Maintaining and regaining soil fertility by organic methods in Embu District, Kenya

INDEX

ABBREVIATIONS AND ACRONYMS

LIST OF TABLES

LIST OF FIGURES

LIST OF PICTURES

1. Introduction
1.1 Intention of the study
1.2 Structure of the study

2. Soil fertility
2.1 Soil fertility and plant nutrition
2.1.1 Primary macronutrients
2.1.2 Secondary macronutrients
2.1.3 Micro (trace)-nutrients
2.1.4 Nutrient deficiency and excess symptoms
2.1.5 The role of soil organic matter (SOM)
2.2 Nutrient cycling in agroecosystems
2.2.1 Soil fertility decline
2.2.2 Soil analytical indicators
2.3 Soil fertility decline in Sub-Saharan Africa
2.3.1 Soil fertility decline in Kenya
2.3.2 Soil Fertility decline in Embu District, Kenya
2.4 Kenyan agriculture and soil fertility management in the 20th century
2.5 Approaches to maintaining and regaining soil fertility
2.6 Soil fertility research in Kenya; examples of the chemical and integrated nutrient management approach
2.6.1 Chemical fertilization: Fertilizer Use Recommendation Project (FURP)
2.6.2 Integrated Nutrient Management: Alliance for Integrated Soil Fertility Management in Africa

3. Organic agriculture
3.1 Principles of sustainability in organic agriculture
3.2 General standards and principles in organic agriculture
3.3 Organic resources in organic agriculture
3.3.1 Nutrient release from organic resources
3.3.2 Quality of organic resources
3.3.2.1 Fertilizer equivalency (FE) value of organic material
3.4 Development of organic agriculture in Kenya
3.5 Soil fertility management technologies and practices in Kenyan organic farming systems
3.6 Manure Management in Kenyan Highlands
3.6.1 Classification of manure
3.6.2 Quality of manure
3.6.3 Effect of cattle manure on soil fertility
3.7 Availability of organic resources
3.8 Organic agricultural research in Kenya
3.8.1 Long-term trial NARL - Kabete (Central Kenyan Highlands)
3.9 Organic farming organizations in Kenya
3.10 Political and social efforts towards organic agriculture and soil fertility

4. Soil microorganisms
4.1 Ecological significance of soil microorganisms
4.2 Microbial inoculants in organic agriculture
4.3 Effective Microorganisms (EM) Technology
4.3.1 Development of EM Technology
4.3.2 The function of EM in agriculture
4.3.3 EM application in agriculture: products
4.3.4 EM dependency on organic matter
4.3.5 Effect of EM on crop production
4.3.6 Effect of EM on soil fertility
4.3.7 Effects of EM on soil health
4.3.8 Effects of EM in the course of time
4.4 EM Technology in Kenya
4.4.1 Evaluation of the EM Technology under Kenyan conditions
4.4.2 Farmers’ statements on EM Technology

5. Area of study
5.1 Geographical location
5.2 Topography
5.3 Natural conditions
5.3.1 Climate
5.3.2 Geology
5.3.3 Soil
5.3.4 Agro-Ecological Zone (AEZ)
5.4 Socio-economic and cultural aspects
5.4.1 Population
5.4.2 Land use system

6. Materials and methods
6.1 Selection of the crops
6.1.1 Selection of maize varieties
6.1.2 Selection of the bean variety
6.2 Experimental Design
6.2.1 Treatments
6.3 Cultivation timeframe
6.4 Experiment accomplishment
6.4.1 Land preparation
6.4.2 Planting
6.4.3 Crop husbandry
6.4.4 Data collection on plant growth parameters
6.4.4.1 Maize
6.4.4.2 Beans
6.5 Soil analysis
6.5.1 Soil sampling
6.5.2 Soil chemical analysis
6.5.3 Soil pH
6.5.4 Soil cation exchange capacity (CEC)

7. Results and discussion
7.1 Results of the analysis of beans (GLP2)
7.1.1 Plant growth parameters of beans (GLP2)
7.1.2 Nodulation of beans (GLP2)
7.1.3 Yield data of beans (GLP2)
7.1.4 Diseases and pests in beans (GLP2)
7.2 Results of the analysis of maize
7.2.1 Plant growth parameters of maize
7.2.2 Maize flowering dates
7.2.3 Yield data of maize (Percentage of dry weights of stover, grains, and cobs)
7.2.4 Analysis of diseases and pests
7.3 Analysis of soil chemical properties
7.3.1 Soil properties before planting
7.3.2 Soil chemical properties before and after the experiment
7.4 Discussion
7.4.1 Meteorological conditions of the short rainy season 2002/2003 and the influence on crop growth and yield
7.4.2 Quality of cattle manure
7.4.3 Soil properties before planting
7.4.4 Beans (GLP2)
7.4.5 Maize varieties
7.4.6 Soil parameters
7.5 Summary

