Abstract Intercropping cotton (Gossypium

hirsutum L.) and cowpea (Vigna unguiculata (L.)

Walp) is one of the ways to improve food security

and soil fertility whilst generating cash income of

the rural poor. A study was carried out to find out

the effect of cotton–cowpea intercropping on

cowpea N2-fixation capacity, nitrogen balance

and yield of a subsequent maize crop. Results

showed that cowpea suppressed cotton yields but

the reduction in yield was compensated for by

cowpea grain yield. Cowpea grain yield was sig-

nificantly different across treatments and the

yields were as follows: sole cowpea (1.6 Mg ha–1),

1:1 intercrop (1.1 Mg ha–1), and 2:1 intercrop

(0.7 Mg ha–1). Cotton lint yield was also signifi-

cantly different across treatments and was sole

cotton (2.5 Mg ha–1), 1:1 intercrop (0.9 Mg ha–1)

and 2:1 intercrop (1.5 Mg ha–1). Intercropping

cotton and cowpea increased the productivity

with land equivalence ratios (LER) of 1.4 and 1.3

for 1:1 and 2:1 intercrop treatments, respectively.

There was an increase in percentage of N fixation

(%Ndfa) by cowpea in intercrops as compared to

sole crops though the absolute amount fixed

(Ndfa) was lower due to reduced plant popula-

tion. Sole cowpea had %Ndfa of 73%, 1:1 inter-

crop had 85% and 2:1 intercrop had 77% while

Ndfa was 138 kg ha–1 for sole cowpea, 128 kg ha–1

for 1:1 intercrop and 68 kg ha–1 for 2:1 intercrop

and these were significantly different. Sole cow-

pea and the intercrops all showed positive N

balances of 92 kg ha–1 for sole cowpea and 1:1

intercrop, and 48 kg ha–1 for 2:1 intercrop. Cow-

pea fixed N transferred to the companion cotton

crop was very low with 1:1 intercrop recording

3.5 kg N ha–1 and 2:1 intercrop recording

0.5 kg N ha–1. Crop residues from intercrops and

sole cowpea increased maize yields more than

residues from sole cotton. Maize grain yield was,

after sole cotton (1.4 Mg ha–1), sole cowpea

(4.6 Mg ha–1), 1:1 intercrops (4.4 Mg ha–1) and

2:1 intercrops (3.9 Mg ha–1) and these were sig-

nificantly different from each other. The LER,

crop yields, %N fixation and, N balance and

residual fertility showed that cotton–cowpea in-

tercropping could be a potentially productive

system that can easily fit into the current small-

holder farming systems under rain-fed conditions.

The fertilizer equivalency values show that sub-

stantial benefits do accrue and effort should be

directed at maximizing the dry matter yield of the

legume in the intercrop system while maintaining

L. Rusinamhodzi (&) Æ H. K. MurwiraTSBF-CIAT Zimbabwe, Box MP 228, Mt Pleasant,Harare, Zimbabwee-mail: l.rusinamhodzi@cgiar.org

J. NyamangaraDepartment of Soil Science and AgriculturalEngineering, University of Zimbabwe, Box MP167,Mt Pleasant, Harare, Zimbabwe

Plant Soil (2006) 287:327–336

DOI 10.1007/s11104-006-9080-9

123

ORIGINAL PAPER

Cotton–cowpea intercropping and its N2 fixation capacityimproves yield of a subsequent maize crop underZimbabwean rain-fed conditions

L. Rusinamhodzi Æ H. K. Murwira ÆJ. Nyamangara

Received: 24 January 2006 / Accepted: 4 July 2006 / Published online: 24 August 2006� Springer Science+Business Media B.V. 2006

or improving the economic yield of the compan-

ion cash crop.

Keywords Cotton cowpea intercropping Æ N2

fixation Æ N balance Æ maize yield Æ rain-fed

conditions

Introduction

Biological nitrogen fixation (BNF) in legumes is a

fundamental process for maintaining soil fertility

and the continued productivity of low-input

cropping systems. Carry-over of N from BNF, e.g.

in roots and stover, can supply the N demand of

subsequent non-N2 fixing crops (van Kessel and

Hartley 2000). BNF therefore can be used in

agricultural systems for replenishing N, which is

often the most limiting growth factor in tropical

soils (Jeranyama et al. 1998). The percentage N

fixation (%Ndfa) and the N contribution from

leguminous crops are influenced by crop species

and a number of environmental factors such as

soil type, water availability and temperature

(Jensen et al. 1997).

