Soil Nutrient Depletion Research Articles

Responses of Spinach to Salinity and Nutrient Deficiency in Growth, Physiology, and Nutritional Value

Salinity and nutrient-depleted soil are major constraints to crop production, especially for vegetable crops. The effects of salinity and nutrient deficiency on spinach (Spinacia oleracea L.) were evaluated in sand cultures under greenhouse conditions. Plants were watered every day with Hoagland nutrition solution, deprived of nitrogen (N), phosphorous (P), or potassium (K) for nutrient deficiency, either with or without 20/10 mm sodium chloride (NaCl)/calcium chloride (CaCl2) for salinity treatment. Salinity significantly decreased shoot fresh weight (FW) and dry weight (DW), leaf relative water content (RWC), and specific leaf area (SLA) relative to controls after 4 weeks of treatment and increased chlorophyll content, maximum photochemical efficiency (Fv/Fm), and photochemical yield [Y(II)]. Nitrogen deficiency greatly reduced shoot FW and DW, SLA, and chlorophyll content, regardless of salt treatment. Y(II) and Fv/Fm were reduced by N deficiency and salinity treatment. Phosphorous and K deficiencies, similarly, decreased shoot FW and DW irrespective of salinity treatment and increased chlorophyll content without salt stress. Phosphorous deficiency increased Y(II) under control and Fv/Fm under both control and salt treatment. Salinity and nutrient deficiency also affected the nutritional value of spinach. Salt stress increased carotenoid and flavonoid contents, and reducing power in full-strength Hoagland solution, and decreased leaf ferrous ion chelating ability (FICA). Nutrient deficiency increased reducing power regardless of salinity treatment. Nitrogen deficiency increased anthocyanin and total phenolic contents, decreased carotenoids and flavonoids regardless of salinity treatment, and increased antioxidant capacity under no-salt conditions. Phosphorous deficiency increased carotenoid and flavonoid contents under no-salt condition and enhanced total phenolic content and reduced FICA and amino acid content under salt stress. Potassium deficiency increased total phenolic, carotenoid, and flavonoid contents and antioxidant capacity under non-salt condition, but decreased FICA regardless of salinity treatment. These results suggest that spinach nutritional value could be improved with only moderately or slightly reduced yield through cultural practices that impose either low fertilizer levels or slight salt stress.

The effect of indole acetic acid on the performance of maize (<i>Zea maysl</i>.)in a Southern Guinea Savanna Zone of Nigeria

Poor soil fertility and nutrient depletion continue to present huge challenges to successful crop production in southern Guinea savanna zone of Nigeria. Consequently, a study was conducted at Ilorin - Nigeria in a southern Guinea savanna zone during the wet seasons of 2012 and 2013 to evaluate the effects of indole acetic acid (IAA), a growth regulator, on the growth and yield of maize and its economic implications. The treatments imposed were five rates of IAA (0, 200, 400, 600 and 800 ppm) using a randomized complete block design replicated three times. A basal NPK fertilizer was applied at 3WAP and IAA was applied at the primary leaf stage foliarly. Data were collected on growth parameters and grain yield of maize. Analyses of variance (ANOVA) for all observations were performed and mean separation was done by using the least significant difference (LSD). Cost benefit analysis was also determined. The result of the experiment indicated that using IAA significantly (P<0.05) increased growth and grain yield of maize from 3300 to 3957.9kgha-1 but the result of economic analysis indicated that the control produced the highest net returns (N225,130 in 2012 and N232,959.00 in 2013) and was more economical because of high cost of indole acetic acid for maize cultivation. Keywords : maize, growth regulator, growth, grain yieldand net returns.

A fragrant neighborhood: volatile mediated bacterial interactions in soil.