8. Future directions and recommendations

REFERENCES

INTERNETREFERENCES

ABBREVIATIONS AND ACRONYMS

Abbildung in dieser Leseprobe nicht enthalten

LIST OF TABLES

Table 1: Essential Macro-and Micro (trace)- nutrients

Table 2: Micronutrients

Table 3: Nutrient deficiency and excess symptoms

Table 4: Nutrient removal by harvested maize and beans

Table 5: Fertility input and output factors of the soil

Table 6: Average nutrient balances of N, P, K (kg ha-1yr-1) of the arable land Of selected countries

Table 7: Kisii District, Kenya: Nutrient budget (kg ha -1 yr-1)

Table 8: Embu District, Kenya: NUTMON balances N, P, K (kg ha-1yr-1)

Table 9: Mean nutrient stocks, balances and relative gains and losses for Embu District

Table 10: Changes of Corg content, pH and available nutrients during the experimental period (%) at Embu A.R.S

Table 11: Manure quality of 299 farms in Kariti, Central Kenya

Table 12: Results of mean grain yield (t/ha) from the long rains 2000 to short rains 2001/2002 at Chuka, Meru

Table 13: Maize yield of the short rains season at RCC-Embu

Table 14: Economic analysis of Soil Fertility trial at TENRI LR 2001

Table 15: MET-Station Embu: Climate data short rains 2003/2004

Table 16: Major soil related crop growth constraints in SSA

Table 17: Recommended maize varieties in the RRC- Embu mandate zone UM2-UM3

Table 18: Recommended bean varieties in the RCC-Embu mandate zone UM2-UM3

Table 19: Experimental design and treatments

Table 20: Cattle manure analysis: samples 17.10.03 (Farm Mutiri)

Table 21: Cropping calendar for Embu District

Table 22: Pest incidents and control measures on experimental plots

Table 23: Effect of various treatments on bean (GLP2) plant growth parameters

Table 24: Effect of various treatments on bean (GLP2) nodulation

Table 24: Effect of various treatments on yield data (GLP2)

Table 25: Effects of various treatments on pests and diseases of beans (GLP2) on a scale from 0 (not affected) to 5 (heavily affected) 19.12.2003 101 Table 26: Selected plant growth parameters of maize: stand count and plant Height

Table 27: Selected plant growth parameters of maize: number of leaves and ear height

Table 28: Maize flowering dates

Table 29: Effect of various treatments of the percentage of dry weights of maize stover, grains and cobs at harvest

Table 30: Effect of various treatments on yield data of maize (Number of ears, shelled grain weight, biomass, yield)

Table 31: Effects of various treatments on diseases and pests of maize on a scale from 0 (not affected) to 5 (heavily affected) 19.12.2003

Table 32: Soil chemical properties %Ntot, NH4, NO3, %Corg, CEC, pH before and after the experiment

Table 33: Percentage of change of soil chemical properties (%Ntot, NH4, NO3, %Corg, CEC)

Table 34: pH value decrease

Table 35: Exchangeable nutrients P, K, Mg, Ca, Na before and after the Experiment

Table 36: Percentage of change of P, K, Mg, Ca, Na before and after the Experiment

Table 37: Micronutrients (Fe, Mn, Zn) before and after the experiment

Table 38: Percentage change of Fe, Ma, Zn before and after the experiment

Table 39: Ranking of the significant soil properties from 1 (best) to 4 (worst)

Table 40: Addition of the ranking of all measured soil parameters

Table 41: Ranking of the soil parameters changes from one (best) to four (worst) results

LIST OF FIGURES

Fig. 1: The Nitrogen Cycle

Fig. 2: Nutrient cycling in agroecosystems

Fig. 3: Nutrient efficiency difficulties in agroecosystems at different scales

Fig. 4: Degradation in a traditional farming system due to land pressure

Fig. 5: Synchrony principle of nutrient release of organic resources

Fig. 6: Organic resource application decision tree

Fig. 7: Decision tree for the determination of manure Nmin from physical characteristics

Fig. 8: Decision tree for the determination of manure C: N ratio from physical characteristics

