626

1122

3014

3021

483

1089

2388

2892

355

754

1953

1902

135 510

1979 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

214

1018

4601

4565

161

995

2746

3521

177

1105

3543

3491

207

724

2270

2535

131 487

1980 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

321

840

5579

5184

174

442

3643

2827

108

399

2689

2418

148

442

1725

2027

189 550

1981 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

6284

4282

3670

2573

211 484

1982 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

416

1185

9353

8742

368

1105

6339

6166

355

1068

6627

4967

315

646

5392

4237

285 466

1983 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

375

890

5914

5787

375

512

5639

5220

150 441

1984 Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

124

436

2384

4082

3220

114

311

1239

1333

757

355

1333

1373

757

121

261

958

1604

990

113 494

Mean Three-leaf

Five-leaf

Shot blade

Soft dough

Harvest

503

1792

5130

4957

422

1343

3808

3706

424

1428

3788

3463

365

1176

3102

2877

167 488

aSome data, especially in 1981 and 1983, were missing; bPPT = precipitation; cPET = potential evapotranspiration.

reduced water uptake by a crop whose early growth is

partly restricted by inadequate nitrogen fertility [6]. The

pattern of water response in the 0.3 - 0.6 m depth under

Cont W (+P) was similar to that of the other cropped

systems. Soil water patterns and contents in the 0.6 - 0.9

m and 0.9 - 1.2 m depths under Cont W (+P) were gen-

erally similar to those under Cont W (N+P). There was

about 45 mm of water in each of these two depths be-

tween spring and the shot blade growth stage. There-

after, water was slowly and gradually lost from these

two depths till harvest, but from the 0.6 - 0.9 m depth

moreso than from the 0.9 - 1.2 m depth (Figure 3(d)).

No water gains occurred in these lower depths between

harvest and fall.

3.2.3. Soil Water Contents during Selected Years

In four selected years of varying precipitation, soil

water contents in the profile varied from the 18-yr mean

pattern (Figure 4). In the period prior to the five-leaf

stage and after harvest, the 0 - 1.2 m depth soil water

content was mainly related to precipitation. For example,

in 1970 the very wet month of June caused the soil water

content to increase from 261 mm at the spring sampling

to 308 mm at the five-leaf stage (Figure 4(a)). However,

in 1976, when spring precipitation was only moderate

see Figure 4(b)); the soil water content was fairly con- (

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

229229

Figure 4. Examples of trends in soil water content in the 0 - 1.2 m depth under spring wheat grown on fallow [F-(W) (N+P)]

for four typical years (values in brackets are the precipitation totals between spring and fall).

stant to the five-leaf stage (Figure 4(b)). Transpiration

during this early part of the growing season was gener-

ally low. Similarly, precipitation dictated the pattern of

soil water trends after the soft dough stage when the crop

was no longer withdrawing soil water. In dry years such

as 1971 and 1973 when little precipitation fell before the

five-leaf stage, the soil water content decreased rapidly

from early spring onwards (Figures 4(c)-(d)). In all sys-

tems the rapid growth between the five-leaf and soft

dough stages (Table 2) was accompanied by a rapid de-

crease of soil water, irrespective of the amount of pre-

cipitation received, as evapotranspiration far exceeded

precipitation.

Soil water distribution in the profile from spring to

fall under a crop, was similar under F-(W) (N+P) as un-

der Cont W (N+P), therefore we show only a couple of

examples for Cont W (N+P) for two dry years (1968 and

1973), and for two wet ones (1970 and 1982) (Figure 5).

In 1968, 177 mm of precipitation fell between spring and

fall, of which almost 90 mm was received outside the

growing season, between harvest and fall. (Figure 5(a)).

Most of the changes in soil water content occurred in the

0 - 0.3 m depth; after an initial slow decrease from

spring to the three-leaf stage, the water content rapidly

decreased to near the wilting point by the shot blade

stage, remained constant till the soft dough stage and

then increased till fall to about 60 mm in response to the

121 mm precipitation received in this late period. The

low spring soil water contents in the 0 - 0.3 m and 0.3 -

0.6 m depths reflect the dry conditions of the previous

year and the small amount of overwinter precipitation

(74 mm between fall and spring) that did not wet the soil

beyond the 0.3 m depth. All depths below 0.3 m gradu-

ally lost small amounts of water till the shot blade stage,

as water use by the crop slightly exceeded the rainfall

replenishment. The small amount of water uptake by the

crop (133 mm) was reflected in the low total dry matter

production [maximum at the soft dough stage, 1306 kg·ha–1

(Table 2)]. The increase in soil water content at the 0.6 -

0.9 m depth between the three- and five- leaf stage may

be a sampling error because there was no precipitation

during this period.

