Soil & Tillage Research 75 (2004) 173–184

Effects of soil management practices and tillage systems onsurface soil water conservation and crust formation on

a sandy loam in semi-arid Kenya

Patrick Gicherua,∗, Charles Gacheneb, Joseph Mbuvib, Edward Mareaa Kenya Agricultural Research Institute, P.O. Box 14733, Nairobi, Kenya

b University of Nairobi, P.O. Box 30197, Nairobi, Kenya

Received 22 July 2002; received in revised form 22 May 2003; accepted 10 July 2003

Abstract

The effect of different soil management practices on crust strength and thickness, soil water conservation and crop perfor-mance was investigated on a ferric lixisol in a semi-arid environment of eastern Kenya.

The study proved that manure and mulching with minimum tillage have a greater effect on the water balance of crustedsoils and maize emergence. There was increase in steady infiltration rates, amount of soil water stored in the soil and betterdrainage. The physical effect of mulch was less important in the rehabilitation of crusted soils in the study site when it wasincorporated into the soil. Manure and surface mulch with minimum tillage should therefore be taken into account in landmanagement and water conservation in the semi-arid areas of Kenya. The response of crops to the improved water availabilitydue to manure with minimum and with conventional tillage and surface mulch was very clear. These management practicesshould be recommended when considering the effectiveness of soil and water management techniques in the study area.© 2003 Elsevier B.V. All rights reserved.

Keywords:Crust strength and thickness; Soil management; Tillage methods; Steady infiltration rates and available soil moisture

1. Introduction

One way to achieve higher rain use efficiency is toincrease infiltration rates of the soil surface in orderto diminish losses of water through erosion. This isimportant especially in the semi-arid areas of Kenyawhere the combination of crust sensitive soils and er-ratic rainfall events result in high risk of water lossesthrough runoff. Crust is a common phenomenon insemi-arid Kenyan soils but very little has been done tounderstand the processes behind their formation andhow to manage them.

∗ Corresponding author. Tel./fax:+254-2443376.E-mail address:kss@iconnect.co.ke (P. Gicheru).

Most of the soils exposed to rain drop impact aresubjected to physical and chemical processes whichchange their properties at the vicinity of the soil sur-face. These processes are liberally called “soil crustingor crust formation” even though the crust layer is ob-served at the soil surface only when the soil has dried.When this phenomenon is disregarded especially inthe semi-arid area, then it may be harmful to agricul-tural production in these areas (Mualem et al., 1990b).

Formation of surface crusts or seals in bare soils isa major problem in many parts of the world (Mbuviet al., 1995). Extensive areas in semi-arid regionswith low soil organic matter content and soils oflow structural stability tend to form crusts at the sur-face (Hoogmoed, 1987; Stroosnijder and Hoogmoed,

0167-1987/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0167-1987(03)00161-2

174 P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184

1984; Bissonnais et al., 1997). This is aggravated byheavy and intensive rainfall.

Deterioration in soil physical properties of the sur-face soils can have a long-term effects on surfacecrusting and water infiltration. Crusts impose severerestriction to water penetration into the soil, cultiva-tion and plant growth (Bately and Davies, 1971).

Soil scientists have documented the mechanismsleading to the formation of surface crusts (Agassi et al.,1981; Eigel and Moore, 1983). Some are of the opin-ion that coarse textured soils are highly sensitive tosurface crusting (Stroosnijder and Hoogmoed, 1984;Kooistra and Siderius, 1986). Sandy loam to silty clayloam soils is the most susceptible to surface sealingand crusting (Courty, 1985). Valentin (1985)relatedthe process of crust formation to low organic mattercontent in arid soils. There is evidence that inappro-priate agricultural management has resulted in the de-terioration of some of the arid and semi-arid areas ofthe world. According to some work done on oxisolsin Brazil, Sombroek (1985)found that soils associatedwith crusting problem had low iron content, high siltcontent and/or higher activity clay minerals.

Water scarcity is the major problem limiting crop pr-oductivity in semi-arid Mbeere district, but little atten-tion has been given to the soil management practicesthat can address this problem for increased crop pro-ductivity. Tillage practices involving annual ploughingwithout other soil management practices and whichhave been widely practised in the lower semi-aridMbeere, are increasingly being recognised to havedeleterious effects on soil conditions (Briggs et al.,1998). To effectively use any rainfall that falls in thesemi-arid areas, appropriate soil management practi-ces and tillage methods are required that not only redu-ces crust formation and enhances rainfall penetration,but also conserve adequate soil water for plant growth.This study aimed at testing different land use manage-ment practices that can reduce crust formation and im-prove water entry into the soil at the on set of the rains.