Most legumes in Zimbabwe are grown as in-

tercrops with cereals (Jeranyama et al. 1998), and

evidence shows that mycorrhizal connections be-

tween the intercropped components may provide

a route of N transfer (He et al. 2003). Although,

any such N benefit to an intercropped non-legume

species would only be significant under low-

yielding conditions (Hardy 1993). In addition,

when parts of the legume senesces and decom-

pose, soil N supply to the associated crop is im-

proved (Rao and Mathuva 2000).

Cowpea (Vigna unguiculata (L.) Walp) is a

grain legume grown in savanna regions of the

tropics and has a long history of cultivation in

Zimbabwe (Reid 1977). Spreading types are

predominant and their leaves, as well as, seeds,

are consumed as an important supplement to the

staple diet of maize. Cotton (Gossypium hirsutum

L.), on the other hand is an important cash crop

for farmers in the smallholder sector that repre-

sents 80% of national production and it is the

second largest foreign currency earner among

agricultural products after tobacco (Nicotiana

tabacum L.) in Zimbabwe (Cotton Research

Institute Report 1993–1994). It is also important

to other sub-Saharan African countries, for

example, in Tanzania cotton is currently rated

third, after cashew (Anacardium occidentale L.)

and coffee (Coffea arabica L.), in terms of foreign

exchange earnings (Myaka and Kabissa 1996).

Although measurements of %Ndfa in sole crop

cowpea are well documented, whether it is en-

hanced or suppressed in cotton–cowpea inter-

cropping systems is poorly understood. The aim

of this study was to quantify the N2 fixation

capacity of cowpea as a sole crop and in two in-

tercropping systems with cotton and its effects on

a subsequent maize crop. The amount of N fixed

and transferred from cowpea to cotton in the

intercropped systems was also quantified.

Materials and methods

Site description

The field experiment was conducted during the

2003/2004 and 2004/2005 cropping seasons at

Kadoma Cotton Research Institute (CRI), Zim-

babwe (29�53¢E, 18�19¢S, Altitude 1156 m). The

soils at CRI are well-drained, reddish brown fer-

siallitics (5E.2) (Zimbabwe), Ferralic Cambisol

(FAO) and Oxic Ustropept (USDA), (Thompson

and Purves 1978; Nyamapfene 1991). The area

was under soybean during 2001/2002 but was

left uncropped fallow during 2002/2003 season.

Soil was sampled randomly using a soil auger

(0–15 cm) and taken to the laboratory for basic

characterization. The land was ploughed during

the dry season and then tilled with a disk plough

just before the start of rains in 2003.

Experimental layout

Cowpea used in this experiment was the short

season, erect type variety called CBC2 released

by Agricultural Research and Extension (AREX)

which was derived from IT18 variety, while cotton

used was the high yielding, small spreading and

medium season variety, Albar SZ9134 (CRI Re-

port 1993–1994). The treatments were (i) sole

cowpea, (ii) sole cotton, (iii) one row of cotton

alternating with one row of cowpea, planted

simultaneously, and (iv) two rows of cotton

328 Plant Soil (2006) 287:327–336

123

alternating with a row of cowpea, planted simul-

taneously. These treatments were laid out in plots

(6 · 9 m) in a completely randomised design

(CRD) with four replicates. Cowpea in sole crop

was sown at a spacing of 0.5 m between rows and

0.15 m within the rows. In intercrops row spacing

for cowpea was adjusted to 1.0 m for the 1:1

(cotton: cowpea) intercrop treatment so that a

row of cotton could fit in-between and 2.0 m for

the 2:1 (cotton: cowpea) intercrop treatment so

that two rows of cotton could fit in-between but

the in-row spacing remained 0.15 m. Cotton was

sown at spacing of 1.0 m between rows and

0.30 m within rows in both sole and intercrops.

