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    Using Principal Component Analysis to Assess Soil Chemical Properties in the Mwea Irrigation Scheme, Kenya: Implications for Rice Agronomic Management
    (2024-03-19) Daniel M. Menge; Ruth N. Musila; Sammy Kagito; Lourine Bii; James Gichuki; Emily Gichuhi; Caroline A. Kundu; Rosemary Murori; Abdelbagi Ismail; Ajay Panchbhai
    Sub-Saharan Africa faces significant challenges as a net importer of rice, with self-sufficiency rates plummeting to as low as 14% in countries like Kenya. The Mwea irrigation scheme shoulders the bulk of rice cultivation, contributing over 80% of the country’s production. However, productivity within the scheme has seen a concerning decline, dropping from 5.6 ─ 6.0 t ha-1 to 1.3 ─ 4.6 t ha-1 between 1977 and 2018. This decline could be attributed to site-specific nutritional deficiencies and/or toxicities, rendering generalized agronomic recommendations ineffective in certain areas. To address this problem, our study aimed to assess soil chemical properties variability within the Mwea irrigation scheme, identify clusters with similar nutritional status, and tailor area-specific agronomic recommendations. During May to September 2020, we collected and analyzed four hundred samples from five sections of the scheme for total organic carbon, soil pH, macro and micronutrients, and exchangeable cations. Principal component analysis was conducted on the mean values of the soil chemical properties to identify significant contributors to variation and establish zones with similar patterns. Principal components 1 to 4 collectively explained 72.2% of the total variability. Cluster analysis revealed four distinct clusters, namely MW, TB, KT, and WU. Within cluster MW, soil pH was below the optimum range for rice cultivation, suggesting a need for liming. Potassium deficiency was observed across all clusters, with rice straw incorporation recommended as a long-term solution. Furthermore, zinc deficiency was noted in cluster WU, necessitating zinc fertilizer application. Conversely, iron toxicity was a concern in cluster MW, suggesting the adoption of alternating wetting and drying techniques and cultivating tolerant varieties. By proving tailored recommendations based on localized soil conditions, we aim to bolster rice productivity within the Mwea irrigation scheme and contribute to regional food security efforts.
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    Comparison of Soluble P in Soil Water Extracts Determined by ion Chromatography, Colorimetric, and Inductively Coupled Plasma Techniques in PPB Range
    (2006-08-18) Pierre Masson; Christian Morel; Eric Martin; Astrid Oberson; Dennis K. Friesen
    The determination of soil solution phosphate ions at trace levels is questionable with colorimetric method due to both limit of sensitivity and possible hydrolysis of organic soluble phosphate (P). A simple procedure was developed to determine orthophosphate at trace levels in soil solutions by single-column, suppressed-ion chromatography with conductivity detection without prior sample concentration. The procedure requires column capacity selection and use of a 500-μL injection loop. The method shows a detection limit of 0.05 μg P L−1 as well as a high resolution with a Dionex AS9SC column. The proposed method was tested by analyzing 45 soil samples (Colombian oxisols) differently P fertilized. Phosphorus levels in solutions were also determined by green-malachite colorimetry and inductively coupled plasma as reference methods. The linear correlation between the various methods showed that analysis of orthophosphate in the ppb range by ion chromatography are likely more acceptable that value obtained with colorimetry, which hydrolyzed organic P and caused systematic error. Examination of coefficients of variation for the IC method calculated from triplicate analyses of five randomized samples (situated between 7.4 and 16.3 μg P L−1) showed value ranged from 2.0 to 6.7%. The procedure allows precise measurements of trace amounts of orthophosphate in the presence of moderate background levels of salts such as chloride, nitrate, and sulfate ions and was adequate for routine analysis.