Fig. 9: Embu District Divisions, Kenya

Fig. 11: Plant height GLP2

Fig. 12: Stover weight GLP2

Fig. 13: Dry matter GLP2

Fig. 14 : Vigour GLP2

Fig. 15: Nodulation GLP 2

Fig. 16: Number of pods GLP2

Fig. 17: Number of seeds GLP2

Fig. 18: Grain weight GLP2

Fig. 19 100 seed weight GLP2

Fig. 20: Plant height H 513

Fig. 21: Plant height EMCO SR 92

Fig. 22: Plant height EMCO SP 92/GLP2

Fig. 23: Shelled grain weight H 513

Fig. 24: Biomass H 513

Fig. 25: Shelled grain weight EMCO SR 92

Fig. 26: Biomass EMCO SR 92

Fig. 27: Shelled grain weight EMCO SR 92/GLP2

Fig. 28: Biomass EMCO SR 92/GLP2

Fig. 29: Yield (t/ha) H 513

Fig. 30: Yield (t/ha) EMCO SR 92

Fig. 31: Yield (t/ha) EMCO SR 92/GLP2

Fig. 32: pH and CEC before planting

Fig. 33: % Nitrogen and Carbon before planting

Fig. 34: Mineral Nitrogen before planting

Fig. 35: Available nutrients before planting

Fig. 36: Available Phosphorus before planting

Fig. 37: Micronutrients before planting

LIST OF PICTURES

Picture 1: EM production, EM Kenya, Embu

Picture 2: EM in Kenya, picture at EM Kenya/TENRI compound, Embu

Picture 3: Livestock unit at the Muturi Farm, Manyatta

Picture 4: Planting of the maize 22.10.2003

Picture 5: First weeding 6.11.2003

Picture 6: EM 5 and EM FPE application

Picture 7: Irrigation of the plots 13.01.2003

Picture 8: Grain moisture measurement

Picture 9: Soil sampling after harvest

Picture 10: Aphids attack on beans

Picture 11: Final plant height measurement

Picture 12: NPK fertilized plots (left) compared to control plots (right) 05.12.2003

Picture 13: Anthesis of the maize

Picture 14: Silking of the maize

Picture 15: Number of cobs and shelled grains for grain moisture determination

Picture 16: Determination of the biomass

1. Introduction

The World Food Summit (FAO) 1996 pointed out, that the issue of the world food security is a matter of global concern for the future. Among developing countries the decrease of food insecurity is a major challenge. In the last decades the per capita food production in Asia and South America grew constantly. In contrast the per capita food production decrease in Sub-Saharan Africa (SSA) sustained (Sanchez et al. 1997, p.2). The agricultural production cannot keep pace with population growth. Besides the demographic pressure, the land depletion in smallholder farms is seen as a main cause for the declining per capita food production in SSA (Sanchez et al. 1997, p.1).

On the basis of the common shift from shifting cultivation systems and bush fallow to continuous cropping the process of soil restoration during the fallow phase is disturbed (Smaling 1993, p. 53). The farms are becoming smaller through intergenerational sub- division. The increasing request on limited land resources resulting from high production pressure is responsible for the physical, chemical and biological land depletion in Africa. Soil fertility decline takes place, when nutrient supply decreases and the chemical, physical and biological soil structure changes for the worse and consequently limits the plant growth. Besides nutrient depletion, the decreasing level of soil organic matter results in soil fertility decline (Donovan 1998, p. 3). According to Sanchez “the soil fertility depletion in smallholder farms is the fundamental biophysical root cause of declining per capita food production in Africa, and soil fertility replenishment should be considered as an investment in natural resource capita” (Sanchez et. al 1997, p. 3).

In the Kenyan economy agriculture plays a major role, although the proportion of the gross domestic product (GDP) value added in agriculture fell from 27.3 percent in 1997 to 16.4 percent in 2002 (World Bank Group 2004). Currently 70% of Kenya’s employment is in the field of agriculture. For 80% of the Kenyan rural population agriculture is the basis for life. For an increase in rural development and wealth as well as the national growth the agriculture sub-sector has to grow at about 4-5 per cent per annum (United Nations 2002, p. 29).

Soil fertility depletion in Kenyan agriculture is a severe constraint. Soil fertility decline in Kenya was estimated for the year 2000 with a depletion of 46 kg N, 1 kg P and 36 kg K (Stoorvogel and Smaling 1990, p. 63). There are three major strategies to maintain and regain soil fertility. The chemical, or mineral fertilization, the integrated nutrient management (INM) which combines inorganic fertilization methods with organic resource application, and the use of sole organic resources, realized in the approach of organic agriculture.

Inorganic fertilization in Kenya was promoted in the 1960s in the course of the worldwide “green revolution” (Lekasi et al. 2001, p. 2). However, in the 1980s the growing soil fertility depletion, excessive use of synthetic agro-chemicals and chemical fertilizers and inadequate natural resource management led to the development of alternative farming technologies (KOFA 2002, p. 5). In the late 1980s the pioneer NGO of organic agriculture in Kenya, the Kenya Institute of Organic Farming (KIOF), was founded. In the course of time organic agriculture was promoted by various organizations in Kenya. The mission statement of the organic agriculture “feed the soil not the crop” approaches declining crop production from the basis, the soil fertility and productivity.

