Vol.2, No.3, 313-317 (2011)
doi:10.4236/as.2011.23042
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
Agricultural Scienc es
Influence of phosphorus on the performance of cowpea
(Vigna unguiculata (L) Walp.) varieties in the Sudan
savanna of Nigeria
A. Singh1*, A. L. Baoule1, H. G. Ahmed1, A. U. Dikko2, U. Aliyu1, M. B. Sokoto1, J. Alhassan1, M.
Musa1, B. Haliru1
1Department of Crop Science, Usmanu Danfodiyo University, Sokoto, Nigeria; *Corresponding author:
2Department of Soil Science and Agricultural Engineering, Usmanu Danfodiyo University, Sokoto, Nigeria.
Received 28 March 2011; revised 24 May 2011; accepted 21 July 2011.
ABSTRACT
Savanna regions of Nigeria are deficient in ni-
trogen and phosphorus, which retard the
growth and yield of crops. Therefore, a study
was conducted in the wet season of 2006 at the
Dry Land Teaching and Research Farm of Us-
manu Danfodiyo University, Sokoto to evaluate
the effect of phosphorus on the growth and
yield of two cowpea varieties sourced from
Republic of Niger. Treatment consisted of four
(4) rates of phosphorus (0, 20, 40, 60 kg·ha–1)
factorialy combined with (2) varieties of cowpea
(KVX303096G and TN5-78) and laid out in a
randomized complete block design (RCBD) rep-
licated three (3) times. Results showed signifi-
cant response to applied P on pods per plant,
grain and stover yield and 100-seed weight with
highest respon se to the application of 60 kg P ha–1.
From this study it can be concluded that
KVX303096G and TN5-78 could both be sown
under Sokoto condition to obtain reasonable
yield of about 1 t·ha–1 of grain and 1.6 t·ha–1 of
stover. Irrespective of the varieties, application
of 60 kg P2O5 ha–1 could be recommended for
higher yi eld of co wpea (1.4 t·ha–1) rela tive to 0 kg
P/ha that yielded 1.0 t·ha–1.
Keywords: Cowpea [Vigna Ung uiculata (L.) Walp.];
Phosphorus; Sudan Savanna; Nigeria
1. INTRODUCTION
Cowpea (Vigna unguiculata (L.) Walp.) is an important
grain legume in the dry savanna of the tropics covering
12.5 million hectares with annual production of about
3.3 million tones [1]. Nigeria is the world’s largest
producer with 2.1 million tones followed by Niger with
650,000 tones and Mali with 110,000 tones [2]. About
64% of the area under cowpea is grown in central and
east Africa. Cowpea is mostly cultivated in mixture with
others crops such as millet and sorghum mostly in
Sahelian and Sudan region.
Cowpea is well adapted to poor fertility and low
rainfall conditions. Cowpea grows best on fertile, loam
soils with rainfall of 760 - 1520 mm during the growing
period, and thrives best on dry areas of Northern part of
Nigeria and transported to the Southern part of Nigeria
[3]. Cowpea is an important crop because of its role in
human and livestock nutrition. It reduces the shortage of
food by making efficient use of water and nutrient. It is a
source of protein and also less expensive than meat.
Cowpea is of vital importance to the livelihood of
several million of people in east and central Africa [4].
Cowpea is an important legume crop in the dry savannas
of Africa, especially West Africa. Out of the 12.5 million
hectare cultivated to cowpea worldwide, Singh et al. [5]
estimated that eight million hectare are in West and Cen-
tral Africa, distributed predominantly between Nigeria
and Niger. In spite of the fact that grain yields are low,
cowpea has continued to be a popular crop among
farmers. This is because cowpea provides high protein
food for people, especially children; it improves and
sustains soil fertility, and provides high quality fodder
for livestock [6,7]. Cowpea contributes to the improve-
ment of soil fertility by the fixation of nitrogen (N) in
the soil (60 - 70 kg·N·ha–1 to the subsequent crop) [8].
The result obtained from a research work carried by IITA
[9] showed that cowpea fixed 240 kg·ha–1 of N. The crop
provides a high proportion of its own requirement,
besides leaving a fixed N deposit in the soil of up to 60 -
70 kg·ha–1. In addition to its role-played in mixture with
cereals, the crop is advantageous in terms of weed

A. Singh et al. / Agricultural Science 2 (2011) 313-317
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
314
control, soil cover, protection from soil erosion and
dispersal of insects [10].