The year 1973 was even drier than 1968, with only 64

mm precipitation well-distributed between spring and

harvest (Figure 5(b)). The soil water content in the 0 -

0.3 m depth decreased almost linearly from spring to

harvest, and then increased slightly from harvest to fall

n response to the late small precipitation events. The i

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

230

Figure 5. Soil water distribution with depth (spring to fall) under stubble wheat [Cont W (N+P)] for two dry (1968 and 1973)

and two wet (1970 and 1982) years (values in brackets are the precipitation totals between spring and fall).

water content of the 0.3 - 0.6 m depth decreased slowly

from spring to the three-leaf stage, then rapidly to the

soft dough stage before levelling off at maturity. This

pattern reflects plant growth and crop water uptake. Be-

cause of low rainfall, the soil water content of the 0.6 -

0.9 m depth remained constant till the five-leaf stage,

reflecting the time it takes for roots to reach this depth.

Thereafter, water loss was linear to the soft dough stage

and more slowly till the fall sampling date. Generally,

between the five-leaf stage and harvest, water losses

from the 0.3 - 0.6 m depth and from the 0.6 - 0.9 m

depth were parallel to each other. In the 0.9 - 1.2 m

depth the soil water content remained relatively constant

from emergence to the shot blade stage, then decreased

slowly, but linearly to harvest and even more slowly till

fall. The marked loss of soil water from all four depths

from spring to harvest, a total of 194 mm, was reflected

in a dry matter production of 3473 kg·ha–1 at the soft

dough stage (Table 2), almost three times higher than in

1968.

The two wet years shown as examples differed in that

50% (142 mm) of the period precipitation received in

1970 occurred during a single week in June (Figure 5(c))

while the 332 mm of precipitation received in 1982 was

well-distributed throughout the period (Figure 5(d)).

Thus, plant dry matter production was much greater in

1982 than in 1970 [6627 kg·ha–1 vs 4202 kg·ha–1 at the

soft dough stage (Ta bl e 2 )] and presumably evapotran-

spiration would also have been much greater in 1982.

The balance between evapotranspiration and precipita-

tion favoured drainage in 1970 as was evidenced by a

rare occurrence under crop when all soil segments to 1.2

m depth (and probably beyond) showed an increase in

water content between the three- and five- leaf stage

(Figure 5(c)). All soil depths then lost water rapidly

between the five-leaf and soft dough stage in 1970, then

they remained constant, except for the 0 - 0.3 m depth

which gained water between harvest and fall, reflecting

the 40 mm of rainfall received in this period.

In 1982, the low spring soil water contents in the 0.3 -

1.2 m depth reflect the lower than normal fall soil water

contents in 1981 and the small amount of overwinter

precipitation (95 mm between fall and spring) that did

not wet the soil beyond the 0.3 m depth. Large amounts

of precipitation received prior to the five-leaf stage (177

mm) markedly increased the soil water content in the 0 -

0.6 m depth and to a lesser extent that in the 0.6 - 0.9 m

depth. However the 80 mm of precipitation received

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

231231

between the shot blade and soft dough stage was more

than counter-balanced by high evapotranspiration, and

consequently the 0 - 0.9 m depth lost significant amounts

of water (82 mm). Between the harvest and fall sampling,

when there is no transpiration, the 50 mm precipitation

received increased the water content in the 0 - 0.3 m

depth by 35 mm, with no wetting below this depth. The

soil water content in the 0.9 - 1.2 m depth remained rela-

tively constant throughout the growing season, suggest-

ing that sufficient water was available for good crop

growth in the upper three depths.

3.3. Dry Matter Production and Water Use

(WU)

We calculated WU and total above-ground dry matter

production between spring and each sampling time for

Cont W (N+P) and Cont W (+P), in order to assess the

influence of N fertilizer on this parameter (Ta b l e 3 and

Figure 6). A regression of dry matter accumulated to

each growth stage vs WU was linear for both treatments

(Figure 6). The y-intercepts were not significantly dif-

ferent (P < 0.05), but the regression slopes were signifi-

Table 3. Effect of N fertilizer on relationship between total dry matter (TDM) accumulation versus water use (WU), potential

evapotranspiration (PET) and relative water use (WU/PET) at Swift Current, Saskatchewan. The values are 18-yr means ρ standard

deviation.