Within each main plot two microplots

(2 · 1 m) received 25 kg N ha–1 as (15NH4)2SO4,

with an enrichment of 10.38% 15N atom excess.

The N-fertilizer was mixed with sufficient sucrose

as a carbon source to give a C:N ratio of 10:1 in

solution, and dissolved in water to allow even

application to the microplots. The area outside

the microplots was fertilized with unlabelled

(15NH4)2SO4 also at 25 kg N ha–1.

Recommended rates for cotton for P

(45 kg P ha–1 as single superphosphate, SSP) and

K (25 kg K ha–1 as Muriate of Potash) and B

(0.5 kg B ha–1 as borax) were applied (Cotton

Training Center 2000).

Cowpea residual N fertility effect in the second

season (2004/2005) was determined by incorpo-

rating all residues and comparing the response of

subsequent maize crop. Aboveground cotton

residues except litter fallen before harvest were

not returned to the soil because the stems were

burnt to control the pink bollworm (Pectinophora

gossypiella). The yields from these plots were

then compared to an N fertilizer response curve

of 0, 30, 60 kg N ha–1 applied as ammonium ni-

trate to calculate N equivalencies. All the plots

received a basal application of 45 kg P ha–1 as

SSP.

Plant sampling and analysis

All plants were sampled (cut at 1 cm above the

ground) when cowpea had reached physiological

maturity and cowpea pods separated from the

leaves and stems. Cotton plants were sampled

at the same time with cowpea and at that time

cotton was between flowering and ball formation

and plant parts were not separated. The 15N

sampling of cowpea and cotton in the first season

were measured by removing the shoots from 1 m2

drying and weighing. The yield plots were 4 m2,

and everything from this was collected at harvest

for grain and dry matter determination. In the

second season, maize yield plots were 4 m2 and all

the above ground material was measured for

grain and dry matter.

The plant samples were first dried at 72�C,

ground in a Wiley mill to pass through a 0.5 mm

sieve and then ball-milled to < 150 microns. Total

N in plant samples was determined using a mod-

ified micro-Kjeldahl procedure. Plant samples

(0.1 g) were digested in 4.4 ml of digestion mix-

ture, which consists of H2O2, H2SO4 and Se cat-

alyst. After digestion, total N was determined

colorimetrically by spectrophotometry (Bremner

and Mulvaney 1982).

The 15N abundance in the ground plant mate-

rial was measured on an ANCA 20-20 GSL iso-

tope ratio mass spectrometer (PDZ Europa Ltd.,

Cheshire, UK) following dry combustion. 15N-la-

belled Plant Reference Material supplied by the

International Atomic Energy Agency (IAEA)

was used as standard.

Land equivalency ratio

The productivity and efficiency of intercropping

was measured by calculating the land equivalency

ratio (LER) (De Wit and Van den Bergh 1965).

The intercrop yields (kg ha–1) were divided by the

pure stand yields for each component crop in the

intercrop (Eq. 1). These two figures were then

added together, values greater than 1 show an

advantage while those less than 1 show a disad-

vantage of intercropping (Sullivan 1998).

LER ¼ Cottonintercrop

Cottonsolecropþ

Cowpeaintercrop

Cowpeasolecrop

" #ð1Þ

Biological N fixation

The %Ndfa (Eq. 2) was measured using the iso-

tope dilution (ID) method. The major assumptions

Plant Soil (2006) 287:327–336 329

123

are that the isotopic composition of the soil N and

its availability to both the fixing and non-fixing

crop is assumed to be similar, any dilution in plant15N isotopic composition is assumed to result from

BNF. The intensity of Ndfa (Eq. 3) is determined

by the extent to which plant 15N isotopic compo-

sition is diluted, this is obtained by comparing 15N

enrichment in N fixing plants and non-fixing le-

gume isolines or non-legumes (McAuliffe et al.

1958).