Phosphorus (P) is among the most needed elements
for crop production in many tropical soils. However,
many tropical soils are P-deficient [11]. Phosphorus,
although not required in large quantities, is critical to
cowpea yield because of its multiple effects on nutrition
[12]. All growing plants require P for growth and
development in significantly large quantity. P deficiency
is the most limiting soil fertility factor for cowpea
production [10]. Phosphorus although not required in
large quantities is critical to cowpea yield (particularly
for improved photoperiod-insensitive cultivars) because
of its multiples effects on nutrition. It not only increases
seed yields but also nodulation. Others workers have
reported that phosphorus application influences the
content of others nutrients in leaves [13] and seed. The
deficiency can be so acute in some soils of the savanna
zone of western Africa that plant growth ceases as soon
as the P stored in the seed is exhausted [14]. Because of
its multiple effects on plant nutrition (not only on
nodulation), a phosphorus fertilizer is recommended to
increase yields (P2O5: 20 - 60 kg·ha–1) [10]. Smyth and
Cravo [15] working on Xanthic Hapludox near Manaus,
Brazil reported that for cowpea critical levels for soil P
was 60 kg·P·ha–1.
Despite the importance of cowpea in human diet and
feed for animals, the yield obtained by most farmers is
very low. In Nigeria the average yield per hectare in the
respective years were 0.75 t·ha–1 in 1990, 0.72 t·ha–1 in
1991, 0.51 t·ha–1 in 1992, 1993 and 1994, 0.49 t·ha–1 in
1995, 0.45 t·ha–1 in 1996, 0.48 t·ha–1 in 1997, 0.41 t·ha–1
in 1998, 0.42 t·ha–1 in 1999, 2000, 2001 and 2002; 0.43
t·ha–1 2003 and 2004 [16]. Because of the rapid increase
in population, there is a high demand for food and
therefore, there is a need to augment the production of
cowpea. N is not critical for legumes because legumes
fix atmospheric N2 through symbiotic association with
strains of Rhizobium sp. Phosphorus is the second most
important nutrient after N and could be used to increase
production of cowpea as most soils in the tropics are P
deficient. Phosphorus is known to increases the yield of
cowpea by increased N2 fixation through nodulation and
utilization of N fertilizer [10]. Thus, this study was con-
ducted to evaluate the effect of phosphorus fertilizer on
yield of cowpea and select cowpea variety responsive to
phosphorus application.
2. MATERIALS AND METHODS
The experiment was conducted in 2006 cropping
season at the dry land farm of Usmanu Danfodiyo
University, Sokoto. Sokoto is located on latitude 13˚01′
N and at longitude 05˚15′ E and lies at an altitude of 350
m above the sea level. It falls in Sudan Savanna agro-
ecological zone. The rainfall starts mostly in June and
ends in October with a mean annual rainfall of about 350
- 700 mm. The maximum and minimum temperature of
Sokoto ranges from 40 to 15˚C, respectively [17]. The
treatments consisted of factorial combinations of four (4)
levels of phosphorous (0, 20, 40 and 60) and twovarieties
of cowpea (KVX303096G and T5-78) (8 treatment
combinations) laid out in a randomized complete block
design (RCBD) replicated 3 times.
The seeds of cowpea varieties were obtained from the
research institute of Niger Republic known as INRAN in
Tahoua State and sown on the 18th of June, 2006. Two
seeds were sown per hill at a depth of 3 - 4 cm and 75
cm apart at a plant-to-plant spacing of 50 cm within a
row. Urea at the rate 15 kg·ha–1 was applied as a starter
dose to all the plots while P2O5 was applied as per
treatment (0, 20, 40, and 60 kg P2O5 ha–1). Weeding was
carried out twice manually at 5 and 8 weeks after sowing.
The plots were sprayed with karate (Lambda cyhorlothrim)
to take care of Aphis craccivora. Harvesting was done
manually at physiological maturity when the pods had
turned yellowish brown.
Data collected on yield parameters were subjected to
analysis of variance (ANOVA) technique using Statisti-
cal Analysis System (SAS) [18]. Significant different
between treatments were further analyzed using least
significant different test (LSD) for mean separation.
