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Soil nutrient status of smallholder farms in MalawiS. S. Snappa

a International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Lilongwe, Malawi

To cite this Article Snapp, S. S.(1998) 'Soil nutrient status of smallholder farms in Malawi', Communications in Soil Scienceand Plant Analysis, 29: 17, 2571 — 2588To link to this Article: DOI: 10.1080/00103629809370135URL: http://dx.doi.org/10.1080/00103629809370135

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COMMUN. SOIL SCI. PLANT ANAL., 29(17&18), 2571-2588 (1998)

Soil Nutrient Status of Smallholder Farmsin Malawi1

S. S. Snapp2

International Crops Research Institute for the Semi-Arid Tropics (ICRISAT),P.O. Box 1096, Lilongwe, Malawi

ABSTRACT

A soil sampling exercise was conducted throughout 60% of the smallholderfarm sector of Malawi, a small country located at the base of the Great RiftValley. Soil samples (1,130) were geo-referenced and analyzed for pH,texture, soil organic carbon (C), phosphorus (P), zinc (Zn), potassium (K),and calcium (Ca) status. Descriptive statistics of soil characteristics wereused to evaluate soil fertility for two agricultural districts in Northern Malawi,two agricultural districts in Central Malawi, and one agricultural district inSouthern Malawi. Generally soils were loamy sands and moderately acid,with "low" to "sufficient" nutrient levels. Over three-quarters of soils sampledhad organic C levels which were greater than 0.8%. This indicated that organicC status was adequate in the main to maintain soil structure, although muchreduced from the non-cultivated state. The organic C data supportedobservations of widespread nitrogen (N) deficiency in Malawi. High spatialvariability of P and Zn values was noted. However, over 60% of soils had a

1Research funded by the Rockefeller Foundation, P.O. Box 30721, Lilongwe 3, Malawi,in cooperation with the Malawi Ministry of Agriculture.2E-mail address: [email protected].

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P status above the critical value of 15 mg kg-1 which was sufficient forsmallholder maize production levels. This finding supported efforts to alterthe current country-wide fertilizer recommendation of 45 kg ha-1 phosphate.New findings reported here were location specific edaphic problems: i)widespread moderate soil acidity in Central Malawi, ii) natural regions inCentral and Southern Malawi which were low in P and Zn, and iii) naturalregions in Southern Malawi were very low in organic C. Researchrequirements were identified. Suggested priorities for technologydevelopment were those appropriate to smallholder farmers, such as combineduse of small amounts of inorganic and organic fertilizers to address verynutrient deficient soils.

INTRODUCTION

Soil fertility status is the foundation of cropping system productivity in thesmallholder agriculture sector of Southern Africa. A case in point is Malawi. Asmall country located at the base of the Great Rift Valley, over 85% of thepopulation is engaged in smallholder agriculture. Use of fertilizer inputs in thissector is about 20 kg ha1 (Heisy and Mwangi, 1996). Fertilizer recommendationsin Malawi have been broadly written, with limited attention to resource variabilityand a cash-constrained farming environment (Snapp and Benson, 1995; Kumwendaet al., 1997). Nitrogen (N) has been shown to be generally the most crop limitingnutrient. Phosphorus (P), sulfur (S), and zinc (Zn) responses have also been shownfor specific crops and regions in Malawi (Brown and Young, 1966; Chilimba,1996; Snapp et al., in press). A better understanding of soil nutrient status inMalawi would facilitate development of targeted recommendations. Technologiesto maximize nutrient use efficiency and improve return on a scarce resource areurgently needed.

Current fertilizer recommendations do not take into account diversity in soiltypes, farmer resources or yield goals. For example, a blanket fertilizerrecommendation of 92 kg N ha"1 N and 45 kg P205 ha"1 ha is used for hybrid maize(Zea mays L.) production throughout the country, where maize is the dominantcrop in Malawi smallholder agriculture (Guide to Agriculture Production, MalawiMinistry of Agriculture and Livestock Development, Lilongwe, Malawi, 1995).Several decades of research in Malawi has addressed maize response to fertilizer(Brown and Young, 1966; Kumwenda et al., 1997). Fertilizer recommendationsfor maize are in the process of being revised based on fertilizer response trialdata. In contrast to fertilizer trials, limited research has addressed soil nutrientstatus and patterns of micro and macro nutrient deficiency (Chilimba, 1996; Wendt,1993). Soil morphological characteristics have been studied in the field, andattempts made to relate parent material to chemical status (Brown and Young,1966; Mandemere and Robertson, 1975).