Change in soil water

(0 - 1.2 m depth)

Precipitation

(Spring to period)

WUa

(Spring to period)

PET

(Spring to period)

Period Treatment (Spring to period) WU/PET TDM

(mm) (kg·ha–1)

Spring to three-leaf Cont W (N+P)

Cont W (+P)

–1 ρ22

–5 ρ21

48 ρ35

47 ρ26

43 ρ28

143 ρ24

0.329 ρ0.157

0.301 ρ0.178

87.0 ρ29.2

78.3 ρ24.6

Spring to five-leaf Cont W (N+P)

Cont W (+P)

–2 ρ34

–11 ρ29

75 ρ48

73 ρ33

64 ρ34

199 ρ24

0.367 ρ0.176

0.322 ρ0.179

424 ρ189

365 ρ166

Spring to shot blade

Cont W (N+P)

Cont W (+P)

20 ρ37

14 ρ34

103 ρ52

123 ρ39

117 ρ37

264 ρ23

0.467 ρ0.141

0.443 ρ0.141

1428 ρ780

1176 ρ559

Spring to soft dough Cont W (N+ P)

Cont W (+P)

58 ρ30

44 ρ28

155 ρ68

213 ρ57

199 ρ51

429 ρ34

0.497 ρ0.132

0.464 ρ0.119

3788 ρ1748

3102 ρ1399

Spring to harvest Cont W (N+P)

Cont W (+P)

60 ρ31

48 ρ28

166 ρ67

226 ρ55

215 ρ52

488 ρ34

0.463 ρ0.126

0.440 ρ0.115

3463 ρ1289

2877 ρ1124

aWU = (Increase in soil water between spring and a later period) + precipitation received in the period.

Figure 6. Effect of N fertilizer on the relationship between dry matter accumulation and water use.

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

232

cantly different at P < 0.10. The equations indicated

early season evaporation (WU intercept) was 39 mm for

Cont W (N+P) and 36 mm for Cont W (+P). The later

season evapotranspiration (WU) efficiency (slope of the

regression line) was 19.2 kg·ha–1·mm–1 for Cont W (N+

P) compared to 16.7 kg·ha–1 ·mm–1 for Cont W (+P).

These slopes are much lower than the 33.0 kg·ha–1·mm–1

slope reported by [42] for winter triticale (xTriticosecale

Wittmack) grown in Colorado, but similar to values 21.3

and 18.9 kg·ha–1·mm–1 reported in [43] from a study

done by [44] which showed a positive effect of P fertil-

izer on water use efficiency of wheat. They observed the

greatest difference in evapotranpiration at any growth

stage to be 12 mm for P treatments, while we observed a

mean maximum difference in WU of 14 mm during the

spring to soft dough stage (Table 3). The early-season

evaporation in the [44] study was 32 mm, while in our

study it averaged 38 mm. We also determined the rela-

tionship between total dry matter produced and WU

relative to potential evapotranspiration (WU/PET) (Ta-

ble 3), but found no significant effect of N fertilizer in

this case.

3.4. Grain Yield, Water Use and Relative

Water Use

The annual grain yields, water use and relative water use

(WU/PET) for each treatment (Table 4) were used to

construct regression equations (Figure 7). The relation-

ships between wheat yield and water use were signifi-

cant (P < 0.05) for all treatments (Figure 7(a)). The wa-

ter use efficiency (i.e., the slope of the regression line)

was slightly greater for well-fertilized wheat grown on

fallow [F-(W) (N+P)] than for well-fertilized wheat

grown on stubble [F-W-(W) (N+P) and Cont W (N+P)],

and the latter slightly greater than for stubble wheat

without N fertilizer [Cont W (+P)]. However, the regres-

sions were not significantly different (P < 0.05), and

therefore we pooled the data and derived a single regres-

sion: Y = 8.91X – 648 (Figure 7(a)). This equation sug-

gests a yield increase of nearly 9 kg·ha–1·mm–1 water

used, and 73 mm of evapotranspiration required before

the first kg·ha–1 of grain is produced in this semiarid

region. These values are generally similar to those re-

ported previously by [24,45,46] for systems in southern

Saskatchewan and Alberta, but lower than the slope

(12.49 kg·ha–1·mm–1) and intercept (132 mm) reported

by [47] for winter wheat in Colorado. A regression of

yield against water use normalized per unit of PET did

not improve the relationship compared to that with water

use alone (Figure 7(b)).