There is increasing evidence that volatile organic compounds (VOCs) play essential roles in communication and competition between soil microorganisms. Here we assessed volatile-mediated interactions of a synthetic microbial community in a model system that mimics the natural conditions in the heterogeneous soil environment along the rhizosphere. Phylogenetic different soil bacterial isolates (Burkholderia sp., Dyella sp., Janthinobacterium sp., Pseudomonas sp., and Paenibacillus sp.) were inoculated as mixtures or monoculture in organic-poor, sandy soil containing artificial root exudates (ARE) and the volatile profile and growth were analyzed. Additionally, a two-compartment system was used to test if volatiles produced by inter-specific interactions in the rhizosphere can stimulate the activity of starving bacteria in the surrounding, nutrient-depleted soil. The obtained results revealed that both microbial interactions and shifts in microbial community composition had a strong effect on the volatile emission. Interestingly, the presence of a slow-growing, low abundant Paenibacillus strain significantly affected the volatile production by the other abundant members of the bacterial community as well as the growth of the interacting strains. Furthermore, volatiles released by mixtures of root-exudates consuming bacteria stimulated the activity and growth of starved bacteria. Besides growth stimulation, also an inhibition in growth was observed for starving bacteria exposed to microbial volatiles. The current work suggests that volatiles produced during microbial interactions in the rhizosphere have a significant long distance effect on microorganisms in the surrounding, nutrient-depleted soil.

Soil property variations in relation to exclosure and open grazing land use types in the Central Rift Valley area of Ethiopia

Land degradation and unsustainable management practices have resulted in soil organic carbon and nutrient depletion, hydrological instability, reduced primary productivity, and low biological diversity. In response to these problems, communities in the Central Rift Valley area of Ethiopia have started to establish exclosures about three decades ago. This study has investigated the variations in selected soil properties (soil textural fractions, bulk density, soil moisture content, pH (H2O) and soil organic carbon content) under two land use types: open grazing land and exclosures in the Central Rift Valley of Ethiopia. Results showed that soil organic carbon varied significantly with land use types (p = 0.040), soil depths (p = 0.010) and the interaction effect (p = 0.039). The soil moisture content showed significant variation (p < 0.0001) only with soil depth. Exclosure land use type has shown an improvement in soil organic carbon against the findings by Mekuria et al. [International Conference on Advances in Agricultural, Biological & Environmental Sciences (AABES-2014), Dubai (UAE) 2014]. Thus, highly degraded open grazing should be designated as exclosure land management zone to restore and rehabilitate severely degraded landscape in the fragile environment of the rift valley area of Ethiopia.

Plant interactions with soils conditioned by different vegetation: A potential explanation of Bromus tectorum L. invasion into salt-deserts?

Invasion by Bromus tectorum L. may condition the soil and increase nutrient availability. We hypothesized that nutrient poor soils of the arid Honey Lake Valley of northeastern California U.S.A., similar in physical and chemical properties, but conditioned by either B. tectorum, Krascheninnikovia lanata, or Artemisia tridentata, would differ as a rooting medium. These soil/vegetation types were placed in equal volumes in replicate 5400 cm3 cylindrical containers, the template removed, and sown to B. tectorum or Leymus triticoides; controls were left unplanted. At harvest, root mass, root nutrient concentrations, and selected soil nutrients were quantified for each soil/vegetation type. Root mass was statistically similar among soil/vegetation types and seeded plants. After harvest, and relative to unplanted controls, B. tectorum invaded soil lost a greater proportion of several nutrients than the other soil/vegetation types suggesting greater nutrient uptake from that soil. Soil/vegetation types planted to B. tectorum had significantly greater depletion of soil nutrients compared to soils planted to L. triticoides. Soil invaded by B. tectorum had greater nutrient availability than similar soil conditioned by A. tridentata or K. lanata and offers a plausible explanation of why this exotic annual grass has been able to invade nutrient poor salt-desert ecosystems.

Long-term impacts of season of grazing on soil carbon sequestration and selected soil properties in the arid Eastern Cape, South Africa

The Karoo biomes of South Africa are major feed resources for livestock farming, yet soil nutrient depletion and degradation is a major problem. The objective of this study was to assess impacts of long-term (>75 years) grazing during spring (SPG), summer (SUG), winter (WG) and exclosure (non-grazed control) treatments on soil nutrients, penetration resistance and infiltration tests. A soil sampling campaign was carried out to collect soil to a depth of 60 cm to analyse bulk density, soil physical and chemical parameters as well as soil compaction and infiltration. Generally, grazing treatments reduced soil organic C (SOC) stocks and C:N ratios, and modified soil properties. There was higher SOC stock (0.128 Mg ha−1 yr−1) in the exclosure than in the SPG (0.096 Mg ha−1 yr−1), SUG (0.099 Mg ha−1 yr−1) and WG (0.105 Mg ha−1 yr−1). The C:N ratios exhibited similar pattern to that of C. From the grazing treatments, the WG demonstrated 7 to 10 % additional SOC stock over the SPG and SUG, respectively. Short period animal exclusion could be an option to be considered to improve plant nutrients in sandy soils of South Africa. However, this may require a policy environment which supports stock exclusion from such areas vulnerable to land degradation, nutrient and C losses by grazing-induced vegetation and landscape changes.