In the early 1990s the liberalization of the Kenyan agricultural sector and thus the cancellation of parastatal subsidies led to increasing costs of inorganic inputs and consequently inorganic fertilizer use declined steadily (Omare and Woomer 2003, p. 168). The focus returned to organic fertilization methods and thus to the utilization of locally available organic resources (Lekasi et al. 2001, p. 2).

Organic resources act as a source of plant nutrients, influence positively soil physical properties and soil biota and thus account for the maintenance of soil fertility. In the mixed farming systems of the Kenyan Highlands crop residues and livestock manure are the most common organic resources. However, the quality of these resources varies widely and the availability of organic resources is limited. Manure in Kenyan Highlands in highly valued and is constantly increasing in price (Lekasi et al. 2001, p. 2). Therefore an efficient use of the mostly low quality organic resources is obligatory. New technologies in the field of natural farming were established to enhance the efficiency of organic resources, and thus increase yields on a sustainable basis that induces a regaining and maintaining of soil fertility (Sangakkara 2001). The organization EM Kenya promotes the technology of Effective Microorganisms (EM) in Kenya. Prof. Dr. Teruo Higa developed the technology of EM in the 1970s at the University of Ryukyus, Okinawa. EM consists of three principal organisms namely, Phototrophic bacteria (Rhodopseudomonas spp.), Lactic acid bacteria (lactobacillus spp.) and Yeast (Saccharomyces spp.) (Higa 2001a). It is an agricultural system, capable of high yield production on a sustainable basis. EM was originally developed for the field of organic agriculture to overcome low production and soil degradation. EM, applied in combination with organic resources, has the capacity to enhance their value by accelerating decomposition and thus releasing greater quantities of nutrients and enhance soil physical properties. Today EM is used in many agricultural and environmental systems such as crop, livestock and aquaculture production fields and in environmental management EM is used for decomposition and recycling of solid and liquid wastes (Sangakkara 2001).

1.1 Intention of the study

The maintenance and restoration of soil fertility in Kenyan Highlands by organic methods is the principal objective of the study. A field trial was carried out during the short rainy season 2003/2004 at the Regional Research Centre (RRC)- Embu, Embu District, to discover the effect of organic methods on crop growth and yields of maize and beans and soil chemical properties. Sole cattle manure, and the combination of cattle manure with the Effective Microorganisms (EM)- Technology were applied to reveal the effects of EM on cattle manure decomposition and thus on crop yields and soil chemical properties. To obtain comparable result for a classification of the organic agricultural techniques, plots with inorganic fertilizer application at recommended rates and of absolute control were established.

1.2 Structure of the study

The thesis introduces issues of soil fertility, plant nutrition, the role of soil organic matter and the nutrient depletion in agroecosystems. The problem of soil fertility decline in Africa is addressed, with a special focus on Kenya and Embu District. Due to the fact that the study is written in the context of the revision of the “Farm Management Handbook of Kenya” the issues of soil fertility depletion, the different approaches of soil fertility replenishment and maintenance in Kenyan agriculture are presented. Following is a description of the major methods of maintaining soil fertility in Kenya, the approach of inorganic fertilization and of integrated nutrient management (INM). The main focus lies on the organic agricultural sector in Kenya. In the “Farm Management Handbook of Kenya” the issue of organic agriculture in Kenya will be taken into consideration, thus the principles, standards, methods, promoters, and significant research works relevant for organic farming in Kenya are expatiated. The quality and fertilizer equivalency as well as nutrient release from organic matter are explained. The manure management in Kenyan Highlands, the quality and the effect on soil fertility is of special interest, because of the use of cattle manure in the experiment.

The function of soil microorganisms and the Technology of Effective Microorganisms (EM) are introduced. The role of EM in agriculture, the effects on crop production, soil fertility and soil health are described and backed up with current research results from all over the world. The research carried out on EM in Kenya is submitted as well as farmers’ statements on EM. The area of study with the relevant information of the geographical location, natural conditions, and socio-economic and cultural aspects is characterized. An explanation of the applied materials and methods of the experiment lead over to the statistical analysis, results and discussion of the various data on plant growth, yields and chemical soil characteristics. The thesis concludes with the results and recommendations concerning the future of organic agriculture in Kenya.

2. Soil fertility

Healthy and productive soils are the foundation for food production on earth. “Soil fertility is the status of a soil with respect to its ability to supply elements essential for plant growth without a toxic concentration of any element.” A sufficient and balanced supply of labile and available elements is needed to guarantee suitable plant nutrition (Foth and Ellis 1997, p. 1). “Soil productivity is the capacity of a soil to produce a certain yield of agronomic crops, or other plants, with optimum management”. The soil productivity is not only dependent on soil fertility, but also soil management practices and all other factors affecting plant growth. Productive soils are always fertile, but fertile soils can be unproductive because of limiting growth factors like drought and unsuitable management practices (Foth and Ellis 1997, p. 2).