2. Materials and methods

2.1. The study area

This study was conducted at Machanga location,Gachoka division of Mbeere district. It is about 45 km

south of Embu town. It has centre coordinates of0◦45′S and 37◦40′E with an altitude of 1067 m abovesea level.

The area falls in agro-climatic zone IV-I and classi-fied as semi-arid. It is fairly hot to very hot (Sombroeket al., 1982). It experiences a bimodal pattern of rain-fall. The first rains of the year traditionally referredto as long rains, come in the months of March toMay with a peak in April. Following the long rains isan extended dry period that extends to mid Octoberwhen the second season starts. The short rains havetheir peak in November and begin to taper off towardsmid December. The average rainfall in the study siteis 600 mm based on a 10-year period, Machanga metsite. The driest month of the year is September with anaverage monthly rainfall of 1.93 mm and the wettestis November with an average rainfall of 183.55 mm.The annual evaporation is 1955 mm.

The soils at the site are classified as ferric lixisolsaccording toFAO (1997)system. The soils are moder-ately deep, well drained brown to dark reddish brown,sandy loam in the topsoil tending to sandy clay at thesubsoil. They tend to compact when dry but friablewhen wet.

They are also highly weathered and show a verygentle clay illuviation, resulting in a clay bulge over atraject of at least 70 cm. The moisture increases downthe profile (Table 1). This is influenced by the clayincrease down the profile.

Table 1Soil properties at the experimental site

Depths (cm)

0–5 5–7 7–23 23–43 43–70

pH (KCl) 5.4 5.41 4.85 5.09 6.14Organic carbon 0.53 0.52 0.48 0.42 0.35Texture LSa LS LS SCb SC

kPa values Soil moisture content (v/v) 199810 ndc 10.6 11.7 16.3 17.920 nd 7.8 9.6 13.0 15.650 nd 5.7 8.0 10.3 13.4

500 nd 3.7 5.6 6.80 8.501500 nd 2.6 4.1 4.30 3.90Bulk density

(kg/m3)1.2 1.4 1.4 1.4 1.4

a Loamy sand.b Sandy clay.c Not determined.

P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184 175

The area is situated in an upland with flat to gen-tly undulating topography. The slopes gradient rangesfrom 0 to 2%.

2.2. Experimental design and layout

The experiment was a factorial randomised com-pletely block design with each block having seventreatments each replicated five times.

The size of the plot was 10 m×10 m while the sizeof each block was 50 m× 10 m. The experiment wascarried out for five seasons starting from short rains(SR) 1998 to long rains (LR) rains 2001. A brief desc-ription for each of the above treatments is given below.

Bare conventional tillage(BC). This is the commontype of land preparation where conventional tools suchas fork jembes and pangas are used for cultivation.This method is popular amount the farmers. The top15 cm of surface soil layer is inverted, resulting inmedium size clods. The previous seasons crop residueand weeds are gathered and sometimes burned beforetillage.

Bare minimum tillage(BM). This is a conservationtillage practice whereby during land preparation weedsare killed by herbicide (Round Up and Lasso atrazine).Round Up was used during post-emergence at the rateof 1.2 l ha−1 and Lasso atrazine at pre emergence atthe rate of 2 l ha−1. In this study, planting was doneby disturbing the soil only where the seed was placed.

Surface mulch and conventional(SMuC). Maizestover was used as mulch at the rate of 3 t ha−1. Theapplication of the mulch in the plot was done ran-domly. Studies conducted byOkoba et al. (1998)inthe study area indicated that farmers can satisfy theother stover needs and still be left with enough toapply as surface mulch. Conventional tillage was ac-complished mainly using hand tools such as hoes andfork jembesbefore residue placement. Digging wasdone to a depth of 10 cm. Only one operation wasdone before planting in all the seasons.

Incorporated mulch and conventional(IMuC). Sur-face mulch and conventional tillage practice was re-peated for this treatment except that maize stover waschopped and incorporated into the soil.