Environmental impacts and constraints associated with the production of major food crops in Sub-Saharan Africa and South Asia

Many environmental factors constrain the production of major food crops in Sub-Saharan Africa and South Asia. At the same time, these food production systems themselves have a range of negative impacts on the environment. In this paper we review the published literature and assess the depth of recent research (since 2000) on crop x environment interactions for rice, maize, sorghum/millets, sweetpotato/yam and cassava in these two regions. We summarize current understandings of the environmental impacts of crop production systems prior to crop production, during production and post-production, and emphasize how those initial environmental impacts become new and more severe environmental constraints to crop yields. Pre-production environmental interactions relate to agricultural expansion or intensification, and include soil degradation and erosion, the loss of wild biodiversity, loss of food crop genetic diversity and climate change. Those during crop production include soil nutrient depletion, water depletion, soil and water contamination, and pest resistance/outbreaks and the emergence of new pests and diseases. Post-harvest environmental interactions relate to the effects of crop residue disposal, as well as crop storage and processing. We find the depth of recent publications on environmental impacts is very uneven across crops and regions. Most information is available for rice in South Asia and maize in Sub-Saharan Africa where these crops are widely grown and have large environmental impacts, often relating to soil nutrient and water management. Relatively few new studies have been reported for sorghum/millets, sweetpotato/yam or cassava, despite their importance for food security on large areas of marginal farmland in Sub-Saharan Africa – however, there is mounting evidence that even these low-input crops, once thought to be environmentally benign, are contributing to cycles of environmental degradation that threaten current and future food production. A concluding overview of the emerging range of published good practices for smallholder farmers highlights many opportunities to better manage crop x environment interactions and reduce environmental impacts from these crops in developing countries.

Corn residue, tillage, and nitrogen rate effects on soil carbon and nutrient stocks in Illinois

Removal of crop residues for use as ethanol production feedstock might deplete soil carbon and nutrient pools in site- and management-specific ways. We investigated the effect of residue removal (RR), tillage (T), and N fertilizer rate (Nr) after five years of continuous corn (Zea mays L.) on total soil carbon (TC) and nutrient stocks at four sites within Illinois. The experimental design was a split–split plot arrangement of treatments in a randomized complete block design with four replications, and all treatments remained in the same place each year. Main plots consist of one of three levels of corn residue removal (RR: full, partial, and none); split plots were two tillage systems (T: chisel tilled and no-till) and split–split plots were four N fertilizer rates (Nr: 67, 134, 201, and 268kgNha−1). The highest TC stocks were found under no-till without residue removal; removing any residue under no-till lowered TC to the levels found under chisel tillage. Removing residue in tilled soils produced higher TC values similar to the levels found with no residue removal and no-till. Residue removal tended to lower P and lowered K and EC in the surface 15cm soil. Tillage decreased the N and K stocks in the surface soil. Increasing the rate of N fertilizer lowered P, K, and pH, generally in an increasing, curvilinear manner, but the response of EC was concave, increasing at the highest N rate used. These responses were closely related to corn grain yields, indicating that the amount of nutrient removed by harvest of grain and residue and the amount of residue retained after harvest affect TC and nutrient stocks in Illinois soils. This information will help producers and policy makers to make better decisions regarding the feasibility of harvesting corn residue, and on agronomic practices that might accompany residue removal in order to prevent soil nutrient depletion.