SOIL NUTRIENT STATUS OF SMALLHOLDER FARMS 2573

Soil types in Malawi are in the main Alfisols, with Oxisols and Ultisols also ofimportance. Soil textures are generally sands and sandy loams (Kumwenda et al.,1997; Snapp et al., 1988). However, it remains a considerable challenge tocharacterize soil fertility of smallholder farming systems in Malawi. Extremes intopography confounded with small field size and diversity in crop managementhistory have led to a situation where soil fertility can vary by orders of magnitudeacross short distances (Snapp and Benson, 1995). Topography in Malawi ischaracterized by a highly dissected and undulating landscape (Brown and Young,1966; Young and Brown, 1962). Texture, soil type and soil organic carbon (C)vary with landscape position. Soil organic C has a regulatory influence on soil N,P, and S availability in the sub-humid tropics (Janssen et al., 1990).

Soil nutrient status characterization provides a foundation for understandinghow to strategically apply fertilizer and organic inputs, and it is a step towardsrevised soil management recommendations. The objectives of this paper were to:i) characterize soil nutrient status (P, Zn, K, Ca), soil texture and pH in keyagricultural regions of Central, Southern and Northern Malawi, ii) quantify therelationship of organic C to other soil characteristics, and iii) develop a spatialunderstanding of soil characteristics.

MATERIALS AND METHODS

A soil sampling exercise was initiated in 1993 by the Maize Commodity Teamin collaboration with the Soil Chemistry and Plant Analysis Team, Department ofAgricultural Research, Malawi Ministry of Agriculture. In cooperation withExtension staff field assistants and district officers, soil samples were collectedfrom over 1,500 smallholder arable fields distributed throughout the country.Information collected at each site included farmer name, village location, croppingsystem for the last two years, fertilizer use history, and slope category. The majorityof fields chosen had not been fertilized in the previous cropping season. Fieldssampled were representative of the dominant cropping systems used bysmallholders, compared to a farm survey conducted by Jones and Sakala (1991).Evaluation of responses on the soil sample data sheet showed that 80% ofsmallholder fields sampled were used to grow maize as a sole crop, or maizeintercropped with minor. The major sole crops in the remainder of fields sampledwere tobacco (Nicotiana tabacum L.), groundnuts (Arachis hypogaea L.), andcotton (Gossypium hirsutum L.).

The agroecosystems of Malawi are dominated by a sub-humid tropical, uni-modal rainfall system, where rainfall varies from 700-1,400 mm annually. Landpreparation in Malawi is almost entirely by hand-held hoes. In the South, a couplemonths after the end of the growing season in July or August, residues and weedsare incorporated in the furrow between ridges. New ridges are then build uponthe former furrow location, in preparation for the next growing season. Ridges


FIGURE 1. Map of Malawi showing the boundaries of the eight agricultural developmentdivisions (ADD). Soil sampling was conducted in five ADDs: Karônga and Mzuzu in theNorth, Lilongwe and Kasungu in the Center, and Blantyre in the South. The shaded areaon the map is Lake Malawi.



are planted with the onset of reliable rains, usually around December. The rainyseason lasts about three to four months. In the Center and North of Malawi, asimilar land preparation and growing season schedule is followed. However,residues are almost always burnt rather than incorporated as they are in the South.

A composite soil sample of 10 sub-samples was collected randomly from the 0-20 cm depth of the ridge. Furrow soil was not sampled. Ridge soil has beenshown to represent the soil environment which crop plants have access to in theMalawi smallholder cropping system (Snapp, unpublished data). Soil was airdried, ground to pass a 2-mm sieve, and analysis conducted. Mehlich 3-extractableP, Zn, K, and Ca were determined, an ammonium fluoride (NH4F) and EDTAbased extradant which is highly correlated with extractants widely used in thetropics, Bray 1 and EDTA (Mehlich, 1994; Wendt, 1995). Soil pH was determinedin 1:2.5 soil/water ratio. Organic C was determined by a wet digestion andcolorimetric method (Anderson and Ingram, 1989). Texture was analyzed bydispersal and hydrometric readings (Anderson and Ingram, 1989).