Manure and conventional(MaC). This is a prac-tice whereby manure is placed randomly at the rateof 3 t ha−1 on the surface and then the plot is conven-tionally tilled hence incorporating the manure into the

soil. In the study area farmers prefer to use goat ma-nure because it is readily available.

Manure and minimum(MaM). In these plots, nodisturbance was done to the soil surface except whenplanting, whereby disturbance was done where theseed was placed. Weeding was done using herbicide(Round Up) the rate of 1.2 l ha−1. Three 3 t ha−1 ofmanure were spread in the plot randomly.

Surface mulch and minimum(SMuC). This is thepractice where the residue mulch is placed randomlyin an undisturbed soil surface at the rate of 3 t ha−1.

The study was conducted from 1998 short rains to2001 long rains. During the first season (1998), all theplots were planted with a uniform crop of maize with-out application of any treatments. The crop spacingfor maize was 75 cm× 50 cm. This was to asses thehomogeneity of the plot in terms of soil fertility. Inorder to arrive at the blocking of the treatments. Theresidue material from the crop harvested during thefirst season was used as residue mulch and this wasdone in all the subsequent seasons.

2.3. Data collection/measurements

Crust strength was measured using a penetrometer(type 1B). It was pushed into the soil at a constant rateand the following formula was used to calculate thepenetration resistance:

CR = I × Cs

AC

where CR is the cone resistance (N cm−2), I the im-pression on the scale (cm),Cs the spring constant(N cm−1), AC the area of cone (cm2).

Crust thickness (mm) was measured using a caliper.Measurements of strength and thickness were based

on five replicates at each sampling site (treatment) andat each selected measurement time during the season.

Whenever crust strength was measured, a corre-sponding crust thickness was done. Replicate mea-surements were taken at least 0.5 m apart within eachtreatment.

Infiltration rates were determined using a discpermeameter. The disc was driven into the soil ap-proximately 4 mm deep, caution being taken that thepenetration was uniform and vertical.

More details on how to use a disc permeameter isgiven by (Perroux and White, 1988).

176 P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184

The degree of aggregation and aggregate size distri-bution were determined by dry sieving. The soil sam-ples were air dried and put on top of a set of sieves of2, 1, 0.5, 0,25 and 0.15 mm, fixed on the vibrax shakerwith a unit timer.

After shaking the samples for 5 min, the weight offractions of the sample retained on the sieves wereweighed and the size fraction on each sieve deter-mined. The mean weight diameter (MWD) i.e. sumof each fraction times the corresponding mean meshsize of the two sieves passing and retaining the frac-tion was determined and the following formula wasused to calculate MWD: MWD= ∑

xiwi, wherexi

is the mean diameter of each size fraction andwi isthe proportion of the total sample weight occurring inthe corresponding size fraction.

Moisture content on a dry weight basis was deter-mined gravimetrically.

2.4. Data analysis

The data collected was then subjected to analysis ofvariance (ANOVA) to evaluate the treatment effects.Step-wise multiple regression analysis on pooled data(average over all seasons) was done to determine themost significant factors to the treatment responses.The statistical package used to analyse the data wasMSTATC (ver. 1-4).

3. Results and discussion

3.1. Soil characteristics

The soil characteristics are given inTable 1. In theupper 5 cm layer clay and silt content range from 17 to29%. The coarse sand fraction accounts for 77% of thetotal sand content. Organic carbon content was verylow (0.44–0.87%). The results indicate that the soil atthe site is loamy sand at the top and sandy clay at thesubsoil with a low water holding capacity. These soilsare prone to crusting and generated runoff even aftera small shower. Clay mineralogy was approximately75% well crystallised illite and 25% kaolinite.

These soils when exposed to rain, aggregate disin-tegration occurs and there follows a clay dispersion.This phenomenon is explained by the results wherebythe soils which were bare had more finer particles

compared to the other treatments. The treatments withmore finer particles (<0.1 mm size class) were proneto soil crusting. A disrupted layer was observed andthis was probably formed by aggregate slaking andthe rearrangement of disrupted aggregates into a com-pacted layer of varying thickness. This phenomenon isexplained very well by the high initial infiltration ratesin the conventional tilled plots which slowed downwith time. This behaviour was attributed to the slakingnature of the disturbed soils. However, the initial infil-tration was slower in the minimum tilled (MT) plotscompared to CT but increased after some seconds andthese rates were maintained at higher rates up to thesteady state. The disrupting mechanism was thoughtto have been controlled by the soil structural stabil-ity and the kinetic energy of the raindrops. It was ob-served that crust thickness and strength increased withaggregate size in these studies, a phenomenon whichwas observed byMoldenhouer and Kemper (1969),Farres, 1978andChiang et al. (1994).

