Assessing Soil Suitability for Irrigated Agriculture in IMP Pilot Sites, Rwanda

CKK Gachene, Dept of LARMAT, UON

IMP Team:MM Malesu D NyoleiA Karuma EK BiamahAR Oduor I MiyukiK Cherogony C Muthuri

M O’Neil J MogoiCKK Gachene

‘Despite the artistic pretensions, sophistication and many accomplishments of mankind, we owe our existence to a six-inch layer of topsoil and the fact that it rains’Anonymous

Vasili Dokuchaev

The Russian School

Soil forming factors

Different Soils

Soil forming processes

Guy Smiths

Genesis is important to the classification partly because it produces the observable or measureable differences that can be used as differentiae. (1952, Belgium)

“time zero of modern soil classification”

Different places – different soils

Different places – different soils

»From hill crest to valley bottom, you will find soils which look different, and also, behave differently

» Variability reflects the soils unique position in relation to the other components of planet earth: at the interface b/w atmosphere, lithosphere, hydrosphere, and biosphere

Soil – the need for reliable information

»Identify and characterise soil landscapes

»Other branches of SS are then applied

to maximise the natural advantages or avoid the difficulties, e.g irrigation, drainage, fertilization

»Information assist in making decisions about LU changes and their impacts, and to monitor and predict impacts of human actions.

• Project purpose: To evaluate the soils’ conditions for irrigated agriculture in IMP Pilot sites

• Study areas: Mugesera sector (Ngoma District); Masaka sector (Kicukiro District); Kibirizi sector (Nyanza District) and Musaza sector (Kirehe District)

(Maimbo et al, 2010).

IMP Pilot Sites

Crops grown Mugesera sector (Ngoma) (1311 ha)

Kibirizi(Nyanza) (411 ha)

Musaza sector (Kirehe) (119 ha)

Masaka sector (Kicukiro) - 586

Banana √ √ √ √Maize √ √ √ √Sorghum √ √ √Cassava √ √ √ √Irish potato √Garden pea √Soybean √ √Sweet potato √ √ √ √Groundnut √ √Beans √ √ √ √Palm oil √Arrowroot √ √ √ √Coffee √ √ √Obelge √ √Pineapple √ √Tomato √ √ √Napier √ √ √ √Pumpkin √ √ √Rice √Tree tomato √Cabbage √Yam √ √Castor oil √Pawpaw √

Methodology• Existing soils’ information• Auger-hole and profile pit

observations• Soils described according to the

FAO guidelines for soil profile description

• All the observations were systematically numbered and geo-referenced culminating in the delineation of the different soil mapping units

• Soils were examined for depth, colour, texture, consistence, internal, drainage, salinity, sodicity, structure, concretions, presence of gravels, mottles, cutans, pores and root distribution within the profile, presence of shallow ground water table

• Each genetic horizon described and sampled for physicochemical analysis

• Soils analysed for texture, OC, TN, pH, EC, P, PSI, K, Ca, Mg, Na, S, Fe, Mn, Cu, B, Zn, Al,CEC, Hp

In addition site characteristics for each of the augerhole and soil profile observations were recorded. Such as: • current land use and soil management practices.• geology, • macro relief, micro/meso relief, • drainage class, • rockiness, • erosion and physiography were also recorded.

Number of augerhole & profile observations per site

Site Augerhole Profile SMU

Masaka 46 10 8

Mugesera 45 10 6

Nyanza 35 10 8

Kirehe 31 6 5

Description of the soil mapping units

Systematic and nomenclature • Every mapping unit in the soil map is identified by a

code.• First letter denotes physiography (U upland; A alluvial

plain)• Second letter geology (S shales; G granite, A recent

alluvial deposits)• Last letter denotes the various soil characteristics (vd

md, etc) • Letter(s) appearing below the code indicate(s) the slope

class.

Soil characteristics of the proposed irrigation pilot sitesFor each pilot site• Description of the individual soil mapping unit

(SMU)• For each of the SMU, acreage, site characteristics

and general soil characteristics are provided• In addition, the same SMU is shown in the soil

map and its accompanying legend which is user friendly for even non soil scientist

• Detailed soil profile description and analytical data are also given for major SMU

A Alluvial plainsAA Soils derived from recent to sub recent alluvial deposits

AA2/A Poorly drained to imperfectly drained, deep, dark brown overlying very dark grey buried horizon, friable to soft, clay loam with shallow ground water table (eutric Fluvisols)

NB: Previous legend had only the soil’s name according to USDA Soil Taxonomy (e.g Haplohumic Eutrorthox)

