LUCID’s Land Use Change Analysis as an Approach
for Investigating Biodiversity Loss and Land Degradation Project
by
Louis N. Gachimbi
KARI-Kabete
P.O.Box 14733
Nairobi, Kenya
E-mail:
November 2002
21
LUCID Working Paper 9
Technical Report of Soil Survey and Sampling Results:
Embu – Mbeere Districts, Kenya
The Land Use Change, Impacts and Dynamics Project
Working Paper Number: 9
By
Louis N. Gachimbi
KARI-Kabete
P.O.Box 14733
Nairobi, Kenya
E-mail:
November 2002
Copyright © 2002 by the:
International Livestock Research Institute, and
United Nations Environment Programme/Division of Global Environment Facility Coordination.
All rights reserved.
Reproduction of LUCID Working Papers for non-commercial purposes is encouraged. Working papers may be quoted or reproduced free of charge provided the source is acknowledged and cited.
Cite working paper as follows: Author. Year. Title. Land Use Change Impacts and Dynamics (LUCID) Project Working Paper #. Nairobi, Kenya: International Livestock Research Institute.
Working papers are available on www.lucideastafrica.org or by emailing .
TABLE OF CONTENTS
List of Tables iv
List of Figures iv
List of Appendices iv
A. INTRODUCTION 1
B. CHARACTERISTICS OF THE SITE 1
C. MATERIALS AND METHODS 4
1. Laboratory methods 4
D. RESULTS AND DISCUSSION 5
1. Land use, soil fertility and soil erosion across AEZ 5
2. Exchangeable bases Ca, Mb and K 15
3. Erosion levels across different land uses and AEZ 15
4. Erosion variation within AEZ 16
E. CONCLUSION 17
F. REFERENCES 18
Appendices 19
LIST OF TABLES
1. Land use , slope altitude and soil types in the different agro-ecological zones
along Embu-Mbeere Transact 1
2. Variation of pH, NPK, soil organic carbon (SOC) and sum of cations
across the AEZ’s 6
3. Variation of pH, Phosphorous (P ppm), Potassium (K me%), soil organic
carbon and erosion class in various land uses and AEZ 7-8
4. Classes of fertility for single elements NPK and soil organic carbon (%C) 9
5. Threshold level of phosphorous, soil organic carbon 14
6. Example of classes for assessment of observed erosion 16
7. Percent erosion classes within AEZs along the EMBU – Mbeere transect 16
LIST OF FIGURES
1. The Embu- Mbeere transect and agro-ecological zonation 2
2. Maize yield trends in Embu-Mbeere Districts 3
3. Rainfall trends in Mbeere District 3
4. Variation of pH, soil organic carbon (SOC), Phosphorous (P)
and sum of cations across the AEZs 9
5. Percent threshold of level of phosphorous (P olsen) soil organic carbon (SOC),
Potassium (K) by land use along Embu- Mbeere Transect 10-14
6. Percent threshold level of phosphorous (P olsen), Potassium (K),
soil organic carbon (SOC) and Nitrogen across AEZ 15
7. Percent erosion rate within AEZs 17
APPENDICES
1: Soil fertility and erosion indicators questionnaire 19
2: pH scale (in 1:1 soil water ratio 21
BOX
Summary of major production constraints and options for increased food production in Embu – Mbeere District 6
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LUCID Working Paper 9
A. INTRODUCTION
The purpose of this study was to examine soil characteristics along the agro-ecological gradient of the southeastern slopes of Mt. Kenya in Embu and Mbeere Districts. The results of the soil analyses will be compared to results from a plant and land use survey, and land use change analyses conducted in the same areas (published in other LUCID working papers). For the soil study, the author conducted a survey of soil erosion indicators and land use histories, and collected soil samples for fertility tests. The study was conducted along two line transects. The main transect runs from the high altitude coffee/tea zone and includes the forest margin (Tropical Alpine, TA) in Embu District, and ends at the Tana River in semi-arid Mbeere District (see Figure 1). The second and smaller transect runs from Ivondo catchment to the Kianjiru Hills west of Kiritiri town in Mbeere District. It includes quadrants within the Mwea National Reserve.
B. CHARACTERISTICS OF THE SITE
Embu and Mbeere Districts lie on the southeastern slopes of Mt. Kenya. Besides differences in ecological conditions along the altitudinal gradient, agricultural systems differ from the upper to lower zones. The districts illustrate a typical agro-ecological profile of the windward side of Mt. Kenya, from the hot, dry lower zones in the Tana River Basin to the cold, wet upper zones. Farms in Embu and Mbeere are well defined and approximately 2 to 5 ha. each. Population density in Embu averages over 132 people per km2. This high density is associated with land fragmentation and a relatively small average farm size of 2.3 ha. (CBS, 1996, GoK/ UNICEF 1990). The elevation ranges from 760 metres in agro-ecological zone (AEZ) LM5 to 2070 meters in LH1. Average annual rainfall increases with altitude from 640 mm to 2000 mm. In a previous study (Jaetzold and Schmidt, 1983) six AEZ were identified in the districts with a number of sub-zones in each main zone. The zones are characterized according to present land use in Table 1. The table shows altitude, slope and soil types.