The impact on nutrient cycles from tropical forest to pasture conversion in Costa Rica

Changes in nutrient and hydrological cycles caused by land disturbance typically lead to detrimental changes to ecosystems. This study utilized a paired, small-catchment approach to examine the effect of deforestation on soils and streams of the tropical Osa Peninsula, Costa Rica. The first catchment had been cleared for pasture and the second consisted of undisturbed tropical wet forest. Soil concentrations of organic matter, total and soil-available phosphorus (P) were higher in the forested catchment with reductions of >33 % of each in the deforested catchment. The effect of deforestation on stream discharge was a 59 % increase in flow during the wet season and a higher Q5:Q95 (percentile flow) ratio showing that the deforested stream yielded shorter duration, higher magnitude flood peaks. The deforested catchment loss of dissolved inorganic nitrogen increased 95 % over the forested catchment. Soluble reactive phosphorus showed a 43 % higher load in the deforested catchment compared to the forested catchment. The molar N:P ratios were low and both streams were well below the level at which N limitation of lotic algal growth has been reported. It, therefore, appears that N is the limiting nutrient in streams in the study area. Soil nutrient depletion in the deforested catchment, accelerated by a changed hydrological regime, is the likely trajectory of soil–water interactions in this tropical ecosystem. Loss of nutrients and organic matter from terrestrial ecosystems will likely lead to long-term impacts on lowland tropical communities. Should deforestation become widespread along this stretch of the Pacific coastline possible eutrophication of receiving transitional and coastal waters may occur.

Monitoring processes of degradation and desertification in Rivne region

This article provides a brief description of soil quality state in Rivne region for the period of 2008– 2012. Over the last decade there has been depletion of soil nutrients, significant reducing the inventories of phosphorus mobile forms and exchangeable potassium, increasing the acidity. Therefore, the primary objective is to find ways of their fertility recreation. The results indicate a slight accumulation of studied toxins and radionuclides in soils of the region and provide an opportunity to develop protective measures to reduce the factor of conversion them into the soilplant system, thereby reducing their harmful effects on the human body.

Nutrient accumulation and export in teak (Tectona grandis l.f.) plantations of Central America

Abstract: In order to assess the nutrient sustainability of teak plantations, a study was conducted to measure the amount of nutrients accumulated by the trees and exported during wood harvest. Three teak plantations (28 stands of different age) were studied in Costa Rica and Panama to assess those questions. Nutrient and biomass accumulation and allocation in different tree components (bole, bark, branches and foliage) were measured in the best performing trees between 1 and 19 years of age. A stand of 150 teak trees ha-1 at age 19 would accumulate (kg ha-1) 405 N, 661 Ca, 182 K, 111 Mg, 33 P, 53 S, 9 Fe, 0.47 Mn, 0.22 Cu, 0.92 Zn, 1 B; whereas the expected nutrient export by timber harvest (bole and bark) is (kg ha-1) 220 N, 281 Ca, 88 K, 63 Mg, 23 P, 39 S, 6 Fe, 0.13 Mn, 0.10 Cu, 0.21 Zn, 0.40 B. Hence, teak nutrition should pay special attention to N and K, together with Ca the nutrients most accumulated by teak. In addition, P and B could also be limiting planted teak forest productivity due to their general soil deficiencies. Proposed models estimate the amount of nutrients removed from the site during timber harvests, information that can be used by plantation managers to avoid soil nutrient depletion, approaching sustainability in forest plantation management.

Physical and Chemical Properties of Four Contrasting Soils under Different Land Use System

This study was undertaken to evaluate the physical and chemical properties of four contrasting soils under two land use systems. The land use types considered were fallow and cultivated. Four soils from Nsukka Hill (Entisol), Nsukka poultry site (Ultisol), Eha-Amufu (Inceptisol) and Ikem (Inceptisol) in Nsukka area of south eastern Nigeria were used for the study. Soil samples from the 0-25cm depth were collected from cultivated and adjacent fallow lands in the four different locations. The soil samples were air-dried at room temperature and then sieved through a 5.00mm sieve. Two hundred and fifty grams (250g) of the sieved sample were further sieved through 2mm sieve and use for the determination of physical and chemical properties of the soils. The result of the study showed that soils under continuous cultivation have low value in all the parameters assessed compared to the adjacent fallow soils. Continuous cultivation decreased the concentration of organic matter (OM), from 2.08-1.87% nitrogen (N), 0.3-0.17% and phosphorous (P) 4.30-3.80mgkg -1 content in all the soils studied. The pH of the soils measured in water and KCl showed low values in cultivated soils with the range 4.10-4.50 and 3.50-3.70 respectively as against fallow soils 4.60-4.90 and 3.20-4.50 respectively. Exchangeable bases, cat ion exchange capacity (CEC) and Base saturation (BS) of the soils decreased following cultivation except for K in Ikem soils, CEC in Nsukka Ultisol and BS in Ikem Inceptisol. Cultivation increased the exchangeable Al 3+ and H + in the Nsukka Entisol and thus increased the acidity. The result of this study is evidence that continuous cultivation causes depletion of soil nutrients and generally affects the physical-chemical properties of soils. This can be remedied through appropriate management Original Research Article