3. RESULTS AND DISCUSSION
3.1. Physico-Chemical Prope rties of the So il
The general chemical and physical properties of the
surface soils (0 - 15 cm) used for the field experiments
are presented in Tabl e 1 . Soils at the experimental site
were largely Sandy and the soil pH (H2O) was moder-
ately acidic (6.5). The organic carbon (OC), total N and
available P in the soil was very low while exchangeable
K in the soil at the experimental site was low. The total
annual rainfall at the experimental site was 604 mm with
peak in August. The rain established in June and ceased
in October coinciding with sowing and harvesting of
cowpea, respectively (Figure 1).
Ta bl e 1 . Physico-chemical characteristic of the soil at the ex-
periment site.
Parameters Value Remarks
Texture Sandy
pH in water 6.50 Mod. acidic
pH in CaCl2 6.19
Total phosphorus (P) (mg·kg–1) 1.80 Very Low
Organic carbon (g·kg–1) 1.58 Very Low
Total nitrogen (N) (g·kg–1) 0.65 Very low
Exchangeable potassium (K) (cmol·kg–-1) 0.65 Low

A. Singh et al. / Agricultural Science 2 (2011) 313-317
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
315315
Figure 1. Monthly rainfall pattern and total annual
rainfall at the Experimental site. (Source: Energy Re-
search Center, Usmanu Danfodiyo University, Soko-
to).
Table 2. Effect of variety and phosphorus on the number of
pods per plant and seeds per pod.
Treatment Pods per plant Seeds per pod
Variety (V)
KVX303096G 52 ± 6.8 9.7 ± 2.8
TN5-78 49 ± 8.4 10.8 ± 3.8
SE 2.1 0.51
Significance level ns ns
Phosphorus (P)
0 43 ± 8.3b 11.1 ± 1.8
20 50 ± 7.8ab 9.9 ± 1.2
40 54 ± 3.4a 10.3 ± 2.4
60 54 ± 3.2a 9.7 ± 1.8
SE 2.9 0.73
Significance level s ns
Interaction
V × P ns ns
3.2. Number of Pods per Plant and Seeds
per Pod
There was no significant effect of the variety on the
number of pods per plant. However, variety KVX303096
G recorded higher number of pods per plant (52) than
the variety TN5-78 (49) (Table 2). Phosphorus had
significant (P < 0.05) effect on the number of pods per
plant. Significantly (P < 0.05) higher pods per plant
were recorded in plots applied with 40 (54) and 60 (54)
kg P ha–1 than 0 (43) and 20 (50) kg P ha–1 (Tab le 2).
This could be due to higher level of P in those plots.
Mokwunye and Bationo [19] have reported that, P is
essential for photosynthesis, pod development and grain
filling in leguminous crops. P is responsible for nodu-
lation in cowpea. Thus higher nodulation resulted in
higher nitrogen fixation and eventually the number of
pods per plant. There was no significant effect of
interaction of variety and phosphorus on pods per plant.
The results of the present study are higher than that
recorded by Okeleye et al. [20] where the number of
pods per plants obtained ranged from 7.3 to 30.8 by ap-
plying a rate of 20 and 30 kg P ha–1 using short and me-
dium duration cowpea varieties.
Both the variety and phosphorus had no significant
effect on the number of seeds per pod. Statistically simi-
lar seed number per pod was recorded in variety TN5-78
(10.8) and KVX303096G(9.7). Interaction between vari-
ety and phosphorus had no significant effect on the num-
ber of seeds per pod.
3.3. Grain and Stover Yield
There was significant effect of phosphorus on the
stover yield of cowpea. Similar to the grain yield, sig-
nificantly (P < 0.05) higher stover yield was recorded in
plots supplied with 60 (2115 kg·ha–1) than with 0 (1411
kg·ha–1), 20 (1482 kg·ha-1) and 40 kg P2O5 ha–1 (1571
kg·ha–1). This could again be attributed to the availability
of P that would have increased the intensity of nodula-
tion and thus nitrogen fixation. Higher nitrogen fixation
would result in higher yield of the crop. Interaction be-
tween phosphorus and variety had no significant effect
on the stover yield of cowpea.