From the multiple regression conducted on the data,the soil parameter that correlated best atP < 0.001with crust strength was crust thickness. This was ex-pected since there is always a strong relationship be-tween the two. The soil physico-chemical propertiesthat correlated(P < 0.05) with crust strength weresoil structural stability (MWD) and organic carbon.During this study, the soil physico-chemical proper-ties that best described soil surface crust formationwere structural stability (MWD) and organic carbon(Table 2).

Another factor apart from aggregate stability, whichwas found to contribute to surface soil crusting wasthe particle size distribution. Silt content was lowercompared to sand in all the treatments. Between 34and 38% of the sand fraction was fine sand in all thetreatments. Silt was in the range of 9–13% in most ofthe treatments.

It means therefore that more than 50% of the particlesize distribution fell into the silt and fine sand fractionrange. Predominance of silt and sand particles in asoil can significantly affect soil behaviour in termsof structure (Carter, 1987). In the case of this study,the soils are very unstable due to the high contentsof fine sand and silt particle size distribution range.According toGreenland (1971)red brown earth soilsin Australia with more than 10% silt and the ratio offine to coarse sand greater than 3 are subject to slaking

P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184 177

Table 2Correlation between crust strength, soil management practices and some physico-chemical soil properties

Treatments Bulk density (r) MWD (r) Crust thickness (r) Organic carbon (r) Infiltration rate (r)

BC −0.16 −0.26 0.25 0.12 0.29BM −0.11 0.01 0.25 −0.04 −0.02IMu C −0.12 −0.25 0.49∗∗∗ −0.05 −0.08MaC −0.11 −0.34∗ 0.32∗ −0.04 −0.32∗MaM −0.18 −0.13 0.50∗∗∗ 0.01 −0.05SMuC −0.0.6 −0.30∗ 0.29 0.03 −0.04SMuM −0.19 −0.27 0.36∗ 0.04 −0.04

∗ P < 0.05.∗∗∗ P < 0.0001.

and breakdown of structure. Soils with relatively highsilt and fine sand have a high tendency for structuralinstability and compaction (Bately and Davies, 1971)especially if organic carbon is low. The soils in thestudy site do full fill above criteria of high fine sandand silt in all the treatments.

The differences in the crust behaviour were appar-ent in both tillage systems and also in the manage-ment practices. However, as shown in the infiltrationrates and MWD results (Figs. 1 and 2) there was anapparent relationship between aggregate stability andcrusting dynamics in the treatments studied. Indeed,the treatments showing the poorest aggregate stabil-ity showed the strongest crusts. It can be concludedtherefore that the soil management practices, which

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

16/

9/98

8/1

2/98

22/

2/99

10/

3/99

6/7

/99

1/1

0/99

3/7

/200

0

3671

7

21/3

/200

1

Time(days)

MW

D(m

m)

BC BM ImuC MaC MaM SMuC SMuM

Fig. 1. Mean weight diameter of soil particles for different soil management practices.

contributed more to the aggregate stability, should berecommended for the management of surface crusts.

3.2. Crust strength and thickness

The results indicate that there were large seasonalchanges in the field measurements of crust strengthand thickness over the two seasons. However, the ini-tial figures of crust strength and thickness were highand this was probably due to the land use practised inthe area by the farmers prior to the experiment. Thesevalues probably had an effect on the final values ofthese two parameters.

Strong temporal variations in both parameters wereevident in all the treatments. This was most apparent

178 P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16/

9/98

8/1

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22/

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3671

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Time (days)

Infil

trat

ion

rate

(cm

/h)

BC BM ImuC MaC MaM SMuC SMuM

Fig. 2. Steady infiltration rates as influenced by different soil management practices over 3-year period.

in BC and MaC soil management practices and tillagemethods. While there were significant differences(P < 0.05) in crust strength and thickness betweensampling times, there were small variations in all thetreatments. However, there was statistical significancebetween the treatments atP < 0.05. There was founda correlation between crust strength and thickness atP < 0.05.

