Chemical Soil Tests Research Articles

The influence of organic transition strategy on chemical and biological soil tests

AbstractSoil testing strategies that include biologically based indicators in organic and alternative farming systems are needed in order to improve recommendations that balance production and environmental goals. In this study, soil samples were collected before and after soils were transitioned from conventional row crop production to organic management using rotations that varied in both their inputs and tillage intensity. Ley-, row crop- and vegetable-based farming systems were implemented using locally specific production practices. Subplots were imposed within each system to allow comparison of farming systems without amendment and with dairy manure- and compost-amendment. Soil analyses included standard chemical tests (0–15 cm) for available phosphorus, exchangeable potassium, calcium, magnesium, pH, total organic carbon (SOC) and total nitrogen (TN). Biological assays (0–15 and 15–30 cm) included particulate organic matter-C and -N (POM-C, POM-N), soil and POM C:N ratios, fluorescein diacetate (FDA) hydrolysis, potentially mineralizable N (PMN) and hydrolysable amino-N + NH4(IL-N). Even though cropping and tillage intensity varied among systems (ley < row crop < vegetable), organic matter and nutrient reserves were not statistically different. Nutrient concentrations tested medium to high, even without compost or manure application. Labile fractions of soil organic matter were more enriched in the deeper sampling depth; whereby, POM stocks within the 15–30 cm depth increased by 20% on average compared to roughly 6% in the surface depth. This and observed changes in other properties demonstrate the multiple benefits derived from use of winter annual or perennial crops. Results from our analyses suggested PMN and POM have particular promise as metrics of change in commercial soil testing facilities to assist recommendations for amendments to balance production and environmental goals.

Composition of characteristic soils on the raised atoll Bellona, Solomon Islands

Knowledge is scarce about the composition of soils on Bellona and other Pacific atolls. Such knowledge is important as it is closely related to sustainable land use of these special soils formed on carbonatic/phosphatic materials. Therefore, the micromorphological, chemical and mineralogical composition of samples from genetic horizons in three dominant Bellonese soils (Malanga, Kenge Ungi and Kenge Toaha) and underlying rock (Tanahu) were investigated. Tanahu mainly consists of dolomite, but this mineral is absent in the three soils, which are dominated by phosphate-containing minerals. The Malanga soil is strongly dominated by Ca and P with minor amounts of Al, F and Fe present in fluorapatite, hydroxyapatite and crandallite. In contrast, Al, Fe and P dominate in the Kenge Ungi and Kenge Toaha soils in accordance with a mineralogy consisting of crandallite together with aluminum oxides (gibbsite/boehmite) and iron oxides, mainly goethite with minor contents of hematite. The observed carbonate for phosphate substitution in the apatites and crandallite is important as it indicates an increased phosphate availability in the soils. All three soil samples contain ≤1% Si and very little K and Mg. Total contents of essential microelements are considered adequate, but the rather high contents of Sr and U, especially in the Kenge Ungi soil may be problematic. Although the composition of the soils suggests substantial fertility and resilience, the lack of K-containing weatherable minerals (silicate minerals) is in line with a very low K (and low Mg) content and explains why fertilization may be needed to sustain future cultivation of these special soils. Due to a possible low bioavailability of Fe, Mn and maybe other micronutrients at the circumneutral pH of these carbonatic/phosphatic soils as well as the rather high contents of Sr and U, it may be recommended to test element availability using appropriate chemical soil tests supplemented by plant experiments to ensure safe and sustainable (optimal) soil use. However, according to the local farmers, the Malanga, Kenge Toaha and Kenge Ungi soils are considered well suited for production of the preferred crops.

Nitrogen mineralization: a review and meta‐analysis of the predictive value of soil tests

Accurate estimation of mineralizable nitrogen (N) from soil organic matter is essential to improve fertilizer management in agricultural systems. Mineralizable N refers to the amount of N in soil that is released during a certain period (ranging from 1 week to the length of a growing season). It has been estimated from increases in inorganic N during incubation or from N uptake by plants grown in a greenhouse or field. Many chemical soil tests measuring extractable organic N (EON) fractions have been proposed to predict mineralizable N. We evaluated the predictive value of these soil tests, using 2068 observations from 218 papers. Meta‐analysis was used to find the best soil test, to analyse differences between field and laboratory experiments, and to determine whether their predictive value is affected by extraction intensity (% of total soil N that is extracted). The concentration of EON was positively related to mineralizable N, explaining on average 47% of the variation. It did not, however, explain more of the variation than total N. Best predictions (57% < R2< 74%) were obtained when EON was extracted with hot CaCl2, acid KMnO4, acid K2Cr2O7, hot water or hot KCl. Extraction intensity was not related to the strength of the above‐mentioned relationship. Predictions of mineralizable N were significantly worse when mineralization was measured in the field compared with measurements under controlled conditions. We found no evidence of a causal and direct relationship between EON and mineralizable N. Accuracy of soil testing may improve when the current ‘single soil test approach’ changes to a more complex approach, which includes different soil tests, soil properties and environmental conditions.