All locations where soil samples were collected were geo-referenced byextension staff using grid overlays on maps. The UTM coordinates systems wasused. A database of soil and site characteristics was developed. Over 1,500fields were sampled, but the geo-referencing system was suspect in some cases (ageopositioning system, GPS, has recently replaced the map grid overlay systemfor greater reliability). A subset of 1,130 soil samples were identified as reliablefrom the Mzuzu, Karonga, Kasungu, Lilongwe, and Blantyre administrative units(agricultural development divisions, ADD) in Malawi. The first two districtsrepresent the North, the second two districts the Center, and the final district theSouthern region of Malawi (Figure 1). Nutrient status, soil texture, and slope ateach site sampled was used to develop descriptive statistics for soil samples fromthese districts which represents about 60% of the land area of Malawi.

Descriptive statistics (mean, mode, and range) from the soil sample databasewere analyzed based on location, for the five agricultural development divisions,and for 88 natural regions in Malawi. The natural regions were defined byagroecological groups as defined through physiography, climate, and landsuitability classes by the Lands Evaluation Project (Venema, 1990). The meanvalues and standard deviation for soil characteristics for each natural region wasevaluated to detect locations which were above or below the mean for the country.A positive, stepwise multiple regression to determine characteristics which predictorganic C was carried out for each district and for all five districts together(Statistica, 1993).

The database is in the process of being continually updated, and it can be relatedto socio-economic or other biophysical databases using the site location information(W. Makumba, personal communication, 1997). This soil database has been usedto chose representative on-farm sites to locate nutrient response trials (Snapp andBenson, 1995).


2576 SNAPP

TABLE 1. Descriptive statistics for soil and site characteristics of 460 samples fromsmallholder fields in Mzuzu agricultural development division (Mzuzu ADD) andKaronga agricultural development division (Karonga ADD) in Northern Malawi. Criticalsoil test values used: sand, 85%; organic C, 0.8%; pH, 5.2; Zn, 0.5 mg kg"1; Ca, 0.2cmolc kg-l; K, 0.2 cmolc kg'1; and P, 13.0 mg kg"1. Multiple regression of soilcharacteristics which contribute to organic C status showed that in Mzuzu ADD soiltexture was the only significant variable [(R=0.40) Y=12.8+(-0.13*sand % X)] and inKaronga ADD soil texture and Ca were significant variables [(R=0.30)Y=4.9+(-0.03*sand % X)+(0.06*Ca X)].

Characteristic

Mzuzu ADDSlopeSandOrganic CPHZnCaKP

%%

mg kg'1

cmolc kg"1

cmolc kg1

mg kg1

Karonea ADDSlopeSandOrganic CpHZnCaKP

%%

mg kg1

cmolc kg1

cmolckg"'mg kg-1

%>Critical

87.272.487.192.598.274.157.9

88.980.187.994.099.588.558.8

Mean

5.175.1

1.25.71.42.30.5

21.7

6.376.1

1.36.21.83.10.7

30.1

Std.dev.

4.610.40.40.51.41.10.5

23.9

4.510.60.50.81.22.20.4

29.6

Median

4.974.0

1.15.71.02.00.3

14.3

5.578.0

1.16.11.52.50.6

20.2

Minimum

1.047.00.24.40.20.10.11.0

1.052.00.44.00.00.10.10.1

Maximum

19.090.03.07.75.25.55.1

184.1

19.094.03.38.36.46.22.5

163.0

Corr. Rorg. C

ns0.40nansnsnsnsns

ns0.15nansns

0.07nsns

•na=not applicable, ns=not significant.

RESULTS AND DISCUSSION

Acidity

Severe soil acidity is not a major edaphic problem for smallholder fields inMalawi. Moderate soil acidity may constrain yields in some areas. The overallmean pH value was 5.8 (Tables 1, 2, and 3). This supports earlier studies inMalawi which report average soil pH values in the range of 5.7 to 6.0 (Weil andMughogho, 1993; Wendt, 1993; Young and Brown, 1962). The critical value foracidity below which maize production is affected in the tropics has been shown tobe between 5.3 and 5.1 (Landon, 1991). Soil acidity did not reduce maize yields