4. DISCUSSION

This rotation study has been ongoing for 40 yr, during

which the first 18 yr discussed in this paper were much

drier than the subsequent 22 years [6]. Thus, it is not

surprising to find that the 18-yr mean amount of water

conserved under summer fallow during the 5.5 mo.

summer period (25 mm) was 16% less than the 31 mm

40-yr average observed for this period by [6]. Annual

analysis of soil water conserved under summer fallow

throughout the summer months showed that the pattern

of soil water accumulation throughout the soil profile

Ta bl e 4 . Growing season precipitation (GSP), potential evapotranspiration (PET), water use (WU)a, relative water use (WU/PET)

and grain yields - effect of cropping frequency and N fertilizer at Swift Current, Saskatchewan (1967-1984)b.

Cont W (N+P) Cont W (+P) F-W-(W) (N+P) F-(W) (N+P)

Year GSP

(mm)

PET

(mm) Yield

(kg·ha–1)

(mm) WU/PET Yield

(kg·ha–1)

(mm) WU/PET Yield

(kg·ha–1)

(mm)WU/PET Yield

(kg·ha–1)

(mm) WU/PET

1967 54 450 1017 147 0.33 820 172 0.38 1139 158 0.35 987 171 0.38

1968 87 466 468 134 0.29 554 124 0.27 429 123 0.26 1264 179 0.38

1969 135 464 1070 177 0.38 1145 166 0.36 1091 181 0.39 1005 216 0.47

1970 244 460 1100 302 0.66 974 278 0.60 1160 296 0.64 1306 361 0.78

1971 124 522 1279 202 0.39 1109 172 0.33 1148 178 0.34 1842 251 0.48

1972 55 536 1038 207 0.39 983 207 0.39 1381 229 0.43 1342 221 0.41

1973 64 503 918 194 0.39 811 162 0.32 802 180 0.36 978 192 0.32

1974 258 464 1413 301 0.65 422 264 0.57 1419 297 0.64 1920 330 0.71

1975 210 457 1750 251 0.55 1636 263 0.58 1607 266 0.58 1539 280 0.61

1976 223 479 1652 273 0.57 1896 269 0.56 2004 300 0.62 2668 322 0.67

1977 227 551 1995 255 0.46 2060 263 0.48 2126 262 0.48 2752 326 0.59

1978 135 510 1017 212 0.42 754 200 0.39 1163 241 0.47 2060 253 0.50

1979 131 487 1529 212 0.44 1285 182 0.37 1618 233 0.48 1923 248 0.51

1980 189 550 960 212 0.38 9412 211 0.38 1279 202 0.37 2013 274 0.50

1981 211 484 1491 241 0.50 1160 253 0.52 1107 250 0.52 2140 297 0.61

1982 285 466 2087 321 0.69 1618 298 0.64 2275 317 0.68 2707 337 0.72

1983 150 441 1804 268 0.61 - - - 1929 241 0.55 - - -

1984 113 494 265 158 0.32 411 171 0.35 2412 147 0.30 1216 227 0.46

Mean

SDc

166

488

1270

492

226

0.47

0.13

1151

456

215

0.44

0.12

1328

543

228

0.47

0.13

1745

603

264

0.54

0.13

aWU = [spring soil water - harvest soil water (0 - 1.2 m depth)] + GSP; bData missing for CONT W (+P) and F-(W) (N+P) in 1983; cρ = standard deviation.

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

233233

Figure 7. Relationship between grain yield and (a) water use and (b) relative water use for wheat grown on fallow or stubble

receiving N and P or P only.

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

234

was related to the initial water content of the soil and the

balance between the amount of precipitation received, its

distribution over time, and the strength of the evapora-

tive demands. The patterns of soil water accumulation

and drying occur first in the uppermost depth and gradu-

ally moved later to lower depths, as is assumed in soil

water simulation models [41,48,49].

Spring soil water contents under wheat grown on fal-

low were 40 mm higher than under stubble crops, and

while water remained constant till the five-leaf stage for

the former, the stubble crop, with lower soil water re-

serves in spring, gained a small amount of water till

about the three-leaf stage due to low transpiration rates

(caused by minimal crop growth) and low evaporation

rates (as a result of low temperatures).