%Ndfa¼�1� %atom excess of fixing plant

%atom excess of non� fixing plant

��100

ð2Þ

Ndfa¼ Total legume N� % Ndfa

100ð3Þ

Nitrogen transfer

When a non-N fixing plant is cultivated in asso-

ciation with an N-fixing plant, N taken up by the

non N-fixing plant in the soil is derived from two

sources: X% (Eq. 4) coming from the Ndfa by

the legume and Y% coming directly from the

soil N (Kurdali et al. 1990). The d15N signatures

of plants relying solely on N2 fixation by bacte-

rial symbionts are similar to those of atmospheric

N2 (i.e. close to 0) as opposed to much wider

range of signatures of plants relying on soil N

sources (Tjepkema et al. 2000). The natural

abundance of the heavy isotope, 15N, is com-

monly expressed as a d15N value, denoting the

relative deviation (in 0/00) from the ratio 15N: 14N

in atmospheric N2. When discussing the 15N

enrichment of natural materials, that is their

natural abundance which is usually a small value

of atom % 15N excess, then the term d15N is used

(Giller 2001a).

Additions of atmospheric N2 or other gaseous

forms of N further modify d15N signatures of

mineral N and organic N pools in soils (Heaton

et al. 1997). The amount transferred (kg N ha–1)

is the calculated using the total N content

(kg N ha–1) of the cotton crop (Eq. 5).

Percentage Ntransferred ðX%Þ

¼ d15Nref:pl � d15Nassociation

d15Nref:pl � d15Nfixation

� � � 100 ð4Þ

Where d15Nfixation is obtained from cowpea,

d15N ref.pl is obtained from cotton sole crop,

d15Nassociation is obtained from cotton in inter-

crops.

Amount of Ntransferred ¼X% � Total N cotton plant

100

ð5Þ

Nitrogen balance

The difference between Ndfa and N exported in

seeds is the amount of N introduced to the soil

system and this has been referred as N balance

(Eq. 6) and this is measured in kg ha–1. The

assumption is that the legume residues will

decompose and N will be added to the soil. Root

fixed N was not measured in this study. Root N

is important in soil N balance calculations and

studies have shown that root N may be 25–41%

of total plant N (Russell and Fillery 1996;

Rochester et al. 1998; Unkovich and Pate 2000).

More recently, Peoples et al. (2001) used an

average of 33% for estimating root N for grain

legume crops in order to calculate soil N bal-

ance. Basing literature from similar studies, the

same figure (33%) was used for calculating N in

roots in this study.

Nitrogen balance ¼ Ndfa�GrainN ð6Þ

Fertilizer equivalence

Crop residue fertilizer equivalencies (FE) were

calculated by comparing residual plot yields fol-

lowing intercrop and residue incorporation, to a

mineral fertilizer response curve experiment in

the same site and season. The response curve

quadratic expression was used to calculate the FE

of the second season crop, by fitting the yield (y)

to calculate the corresponding rate of N applica-

tion to achieve the observed yield (Equation 7).

330 Plant Soil (2006) 287:327–336

123

To compare the FE, FE% (Eq. 8) was calculated

by dividing FE by the actual amount of N applied

in crop residues and expressed as a percentage

(Mutuo et al. 1999).

FE ¼ �b �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðb2 � 4acÞ

p2a

ð7Þ

%FE ¼ FE

N applied� 100 ð8Þ

Statistical analysis

Analysis of variance (ANOVA) using Genstat

(2002) was used to identify treatment effects.

Variables on which statistical analyses were per-

formed are cowpea grain and total dry matter

yield, amount of N fixed (Ndfa), cotton lint and

total dry matter yield, LER, maize grain and total

dry matter yield. Treatment differences were

identified using standard error of differences

between means (SED).

Results

Soil characterization

Soil organic carbon (OC) was 1.5% and shows

that the soil has a good capacity to meet long-

term crop nutrient demands. The pH was greater

than 5 and provides a suitable environment for

crops that are commonly grown in Zimbabwe.

Available N of 28.4 mg kg–1 (Table 1) falls within

low category (15–30 mg kg–1), (Mashiringwani

1983) and more N can be released by minerali-

zation from OC (Cooper and Fenner 1981). The

sodium absorption ratio (SAR) of 13 is high but

the pH of 6.5 means that the soil is normal

according to the categories given by Brady (1990).