TABLE 2. Descriptive statistics for soil and site characteristics of 450 samples fromsmallholder fields in Lilongwe agricultural development division (Lilongwe ADD) andKasunga agricultural development division (Kasunga ADD) in Central Malawi. Criticalsoil test values used: sand, 85%; organic C, 0.8%; pH, 5.2; Zn, 0.5 mg kg"1; Ca, 0.2cmolc kg'1; K, 0.2 cmolc kg"1; and P, 13.0 mg kg'1. Multiple regression of soil characteristicswhich contribute to organic C status showed that in Lilongwe ADD soil texture and Cawere significant [(R=0.43) Y=3.3+(-O.O3*sand % X)] and in Kasunga ADD soil texture,Ca, and slope were significant [(R=0.46) Y=6.6+(-0.07*sand % X)+(0.03 slope %X)+(0.01*CaX)].

Characteristic

Lilonewe ADDSlopeSandOrganic CPHZnCaKP

%%

mg kg-1

cmolc kg'1

cmolc kg'1

mg kg'1

Kasunça ADDSlopeSandOrganic CpHZnCaKP

%%

mg kg'1

cmolc kg"1

cmolc kg'1

mg kg'1

%>Critical

84.890.666.197.1

100.088.564.3

88.891.899.291.9

100.083.659.3

Mean

4.774.7

1.65.41.44.10.4

31.3

4.878.11.75.91.51.90.5

24.2

Std.dev.

4.59.20.60.41.02.80.2

29.7

4.49.10.80.41.90.80.4

21.5

Median

4.578.0

1.55.41.33.30.4

21.4

4.578.0

1.65.91.21.70.4

17.5

Minimum

1.052.00.44.70.20.80.10.6

1.058.00.34.30.10.20.10.1

Maximum

19.092.03.36.86.0

21.01.1

130.2

19.098.04.67.25.76.01.5

81.0

Corr. Rorg. C

ns0.25nansns

0.15nsns

0.080.11nansns

0.05nsns

"na=not applicable, ns=not significant.

in 20 on-farm trials where pH was at least 5.2 (W.D. Sakala, J.D.T. Kumwenda,and Snapp, unpublished data, 1997). On this basis, 5.2 was chosen as the criticalvalue to evaluate pH data in this study.

The pH was >5.2 for 87% of the soils tested in the North (Table 1) and for 7 8 %of the soils in Southern Malawi (Table 3). In Lilongwe ADD, which contains theCentral mid-altitude plain of Malawi, only 66% of soils tested were >5.2 (Table2). This is in sharp contrast with Kasungu ADD, also in Central Malawi, whereover 90% of pH values were greater than 5.2. The average acidity of soils inLilongwe ADD was 0.3 units lower than any other region. This was somewhatsurprising given that Alfisols are the dominant soil type in Lilongwe ADD.


2578 SNAPP

TABLE 3. Descriptive statistics for soil and site characteristics of 220 samples fromsmallholder fields in Blantyre agricultural development division (Blantyre ADD) inSouthern Malawi. Critical soil test values used: sand, 85%; organic C, 0.8%; pH, 5.2;Zn, 0.5 mg kg1; Ca, 0.2 cmolc kg1; K, 0.2 cmolc kg1; and P, 13.0 mg kg-1. Multipleregression of soil characteristics which contribute to organic C status showed that onlysoil texture was a significant variable [(R=0.43) Y=14.6+(-0.16*sand % X)].

Characteristic

SlopeSand %Organic C %PHZnCaKP

mg kg-l

cmolc kg'1

cmolc kg-l

mg kg"1

%>Critical

80777890999977

Mean

7.472.3

1.25.91.73.01.1

45.5

Std.dev.

5.811.50.50.41.32.10.9

42.3

Median

4.570.0

1.15.81.32.40.9

38.2

Minimum

1.052.00.35.10.00.20.00.9

Maximum

19.090.02.96.99.69.5

13.0119.0

Corr.Rorg. C

ns0.43nansnsnsnsns

"na=not applicable, ns=not significant.

Agronomic practices may have contributed to soil acidity. For example,widespread burning of residues, continuous maize production without fallowingland or rotating crops, and use of acidifying fertilizers (Jones and Sakala, 1991).