Variety had no significant effect on the grain yield of
cowpea. Statistically similar grain yield was recorded in
both KVX303096G (1120 kg·ha–1) and TN5-78 (1074
kg·ha–1) (Table 3). This indicated the similar performance
of the two varieties under Sokoto agro-ecology.
There was significant (P < 0.05) response to applied P
on the grain yield of cowpea. Significantly higher grain
yield was recorded in plots applied to 60 (1353 kg·ha–1)
than 0 (1017 kg·ha–1), 20 (1067 kg·ha–1) and 40 kg P ha–1
(951 kg·ha–1). Application of 20 and 40 kg P2O5 ha–1 was
Ta b le 3 . Effect of variety, phosphorus and their interaction on
the grain and stover yield of cowpea.
Va ri e ty P levels
(kg·ha–1)KVX303096GTN5-78
Mean phosphorus
levels (Main effect P)
Grain yield (kg·ha-1)
0 928 ± 239 b 1106 ± 188 ab 1017±216 b
20 1165 ± 179 ab968 ± 218 b 1067±209 b
40 914 ± 82 b 988 ± 298 b 951±200 b
60 1472 ± 266 a 1235 ± 61 a 1353±216 a
Means
Va ri e ty 1120 ± 294 1074 ± 211
SE SEVa ri e ty = 62.6; SEphosphorus = 88.6;
SEVariety × Phosphorus = 125.28
Stover yield (kg·ha–1)
0 1314 ± 261 1509 ± 522 1411 ± 384 b
20 1753 ± 665 1546 ± 202 1649 ± 454 ab
40 1595 ± 416 1881 ± 539 1738 ± 458 ab
60 2072 ± 265 2158 ± 284 2115 ± 250 a
Means
Va ri e ty 1684 ± 468 1773 ± 449
SE SEVa ri e ty = 100.1; SEphosphorus = 141.5;
SEVariety × Phosphorus = 259.9
Values following ± are standard deviation of the means. Means in a column
for Phosphorus and across row and column for variety x P interaction fol-
lowed by same letter (s) are not significantly different at 5% level, ns = Not
significant, *Significant at 5% level.

A. Singh et al. / Agricultural Science 2 (2011) 313-317
Copyright © 2011 SciRes. Openly accessible at http://www.scirp.org/journal/AS/
316
not different from those plots that were not applied with
P (0 kg P ha–1). This indicated that P became a limiting
factor at later stage of plant growth. Higher yield re-
corded with 60 kg·ha–1 was attributed to higher avai-
lability of P that is responsible for effective nodulation
in cowpea. Okeleye and Okelana [21] also observed sig-
nificantly increased nodulation, grain yield, and total dry
matter for cowpea varieties in response to P application.
Variety had no significant effect on the stover yield of
cowpea as both varieties recorded 1600 kg·ha–1 of cow-
pea stover (Table 3).
There was significant effect of phosphorus on the
stover yield of cowpea. Similar to the grain yield,
significantly (P < 0.05) higher stover yield was recorded
in plots supplied with 60 (2115 kg·ha–1) than with 0
(1411 kg·ha–1), 20 (1482 kg·ha–1) and 40 kg P2O5 ha–1
(1571 kg·ha–1). This could again be attributed to the
availability of P that would have increased the intensity
of nodulation and thus nitrogen fixation. Higher nitrogen
fixation would result in higher yield of the crop. In-
teraction between phosphorus and variety had no sig-
nificant effect on the stover yield of cowpea.
3.4. Harvest Index and 100-Seed Weight
Harvest index is the proportion of grain in the total
aboveground biomass of the crop expressed in percent-
age and it ranged from 36% to 40%. This indicated that
only 36% to 40% of the photosynthate was translocated
to the grain. Variety did not have significant influence on
the HI of the crop. KVX303096G recorded statistically
(P > 0.05) similar HI with TN5-78 variety (Ta b le 4). P
also did not have significant influence on the HI of the
crop implying that HI is a genetic trait and will only be
influenced by variety differences. Effect of interaction
between variety and phosphorus was not significant
(Table 4).
There was no significant effect of variety on the hun-
dred seeds weight (Figure 2). The two varieties recorded
similar 100-seed weight of about 20.2 g. Effect of phos-
phorus on 100-seed weight was significant (P < 0.05)
Table 4. Effect of variety, phosphorus and their interaction on
harvest index of cowpea.