Yield of intercrops

Cowpea grain yield was higher in the sole

crop (1.4 Mg ha–1) than in the intercrops at

1.1 Mg and 0.7 Mg ha–1 for 1:1 and 2:1inter-

crop treatments, respectively. Intercrops were

more productive than sole crops as shown by

land equivalence values (LER) which were

greater than 1 in both cases (Table 2). The

same trend was observed for cotton lint yield

with sole cotton recording 2.5 Mg ha–1 but

the 2:1 intercrop treatment yielded higher

lint (1.5 Mg ha–1) than the 1:1 intercrop

(0.9 Mg ha–1) treatment (Table 2). These yields

were significantly different (P < 0.05). Both

crops recorded higher total aboveground dry

matter yield in sole crops compared to inter-

crops. Cowpea total dry matter yields were

4.7 Mg ha–1 for sole cowpea, 3.8 Mg ha–1 for

1:1 cotton–cowpea intercrops and 2.2 Mg ha–1

for 2:1 cotton–cowpea intercrops. Cotton yields

were 12.3 Mg ha–1 for sole cotton, 6.5 Mg ha–1

Table 1 Soil properties from the experimental site

Sand(%)

Clay(%)

Silt(%)

pH(CaCl2)

OC(%)

N (mg kg–1) AvailableP (%)

Ca(cmol+ kg–1)

Mg(cmol+ kg–1)

K(cmol+ kg–1)

Na(cmol+ kg–1)

Mineral Total

42 37 21 6.5 1.5 28.4 1880 0.03 8.4 7.7 0.3 3.7

Table 2 Effect of sole crops and intercrops on total dry matter (TDM), grain and lint yield, %Ndfa and Ndfa

Treatment CowpeaTDM(Mg ha–1)

CottonTDM(mg ha–1)

Cowpeagrain yield(mg ha–1)

Cotton lintyield (mg ha–1)

LandEquivalenceratio (LER)

%Ndfa Ndfa (kg ha–1)

Sole crop 4.7 12.3 1.4 2.5 1.0 73 1381:1 intercrop 3.8 6.5 1.1 0.8 1.4 85 1282:1 intercrop 2.2 9.5 0.6 1.5 1.3 77 68SED 0.3 0.8 0.2 0.4 0.01 3 27

Plant Soil (2006) 287:327–336 331

123

for 1:1 intercrop treatment and 9.5 Mg ha–1 for

2:1 intercrop (Table 2).

Biological nitrogen fixation

The amounts of Ndfa were significantly different

(P < 0.05) among treatments (Table 2). The

highest %Ndfa (85%) was recorded for 1:1 cot-

ton/cowpea intercrop, the lowest percentage

(73%) was recorded for sole cowpea treatment

and 2:1 cotton/cowpea intercrop treatment had

77%. The highest amount of Ndfa (138 kg N ha–1)

was however obtained from the sole cowpea crop,

the 1:1 cotton/cowpea intercrop had 128 kg N ha–1

and the 2:1 cotton/cowpea treatment had the

lowest amount of 68 kg N ha–1.

Nitrogen balance

The N balances of all the treatments were positive

(48–92 kg N ha–1) (Table 3). The 1:1 cotton/

cowpea intercrop treatment had the highest N

balance of all the treatments. Uptake from the

soil ranged between 15 and 38 kg N ha–1 (Ta-

ble 3). The amount of N transferred between

crops was insignificant with the 1:1 intercrop

having the highest amount of 3.5 kg N ha–1

while

the 2:1 intercrop recorded only 0.5 kg N ha–1.

The intercrop system’s planting arrangement and

fixation capability had an effect on the percentage

of N in the associated cotton crop coming from

N2-fixation by cowpea.

Yield of maize after intercrops

Previous crop residues had a significant effect on

the following maize crop yields, with maize fol-

lowing sole cotton having a significantly lower

yield than either sole crop cowpea or the two

intercrop treatments (P < 0.05) (Table 4).

Following sole cotton, maize yields had a

highly negative %FE (–27%) that was signifi-

cantly lower than the other three treatments. Sole

cowpea (27%), 1:1 intercrop (32%) and 2:1

intercrop (29%) showed no significant differences

between them.