The soil fertility status is influenced by many chemical, physical and biological factors. Soil chemical properties play a vital role in soil fertility determination. Adequate nutrient supply in correct proportions are responsible for plant growth. The range of acidity, the soil pH influences soil nutrient availability. The soil structure and thus the physical soil fertility is an important factor. Soil depth, and thus a good rooting depth as well as a good aeration are major attributes of soil fertility. Availability of water and a well functioning drainage are also important factors. The mineral composition influences the soil structure, the nutrient holding capacity and the nutrient release by weathering. Soil organic matter also influences soil structure, soil life, water retention, nutrient holding capacities and the release of nutrient by decomposition. Another vital parameter of soil fertility is the biological aspect, the soil biota. Soil microorganisms influence nutrient availability, the soil structure and water retention. Furthermore they are responsible for the process of decomposition and soil health status (Eyhorn et al. 2002, p. 51).

The approach of maintaining and regaining soil fertility is based on these chemical, physical and biological soil properties. The soil nutrient concentration depends on chemical soil factors. The conditions that permit release and uptake of essential nutrients depend on the physical properties. The biological factors determine the life of soil fauna (Donovan and Casey 1998, p. 11). The balance of chemical soil fertility can be achieved by application of mineral or organic fertilizers. The essential nutrients required can be provided by both sources. The physical soil fertility depends mainly on soil organic matter, maintained by the application of organic resources.

2.1 Soil fertility and plant nutrition

Besides air, water and light, plants need nutrients for their growth. Fertile soils have the ability to supply labile and available nutrients in a sufficient and balanced manner suitable for plant growth. Plant nutrients are mainly supplied from soil reserves, mineral fertilizers, organic sources, atmospheric nitrogen through biological fixation, aerial deposition caused by wind and rain, as well as irrigation, flood or groundwater, and sedimentation (FAO 1998, p. 3).

Three of the 16 essential nutrients, the non-mineral nutrients carbon (C), hydrogen (H) and oxygen (O) are supplied from water and air, although some carbon can be extracted by plants from soil. The soil environment provides the other 13 mineral nutrients, which are subdivided into macro- and micro (or trace)-nutrients, based on the elemental contents of plants.

Table 1: Essential Macro-and Micro (trace)- nutrients

Abbildung in dieser Leseprobe nicht enthalten

(Adapted from Van Reuler and Prins 1993, p. 16)

The essential macronutrients are required by plants in greater quantities than micronutrients (Muriuki and Qureshi 2001, p. 10). Micronutrients are also found in smaller quantities in plants. Plants dry matter content 0,3-50 mg/kg of micronutrients and 2-30 g/kg of macronutrients (IFA 1996-2004). However there are additional nutrients such as sodium (Na) that accomplish beneficial effects for plant growth, but are not essential (Motavalli et al. 2004).

To be usable by plants the nutrients must be available in the soil solution in an ionic form. The dry matter production of plants and thus the plant growth depends on nutrient supply. Three stages of nutrient supply are defined; the deficiency stage, the adequate stage and the toxic stage. Optimum growth is reached at the stage of adequate supply of nutrients. Deficient supply as well as toxic, nutrient oversupply, affects plant growth (Muriuki and Qureshi 2001, p. 11).

2.1.1 Primary macronutrients

Nitrogen (N)

The element nitrogen (N) is one of the most important nutrients concerning plant growth and crop production. It is needed for the building up of chlorophyll, that is responsible for the green colour of the leaves and thus enables plants to gain the required energy for nutrient uptake and plant growth (Eyhorn et al. 2002, p. 96). Nitrogen is also a constituent of proteins, nucleic acids and amino acids (Fig.1). By rhizobia microbes nitrogen can be fixed from the atmosphere. Especially legumes, like beans, are capable of biological N fixation due to a symbiosis between legume and rhizobium. On the roots of legumes rhizobia bacteria infect nodules. From atmospheric nitrogen, ammonium is produced that is usable for plant uptake. The fixation process is controlled by amino acid cycling between the plant and the bacteria. The bacteria acts like an organelle on which the plant depends on. This symbiotic relationship between legumes and rhizobia produces around 65% of the global nitrogen, which can be used for further plant growth, when the legume dies, or for animal feed, in case the legume is eaten (Science Line 2003).