Table 1. Land use, slope, altitude and soil types in the different agro-ecological zones along the Embu-Mbeere transect. Source: Jaetzold and Schmidt, 1983.
AEZ
/Land use
/Land use (1983)
/Main soil types
TALH1
UM1
UM2+3+4
LM3
LM4
LM5 /
Forest
Tea, maize/ beans/dairyTea/coffee/dairy/food crops
Coffee/dairy/food cropsMaize/tobacco/sorghum/food
Crops
Maize/free range dual purpose
Cattle/food crops
Maize/sorghum/dual purpose
Cattle / TA - Forest
LH1 – Tea/Dairy
UM1- Tea/Coffee/Diary
Tea
LM3 - Tobacco/Food crops
LM4 - Livestock/Shifting cultivation
LM5- Livestock/shifting cultivation / Eutric Astosols
Eutric Astosols
Andosols/Nitisols
Nitisols-Ferrasols
Arenosols
Regosols/Ferralsols
Lithosols/Cambisols
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LUCID Working Paper 9
Figure 1. The Embu-Mbeere Transect and Agro-Ecological Zones
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LUCID Working Paper 9
Figure 2. Maize yield trends in Embu and Mbeere Districts
Figure 3. Rainfall trends in Mbeere District.
The major problems of soil fertility management along the transect were detailed by District Agricultural Officers as follows:
· Human population has to date increased faster than the increase in crop production. It has increased from 66 persons per km2 to 132 persons per km2 in 1969 and 1996 respectively (CBS 1969, CBS 1996).
· The population increase has led to pressure on land such that cultivated land per capita has been decreasing leading to people migrating from high potential upper midland zones to low potential areas. This has led to reduced crop yields as exemplified in Figure 2 (maize yield trends according to Embu and Mbeere District Agricultural Office annual reports (DAO, 1980-2001).
· Crop yields in many areas remain low and most farmers cannot purchase inputs due to their high prices. Farmers use fertilizers and farmyard to improve soil fertility.
C. MATERIALS AND METHODS
Sampling was stratified according to AEZ and land use class. Along the transect line within each of the six AEZ, the location of at least four points were randomly selected by computer (the points identified along the transect on Figure 1). Each of these points served as a midpoint for a kilometre long sub-transect that was perpendicular to the main transect. Twenty-two sub-transect points were thus identified, along which quadrants were chosen representing different land use classes. If a land use class was quite prominent in the AEZ but not sufficiently represented in a sub-transect, supplemental sampling was conducted. The one-kilometre length provided a distance long enough to include all major land use types and variability in soil units and landscape forms.
Each land use class was represented in the sub-transects with at least three quadrants. In the quadrants, vegetation species were surveyed and soil samples were collected. Composite samples from 0-20 (top soil) and 20-30 cm (subsoil) were collected. A total of 320 soil samples were collected but due to budget constraints only the topsoil samples have been analysed. Since soil properties and laboratory measurements have inherent variation, it is necessary to sample at least in triplicate. It was therefore decided to collect soil from three locations in each quadrant and pool them for the analyses. Standard sample sizes were used for the individual samples. The choice of analyses followed the objectives of the study concerning soil erosion and productivity (soil nutrient amounts and related assessments) and their variability within the zones. Other information to be collected included the following:
1. Soil erosion indicators and their qualitative or quantitative assessments (see Annex 1).
2. Information on land use history using a standard questionnaire.
Given the above, we developed a form and questionnaire that makes use of existing information, requires a minimum of resources, and results in a quantitative description of the variability of soil across AEZ and land uses.
C.1. Laboratory methods
The International Centre for Research in Agroforestry (ICRAF) analysed the soil samples using the following methods:
· Air drying, breaking up of aggregates by careful pounding with pestle and mortar, sieving through 2mm sieve. Only soil that passes the sieve is analysed.
· pH: 2.5.1 solution: soil ratio: dionised water;
· EA, Ca, Mg: 10:1 solution: soil ratio, 1MKCL extraction, analysis by NaOH titration (EA) or AAS (Ca, Mg);
· K and P: 10:1 soil: solution ratio, 0.5M NaHCO3 + EDTA, pH 8.5 (modified Olsen) analysis by flame photometer (K) or colorimetrically by molybdenum method (P).
· Total organic carbon was determined by an improved cromic acid digestion.