Average pH values were higher in the North than in the rest of Malawi; however,the highest variability of pH and the lowest pH (4.0) also occurred in the North.This is presumably due to localized Oxisol soils found in the Northern region(Young and Brown, 1962). Oxisols are highly weathered and generally acidic.Excess hydrogen (H+) ions are not necessarily responsible for yield reductions inacid soils. Acidity effects are mediated by a complex of aluminum (Al) toxicityand nutrient imbalances. Aluminum becomes available if pH falls below about5.4 and causes toxicity problems in soils with acidity <4.8, if exchangeable Al ishigh. Aluminum toxicity has rarely been observed in Malawi, although it mayoccur in Southern Malawi tea estates and in isolated high altitude areas in CentralMalawi (Brown and Young, 1966; Aggarwal et al., 1997).

Moderate acidity is widespread in Malawi, in contrast to the small proportionof very acid sites (Tables 1,2, and 3). Amendment of soil pH with lime has notbeen shown to consistently enhance yields of maize for moderately acid soils(Landon, 1991; Young and Brown, 1966). Use oflime is in any case prohibitivelyexpensive due to transport costs for the vast majority of smallholder farmers.Yet, it is possible that the wide-spread moderate acidity in Malawi is associatedwith reduced N and P availability. Janssen et al. (1990), working in Kenya on theQUEFTS model, found support for a negative relationship of pH to N and P uptake



by maize at moderate acidity levels. However, a global soil and plant nutritionsurvey found little relationship of pH to N and P availability (Sillanpaa, 1982).

The extent to which moderate acidity influences performance of major crops(e.g., maize, groundnut, and tobacco) in Malawi deserves more research attention,with a focus on organic inputs to ameliorate soil pH and nutrient availability.Lilongwe ADD includes the fertile Central plain which is responsible for about40% of cereal grain production in Malawi. The diagnosis of moderate to severeacidity in this "bread basket" of Malawi is a cause for concern (Table 2). Organictechnologies such as mulching and incorporation of organic materials showpotential as alternatives to liming, to ameliorate soil pH, and enhance nutrientavailability in acidic soils (Pocknee and Sumner, 1997; Ruhigwa et al., 1993).Mulch added at 21 ha1 to an Oxisol soil in Nigeria increased pH by 0.1 unit, andavailable P by 40%. Mulch added at 61 ha1 was even more effective, it increasedpH by 0.5 units and available P by 85% (Lai, 1981). Cation and N content oforganic inputs has been shown to be closely related to ability to reduce soil acidityand enhance soil N availability (Pocknee and Sumner, 1997). This relationshipremains to be investigated for soils and residues of smallholder farms in Malawi.

Texture

Sand content in the Center and Northern regions was about 76% (Tables 1 and2). Average sand content in the Southern region was slightly lower, 72% (Table3). These results concur with descriptions of Malawi soils as generally sandy intexture and well drained (Brown and Young, 1966; Wendt, 1993; Young andBrown, 1962). Soil samples were obtained from the primary maize productionfields of smallholder agriculture. Average sand content reported here would havebeen much lower if the hydromorphic soils along drainage lines had been includedin the sample. These soils are used for grazing livestock and winter vegetableproduction, and they have a high clay content in the range of 30 to 60% (Youngand Brown, 1962; Brown and Young, 1966). Soil texture class was determinedfor the soils reported here and soils in the North were largely sandy loams orsands. The remainder were sandy loams, and sandy clay loams. A similar patternwas observed in the Center and South, although sandy loams were generally muchmore common than in the North.

Soils which are more than 85% sand tend to be excessively drained, low in soilorganic C, and very low in nutrient content (Landon, 1991). Tables 1, 2, and 3show that about one-sixth of the soils sampled had sand content >85%. Waterand N losses can be high form these soil types. Results from a simulation modeland field trials conducted in Central Malawi suggest that N losses from leachingin sandy soils are on the order of 50 kgN ha1 annually, compared to approximately20 kg N ha1 annually from sandy loam soils (Thornton et al., 1995). This indicatesthe importance of agronomic technologies to reduce N and water loss. Applicationof N fertilizer using multiple splits, adjusted for the rainfall pattern, is a promisingapproach shown to reduce N losses in nearby Zimbabwe (Piha, 1993).