Soils developed on recent alluvial depositsMapping unit AA1/A : 4.87 haParent material : Alluvial depositsMacro – relief : Flat to very gently undulating, slopes 0 - 2%Erosion : SlightRockiness / stoniness : NoneVegetation : None, cleared from cultivationLand use : Cultivation, mainly bananas that are well mulched

Soils, general:The soils are well drained to moderately drained, deep, clay soils of varying colours and consistence. They have moderately to weakly developed subangular blocky structure. The pH is moderately to slightly acidic (5.0 – 6.6). The N and OC are moderately high (0.25% and 3.02% respectively). Topsoil CEC is moderately high (20.62 m.e/100g) while Hp is below the critical level (< 0.4 m.e). P is very low (< 6.0 ppm) while topsoil is well supplied with K (0.30 Cmol/kg).

For the description of a representative profile with analytical data, see Appendix V, profile No. KiP1

General site information for Mapping unit AA1/AMapping unit : AA1/ASoil classification : Dystric FluvisolObservation no/date : Kip1GPS coordinate : 0236005/9747126District : KireheParent material : alluvial depositsPhysiography : uplandRelief, macro : very gentle undulatingSlope at site/position : < 2%Present land use : banana, arrowrootErosion type : nilRockiness : nilEffective soil depth : 175 cmDrainage class: moderate-well drainedAltitude: 1389masl

Ap 0-20 Dark reddish brown (5YR 3/4, moist), clay; weak, coarse subangular blocky structure; firm when moist, sticky and plastic when wet; many fine pores; few medium roots; clear and irregular transition to:

AU1 20-35 dark reddish brown (2.5YR ¾,moist),clay; weak to moderate, medium subangular blocky structure; firm when moist, sticky and plastic when wet; many fine pores; few medium roots; clear and smooth transition to:

AU2 35-55 dark reddish brown (2.5YR ¾,moist),sandy loam; weak to moderate, medium subangular blocky structure; friable when moist, slightly sticky and slightly plastic when wet; very many very fine pores; clear and smooth transition to:

AU3 55-75 dark reddish brown (2.5YR 2.5/4,moist),clay; moderate, medium subangular blocky structure; firm when moist, sticky and plastic when wet; few fine pores; abrupt and smooth transition to:

AU4 75-120 dark reddish brown (5YR 3/4,moist),clay; moderate, medium subangular blocky structure; very firm when moist, sticky and plastic when wet; few fine pores; clear and smooth transition to:

AU5 120-17 Strong brown (5YR 4/6,moist),clay; moderate, medium subangular blocky structure; very firm when moist, sticky and plastic when wet; few fine pores;

Profile description

Horizon Ap AU1 AU2 AU3 AU4 AU5Depth(cm) 0-20 20-35 35-55 55-75 75-120 120-170pH (H2O) 6.649 5.039 4.998 5.038 4.993 5.024OC % 3.02N % 0.25Exch.acidity (Hp) me% 0.39 1.08 1.08 1.37 0.88 0.69

P (ppm) 5.76 1.68 1.70 1.21 0.59 0.27K (ppm) 120.80 59.21 42.37 55.45 52.15 44.85Ca(ppm) 2540 595 272.80 539.30 794.10 812.90Mg(ppm) 654.20 246.50 104.40 230.20 225 230.60Mn (ppm) 285.10 34.23 57.98 91.13 328.80 157.30S (ppm) 11.69 25.01 25.80 17.96 15.71 19.77Cu(ppm) 1.53 2.01 1.18 2.32 3.20 2.49B(ppm) 1.11 0.21 0.17 0.22 0.23 0.12Zn (ppm) 6.82 1.06 1.86 1.02 1.61 2.39Al (ppm) 789.60 1152 779.40 1298 1364 905Na (ppm) 21.54 15.19 19.58 15.88 53.63 24.49Fe (ppm) 104.80 101.50 84.33 146.10 317.80 169.60CEC (meq/100g) 20.62 10.74 5.11 9.89 13.09 12.83

EC(S) Us/cm 148 77 61 47 68 93P sorption index (PSI) (ppm) 27.38 76.81 59.08 106.43 86.49 81.99

TextureSand % 49 49 90 32 34 34Silt % 12 16 6 26 24 22Clay % 39 35 4 42 42 44Textural class SC SC S C C C

Lab Data: Observation No: KIP1 Mapping unit: AA1/A

• Acri rhodic Ferralsols• Shallow soils – petroferric phase or skeletic

phase• Mostly deep clay soils, > 55% clay. Most of

Nyanza soils characterised by SCL/SC topsoil over clay soil

0 10 20 30 40 50 60 70 80 900

100

200

300

400

500

600

Infiltration rate (mm/hr)