When the percentage emergence of maize seedlingswas evaluated, it was found that seed emergence wasrestricted by crusting in minimum tilled treatmentswhereby the percentage emergence was 83, 66, 74, 71,70, 73, and 66 and for BC, BM, IMuC, MaC, MaM,SMuC, and MaM, respectively. This is in agreementwith work done byNjarui et al. (2000)in Australiawhere seedlings emergence was restricted at between0.61 and 1.98 kg cm−2.

All the soil management practices and tillage meth-ods influenced the decrease of crust strength and thick-

Table 3Effect of tillage methods on crust thickness (mm) and strength (kg cm−2)a

Trts Crust thickness Crust strength

Conventional tillage Minimum tillage Mean Conventional tillage Minimum tillage Mean

Bare 2.10 cd 2.52 a 2.31 a 2.44 b 3.02 a 2.73 aManure 1.72 e 2.20 bc 1.96 c 1.77 d 2.41 b 2.09 bSurface mulch 1.96 d 2.30 b 2.13 b 2.22 bc 2.10 c 2.16 b

Mean 1.93 b 2.34 a 2.14 b 2.51 a

a Means followed by the same letter are not statistically significant at 5%.

ness over time. It was established that crust water con-tent during the sampling time had an influence on cruststrength. The treatment with the strongest crust hadthe least water content. To explain these seasonal dif-ferences, an analysis of variance (ANOVA) was doneto show the effects of soil management practices andtillage methods on crust strength and thickness. It wasestablished that minimum tillage had stronger cruststhan conventional tillage atP < 0.001. However, bareand surface mulched plots had stronger crusts thanmanured plots (Table 3). When an ANOVA was doneon crust thickness data during the same period, it wasfound that there were significant differences withinthe tillage practices atP < 0.001 and between thesoil management practices. Manure and surface mulchplots were found to differ with bare plots significantlyas seen inTable 3.

It can be deduced from the results that the crustthickness and strength were influenced by both tillage

P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184 179

system and management practices and in effect theseaffected the seedling emergence and hence maizeyields in all the seasons. During the seasons, weedingwas carried out manually with hand hoes in the con-ventional tillage treatment, and therefore this tillagemethod destroyed the crusts. These crusts howeverquickly re-established after a slight shower as wasobserved in the field. This phenomenon has also beenobserved in West Africa (Rockstrom and Valentin,1997). Minimum tillage was done using herbicidesand therefore crusts were not disturbed at all. Thiscould explain why minimum tillage had strongercrusts than conventional tillage. Manure was able toimprove the organic carbon values during the studyperiod. This in essence improved the structural stabil-ity of the soils within the experimental site. Duringthe study MaC and MaM treatments had the most sta-ble aggregates. These management practices thereforehad the lowest risk of crust formation.

When the soil management practices were consid-ered, it was clear that manure and surface mulch hadless strong crusts as compared to bare plots. In themanured plots, it was observed that earthworm ac-tivity was higher than in the other treatments. Theyassisted in working out the soil and hence destroy-ing the crusts. Earthworms are known to perforate al-ready formed surface crusts thus enhancing infiltration(Valentin et al., 1991). The surface mulch on the otherhand can trap the raindrop energy and hence reducetheir impact on the soil surface and this determinesthe type of crust formed. Bare plot had the strongestcrusts and this could be attributed to lack of distur-bance of the crusts, poor soil structure, low organicmatter and high rainfall kinetic energies impacting onthe soil surface.

The studies established that there were large sea-sonal changes in field measurements of crust strengthand thickness over the season. All the soil managementpractices and tillage methods managed to influence thedecrease of both parameters within the two seasons.Minimum tillage was found to have stronger cruststhan the conventional tillage. However, bare and sur-face mulched plots had stronger crusts than manuredplots. When an analysis of variance was done for crustthickness, it was found that there were no significantdifferences when tillage methods were compared al-though there were significant differences within thesoil management practices. It can be concluded from

the results that both parameters were influenced bytillage methods and soil management practices. How-ever, manure had weaker crusts than surface mulchand bare treatments.

3.3. Aggregate size distribution

The results show that the MaC, MaM and surfacemulch with conventional tillage contained the high-est proportion of >2 mm dry aggregates which weresignificantly greater atP < 0.05 than in BC, SMuM,IMuC and BM, respectively (Fig. 3).