Discussion

The %Ndfa by cowpea was higher in intercrop

treatments than in sole crop indicating a higher

productivity of the cotton/cowpea intercropping

system. This was due to increases in competition

for soil N that led to better nodulation. Marschner

(1995) reported that growing a non-legume with a

legume reduced the amounts of mineralized N in

the soil and legumes responded by fixing more N

Table 3 Total N in plant, N uptake from soil, Ndfa, N removed in grain and N balance in soil after intercrops as comparedto sole cowpea

Treatment Total N inplant (kg ha–1)

Ndfa(kg N ha–1)

N uptakefrom soil(kg N ha–1)

Grain Nremoved(kg N ha–1)

N balance(kg N ha–1)

Sole cowpea 189 138 51 46 921:1 Intercrop 150 128 22 36 922:1 intercrop 88 68 20 20 48SED 36 27 12 5 22

Table 4 Effect of residual fertility (N) on stover and grain yield of maize after growing cotton and cowpea intercrops aswell as sole crops as compared to three mineral fertilizer levels at CRI

Yield Previous crop Fertilizer level (kg N ha–1) SED

Cotton Cowpea 1:1 2:1 0 30 60

Grain (mg ha–1) 1.4 4.6 4.4 3.9 2.3 4.9 6.3 1.2Stover (mg ha–1) 4.3 8.6 8.1 7.3 5.9 8.8 11.2 1.6

332 Plant Soil (2006) 287:327–336

123

than they do in a pure stand, so long as the le-

gumes dominated the mixture. The Ndfa was

however lower in intercrop treatments than in

sole crop due to the lower density of the fixing

plant in intercrop than in sole cropping. The

greater density in cowpea sole crop increased the

dry matter yield per unit area and hence the total

amount of N fixed.

This study made use of the isotope dilution

method (ID). A great advantage with the use of15N isotope dilution technique to estimate N2

fixed is its ability to give an integrated estimate of

N fixation over a growing season or longer

(McAuliffe et al. 1958). It is the only method that

can distinguish between soil, fertilizer and fixed N

in field-grown crops. Estimates made using the

isotope dilution technique have established that

most legumes derive a large proportion of their N

from fixing atmospheric N2 and, in general, this is

in excess of 70–80% of their total N requirements

(Eaglesham et al. 1977) and results reported here

are close to that range. Errors often result from

differences in the pattern of N uptake between

reference and N2-fixing plants (Giller and Witty

1987). The addition of mineral N to the soil is

likely to disturb the process under measurement

as this added amount can constitute a significant

fraction of the mineral N available to the legume

(McNeill et al. 1996). The major problem with the

ID method is that 15N uptake is affected by var-

iation in rooting depth and timing of N uptake

between fixing and non-fixing crop (Giller,

2001a). Cowpea has a deep rooting system

(Giller, 2001a), and so is cotton but initial growth

is slow (Cotton Training Center 2000). Due to its

slow initial growth, the peak of 15N uptake could

have been different between cotton and cowpea

because available soil 15N rapidly declines due to

plant uptake, other losses and dilution due to

mineralization of N with smaller enrichment from

soil organic matter (Giller, 2001a). This could

have resulted in lower enrichment of 15N in the

cotton reference crop and an underestimate of

N2-fixation.

The LER obtained for cotton and cowpea in-

tercrops were all greater than one (>1.0). This

meant that the land requirement for the inter-

crops was lower than that for monocrops. Inter-

cropped legumes and cotton used plant growth

resources more efficiently than pure stands, thus

supporting results of Fukai and Trenbath (1993)

and Rao et al. (1987) on intercropping systems

with legumes and non-legumes. They attributed

the advantage of intercropping to different

aboveground and belowground growth habits and

morphological characteristics of the crops, and to

the higher efficiency in the utilization of water

and radiation energy. Cowpea rooting system is

more extensive than those of soybean (another

potential companion with cotton and also a crop

gaining prominence in Zimbabwe) and it can ex-

plore deep soil profiles for moisture (Miller 1998).

Cotton rooting on the other hand depends on

moisture availability. Under conditions of little

moisture, the main root system usually develops

to a considerable depth of up to 2.0 m (Monks

1999).

Although %Ndfa was high in the intercrop

systems (>75%), the associated cotton crop did

not get much of the N fixed by cowpea. Little

transfer was observed in both cases although it

was a little higher for 1:1 intercrop than for 2:1.