Infiltration rate (mm/hr)

Sieve size OC Mn Al Fe clay

2mm 0.079 0.047 0.180 0.030 0.228

1mm 0.126 0.239 0.039 0.016 0.063

0.5mm 0.208 0.122 0.002 0.005 0.380*

0.25mm 0.474* 0.415* 0.243 0.339* 0.242

0.125mm 0.142 0.149 0.050 0.094 0.102

0.063mm 0.079 0.098 0.253 0.239 0.165

% weight of WSA of 0.25mm in diameter was the best measure of AS for Rwanda soils

Site %OC %TN

Kirehe 3.06 0.28

Mugesera 1.83 0.15

Masaka 1.90 0.14

Nyanza 1.68 0.13

cm K P Hp CEC0-20 0.310 5.78 0.39 20.6235-55 0.109 1.70 1.08 5.1155-75 0.142 1.21 1.37 9.89

Mean of topsoil and subsoil soil chemical propertiesSoil parameter Topsoil Subsoil

%OC 2.02 -

%TN 0.16 -

pH 5.77 5.27

Hp (me%) 0.95 1.48

P (ppm) 5.25 4.40

K (meq/100g) 0.43 0.23

Ca “ 6.33 3.05

Mg “ 2.15 1.37

Na “ 0.10 0.11

CEC “ 11.84 8.13

PSI (ppm) 84.03 145.40

Soil chemical properties across sitesSoil parameter

Min Max Mean Remarks

Top pH 4.45 7.33 5.77 Mod acidSub 4.39 7.22 5.27 Mod acidTop Hp (exch

acidity)0.14 4.21 0.95 < 0.5

negligibleSub 0.10 4.31 1.49 “Top P (ppm) 1.66 18.32 5.25 <20 lowSub 0.01 56.73 4.40 “Top K (meq) 0.08 2.05 0.43 <0.2 low

Sub 0.03 1.38 0.23 “

Soil chemical properties across sitesSoil parameter

Min Max Mean Remarks

Top Ca (meq) 0.56 21.66 6.33 <2.0 low

Sub 0.40 15.61 3.05 “

Top Mg (“) 0.18 7.03 2.15 <1.0 low

Sub 0.15 5.73 1.37 “

Top Na (“) 0.02 0.50 0.11 0 – 2 mod

Sub 0.03 0.35 0.11 “

Top CEC (“) 1.78 32.94 11.85 <12 low

Sub 1.56 21.90 8.13 “

Suitability of soils for irrigation

Land evaluation• LE follows closely the FAO ‘Framework for Land Evaluation

• Based on the assumption that LE can only be done correctly for a well defined LUT if the LQs are properly specified and rated

• These LQs are used to establish specifications for each land

suitability class for a specific LUT, in this case, smallholder irrigation.

Land Suitability Classes

Class S1.1: Highly suitableLand having no significant limitations to sustained application of a given use

Class S1.2: Moderately suitableLand having limitations which are moderately severe for sustained application of a given use

Class S1.3: Marginally suitableLand having limitations which on aggregate are severe for sustained application of a given use and will so reduce productivity or benefits, or increase required inputs that this expenditure will be only marginally justified.

Class NS: UnsuitableLand which has qualities that appear to preclude sustained use of the kind under consideration.

Land qualities (LQs)

A LQ is an attribute of the land that acts in a distinct manner on the suitability of the land for the use under consideration.For every soil mapping unit, a rating of the relevant LQs is made. The following LQs are used:

1. slope2. soil moisture storage capacity3. drainage condition4. soil fertility (K, Ca, Mg, P)5. depth

Land Quality Ratings - slope

Slope Rating

0 –2 1

2 – 3 2

>3 3

The SMSC is estimated using correlations between water and clay content and equations derived from these graphs. Estimated easily available moisture (e.e.a.m) per 10 cm of soil for various textures Texture LS SL SCL SC SiC Ce.e.a.m (mm) 3.2 4 5.5 8 10.5 11.2 Soil moisture storage capacity is considered as the total productive available moisture which is a function of depth and texture. The ratings are as follows:

SMSC

SMSC in mm for 100 cm depth Ratings> 100 170 – 100 240 – 70 3< 40 4

Drainage Classrating

Well drained 1Moderately well drained 2Imperfectly drained 3Poorly drained 4

Fertility

Sub rating CEC (meq/100g)