2580 SNAPP

Organic Carbon

The Northern and Southern regions of Malawi had the lowest soil organic C, onaverage 1.2% (Tables 1 and 3). Soil organic C values of the Center of the countrywere higher, about 1.6% (Table 2). Weil and Mughogho (1993) and Wendt ( 1995)also found mean organic C levels of about 1.5% in soils from Malawi. Minimumand maximum values across the country ranged from 0.2 to 4.6% organic C. Thisraises the question of what is an adequate level of soil organic C. Data from long-term trials in the sub-humid tropics suggest that a level of soil organic C between0.8% to 1% is sufficient to support crop production and prevent soil degradation(Araki, 1993; Pieri, 1995). This is for soils with sand content above 60%, as istypical of the topsoil in Malawi. This is a minimal soil organic C level, belowwhich soil structure is severely impaired. Based on this rational the critical valueof 0.8% was used to evaluate the soil organic C data. About 80% of the soilstested had organic C values above the critical value of 0.8% (Tables 1,2, and 3).The mean value for natural regions with low organic C levels was 1.0%, and thusjust above 0.8% (Table 4).

The data indicate that the majority of smallholder fields in Malawi have adequateorganic C to prevent soil physical degradation. This suggests that agronomictechnologies to enhance soil organic C levels are not recommended on a largescale. Although, erosion prevention measures must be taken to preserve theremaining soil and the wide-spread practice of residue burning evaluated for itslong-term effects on soil C. As a cautionary note, the levels observed here onsmallholder farms were severely depleted compared to the 1.8 to 3.5% organic Cfound under natural vegetation in Malawi and at research stations such as Chitedze(Brown and Young, 1966; Snapp et al., 1998).

Building soil organic matter has been advocated as a critical need throughoutsub-Saharan Africa. However, it is difficult to produce sufficient organic materialsto increase soil organic matter in the semi-arid and sub-humid tropics, given thatland and labor are often in short supply and the rapid C mineralization rate in thisenvironment (Snapp et al., 1998). A more realistic goal may be to promoteincorporation of selected legumes in smallholder cropping systems. This willincrease the amount of high quality organic residues and N from biological nitrogenfixation and enhance crop productivity and biological nutrient cycling efficiency,without necessarily increasing soil organic C. Notable exceptions to this strategywere hilly regions in Ntcheu, Nkhotakota and Michinji. These were natural regionswith very low C (Table 4), where management to improve soil organic C levelsshould be implemented.

Multiple regression analysis was used to evaluate the relationship of organic Cto other soil characteristics for the entire soil database. Positive, stepwise multipleregression showed that sand and calcium were significant contributors towardspredicting organic C, where R=0.61 [Y=4.4+(-0.04*XS)+(0.03*XCa) andY=organic C (%), where: XS=sand (%), and XCa=Ca (cmolc kg"1)]. Sand, and to


aiO

TABLE 4. Soil characteristics and distinctive groupings higher and lower than countryside means by natural regions in Malawi.United States Department of Agriculture terminology equivalent for FAO system soil family names is Udic Rhodustalfs and OxicHaplustalfs for Chromic Luvisol and Haplic Lixisols, respectively.

Natural region description

Lilongwe PlainUpper Bua PlainsMichinji HillsNkhotakota Scarp ZoneNtcheu Scarp ZoneNtcheu RegionChikwawa-Thyolo EscarpmentChileka RegionMichinji HillsDedza HillsDedza Scarp Zone

Soil analyses(mean)

High org. C 2.0%High org. C 2.5%Low org. C 1.1%Low org. C 1.1%Low org. C 0.9%Low org. C 1.0%Low P 15 mg kg1

Low P 5 mg kg"1

Low Zn 1.1%Low Zn 0.9 mg kg1

Low Zn 0.8 mg kg'1

Moisture(mm)

850900800

1,1301,020

940960980800910900

Elevation(m)

1,2001,1501,3101,2301,230

970480690

1,3101,340

860

Soil class(FAO system)

Chromic LuvisolChromic Luvisol, Haplic LixisolHaplic Lixisol, Gleyic CambisolHaplic Lixisol, Chromic LuvisolHaplic Lixisol, Chromic LuvisolHaplic Lixisol, Chromic LuvisolEutric Cambisol, Chromic LuvisolChromic Luvisol, Eutric CambisolHaplic Lixisol, Gleyic CambisolHaplic Lixisol, Chromic LuvisolHaplic Lixisol, Chromic Luvisol

1O

>

oam73

to00


2582 SNAPP

some extent Ca, also were predictors of soil organic C when the regression analysiswas conducted separately by agricultural district. There was one addition, a slopevariable from the site description characteristics, which was a significant variablepredicting soil organic C in the Kasungu district (Table 2).