These results show that MaC, MaM and SMuC hadthe most stable aggregates among the treatments hencea stable soil structure.

Treatment trends were similar for other aggregatesizes between 2 and 0.25 mm with SMuM having 37%of all aggregate between this range followed by BC35%, IMuC 34%, SMuM 33%, MaM 32% and BMand MaC 31%. The trends in the wind erodible frac-tion reversed (0.25–0.1 mm size range) such that BChad the highest (26%) followed by SMuM (13%), BM(12%) and IC (10%), while all the other treatmentshad an average of 7%. This shows that BC had thepoorest structural stability.

Fig. 3. Aggregate size distribution (mm) and percentage aggregate(per size) for the treatments.

180 P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184

In most of the treatments the majority of the ag-gregates were in the >2 mm size. However, MaC andMaM management practice had a significantly higherpercentage than the other treatments in this aggregatesize fraction and consequently fewer micro-aggregatesin the <0.1 mm classes. This is in agreement with(Cunha et al., 1996) who found that higher percent-age of micro-aggregates were in no till and mini-mum tillage. The results also varied slightly betweenfractions than between management practices espe-cially in the 2–0.25 mm fraction. In general the beststructural stability was recorded for the size fraction>2 mm and the relative increase in macro-aggregationunder the management practices and the relative de-crease in small size aggregates<0.1 mm supportsthe conceptual model ofTisdall and Oades (1982)which proposes that small aggregates serve as build-ing blocks for larger aggregates. At the same time,the variability of the results decreased in aggregateclass indicating larger aggregates were more hetero-geneous than the smaller ones. Although there wereno significant differences between the treatment in thesize range 2–0.25 mm, there was a tendency of ma-nure treatments to show lower values than the othertreatments.

3.4. Mean weight diameter (MWD) of aggregates

When mean weight diameter was considered as ameasure of aggregate stability, it varied significantlyamongst the management practices. The averageMWD ranged from 0.32 to 0.26 mm within the sea-sons with MaM having the highest and BC havingthe lowest (Fig. 1).

On average, under manure and minimum tillageMWD of aggregates increased significantly during the3 years of the study period.

These results have shown that rapid temporalchanges in the relative stable aggregates, as expressedby the MWD parameter, are mainly related to highvalues of >2 mm aggregates among the managementpractices. Given the slightly greater organic carboncontents observed in MaM, MaC and SMuC com-pared to the other treatments, these same treatmentshad the best structural stability. The data indicate thatminimum and conventional tillage with manure couldimprove aggregate distribution and stability for theseloamy sandy soils over the 3-year period.

The treatment with the least organic carbon con-tent had the lowest MWD indicating that aggregatestability was the poorest in latter treatment. TheMWD ranged from 0.32 to 0.26 mm suggesting thatthese soils are in very unstable category. AccordingBissonnais and Arrouays (1997)stable soils haveMWD ranging from 0.7 to 1.5 mm and unstable soilsfrom 0.3 to 0.6 mm in France.

3.5. Infiltration

Infiltration rate is a surface phenomenon, influencedby soil chemical and physical characteristics and man-agement practices. All data show clear relationshipbetween the steady infiltration rates and soil manage-ment practices (Fig. 2). The steady infiltration rates inmanure treatments with minimum tillage was 34, 25,24, 17.8, 23 and 6% higher for BC, BM, IMuC, MaCSMC and SMuM, respectively.Kelly (1973) foundtenfold lower steady infiltration rates in bare areas thanin littered (mulched) areas (double dug).

Manure and surface mulch with minimum tillageshowed significant effect(P < 0.05) in all the years,followed by SMuM, BM and BC. Manure was instru-mental in improving structural stability and the steadyinfiltration rates of the crusted soils of the experimen-tal site. The increased infiltration rates promoted theavailable soil water. This study shows that the steadyinfiltration rates of these soils were strongly influencedby soil management practice rather than tillage. Thebare in both minimum and conventional tillage had rel-atively slow infiltration rates whereas the MaM, MaCand SMuM plots had rapid to very rapid steady infil-tration rates. Organic carbon, crust strength and bulkdensity explained most of the variation in steady in-filtration rates in the site.