1 > 16

2 12 – 16

3 6 – 12

4 2 – 6

5 0 - 2

Available nutrients

Sub – rating P (ppm) K Ca Mg1 > 200 > 3.5 > 20 > 122 80 – 200 2.0 – 3.5 10 – 20 6 – 123 20 – 80 1.0 – 2.0 6 – 10 3 – 64 0 – 20 0.3 – 1.0 2 – 6 1 – 35 < 0.3 < 2 < 1 

Soil depth

Rating Soil depth (cm)

1 > 120

2 80 - 120

3 50 - 80

4 25 - 50

5 < 25

Suitability class

Soil mapping Area (ha)

Constraints/ limitations

S1.1 USvd/A; USvd/BC; USd/C; (AA/A)

1006.68 (+ 8.84)

Fertility, AA/A highly suitable if put under paddy rice (see also below under NS)

S1.2 USd/D 52.19 Fertility, requires intensive labour for levelling land due to slopes (> 16%)

S1.3 USs/C 243.06 Soil moisture storage capacity, fertility, can be utilised for crops with rooting depths of < 50 cm

NS AA/A* 8.84 Seasonally flooded, drainage limiting for most crops, except for paddy rice

Potential land suitability classification for smallholder irrigation of commonly grown crops in Mugesera site (*depending on the land utilization type to be considered)

Soil mapping Area (ha) Constraints/ limitations

USvd/A; USvd/BC; USd/C; (AA/A)

1006.68 (+ 8.84)

Highly suitable, soil fertility is the only limiting factor (see 3.5.1), AA/A is also highly suitable if put under paddy rice (see also below under NS)

USd/D 52.19 Moderately suitable, fertility needs to be addressed, requires intensive labour for levelling land due to steep slopes (> 16%)

USs/C 243.06 Marginally suitable, soil moisture storage capacity limiting due to depth, fertility is an issue, can be utilised for crops with rooting depths of < 50 cm. This mapping unit was however found to be suitable for palm trees intercropped with tomatoes in the southern side of the area and this should be considered as one of the most promising land utilization of the mapping unit despite its depth limitations.

AA/A* 8.84 Highly suitable for paddy rice. Seasonally flooded and drainage may be limiting for most of the other crops

Potential land suitability for Mugesera site

Conclusions and Recommendations

• Main constraint to crop production is soil fertility• Soils are strongly acidic (pH 5.3 – 5.6)• N, P, K and CEC are quite low. • Can be improved by applying N, P and K containing

fertilizers. Any acidifying fertilizer should be avoided

NB: most of the crops - bananas, sweet potatoes, pineapple and napier - grown by the farmers are heavy consumers of K explaining the low levels of K in most of the soils

• However the beneficial effect of heavy mulching of bananas was clearly demonstrated in some parts of Kirehe and Mugesera where the topsoil had relatively higher OM and K levels than in the subsoil

• This practice is therefore recommended if the current system of banana production is to be maintained.

• Exchangeable acidity was on average > 1.0 meq indicating liming requirements. Future lime applications should be based on Hp

• P sorption perhaps the most important process controlling P availability in these soils

• Dominant sorption reactions are those with Fe-Al or Mn oxides and hydroxides and Ca – carbonates. Contributing factor to reduced effectiveness of added phosphates

• Above may necessitate large applications of P fertilizers to achieve good crop yields

• For most crops, the amount of P in equilibrium with 0.2 mg P/litre has been shown to be the threshold over which no response to P is observed

NB: application of a single high dose of P and the

measurements of the amount of P sorbed by the soil

to get an estimate of P sorption capacity. Index

calculated is the PSI

Soil properties Correlation coefficient, r Number of observation

% organic carbon

% total nitrogen

pH

Hp meq

P ppm

K ppm

Ca ppm

Mg ppm

Mn ppm

S ppm

Cu ppm

B ppm

Zn ppm

Al ppm

Na ppm

Fe ppm

CEC meq / 100g

EC(S) uS / cm

- 0.376 *

- 0.348*

- 0.650**

0.720**

- 0.144

- 0.346**

- 0.604**

- 0.624**

- 0.414**

0.305**

- 0.166*

- 0.532**

- 0.378**

0.488**

- 0.079

- 0.274**

- 0.569**

- 0.462**

35

35

147

147

147

147

147

147

147

147

147

147

147

147

147

147

147

147

Table 2: Relationship between PSI and some soil chemical properties (across the sites) * and ** indicate significance at 0.05 and 0.01 respectively

• Farmers be encouraged to use both organic and inorganic fertilizers.

• Handling and managing the locally available organic fertilizers such as FYM and composting is quite a challenge.

What next with most of the traditionally grown crops?

Proper consultation on the type of crops to be grown with the primary stakeholders is strongly recommended especially in Kirehe before commencement of the irrigation.

Murakoze cyane