Harvest index (%) P levels
(kg·ha–1) KVX303096G TN5-78
Mean phosphor-
rus levels
0 41.2 ± 1.8 43.0 ± 6.6 42 ± 4.4
20 40.9 ± 6.4 38.4 ± 6.9 40 ± 6.1
40 37.0 ± 5.7 35.2 ±13.7 36 ± 9.4
60 41.4 ± 1.4 36.6 ± 3.7 39 ± 3.7
Means Variety 40.0 ± 4.2 38.0 ± 7.9
SE SEVa r ie t y = 2.10; SEphosphorus = 2.97;
SEVariety × Phosphorus = 4.196
Values following ± are standard deviation of the means.
Figure 2. Main effect of variety on 100-seed weight of
cowpea.
Figure 3. Main effect of phosphorus levels on 100-
seed weight of cowpea. Bars with same letter (s) are
not significantly different using LSD at 5% level.
Table 5. Interaction effect of variety x phosphorus on 100-
grain weight of cowpea.
100-grain weight (g) Phosphorus levels
(kg·ha-1) KVX303096G TN5-78
0 18.32 ± 0.06 b 18.59 ± 0.54 b
20 20.51 ± 1.05 a 20.60 ± 0.23 a
40 21.50 ± 0.57 a 20.54 ± 0.93 a
60 20.30 ± 0.73 a 20.98 ± 1.31 a
SE 0.439
Values following ± are standard deviation of the means. Means across rows
and columns followed by same letter (s) are not significantly different using
LSD at 5% level.
with 40 kg·ha–1 (54.17 g) and 60 kg·ha–1 (54.17 g)
recording significantly higher 100-seed weight than 0 kg
P ha–1 (42.83 g) (Figure 3). The interaction of variety
and phosphorus on 100-grain weight of cowpea was
significant (Table 5). For both variety KVX303096G and
TN5-78, application of 20, 40 and 60 kg P ha–1 resulted
in similar 100-grain weight (Table 5). For all levels of P
including control, the two varieties were at par. These
results coincide with the results recorded by Okeleye et
al. [20] where the weight of 100 seed of cowpea ranged

A. Singh et al. / Agricultural Science 2 (2011) 313-317
Copyright © 2011 SciRes. http://www.scirp.org/journal/AS/
317317
from 13.50 to 39.7 g by the application of 20 and 30 kg
P ha–1. Phosphorus intervenes in the formation of seed
and improves seed quality [21].
Openly accessible at
4. CONCLUSIONS
From this study it can be concluded that KVX303096G
and TN5-78 could both be sown under Sokoto condition
to obtain reasonable yield of about 1 t·ha–1 of grain and
1.6 t·ha–1 of stover. Irrespective of the varieties, applica-
tion of 60 kg P2O5 ha–1 could be recommended for higher
yield of cowpea (1.4 t·ha–1) relative to 0 kg P2O5 ha–1
that yielded 1.0 t·ha–1. However, 60 kg P2O5 ha–1 may not
be the optimum as further application of P may or may
not increase the yield of cowpea. Therefore it is subject
to investigation.
REFERENCES
[1] FAO (Food and Agriculture Organisation) (2005) Cow-
pea production data base for Nigeria 1990-2004.
http://www.faostat.fao.org/
[2] IITA (International Institute of Tropical Agriculture)
(2003) Crop and farming systems.
http://www.iita.org/crop/cowpea.htm
[3] Adegbola, A.M. and Akinsanmi, A.K. (1971) Agricul-
tural sciences for West African schools and colleges. 3rd
Edition, Oxford University, Oxford.
[4] Bressani, R. (1985) Nutritive value. In: Singh, S.R. and
Rachies, K.O., Eds., Cowpea Research Production and
Utilization, John Wiley and Sons, New York.
[5] Singh, B.B., Chambliss, O.L. and Sharma, B. (1997)
Recent advances in cowpea breeding. In: Singh, B.B.,
Mohan-Raj, D.R., Dashiell, K.E. and Jackai, L.E.N., Eds.,
Advances in Cowpea Research, Copublication of Interna-
tional Institute of Tropical Agriculture (IITA) and Japan
International Research Center for Agricultural Sciences
(JIRCAS). IITA, Ibadan, Nigeria, 30-49.