Microstructure and Chemical Properties of Cement Treated Soft Bangladesh Clays

This paper examines the relationship between the microstructure and chemical properties of soft Bangladesh clays due to cementation. The microstructure was investigated using X-ray diffraction, scanning electron microscopy, pH measurement, specific surface area and soil chemical tests. The results indicate that a multitude of changes occurred in the properties and behavior of cement-treated clays that can be explained by the interaction between four underlying microstructural mechanisms. That is, it is suggested that the hydrated lime is formed by hydration, which causes the flocculation of the little clay particles, by the preferential attack of the calcium ions on kaolinite rather than on illite and monmorillinite in the pozzolanic reaction, by surface deposition and shallow infilling by cementitious products such as calcium silicate hydrate and calcium alumino silicate hydrate (CSH and CASH) on clay clusters, and finally, by the presence of water trapped within the clay clusters. The chemical properties of the cement-treated clays were found to depend on the plasticity of soil.

Site-specific management of limiting nutrients in peanut (Arachis hypogaea) on the Texas High Plains

Nutrient data obtained from soil chemical tests were analyzed in an activity analysis model to identify limiting factors in peanut production on the Texas High Plains. A production function was estimated for the study field, and limiting factors were identified at individual sites where the production function indicated that yield did not respond. The estimated production function also enabled us to conduct a cost-return analysis of variable- and blanket-rate fertilizer applications. The results showed that peanut yields did not respond to most of the nutrients included in the study, which confirmed conclusions from previous studies in the study region. Calcium and nitrogen were the only two limiting factors identified in this study. Significant economic returns could be obtained by site-specific fertilizer application. The average economic return from variable-rate calcium fertilizer application was $27.84 ha−1 and from blanket-rate it was $10.73 ha−1. The return from variable-rate nitrogen fertilizer application was about $20 ha−1 and from a blanket-rate it was about $14 ha−1. There seems to be quite a strong economic incentive to adopt variable-rate application for calcium and nitrogen fertilizer application.

Initiatives of the Australasian Soil and Plant Analysis Council to Assess the Measurement Quality of Common Methods for Soil and Plant Analysis in Australasia

The Australasian Soil and Plant Analysis Council Inc (ASPAC) commenced its not‐for‐profit interlaboratory proficiency programs (ILPPs) in 1990 and issued its first soil program report in 1993. ILPPs target soil and plant chemical testing laboratories across Australasia. These ILPPs contribute to ASPAC's goals to promote excellence in all aspects of soil and plant analysis and to encourage adoption of preferred methods and protocols used in soil and plant analysis within Australasia. The ASPAC recently re‐engineered its ILPPs to increase both their frequency and the number of samples assessed in a program year, the first of which was completed in 2004–5. This article provides more details on these new ILLPs for soils and plants, including how ASPAC's method‐specific certification now operates. More than 35 laboratories undertake the most popular soil tests but less than half report results for more specialized or less common tests. Also included is an outline of comparative results from a pilot assessment of Mehlich 3 soil‐test parameters using four homogeneous samples (and results) obtained from the North American ILPP. For the most part, results were wider apart than expected, which may be confounded by storage and handling changes in the samples that also had to pass strict quarantine protocols in Australia and New Zealand. The ILPPs of ASPAC are overseen by ASPAC's Laboratory Proficiency Committee, comprised of a convenor and three others. A Technical Advisory Group operates between ASPAC and its proficiency service provider, who is now located in New Zealand.

Variation of Properties in Surface Soils from a Prior Lake‐Bed (Lake Askuris, Greece) Farmed for Over 90 Years