Infiltration rates increased irregularly in all the treat-ments between 1998 and 2001. The results proved thatminimum and conventional tillage with manure andsurface mulch are key to improvement of infiltrationrates of crusted soils of the research area. Throughoutthe experimental period, manure and mulch placedon the surface with both minimum and conventionaltillage were found to improve the steady infiltrationrates of soil. The results are in agreement with thoseof Maurya and Lal (1980)that mulch cover of maizestover and weeds improved infiltration rates morethan ploughed furrows. The low infiltration rates in

P.G

iche

rue

ta

l./So

il&

Tillage

Re

sea

rch7

5(2

00

4)

17

3–

18

4181

Table 4Available soil water content (V/V) as influenced by soil management practices (0–7 cm)a,b

Trts Dates Mean

16 September1998

8 December1998

22 February1999

10 March1999

13 May1999

6 July1999

1 October1999

24 February2000

3 July2000

7 October2000

12 February2001

21 March2001

BC 5.98 5.92 4.06 12.02 5.24 5.62 5.04 3.74 4.90 5.22 6.60 6.14 5.87 dBM 6.64 7.24 4.72 9.10 5.78 5.38 5.06 4.32 5.38 7.22 7.96 7.38 6.35 cImuC 5.52 5.58 4.70 10.12 5.86 6.34 4.80 4.42 5.18 5.88 8.00 6.54 6.08 cdMaC 6.70 7.00 7.32 13.20 7.26 6.98 5.82 5.84 5.84 7.46 9.02 7.74 7.52 aMaM 6.68 7.14 7.10 13.54 7.22 6.98 6.40 6.08 5.98 7.62 9.02 7.74 7.63 aSmuC 5.10 6.30 5.26 12.04 5.76 5.36 4.92 4.92 5.16 5.78 7.32 6.30 6.19 cdSmuM 6.68 7.22 6.30 13.50 6.54 6.94 5.78 4.76 5.82 7.54 7.90 7.18 7.18 b

Mean 6.19 e 6.63 cde 5.64 f 11.93 a 6.24 de 6.23 de 5.40 f 4.87 g 5.47 f 6.67 cd 7.97 b 7.00 c

a CV = 13.76%, LSD0.05 (date) = 0.433, S.E.(date) = 0.7169, LSD0.05 (TRT) = 0.3307, S.E.(TRT) = 0.1188, LSD0.05 (date× TRT) = 1.146.b Means followed by the same letter are not statistically significant at 5%.

182P.

Gich

eru

et

al./S

oil

&T

illageR

ese

arch

75

(20

04

)1

73

–1

84

Table 5Available soil water content mean values for the five seasons as influenced by soil management practices (7–23 cm)a,b

Trts Dates Mean

16 September1998

8 December1998

22 February1999

10 March1999

13 May1999

6 July1999

1 October1999

24 February2000

3 July2000

7 October2000

12 February2001

21 March2001

BC 7.62 6.20 6.60 11.20 7.74 5.98 7.12 4.12 3.70 4.98 5.24 4.80 6.28 bBM 6.92 5.82 8.14 10.28 6.82 6.82 6.86 4.48 4.30 6.04 7.28 6.72 6.71 bIMuC 7.12 5.96 6.32 10.80 7.08 6.70 7.26 5.90 4.16 5.56 7.12 5.54 6.63 bMaC 7.16 6.10 8.86 14.84 8.30 6.88 8.14 7.68 4.50 7.52 7.72 7.40 7.93 aMaM 7.42 7.10 9.88 15.22 7.34 7.24 8.92 7.04 5.12 8.14 8.50 8.16 8.34 aSMuC 5.32 6.74 8.92 12.70 7.96 5.54 7.90 4.90 5.08 5.12 5.82 6.10 6.84 bSMuM 7.24 6.40 6.44 10.62 6.66 6.06 7.34 3.82 4.46 5.66 7.38 7.20 6.61 b

Mean 6.97 cd 6.33 de 7.88 b 12.24 a 7.41 bc 6.46 de 7.65 bc 5.42 f 4.47 g 6.15 ef 7.01 cd 6.56 de

a CV = 22.26%, S.E.(date) = 0.4992, S.E.(TRT) = 0.2025, S.E.(date×TRT) = 0.7014, LSD0.05(date) = 0.7379, LSD0.05(TRT) = 0.5636, LSD0.05(date×TRT) = 1.952.b Means followed by similar letters are not significantly different at 5.