N applied as organic matter such as animal manure, compost and green manure is converted to a plant available form with the help of soil microorganisms by the process of mineralisation. The mineralisation of organic N takes place in two stages. From the organic matter, amino acid is released (proteolysis) and then reduced to ammonia (ammonification). The activity of the soil microorganisms involved in mineralisation requires carbon (C). Thus the mineralisation of organic N is controlled by the carbon to nitrogen (C: N) ratio of the organic matter. C: N ratios of 25 promote mineralisation. During the process of the nitrification soil microorganisms convert ammonium (NH 4 +) to nitrate (NO3-). High temperatures with an optimum from 25-30 C°, neutral to slightly acid pH and good soil aeration support nitrification. The following equation shows that nitrification results in the release of hydrogen ions (H+) and therefore promotes soil acidification.

Abbildung in dieser Leseprobe nicht enthalten

Plants take up both, nitrate and ammonium ions. Plant growth responds more rapid to nitrate than to ammonium but during the course of crop growth this effect adjusts and the effect of nitrate and ammonium stays the same.

Fig. 1: The Nitrogen Cycle

Abbildung in dieser Leseprobe nicht enthalten

(Muriuki and Qureshi 2001, p. 16)

In alkaline soil environments (pH > 7) ammonium can be lost through volatilisation as ammonium gas (NH 4 +). During the process of denitrification that favours high soil temperatures, high soil moisture and a neutral pH, anaerobic soil microorganisms convert nitrate to gaseous forms such as nitrogen gas (N2) and nitrous oxide (N2O). Nitrate also can get unavailable for plants through immobilization, leaching or soil erosion (Muriuki and Qureshi 2001, p. 17).

Phosphorus (P)

Phosphorus is essential in the metabolism processes of plants related to transport of energy. P is involved in vital processes such as photosynthesis, respiration, energy storage and transfer, cell division, cell enlargement and several other processes (Government of Newfoundland and Labrador, not dated). It is constituent of nucleic acids, phospholipides and energy transfer molecules like andenosine tri-phosphate and andenosine di-phosphate (Muriuki and Qureshi 2001, p. 18). P is essential for plant growth. It improves early growth and formation of roots, flowering and riping of plant seeds. In livestock nutrition P plays an essential role for bone growth and for the metabolism (Eyhorn et al. 2002, p. 96).

Besides rock phosphate, organic resources like manures are commonly used as sources of phosphates. By mineralisation P is released from organic matter into the soil solution. The soil solution contains very small amounts of P because it is immobilized by soil microorganisms and plants, or available P is fixed to through adsorption and precipitation. Many tropical soils show P deficiency. This fixation takes place when soil minerals and free cations of aluminium, iron and calcium react with P. The process of adsorption and precipitation depends on soil pH. For plant uptake P is most mobile at pH 5.5 to pH 7.5 (Muriuki and Qureshi 2001, p. 18). Plants use P as primary orthophosphate ions H2PO4 and HPO42- ions. P absorbed onto soil particles can only be dissolved in very small quantities, thus most phosphates usable for plants are bound to soil organic matter or incorporated to soil microorganisms. Plant uptake of P can be increased by the colonisation of plant roots with mycorrhiza (Eyhorn et al. 2002,

p. 96).

P is the most immobile major nutrient and due to its tendency to react with soil minerals, and therefore be unavailable for plant roots, fertilizers should be placed close to the roots (Muriuki and Qureshi 2001, p. 18). P deficiency can be observed at overall stunted plants. Plant leaves turn to a purple or reddish colour. Severe P deficiency results in dead areas on the leaves, fruits and stems (Government of Newfoundland and Labrador, not dated).

Potassium (K)

Potassium plays an important role in photosynthesis, the carbohydrate metabolism and protein synthesis, as well as in cation - anion balance and water balance through osmoregulation. K is an essential nutrient for growth of roots and shoots (Muriuki and Qureshi 2001, p. 19). Plants ability to develop resistance to diseases is also increased by K. The optimum ratio of potassium and nitrogen in plants is 1:1. Potassium is also an essential element for livestock (Eyhorn et al. 2002, p. 97).

K is used by plants in the ionic form K+. It is mostly taken up during the vegetative stage and in cereals at the time between tillering and ear emergence. Weathering of potassium containing minerals of the chemical underground are the main source for potassium replenishment under natural conditions. Inorganic fertilizer and organic materials like manure and crop residues also provide and contain potassium. Because of the interaction of the positive charge of potassium ions and the negative charge of clay particle, K is held onto the soils exchange complex. Potassium is also strongly absorbed into the interlayer structure of 2:1 clays such as vermiculite, illite and montmorillionite. The major amount of K in soils is not readily available for plants, because of its strong incorporation in chemical particles. More easily available are the amounts of K that are absorbed onto the surface of chemical particles. Acid soils with pH below 4.5 lead to higher K fixation and restricting plant availability than soils with a pH > 7. Of all essential nutrients K has the least strong affection to clay colloids and is therefore prone to leaching (Muriuki and Qureshi 2001, p. 19). K deficiency is recognized in premature death of older plants first, because potassium is highly mobile in plants and mostly needed in new tissues. The need for potassium depends highly on the cultivated crops (Eyhorn et al. 2002, p. 97). In maize cultivation K deficiency leads to deformation of the cobs and affects the grain filling (Muriuki and Qureshi 2001, p. 19).