Soil water changes under cropped systems mainly

showed the influence of precipitation during the periods

prior to and after the expansive growth of the crop had

occurred (five-leaf to soft dough stages). During the lat-

ter period soil water was rapidly lost in all cropped sys-

tems despite precipitation. The rate of water loss in this

period was greater for F-(W) (N + P) (1.87 mm·day–1)

than for Cont W (N + P) (1.23 mm·day–1) which was

greater than for Cont W (+P) (1.17 mm·day–1).

Under a crop, soil water is used progressively from

the uppermost depths, but rarely from the 0.09 - 1.2 m

depth, which suggests that deeper rooting crops like

winter wheat and/or safflower may be needed in the ro-

tation to extract more plant available water. Water re-

charge between harvest and fall primarily occurs in the 0

- 0.3 m depth. On one rare occurrence when 142 mm of

precipitation was received in early June, 1970, there was

evidence that drainage beyond 1.2 m may have occurred

under a cropped situation. This supports our findings

that there has been little NO3-N leaching from Cont W

after 37 yr in this study [50].

Regression analysis showed that withholding N from

Cont W was reflected in a reduced rate of above-ground

plant biomass production. This supports similar findings

by [44] regarding the influence of P on wheat production.

Regression analysis also showed that grain yields were

directly related to water use, with rates not significantly

influenced by cropping frequency nor fertilizer applica-

tions, although there was a tendency for a greater water

use efficiency for wheat grown on fallow than on stubble,

and for well-fertilized Cont W than for Cont W receiving

only P fertilizer.

In addition to the soil and plant parameters described

in this paper, measurements and analyses are available

for soil NO3-N [50,51] and bicarbonate extractable P [52,

53] at the same depths and with the same frequency as

the soil water content measurements. Analysis of total

organic N and C in the upper two soil depths were made

at irregular time intervals [54]. N and P concentrations in

the above-ground biomass (grain and straw) were meas-

ured at different phenological growth stages, for both

combine- and hand-harvested data [31,55]. The meas-

ured daily meteorological parameters include precipita-

tion, maximum and minimum temperatures, global ra-

diation and class A pan evaporation. Windspeed and

relative humidity data may be obtained from a nearby

weather station, located approximately 5 km NE of the

experimental plots. Soil and crop management informa-

tion on seedbed preparation, seeding, fertilizer and her-

bicide application, tillage operations and harvesting are

also available [28,56].

While several investigators [40,41,57-60] have used

these long term rotation data for testing and validating

their models, there is a wealth of information left to be

explored. For example, the measured P data have not

been used in any modelling exercise. We know of no

similar data sets where such detailed measurements have

been made for such a lengthy period. Soil and crop mod-

ellers are therefore invited to have a look at these in-

valuable data sets, and start using them.

5. SUMMARY

The effects of cropping frequency and N fertilization

on trends in soil water content and water use were

quantified using a long-term (18-yr) field experiment in

which multiple samplings were made each year. The

main findings of this study were:

1) In most years precipitation increased stored soil

water during non-cropping periods, (i.e. overwinter and

during summer fallow), wetting surface soil layers first

and then the lower layers. However, in very dry years

summer fallow treatments actually lost soil water.

2) A growing spring wheat crop used stored soil water

first from the surface layers and then gradually over time

from lower depths. Rarely was water extracted from

below 90 cm.

3) Soil water distribution with depth and over time

was different in dry years compared to wet ones.

4) Nitrogen fertilization improved (i.e., increased) the

slope of the water use/dry matter function.

5) The water use/grain production function was similar

to those previously reported for spring wheat in Saska-

tchewan and Alberta; it did not vary with cropping

intensity, nor with N fertilization.

6) The 18 years of detailed measurements of soil and

plant parameters under various cropping systems provide

researchers with a unique and invaluable source of

information for developing and testing soil-crop-mana-

gement simulation models. A copy of the data, including

daily weather data, can be obtained from Dr. R. P.

Zentner, Semiarid Prairie Agricultural Research Centre,

R. de Jong et al. / Agricultural Science 2 (2011) 220-237

235235

Agriculture and Agri-Food Canada, Swift Current, SK,

S9H 3X2, Canada (zentnerr@agr.gc.ca).

6. ACKNOWLEDGEMENTS

The authors acknowledge the technical assistance of staff at the

Semiarid Prairie Agricultural Research Centre in Swift Current for

maintaining the long-term crop rotation experiment and wish to thank

Valerie Kirkwood for stenographic assistance.

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