D. RESULTS AND DISCUSSION
The distribution of soil is largely determined by parent material and physiography. Lower highland/ upper midland zones have moderately deep to very deep soils while the hills and ridges have shallow sandy stony soils (as detailed in Table 1). This is as reflected in the Eutric Andosols in the forest, Andosols/ Nitisols lower highland zones, and Lithosols/ Combisols in the lower elevation zones. Soils developed from volcanic material contain low percentages of sand and are mainly clays and clay loams. Most of these soils are deep and well drained, but areas along river plains and bottomlands where alluvial deposits are common have imperfectly drained soils.
D.1. Land use, soil fertility and soil erosion varying by AEZ
Land use varies within and across AEZ’s. Crops, for example, vary from, in the higher areas, tea, coffee, maize and beans to, in the lower zones, drought resistant crops like cow peas, sorghum and millet. Zero-grazed dairy cattle in the upper zones are replaced by free-range crossbreed dual-purpose animals in the lower areas. As the altitude decreases from the forest zone of 2000 meters elevation to the lower zones of 640 meters, other factors also vary including human population density, plant cover, rainfall amounts and intensity, and soil type. These factors contribute immensely to varying levels of soil fertility and crop yields.
Previous studies (Onduru et al 2002) as detailed in Box 1 have identified major crop production constraints and options for increased food production. During the survey, it was found that different soil conservation or improvement measures have been devised, installed and been constantly improved. Major soil and water conservation practices in the districts include terracing (fanya juu, cut off drains and retention ditches), stone lines, grass strips, trash lines, unploughed strips, river bank protection, contour bunds, negarimsnegarims, semi-circular bunds, basins/9 seeds in a hole, and plating pits. In addition to the above practices, gully control using vegetative materials, gabbions and mixed materials are also being practiced. Figure 4 shows thatThe dominant soil and water conservation practices in the districts at the farm level were fanya juu method of terracing and the use of trash lines in the lower midland zones, and channel terraces in the upper midland zones though they were not well maintained.
Box . Summary of major crop production constraints and options for increased food production
Constraints
· Limited accessibility to high yielding varieties.
· Low and variable rainfall.
· Declining soil fertility (low use of inputs).
· Exacerbation of pests and diseases.
· Poor market infrastructure.
Options and opportunities
· Increasing land under cultivation.
· Community bulking of high yielding seed varieties (seed bank).
· Addressing soil fertility constraints. recycling of manure, use of crop residues, use
of green manures and cover crops and adoption of agroforestry. Use of combinations of organic and inorganic inputs.
· Efficient water use: timing of planting, water harvesting and soil moisture retention techniques.
· Land intensification through irrigation where possible.
· Farm mechanization through increased use of oxen draft power to reduce labour
demands for land preparation and weeding.
· Enhanced skills in integrated crop management (good farming practices, pest management, soil fertility management etc.).
Chemical fertility in agronomic terms differs with land use and AEZ. All fertility measurements (nitrogen, soil organic carbon and phosphorus) from the mass analysis (Mehlich et al 1964) range from adequate in the upper zones, to moderate to low in the lower zones. The results are presented in Tables 2 and 3, and in Figures 4 and 5a to 5j. The tables present the average of the sampling results by land use class in the different AEZ.
Figures 5a to 5j show the results as a percent of the threshold level. The percents have been calculated and plotted on one graph by dividing laboratory measured values by specific critical values set out by Mehlich et al (1964) and modified by Legger (1978), then multiplied by 100. Table 4 shows the classes and critical value defined for available P, K, N and C.
Table 2. Variation of pH, Hp, N, P, K, Soil Organic Carbon (SOC) and sum of cations across AEZ's.
Zone No / AEZ / pH (me) / Hp / Ca (me%) / Mg (me%) / K (me%) / Ca +K + Mg / P (Olsen) (ppm) / SOC (%) / N (%)1 / TA / 4.0 / 4.5 / 0.3 / 0.2 / 0.2 / 0.7 / 6.0 / 6.6 / 0.6
2 / LH1 / 4.7 / 2.5 / 2.0 / 0.6 / 0.5 / 3.1 / 16.5 / 3.2 / 0.4
3 / UM1 / 4.4 / 2.4 / 1.2 / 0.4 / 0.3 / 1.9 / 10.1 / 2.3 / 0.3
4 / UM2+3+4 / 5.6 / 0.6 / 6.0 / 2.4 / 0.6 / 9.0 / 11.5 / 1.6 / 0.2
5 / LM3 / 6.2 / 0.2 / 4.6 / 1.4 / 0.4 / 6.4 / 4.3 / 1.0 / 0.1
6 / LM4 / 6.1 / 0.0 / 5.4 / 2.4 / 0.2 / 8.0 / 0.8 / 0.7 / 0.1
7 / LM5 / 6.9 / 0.0 / 6.2 / - / 0.2 / - / 6.0 / 0.7 / 0.1
Table 3. Variation of pH, phosphorus (P ppm), Potassium (K me %), soil organic carbon (SOC %) and erosion class in various land uses and AEZ.
(a) Zone TA – Tropical Alpine
Land use
/pH
/P Olsen
/SOC
/K
/Erosion class