2.1.2 Secondary macronutrients

Calcium (Ca)

Calcium is needed for the growth of meristematic tissue, for cell elongation and cell division. It is an essential part of the cell wall structure. Ca is an essential nutrient for root health, growth of new roots and root hairs, and the development of leaves (NSW Agriculture 2002). Dolomitic lime, gypsum and superphosphate are sources of soluble calcium (Ca² +). Lime also induces pH increase. High nitrification rates in humid areas leads to Ca deficiency and to soil acidification. Ca² + and Mg² + are leached with NO3-. The plant uptake of cations such as Ca also leads to soil acidification, because of H+ release. Ca also induces a good soil structure due to its promotion of the floccation process of soil colloids (Muriuki and Qureshi 2001, p. 20).

Magnesium (Mg)

Magnesium is an essential component of chlorophyll and is important for photosynthesis. It also activates plant enzymes needed for growth and is vital for the formation of proteins. Deficiencies occur mainly on sandy acid soils in high rainfall areas. High applications of K in fertilisers can also result in magnesium deficiency. Especially banana growers need to watch magnesium levels because bananas are big potassium users (NSW Agriculture 2002).

Sulphur (S)

Sulphur, essential for protein synthesis, is a key element of amino acids in plant proteins. Energy-producing processes in plants are positively influenced by sulphur. Enzyme and vitamin development are promoted as well as the chlorophyll formation. Root and seed production are improved. In soils high in organic matter sulphur deficiency is not a problem. However sulphur leaches easily (NSW Agriculture 2002).

2.1.3 Micro (trace)-nutrients

Table 2: Micronutrients

Abbildung in dieser Leseprobe nicht enthalten

(Adapted from Muriuki and Qureshi 2001, p. 23-24)

Nutrient deficiency or excess leads to various symptoms. In the following table most important symptoms of nutrient excess and deficiency are described.

2.1.4 Nutrient deficiency and excess symptoms

Table 3: Nutrient deficiency and excess symptoms

Abbildung in dieser Leseprobe nicht enthalten

(Adapted from Motavalli et al. 2004)

Besides the soil nutrients, the soil organic matter (SOM) plays a crucial role for the production capacity of the soil. The maintenance of the soil quality, and thus the physical, chemical and biological properties depend on organic matter.

2.1.5 The role of soil organic matter (SOM)

Soil organic matter (SOM), built by the decomposition of organic resources with a fraction stabilized as SOM, plays a crucial role for the production capacity of the soil. The maintenance of the soil quality, and thus the physical, chemical and biological properties depend on organic matter. Organic matter improves the soil structure, increases the water holding capacity, permeability and the aeration (Hilhorst et. al. 2000, p. 5). Beyond it soil organic matter supplies plant nutrients, especially N, S and P and serves as a source of energy for microorganisms (Prasad 1997, p. 13).

The basis for soil organic matter are plant and animal residues, available in different gradations of decomposition from fresh material to well-decayed soil humus. The main share of soil organic matter originates from plant tissue. Animal tissue comes from plant products as well. 10-40% of plant residues consist of dry matter. The other 60- 90% are moisture. Plant dry matter contains mainly carbon (about 40%) and oxygen (about 40%). Less than 10% is hydrogen. The share of inorganic elements, less than 10% of the total dry weight, contains the macro-and micronutrients (Prasad 1997, p. 49). Besides the release of plant nutrients, the decomposition of plant residues produces CO2, energy, water and soil humus. According to Prasad “soil humus is a combination of the residues of the added organic matter, as well as the resynthesized microbial tissue that is resistant to microbial action” (Prasad 1997, p. 50).

The most important properties of soil humus concerning soil fertility management are:

- Humus particles form clay-humus complexes with clay and other silicate surfaces
- Storage and release of soil N
- Buffering capacity
- Cation and anion exchange capacity
- Absorption of pesticides and other chemicals (Prasad 1997, p. 52)

There is only little leaching of soil organic matter, because of the strong association with clay and the insolubility of salts of divalent and trivalent cations with SOM (Prasad 1997, p. 17). The soil organic matter content depends on many exogenous factors.

Climate, soil texture, agricultural practices like cultivation, fallowing and tillage, as well as the varying application of fertilizers and manures determine the SOM content (Prasad 1997, p. 57-63).