[6] Tarawali, S.A., Singh, B.B., Peters, S.M. and Blade, S.F.
(1997) Cowpea haulms as fodder. In: Singh, B.B., Mohan
-Raj, D.R., Dashiell, K.E. and Jackai, L.E.N., Advances
in Cowpea Research, Copublication of International In-
stitute of Tropical Agriculture (IITA) and Japan Interna-
tional Research Center for Agricultural Sciences (JIR-
CAS), Ibadan, Nigeria, 313-325.
[7] Singh, B.B. and Tarawali, S.A. (1997) Cowpea: An inte-
gral component of sustainable mixed crop/livestock
farming systems in West Africa and strategies to improve
its productivity. In: Renard, C., Ed., Crop Residues in
Sustainable Mixed Crop-Livestock Farming Systems,
CAB International in Association with the International
Crops Research Institute for the Semi-arid Tropics (IC-
RISAT) and the International Livestock Research Insti-
tute (ILRI), 79-100.
[8] Rachies, A.K. (1985) Problems prospects of cowpea
production in Nigeria Savannah. Tropical Grain Legumes
Bulletin, 32, 78-87.
[9] IITA (International Institute of Tropical Agriculture)
(1973-1974) Annual Report.
[10] Reamaekers, R.H. (2001) Crop production tropical Africa.
DGIC, Belgium, 334-34.
[11] Smyth, T.J. and Cravo, M.S. (1990) Critical phosphorus
levels for corn and cowpea in a Brazilian Oxisol. Agro-
nomy Journal, 82, 309-312.
[12] Adetunji, M.T. (1995) Equilibrium phosphate concentra-
tion as an estimate of phosphate needs of maize in some
tropical Alfisols. Tropical Agriculture, 72, 285-289.
[13] Muleba, N. and Ezumah, H.C. (1985) Optimizing cul-
tural practices for cowpea in Africa. In: Singh, S.R. and
Rachie, K.O., Eds., Cowpea Research, Production, and
Utilization, John Wiley and Sons Ltd, Chichester, 289-
295.
[14] Kang, B.T. and Naggos, M. (1983) Phosphorous re-
quirement of cowpea. IITA Ibadan, Annual Report, 79-
115.
[15] Mokwunye, A.U., Chien, S.H. and Rhodes, E. (1986)
Phosphorus reaction with tropical African soils. In:
Mokwunye, A.U. and Vlek, P.L.G., Eds., Management of
Nitrogen and Phosphorus Fertilizers in Sub- Saharan Af-
rica, Martinus Nijhoff, Dordrecht, 253-281.
[16] FAOSTAT (2004) Cowpea production data base for Ni-
geria 1990-2004.
http://www.faostat.fao.org/cowpeayield/cowpeaproductio
n
[17] Arnborg, T. (1988) Where savannah turns into desert.
International rural development center. Swedish Univer-
sity of Agriculture Sciences Rural Development Studies.
[18] SAS (2003) Statistical analysis system. SAS release 9.1
for Windows, SAS Institute Inc. Cary.
[19] Mokwunye, A.U., Bationo A (2002) Meeting the phos-
phorus needs of the soils and crops of West Africa: The
role of indigenous phosphate rocks. In: Vanlauwe, B., J.,
Diels, Sanginga, N. and Merckx, R., Eds., Integrated
Plant Nutrient Management in Sub-Saharan Africa: From
Concept to Practice, CABI/IITA, Cromwell Press, Trow-
bridge, 209-224.
[20] Okeleye, K., Ariyo, O.J. and Olawe, V.I. (1999) Evalua-
tion of early and medium duration (Vigna unguiculata L.)
cultivars for organic trails and grain yield. The Nigerian
Agricultural Journal, 30, 1-11
[21] Okeleye, K.A. and Okelana, M.A.O. (1997) Effect of
phosphorus fertilizer on nodulation, growth and yield of
cowpea (Vigna unguiculata) varieties. Indian Journal of
Agricultural Science, 67, 10-12.
[22] Kang, B.T. and O.C. Osiname (1979) Phosphorus re-
sponse of cowpea grown on alfisols of southern Nigeria.
Agronomy Journal, 71, 873-877.