Crop-sequence effects on productivity in a wheat-based cropping system at Wongan Hills, Western Australia

Western Australian grain production is dominated by wheat, but growing wheat continually in unbroken sequences leads to increasing problems with soil nutrient depletion, root and leaf disease build-up, high weed burdens, and possibly other less well-defined production constraints. These can adversely affect both production and grain quality. Including breaks in the crop sequence in the form of break crops, pasture, or fallow can reduce these problems, but these breaks can be expensive to implement, in terms of both direct cost and forgone revenue. It is therefore critical to predict the response of subsequent wheat crops to a break in order to choose crop sequences rationally. We conducted a 4-year experiment at Wongan Hills, Western Australia, evaluating how wheat productivity in a wheat-based cropping sequence is affected by including wheat, barley, lupins, triazine-tolerant and Roundup Ready® canola, oaten hay, volunteer pasture, serradella pasture, and chemical fallow. Wheat yield responded positively to fallow, lupins, oaten hay, volunteer pastures and serradella but not to barley or canola when compared with continuous wheat. Responses depended on seasonal conditions; in a dry year, a very large response occurred after fallow but not after lupin or serradella, whereas in a wetter year, there were large responses after these crops. Fallowing, cutting hay, crop-topping lupins, and spray-topping volunteer and serradella pasture all reduced seedset of annual ryegrass dramatically, and reduced weed competition was a major contributor to the observed break crop responses. Nitrogen fixation by lupins and serradella and water storage by fallow in a dry year were also important, but soilborne diseases did not contribute to wheat yield responses. Some yield responses persisted for at least 3 years, and the contribution of effects of weed competition to yield responses increased over this time. These results emphasise the importance of understanding which productivity constraints are present in a cropping system at a given time when deciding whether a break is necessary and which is the most appropriate break. The results also emphasise the importance of managing the wheat crop after a break to maximise the response and its longevity.

Effects of organic and conventional pesticides on plant biomass, nematode diversity and the structure of the soil food web

Due to the uncertain future of the soil fumigants most commonly used in the EU, there is a need to develop new integrated pest management programmes to control crop diseases. Different nematode management practices, such as solarisation and the use of ecological nematicides, including nematophagous fungi, are used to control populations of plant-parasitic nematodes, one of the most common pests affecting crops. The objective of this study was to determine the effects of organic (neem seed paste and a mixture of nematophagous fungi) and conventional (oxamyl and fenamiphos) nematicides on soil physical chemical properties, soil biodiversity and plant biomass. Such effects were investigated in two types of habitats: low diversity soils from an agricultural farm and high diversity soils from a natural vegetation area. The greater effect was observed with the neem treatment, which induced a large boost of dauer juveniles in the nutrient-depleted soil, while the same treatment induced an increase of populations of less opportunistic, generalist bacterivore nematodes in the pine forest soil, rich in organic matter. We have studied the effects of different biological and chemical nematicides on the whole soil nematode community through ecological indices and their relationship with plant biomass, but further research is needed to improve understanding of the effect of these products on nematode assemblages.

The soil microbial community predicts the importance of plant traits in plant-soil feedback.