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    Phosphorus Transformations in an Oxisol under Contrasting Land-use Systems: The Role of the Soil Microbial Biomass
    (2001) Astrid Oberson; Dennis K. Friesen; I.M. Rao; S. Bühler; Emmanuel Frossard
    It is generally assumed that phosphorus (P) availability for plant growth on highly weathered and P-deficient tropical soils may depend more on biologically mediated organic P (Po) turnover processes than on the release of adsorbed inorganic P (Pi). However, experimental evidence showing the linkages between Po, microbial activity, P cycling and soil P availability is scarce. To test whether land-use systems with higher soil Po are characterized by greater soil biological activity and increased P mineralization, we analyzed the partitioning of P among various organic and inorganic P fractions in soils of contrasting agricultural land-use systems and related it to biological soil properties. Isotopic labeling was used to obtain information on the turnover of P held in the microbial biomass. Soil samples were taken from grass–legume pasture (GL), continuous rice (CR) and native savanna (SAV) which served as reference. In agreement with estimated P budgets (+277, +70 and 0 kg P ha−1 for CR, GL and SAV, respectively), available P estimated using Bray-2 and resin extraction declined in the order CR > GL > SAV. Increases in Bray-2 and resin Pi were greater in CR than GL relative to total soil P increase. Organic P fractions were significantly less affected by P inputs than inorganic fractions, but were a more important sink in GL than CR soils. Extractable microbial P (Pchl) was slightly higher in GL (6.6 mg P kg−1) than SAV soils (5.4 mg P kg−1), and significantly lowest in CR (2.6 mg P kg−1). Two days after labeling the soil with carrier free 33P, 25, 10 and 2% of the added 33P were found in Pchl in GL, SAV and CR soils, respectively, suggesting a high and rapid microbial P turnover that was highest in GL soils. Indicators of P mineralization were higher in GL than CR soils, suggesting a greater transformation potential to render Po available. Legume-based pastures (GL) can be considered as an important land-use option as they stimulate P cycling. However, it remains to be investigated whether crops planted in pasture–crop rotations could benefit from the enhanced Po cycling in grass–legume soils. Furthermore, there is need to develop and test a direct method to quantify Po mineralization in these systems.
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    Drought Response of Dry-seeded Rice to Water Stress Timing and N-Fertilizer Rates and Sources
    (2006-07) Ernesto Guttierez Castillo; To Phuc Tuong; Upendra Singh; Kazuyuki Inubushi; Jaime Padilla
    Dry seeding has been identified as an option for increasing cropping intensity and productivity in rainfed ricelands. Managing drought and nutrients are important for increasing yield, but the interactive effects of drought and nutrients on dry-seeded rice (Oryza sativa L.) growth have not been systematically investigated. Two experiments were carried out in 1994 and 1995 to analyze the effects of N fertilizer rate and the use of controlled-release fertilizers (CRFs) on the growth and yield of dry-seeded rice grown on a silty clay loam (Typic Tropaquept) subjected to water stress at different crop stages. In both years, in the main plots, rice was subjected to water stress at four different stages of development. The subplots were designed to compare the effect of the application of prilled urea and CRFs Osmocote (1994) and Polyon 12 (1995). Four N rates (0, 60, 120 and 180 kg ha−1 ) were imposed on rice in the sub-subplots (1994 only). The N fertilizer source did not affect any of the measured parameters. Irrespective of the N the fertilizer rates, grain yield and total dry matter accumulation of rice plants stressed at the flowering stage (WSFL, 1994) and panicle initiation stage (WSPI, 1995) were significantly lower than those of well-watered plants and plants stressed at the vegetative stage. Water stress during the grain-filling stage reduced the grain yield in 1995 when the stress was severe. Application of N fertilizer increased the yield compared with zero N in all water treatments, except for the WSFL plants whose yield did not change. The WSFL treatment also significantly reduced agronomic N-use efficiency.
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    Integrated Nitrogen Fertilization for Intensive and Sustainable Agriculture
    (2005) Upendra Singh
    Increased efficiency of land use and resource-use are critical for agriculture to feed the ever increasing population and yet remain sustainable for the future generations. Mineral fertilizers, particularly, nitrogen (N) fertilizer has played a very important role in meeting the increased food demand and in saving millions of hectares of marginal and wildlife reserves from coming under cultivation. Apart from higher yields per unit land, N application also results in higher protein content. Improper use of N fertilizers due to high application rates, incorrect source and method of application, and poor timing of application have led to air and water pollution and economic losses. Integrated and balanced N fertilization results in a win-win situation with intensive and sustainable agriculture that feed the world without harming the environment. Since multitude of management, soil, crop, and weather-related factors control soil and plant N dynamics, N management recommendations based on field trials alone are too costly and time-consuming. Dynamic N management recommendations that are both site— and season-specific combine field trials with decision support systems (DSS) that simulate crop growth and N transformations as a function of above-mentioned factors. Innovative N fertilizer products and N efficient plant types will further improve N use efficiency.