This could be explained by the fact that in 1:1

intercrop, the cotton crops are surrounded by two

rows of cowpea crops whereas in 2:1, two rows of

cotton will share a row of cowpea and hence

transfer is very low. Snoeck et al. (2000) were able

to show that under field conditions; roughly 30%

of N effectively fixed by a legume was transferred

to the associated non-legume. They reported that

N transfer was high because it was enhanced by

the interactions of roots and root exudates. The

low value (3%) for N transfer reported in this

study could be because the period of the study was

short (2 months) compared to at least two seasons

for Snoeck et al. (2000). The crops they used for

their study were coffee (Coffea arabica (L.) and

legumes, Flemingia macrophylla (Willd.) Merr.,

Desmodium intortum (Mill.) Urb., Leucaena leu-

cocephala (Lam.) De Wit., Leucaena diversifolia

(Schelecht.) Bentham, Calliandra calothyrsus

(Meissn.) and Erythrina abyssinica (Lam.).

The higher N balance (92 kg N ha–1) for the

cowpea sole crop and 1:1 intercrop could be ex-

plained by a higher amount of N fixation but far

much lower removal of N in the grain of

46 kg N ha–1 and 36 kg N ha–1, respectively. This

figure includes the contribution of roots to N

Plant Soil (2006) 287:327–336 333

123

balance and represents the maximum possible

amount of N added to the soil after growing sole

cowpea or 1:1 intercrops.

There is evidence of soil fertility improvement

in cropping systems that includes legumes such as

cowpea as shown by the yield of the succeeding

maize crop especially where cowpea plant density

was high (Giller 2001b). In rotation, legume res-

idues have a longer time to decompose as com-

pared to intercropping releasing more N that lead

to better yield of the following cereal (Jeranyama

et al. 2000). Increase in yield of crops following

cowpea has also been attributed to reduced

moisture stress in rotations (Rao and Mathuva

2000), reduction in Striga hermonthica (Reddy

et al. 1994) and also the ability of cowpea to

access sparingly soluble P (Vanlauwe et al. 2000).

However for the benefits to be fully realized the

legume should produce more residue and careful

management of the residue is required.

The fertilizer equivalence (FE) values for the

previous crop residues showed that cowpea resi-

dues contribute more to the N nutrition of the

following crop than cotton residues. Cadisch and

Giller (1997) reports that legumes are able to

provide more mineral N in the short term than

residues of other crops because of their better

chemical composition which has a major influence

on the rates of decomposition and N release. The

reduction in yield obtained in plots that had sole

cotton could be explained by the poor quality cot-

ton residues which caused immobilization of soil

available N (data not shown). The higher maize

grain yields from plots previously under intercrops

than from fallow indicated an obvious incentive to

integrate legumes in the existing cropping systems.

The positive effect of the previous cowpea ob-

served in these trials could have been partly due to

the previous cowpea absorbing less soil N than the

previous cotton, which is referred to as sparing ef-

fect (Peoples et al. 1995). Additionally, there was

probably additional N fixed and left in the soil for

the subsequent maize crop.

Conclusions

The intercropping of cotton with a short season

cowpea variety is a potentially productive system

as shown by high LER values. Nitrogen fixation

capacity of cowpea is enhanced in the intercrop

compared to sole crops. Nitrogen transfer from

cowpea to cotton does occur in cotton/cowpea

intercrops but it is of little significance to the

overall nutrition of the cotton crop. The positive

N balance in the intercrop indicated a further

advantage, and this N could be used to produce a

subsequent high yielding crop such as maize

which has high nutrient demands. This is shown

by the improvement in the grain and stover yield

of maize following intercrops without additional

N fertilizer. The benefits mentioned in this study

accrued without substantial additional costs ex-

cept for cowpea seed, farmers would be able to

save on N fertilizer input and should now divert

the little resources they have on securing P fer-

tilizer as it is the second most limiting element to

crop production.

Acknowledgements Special thanks to N. Mutasa andR. Dhliwayo from Kadoma Cotton Research Institute’sCrop Productivity Unit for letting me use their land andassistance with laying out the trials especially during thefirst season. Financial support from TSBF-CIAT throughthe IFAD Project is greatly appreciated.

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