Reciprocal interaction between plant and soil (plant-soil feedback, PSF) can determine plant community structure. Understanding which traits control interspecific variation of PSF strength is crucial for plant ecology. Studies have highlighted either plant-mediated nutrient cycling (litter-mediated PSF) or plant-microbe interaction (microbial-mediated PSF) as important PSF mechanisms, each attributing PSF variation to different traits. However, this separation neglects the complex indirect interactions between the two mechanisms. We developed a model coupling litter- and microbial-mediated PSFs to identify the relative importance of traits in controlling PSF strength, and its dependency on the composition of root-associated microbes (i.e. pathogens and/or mycorrhizal fungi). Results showed that although plant carbon: nitrogen (C:N) ratio and microbial nutrient acquisition traits were consistently important, the importance of litter decomposability varied. Litter decomposability was not a major PSF determinant when pathogens are present. However, its importance increased with the relative abundance of mycorrhizal fungi as nutrient released from the mycorrhizal-enhanced litter production to the nutrient-depleted soils result in synergistic increase of soil nutrient and mycorrhizal abundance. Data compiled from empirical studies also supported our predictions. We propose that the importance of litter decomposability depends on the composition of root-associated microbes. Our results provide new perspectives in plant invasion and trait-based ecology.

Mycorrhizas effects on nutrient interception in two riparian grass species

Effects of arbuscular mycorrhizal (AM) fungi on plant growth and soil nutrient depletion are well known, but their roles as nutrient interceptor in riparian areas are less clear. The effects of AM fungi on growth, soil nutrient depletion and nutrient leaching were investigated in columns with two riparian grass species. Mycorrhizal and non mycorrhizal (NM) plants were grown in a mixture of riparian soil and sand (60% and 40%, w/w respectively) for 8 weeks under glasshouse conditions. Mycorrhizal colonization, AM external hyphae development, plant growth, nutrient uptake and NO 3 , NH 4 and available P in soil and leachate were measured. Mycorrhizal fungi highly colonized roots of exotic grass Phalaris aquatica and significantly increased plant growth and nutrient uptake. Columns containing of AM Phalaris aquatica had higher levels of AM external hyphae, lower levels of NO 3 , NH 4 and available P in soil and leachate than NM columns. Although roots of native grass Austrodanthonia caespitosa had moderately high levels of AM colonization and AM external hyphae in soil, AM inoculation had no significant effects on plant growth, soil and leachate concentration of NO 3 and NH 4 . But AM inoculation decreased available soil P concentration in deeper soil layer and had no effects on dissolved P in leachate. Although both grass species had nearly the same biomass, results showed that leachate collected from Austrodanthonia caespitosa columns significantly had lower levels of NO 3 , NH 4 and dissolve P than leachate from exotic Phalaris aquatica columns. Taken together, these data shows that native plant species intercept higher nutrient than exotic plant species and had no responsiveness to AM fungi related to nutrient leaching, but AM fungi play an important role in interception of nutrient in exotic plant species.

The Impact of Soil and Water Conservation Program on the Income and Productivity of Farm Households in Adama District, Ethiopia

Land degradation due to soil erosion and nutrient depletion is one of the main problems constraining the development of the agricultural sector in Ethiopia. As part of intervention activities a number of soil and water conservation (SWC) practices have been promoted to smallholder farmers living in highly degraded and drought prone areas of the country. This study was conducted to assess the impact of SWC intervention on the livelihood of smallholder farm households in terms household income and productivity. To meet this objective primary data was gathered in 2012 from 101 SWC program participants and 115 non-participants that were randomly selected from 3 intervention area and 3 counterfactual villages respectively. Descriptive and inferential statistics and propensity score matching (PSM) models were used to address the stated objectives. Results of the descriptive statistics showed that before matching there was statistically significant difference between program participants and their counterfactual households in terms of sex and age of household head, family size and farm size generally in favor of program participants. Results of the PSM model revealed that SWC intervention did not result in significant difference between program participant and nonparticipant households in terms of total crop and household income, and crop yield. However it was to be noted that there were positive trends which all together should guide SWC policy makers to identify important factors influencing the contribution of such a program and reconsider the design and implementation of the interventions Keywords : Soil and water, Conservation practices, Impact, Oromia region