2.2 Nutrient cycling in agroecosystems

The major processes of nutrient cycling in agricultural systems are presented by Van Noordwijk in the following scheme (Fig. 2). The uptake efficiency, the utilization efficiency, the replenishment efficiency and the application efficiency. The schematic view shows in the lower left corner the uptake efficiency, the plant nutrient uptake from added organic and inorganic fertilizers as well as from soil. In the upper left arc the utilization efficiency and thus the internal redistribution and yield formation are presented. The replenishment efficiency in the upper right corner includes the harvest removal, and their exchange for external products and the recycling of the crop residues in the cycle. The lower right arc shows the application efficiency, the nutrient input by external products and by recycled on- farm products (Van Noordwijk 1999, p. 13).

Fig. 2: Nutrient cycling in agroecosystems

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(Van Noordwijk 1999, p. 13)

2.2.1 Soil fertility decline

Soil fertility decline takes place, when nutrient supply decreases and the chemical, physical and biological soil structure changes for the worse and consequently limits plant growth. By harvesting of agricultural products substantial nutrients are removed, which need to be replenished. Nandwa, S.M. et al., describe the nutrient removal by harvested maize and beans as follows (table 4). For the example a maize yield of 4 t/ha and a bean yield 1.3 t/ha is assumed.

Table 4: Nutrient removal by harvested maize and beans

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(Nandwa et al. 1999, p. 213)

Beside nutrient losses through harvest removal, further processes prevent efficient nutrient use and result in nutrient losses of agroecosystems (Fig. 3). The chemical occlusion as well as soil physical and biological processes are able to prevent and limit nutrient uptake by plants in the rhizosphere. The horizontal nutrient transfer stands horizontal nutrient exchange on farms resulting in enrichment and depletion areas (e.g. that which can be caused by farmers’ practices). Soil losses through erosion and deposition processes are responsible for huge nutrient losses in many agrological systems. Through the process of leaching nutrient transfer to deeper soil layer occur and out of reach for shallow rooting crops, such as maize on farm scale. Gaseous losses to the atmosphere are especially important for N and S. Dust or ash particles are also included in the losses to the atmosphere although a deposition follows. On the landscape scale the export of harvest products is included. The last step on society scale is that nutrient outflows can’t be replenished by external inputs (Van Noordwijk 1999, p, 14).

Fig. 3: Nutrient efficiency difficulties in agroecosystems at different scales

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(Van Noordwijk 1999, p, 14)

Besides nutrient depletion, the decreasing level of soil organic matter and a decrease in soil pH results in soil fertility decline (Donovan and Casey 1998, p. 3). In traditional farming systems land pressure leads to soil fertility decline (Fig. 4).

Fig. 4: Degradation in a traditional farming system due to land pressure

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(Pieri 1992, p. 89)

Figure 4 shows the degradation of farming systems when pressure on land approaches saturation. The pressure on land increases by growing population and thus an increase in cultivated land. A decrease of reserves due to shorter periods of fallow, and thus a decrease of soil fertility result from this pressure. The process goes on with a shortage in animal fodder and thus either a removal of stover from the field or less livestock keeping. Both result in a lower soil organic matter contents and a deterioration of the soil structure.

2.2.2 Soil analytical indicators

There are various approaches for the determination of the soil fertility status in Kenya, taking soil chemical, physical and biological soil characteristics into account. Yet chemical properties are principally used. For the evaluation of the soil fertility status chemical properties, in the first place critical soil macronutrient levels (N, P, K etc.) are taken into consideration. For example the soil analysis included in the Fertilizer Use Recommendation Project (FURP) in Kenya concentrates on the macronutrients K, P, Ca and Mg (FURP 1994, Volume 6, p. 15). However, out of many approaches, following examples of soil fertility evaluation are also applied in Kenya’s soil fertility and productivity research.

The Quantitative Evaluation of the Fertility of Tropical Soils (QUEFTS) model calculates N and P plant-supply from a combination of soil test values. N supply is determined from organic carbon, pH, temperature and texture; P supply is calculated from organic carbon, total and available P and pH (Nandwa and Bekunda 1998, p. 8). Braun et al. reviewed the Kenyan Highland’s soil productivity on following indicators; organic carbon, nitrogen (total N, N supply, N balance, N leaching, N gaseous losses, N accumulation at depth), phosphorus (total P, P supply, P balance, P fixation, easily extractable P), soil acidity, soil rootability, soil erodibility and relief (Braun et al. 1997, p. 18). Soil organic carbon also plays a major role as an index for sustainability in land use systems. It is used as an indicator of a long-term trial as well as data about the soil microbial biomass (Kapkiyai et al. 1999, p. 1776). Nutrient balances, estimating the nutrient in- and outflows of the soil, are applied at various scales.