Twenty‐seven soil samples were collected from surface horizons, in order to include representative soil types. Three samples per site were taken and mixed from the examined area, in spring 2005. The study area occupies 820 ha of the now‐drained (1910), partially levelled, and continuously farmed Lake Askuris in Greece. The area, which is close to the village of Kalipefki (Longitude: 22° 27′ 22″ E; Latitude: 39° 57′ 39″ N) and to Mount Olympus, is elevated (≈1000 m) with a Mediterranean climate of mild, rainy winters and hot, dry summers. In addition to clay content, the soils were chemically characterized using 22 soil chemical tests, including pH, total carbon and organic matter, total nitrogen, total sulphur, exchangeable cations, cation exchange capacity, organic and extractable phosphorus (P), and pseudo‐total and diethylenetriaminepentaacetic acid (DTPA) extractable trace elements. All of the soil data were skewed, with population mean values higher than the median for clay percentage, cation exchange capacity, carbon/nitrogen ratio, exchangeable calcium (Ca) and magnesium (Mg), DTPA iron (Fe), and total zinc (Zn). A geographic information system and kriging was used to prepare spatially explicit maps (at least two or three level of fertility) for the exchangeable basic cations, the trace elements, phosphorus, organic matter, total nitrogen (N), and total sulfur (S). It is expected these contemporary baseline data will assist the local community and farmers as they consider future nutrient management plans along with other land use options that could include re‐flooding the lake bed.”

Phosphorus Fertility Assessment of Intensively Farmed Areas of Catchments Draining to the Great Barrier Reef World Heritage Area; 2: Potential of Soils to Release Soluble Phosphorus

The potential of fertilized cropping soils (sugarcane, vegetables, fruit trees) in river catchments that drain to the Great Barrier Reef World Heritage Area in northeastern Australia to release soluble phosphorus (P) was assessed using soil chemical tests on typical soils (0–10 cm) collected in 2003. Tests included total P, diagnostic soil P tests, estimates of P buffering, and soluble and redox sensitive soil P. Routine soil P tests proved poor predictors of CaCl2 P (0.005 M). Values≥0.20 for the Mehlich‐3 P saturation ratio separated (P<0.01) lower from higher concentrations of CaCl2 P. The ratio of Mehlich‐3 Fe to [Mehlich‐3 Fe+Mehlich‐3 Al] is offered as a possible simple index of redox sensitive P. Part 1 (Rayment and Bloesch 2006) describes the soil survey in more detail.

Ideal and saturated soil fertility as bench marks in nutrient managementII. Interpretation of chemical soil tests in relation to ideal and saturated soil fertility

In a previous paper (Part 1), the ideal soil fertility and the saturated soil fertility were expressed on a relative scale, called soil fertility grade (SFG). In the current paper (Part 11), the relation between SFG and soil test values is discussed. The required uptake of nutrients from the soil is translated into soil organic carbon, P-Olsen, exchangeable K, and pH (H2O) using relationships developed for a model on Quantitative Evaluation of the Fertility of Tropical Soils (QUEFTS). Target soil test values were calculated for target yields between 2 and 10 Mg ha(-1) season(-1). The required uptake of soil nitrogen is a function of target yield, and it is linearly related to soil organic carbon. Results of the calculations indicate that when target yields are less than 7-8 Mg ha(-1), stover must be incorporated to maintain soil organic carbon above the critical level of 6 g kg(-1). When yields are below 2 Mg ha(-1), also organic sources from outside the field have to be brought in. The interpretation of chemical soil test values according to the ISF-SSF framework may be rather difficult in practice, as is demonstrated with eight African soils. The major reason is that the soil supplies of N, P and K seldom are in the same proportions as in ISF-SSF. For none of the used African soils replacement input or a neutral nutrient budget would be the best management option. Replacement input will often lead to inefficient use and even waste of nutrients. Optimum soil test values depend on target yield, but the ratios of soil test values do not depend on target yield. Therefore key values were established for the ratio of soil organic carbon to P-Olsen and for the ratio of soil organic carbon to the square root of exchangeable K. Based on these key values, a new classification scheme with recommended input ratios is presented. The scheme has six classes for N and P ratios, and seven classes for N and K ratios. (c) 2006 Elsevier B.V. All rights reserved.

On seasonal fluctuations of available Olsen P in soils

The likelihood of a strong seasonal effect on the analytical results of soil chemical tests has been substantiated by several findings. The occurrence of such a trouble would heavily affect the routine work of chemical laboratories dealing with hundreds or thousands of soil samples every year and would possibly interfere even with current timing of soil sampling and testing in agronomical research. Soils from four different experimental fields were sampled and analyzed for Olsen P test through the growing season of both spring-summer crops (maize) and autumn-spring crops (soft wheat). To avoid the cross effect of recent P applications, samples were collected only from the control plots, with no P applied. In a first long-time experiment the level of Olsen P was followed in three soils during two years, while in a second experiment the study considered two different soils for a couple of years each. The amount of extractable (available) Olsen P showed significant fluctuations-with-time in four years out of six. The observed variations seems entirely due to P absorption by crops, as they appeared in the final samplings during the cropping season. The amount of decrease was nearly always small, less than 1-1.5 mg kg-1 soil, and consistent with the reduction of the available pool which is to be expected as crops gradually take up nutrients. As far as Olsen P is concerned, no evidence was found for the occurrence of erratic seasonal variations such as those previously reported. As soil samples for advisory purposes are usually taken at different times of the year, it seems reassuring that no complications are to be expected about information needed to interpret the results of chemical tests.