Overall the influence of Ca was limited; sand content was the major predictorof soil organic C content. Sand is negatively related to organic C (Figure 2).Sandy soils are generally low in soil organic C due to the lack of physical andchemical protection from mineralization processes (Parton et al., 1987; Pieri, 1995).Soil Ca availability has not been generally found to be related to organic C.However, organic C is related to soil cation exchange capacity (Landon, 1991).This may be a factor influencing the relationship of Ca and organic C observedhere.

Phosphorus

Soil P status in Malawi was generally high, about 60% of soils were above thecritical value of 15 mg kg'1 (Tables 1, 2, and 3). This finding is supported by aglobal survey of soils which indicated that, in general, Malawi soils are sufficientin P for maize production, and have a higher P status than surrounding countries(Sillanpaa, 1982). There were two natural regions in the South which weremarkedly low in P, Chileka and Chikwawa-Thyolo (Table 4). The variability isdemonstrated by a comparison of means and standard deviations for P in theNorth (Table 1 ), Center (Table 2), and the South (Table 3). The standard deviationwas as high as the mean for most of the country.

Variability of extractable P in Malawi has been noted by others, such as Youngand Brown (1962) and Wendt (1993). This variability is not surprising due to thetremendous spatial heterogeneity of topography in Malawi. R. Weil and S.Mughogho (USAID final report, 1992) reported extensive inter-fingering of lightand dark red soils at a scale of less than 0.5 kilometers. Further, average smallholderfield sizes are in the range of 0.5 to 1.5 hectares in Central Malawi, and smaller inSouth (Snapp et al., 1998). This leads to a wide range of fertilizer and agronomicmanagement histories across the landscape.

It is also must be considered that the extractable P values reported here werenot correlated closely with crop response to P fertilizer. Less than one-third ofon-farm trials showed the expected response of maize to P fertilizer for soils withlow extractable P (Chilimba, 1996). Wendt (1993) found similar results toChilimba: available soil P, as indicated by extractants Mehlich 3, Bray 1, andOlsen, identified correctly the majority of P sufficient sites, but not low P sites.Only a few fields showed maize responses to P fertilizer, and 20 kg ha1 phosphatewas the maximum level at which a maize response was obtained. The datapresented here and the literature on maize response to P fertilizer both suggestthat P response and soil P levels are highly variable. However, crop response toP and available soil P do not always concur.



65 75

Sand (%)

Regression95% confid.

FIGURE 2. A linear regression of soil samples from Central Malawi showing the negativerelationship of sand content soil and organic C.

The generally sufficient P status in Malawi supports recent efforts to evaluateblanket fertilizer recommendations for hybrid maize of 45 kg P2O5 ha1, and developlower P recommendation rates (Kumwenda et al., 1997). One problem with overapplication of P has been noted by Wendt (1993, 1995): P fertilizer can reduceZn availability under Zn-deficient conditions. This potentially is a serious problemin low Zn areas such as the Dedza region in Central Malawi (Table 4). Phosphatefertilizer will generally still be required for high P demanding crops such ascommon bean. This was shown by the application of small amounts of P, e.g., 20kg P205 ha"1, to maize/bean intercrops which consistently enhances yields by 30 to100% in Central Malawi (Aggarwal et al., 1997).

Potassium

Potassium levels in the North and Center were over the critical value for about80% of the soils tested (Tables 1 and 2). Potassium status was even higher in theSouth where soil analysis indicated that over 99% of soils had sufficient K (Table3). The mean K value for the soils tested here was about 3 fold higher than thecritical value. Generally, maize fertilizer trials have indicated no response to K inMalawi smallholder fields (Kumwenda et al., 1997). The data presented here andthe literature indicate that Malawi soils appear to be broadly K sufficient.

Calcium

Almost 100% of soils in Malawi have sufficient Ca as indicated by soils analysis(Tables 1, 2, and 3). Studies have indicated low cation exchange capacity in


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Malawi soils, a characteristic which was not studied here, but one expected toinfluence Ca availability (Mwandemere and Robertson, 1975). Further, the Cacritical value used here was developed for maize. Crops such as groundnuts havea high demand for Ca and some soils in Malawi may be Ca deficient with regardsto groundnut production. Calcium supplying soil amendments, gypsum and lime,have been shown to be inconsistent in enhancing groundnut pod filling and yieldsin Central Malawi (Nyirenda et al., 1992). Calcium uptake in groundnuts isinfluenced by water availability as well as soil Ca supply. This may explain theuneven response of groundnuts to Ca nutrition in Malawi. Overall, soil Ca statuswas very high in the soils studied here and amendment with Ca does not appear tobe required.