Live fences – a hidden resource of soil fertility in West Kenya

AbstractIn the highlands of Western Kenya, intensified land use combined with low use of mineral fertilizers at farm level is driving soil nutrient depletion and declining yield levels. All farms and individual field crop plots are surrounded by life fences and the area under such hedge structure is estimated to cover 3–5% of the total farmland area. The land below the hedges is not tilled and may receive nutrient and carbon inputs by the occasional addition of field crop residues and litter fall. We hypothesized that the areas covered by live fences represent not only an important land resource, but are also largely untapped sites characterized by high soil fertility in an otherwise largely degraded environment. We characterized physico‐chemical attributes of top soils collected on farmland and from adjacent live fences composed of five different fencing species in representative environments of Kakamega district (sandy Acrisol and bimodal rainfall distribution vs. clay Ferralsol and monomodal rainfall distribution). In addition, the maize production potential of these soils was assessed in a supplementary pot experiment. Concentrations and total amounts of soil C, N, N supplying capacity, exchangeable K, as well as aggregate stability tended to be higher in hedge structures than in field crop soil. The amount of labile (permanganate‐oxidizable) carbon and the carbon management index in fence lines on Acrisol were similar to those of the adjacent Kakamega rain forest reserve, while they were reduced by &gt; 50% in crop fields. These trends were reflected in the biomass and N and K uptake by maize in potted soil. Effects were generally larger in Acrisol than in Ferralsol and were most pronounced with Tithonia diversifolia. We conclude that live fences are soil fertility hotspots that may be valorised in the future by replacing the generally unproductive fencing species with economical perennial crops.

English

Soil nutrient depletion and Xanthomonas wilt (Xanthomonas campestris pv. musacearum) are major causes of declining productivity in smallholder banana systems in East and Central Africa. This study examined the interactive effect of fertilizer and inoculum concentration on development of Xanthomonas wilt. Fertilization significantly (p&lt;0.01) increased the plant height, plant girth and leaf area in banana compared to control without fertilizer. Despite this, between 9 and 21 days post inoculation (dpi) all inoculated plants had exhibited typical disease symptoms (that is, chlorosis, necrosis and wilting of leaves). No significant reduction (p&gt;0.05) in disease incidence, wilt severity index or mortality could be associated with increasing fertilizer amounts. Interestingly, there was a highly significant (p&lt;0.01) overriding effect of inoculum concentration on the ability of fertilizer to reduce wilt severity index and mortality in banana. Plants inoculated with 106 to 1012 cfu mL-1 developed twice as much disease compared to 104 cfu mL-1 inoculations. Average mortality of 9.2% for 104 cfu mL-1 inoculated plants provides evidence of the potential to cause latent infections. Low bacterial loads are implicated in recent field resurgence of Xanthomonas wilt in banana orchards where disease had been successfully contained. &nbsp; Key words: Banana, fertilizer, inoculum concentration and Xanthomonas wilt.

English

The rate at which soil ecosystem is being degraded in crop production systems in the tropics is alarming. This study therefore attempts to assess the physico-chemical properties of soil and crop yield under agroforestry in the traditional farming systems. This was a researcher-designed, farmer-managed participatory experiment. Two farming systems (cashew/maize intercropping and sole maize cropping) were used. The two farm locations were in Wasangari village, near Saki.&nbsp; From 5 ha cashew plantation established in 1998 using a plant spacing of 9.0 &times; 9.0 m2 by the collaborating farmer, two plots of 20 &times; 20 m2 were mapped out for maize, intercropped at a plant spacing of 90 x 40 cm2 in 2002. Also, to a fallowed land since 1998 adjacent to cashew plantation but cultivated to sole maize in 2002 using the same plant spacing, two plots of 20 &times; 20 m2 were mapped out. The maize seedlings in the 4 plots were thinned to 2 stands per hole 2 weeks after planting (WAP) to give a plant population of 55,555 plants/ha. The two collaborating farmers weeded their farms 2 times (2 and 5 WAP) using hoe. The experiment was conducted over two planting seasons. Values of the soil nutrients (0 to 15 cm) evaluated before maize introduction in cashew/maize plots were significantly (p&lt; 0.005) higher than those from sole maize plots. Also, the mean yields of maize in the intercropped plots (1.34, 1.02 t ha-1) were significantly (p &lt; 0.05) higher than mean yields in sole maize fallowed plots (1.05, 0.81 t ha-1) in the early and late season studies respectively. The study demonstrated that intercropping maize with cashew, in the early stage serves as additional source of income to traditional farmers in the tropics. &nbsp; Key words: Agroforestry, traditional farming system, cashew, soil nutrient depletion, litter falls,cashew/maize intercrop.