P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184 183

BC, IMuC and BM was attributed to poor aggregatesize distribution and stability, thereby causing surfacecrusting. The study shows that manure and surfacemulching had a stronger influence on soil structure interms of enhanced water entry into the soil, causingincreased water availability.

3.6. Available soil water content

The effect of tillage systems and soil manage-ment practices on gravimetric water content in thetwo depths (0–7 and 7–23 cm) was monitored fromSeptember 1998 to March 2001. Data presented aremeans of minimum and conventional tillage and sevensoil management practices.

The treatments had almost similar available water inthe 0–25 cm soil depth when the studies were started inSeptember 1998. The difference between MT and CTon soil water did not vary through cycles of rechargeand drying (Tables 4 and 5). The difference in soil wa-ter appears to be more related to infiltration and evap-oration owing to management practices rather than theinitial loss of water through runoff.

Soil water content was greater in MT treatmentsthan in CT treatments on all the dates sampled withinthe experimental period. The rainfall influenced thesoil water throughout the study period.

The MT treatments enhanced water uptake into thesoil after the onset of the rains compared to CT. Whenthe amount of water for each management practicewere averaged for all the seasons, a significant differ-ence was found at(P = 0.05). MaM and MaC hadsignificantly higher soil water content than the othertreatments. Soil water content was higher under MaM,SMuM and MaC than in the other treatments dur-ing most of the study period and in 0–7 and 7–23 cm(Tables 4 and 5). The higher water content found un-der MaM and SMuM was attributed both to highersteady infiltration rates, better cover which reducedthe rainfall kinetic energy and also to better structuralstability contributed to by higher organic carbon. Thebare crusted soil surface favoured high evaporationand runoff hence low steady infiltration rates.

The available soil water ranged from 6.19% vv in1998 to 7.0% v/v by 2001 with manure and mini-mum tillage having the highest soil water followed byMaC. The lowest average available water was foundin the BC. The MaM and SMuM, MaC treatments

commonly had higher soil water contents than BC,BM, SMuC and IMC in both 0–7 and 7–23 cm depth.The differences in soil water between the manage-ment practices appear to be as a result of the differ-ences in the effects of soil management practices i.e.manure and surface mulch and tillage systems on in-filtration which was attributed to improved soil struc-ture whereby manure with minimum tillage was moresuperior in both tillage systems followed by surfacemulch with minimum tillage. MaM and SMuM treat-ments often showed greater soil water content thanthe other treatments indicating higher infiltration ratesand also deeper water movement. The differences inthe effects of the treatments on soil water content isas a result of steady infiltration rates, poor structuralstability and high incidence of soil crusting.

Manure and surface mulching increased water stor-age capacity hence more available water in the soil.Water storage as reflected by the available water onmanure and surface plots is related to the amount ofwater input by infiltration and was significant atP <

0.05 compared to the other treatments.The increased moisture in the soil under MT com-

pared with that under CT in the present study agreeswith results reported byEhlers (1975)whereby in-creased soil water content under reduced tillage re-flected the decrease in evaporation as well as enhancedinfiltration. Another reason which could have causedthe differences in soil moisture characteristics in the0–25 cm soil depth was the reorganisation of soil struc-tural units owing to tillage and difference in porositydue to the slaking nature of these soils.

A high proportion of aggregates less than 0.1 mmoccurred in BC, BM, IMuC and SMuM treatment andthis was associated with high crusting, low infiltrationand hence low soil water content.

4. Conclusions

This study therefore confirms that manure andmulching with minimum tillage have a greater effecton the water balance of crusted soils, maize emer-gence and yield. It increases the infiltration rates,amount of soil water stored in the 0–25 cm soil depthand better drainage. The physical effect of mulchwas less important in the rehabilitation of crustedsoils in the study site when it was incorporated into

184 P. Gicheru et al. / Soil & Tillage Research 75 (2004) 173–184

the soil. Manure and surface mulch with minimumtillage should therefore be taken into account in landmanagement and water conservation in the semi-aridareas of Kenya. Indeed, it is clear that the response ofcrops to the improved water availability due to MaM,MaC and SMuM is recommended when consider-ing the effectiveness of soil and water managementtechniques in the study area.

The results reported here, serve to emphasise thepotential value of soil management practices with ma-nure and surface mulch with minimum tillage as toolsfor avoiding crop losses from drought in the area un-der study through soil moisture conservation.

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