Zinc

Zinc appears to be sufficient throughout most of Malawi. Yet, soil Zn statuswas highly heterogeneous, as indicated by high standard deviations. Mean valuesfor Zn were above the critical level of 0.8 mg kg'1 for about 94% of soils in theNorth and Center (Tables 1 and 2), and 90% in the South (Table 3). A globalstudy by Sillanpaa ( 1982) indicated mean Zn values which were generally sufficientin Malawi, however, the study also indicated isolated areas of very Zn deficient.

In Malawi, Zn fertilizer responses in maize fertilizer trials have been noted forthe Dedza hills region (Kumwenda et al., 1997; Wendt, 1993). The same regionwas identified as Zn deficient through statistical analysis of soil Zn per naturalregions (Table 4). Soil Zn values for Dedza region were about 0.8 mg kg"1,whereas values for the rest of the country were almost double this. The soilparent material in Dedza is igneous granite, generally low in total Zn (less than 40mg kg1), which could help explain low availability of Zn in sandy soils of theregion (Tisdale et al., 1985). Another important factor could be the cooltemperatures in this high altitude region. Cool temperatures tend to slow plantgrowth and enhance Zn deficiency (Tisdale et al., 1985). Taken together, the dataconfirmed earlier suggestions that Zn fertilizer should be made available tosmallholder farmers in the Dedza area (Wendt, 1993).

CONCLUSIONS

The majority of Malawi soils were loamy sands and moderately acid, withsufficient to low nutrient levels. Organic C levels were adequate to maintain soilstructure in the main, although much reduced from the non-cultivated state. Lowsoil organic C suggested that N supply was inadequate throughout most of thecountry. Inadequate N supply was also indicated by the literature whichdemonstrated consistent crop responses to N (Brown and Young, 1966; Chilimba,1996).

Widely deficient N status was in contrast to soil K and Ca levels which weresufficient throughout most of the country. Soil P, on the other hand, was highly



variable. The mean P status in Malawi was well above soil P levels generallyobserved for soils in Southern Africa. This indicates P-sufficient soils, anobservation supported by the limited response of maize to P fertilizers in Malawi(Wendt, 1993). However, the data reported here indicated that local P deficiencieswere severe in some areas of Malawi. Soil Zn was also heterogeneous. In general,soil Zn was adequate, but there were isolated areas of severe Zn deficiencies.Zinc deficiencies occurred in cooler, high altitude areas with igneous granite parentmaterial.

Soil characteristics reported here were typical of the range reported previouslyfor soils in Malawi. The data highlighted the need for action on fertilizerrecommendations: i) Zn fertilizers should be made available to farmers in the Zndeficient Dedza region, ii) generally observed high soil P status strengthened earlierreports by Wendt (1993) and Kumwenda et al. (1997) that new fertilizerrecommendations should halve P fertilizer rates (e.g., a reduction from 45 to 23kg phosphate per ha for hybrid maize), and iii) the extent of low organic C, sandysoils reinforced the importance of fertilizer recommendations which improve Nefficiency. This includes new, targeted, and flexible approaches to fertilizer useand developing alternative, legume-based N sources for smallholder farmer.Quality of organic matter and nutrient cycling efficiency are areas of activeinvestigation in Malawi (Jones et al., 1996; Snapp et al., 1998). More attention iswarranted to use of combined inorganic and organic fertilizers, and splitapplications of small amounts of fertilizer.

New findings reported here were the broad extent of moderate soil acidity inthe Lilongwe plain and the delineation of natural regions with low C and low P.The data suggest that new research initiatives should be undertaken on the use oforganic materials to ameliorate moderate acidity and enhance N supply in CentralMalawi. Technologies are required to address problem soils with very low organicC and P, including combined use of inorganic and organic fertilizers. Priorityshould be given to organic materials which can be easily produced by smallholderfarmers and by-products of crops widely grown, such as residues from groundnuts.

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