Cadmium in Sugar Cane and Vegetable Systems of Northeast Australia

Long‐term agricultural sustainability and food quality are threatened by a range of chemicals capable of persisting in the environment and entering the food chain at concentrations that can pose threats to human health. The heavy metal cadmium (Cd) is one of these, despite the existence of Cd food standard developed by Food Standards Australia New Zealand and standards for Cd in fertilizer. This article collates information from northeast Australia (mostly Queensland) by river catchment on historical inputs of Cd as an impurity in P fertilizers and on total concentrations in a broad selection of soils used primarily for sugar cane and vegetable production. How Cd partitions by soil depth and how it recycles in sugar cane production systems is explored in the context of sustainability. Actions by state governments to regulate the maximum permitted concentrations and encourage the fertilizer industry to lower Cd impurities in commercial products have helped reduce the quantity of Cd entering the two cropping systems. The study also reports briefly on the assessment of four soil chemical tests for Cd to assess the likelihood of edible vegetables grown on nonsaline soils from Queensland exceeding a concentration of 0.025 mg Cd kg−1 fresh weight. The order of accumulation of Cd (fresh weight basis) in the vegetables studied was as follows: leaf vegetables>roots and bulbs>vegetable stems and fruits>seed vegetables≈brassica. Silverbeet accumulated the most Cd, a finding in accord with international reports.

Soil Analysis for Organic Farming

Organic farming according to EU 2092/91 aims at closed nutrient cycles, which means that external nutrient inputs are kept to a minimum. By comparison, conventional farming is input orientated, focusing on high crop productivity. Soil analytical methods for organic farming comprise physical, biological, chemical, and energetic soil tests. The spade diagnosis is an old field test to obtain “in situ” information about soil fertility, which has experienced a renaissance in organic farming. At the laboratory level, organic farming uses the standard chemical analytical methods used in conventional agriculture comprising the determination of organic matter, humus dynamics, pH, soluble plant nutrients and reserve fractions. Special attention is paid to phosphorus, which is determined in three different extracts (acetic, lactic, and citric acid) to assess phosphorus dynamics, which is an indicator of the turnover of organic matter. Compared with conventional agriculture, biological methods are of particular interest in organic farming. Soil biological methods, such as the release of CO2, nitrification, dehydrogenase, amylase and protease activities, reflect the biological activity of soils. Problems of biological methods are the high spatiotemporal variability in dependence on biotic, pedogenetic, and climatic conditions and the lack of interpretation schemes for a transformation into practical recommendations. The soil chroma test is a method that aims to analyze energetic processes in soils. It is a so‐called “picture forming” method and characteristic for the biodynamic school of organic farming.

The story in the floors: chemical signatures of ancient and modern Maya activities at Aguateca, Guatemala

Abstract Soil chemical analysis has been one of the most active and promising areas among the recent innovations in household archaeology. Ancient inhabitants unintentionally left chemical imprints of daily activities, providing important clues as to past practices and space use, which are difficult to judge from artifactual data alone. Soil chemical testing in the Maya region is particularly promising given the highly calcareous nature of the soils derived from the carbonate geology of the region, as calcium ions and soil alkalinity render phosphorous, iron, and other metallic ions insoluble. This paper reports the results of soil chemical analysis of modern and ancient residential structures at the archaeological park of Aguateca, Guatemala. An ethnoarchaeological study of the guards’ living quarters and archaeological data from rapidly abandoned Classic-period residences provided opportunities to refine an understanding of the relationship between human activities and soil chemical signatures. Both cases exhibited good correlations of high phosphorous concentrations in soils with food processing, consumption, and disposal. High levels of heavy metals in the modern structures probably derived from the filing of machetes and the disposal of flashlight batteries, whereas the use of mineral pigments and craft activities appear to have contributed to the concentrations of these elements in archaeological contexts.

The story in the floors: chemical signatures of ancient and modern Maya activities at Aguateca, Guatemala

Soil chemical analysis has been one of the most active and promising areas among the recent innovations in household archaeology. Ancient inhabitants unintentionally left chemical imprints of daily activities, providing important clues as to past practices and space use, which are difficult to judge from artifactual data alone. Soil chemical testing in the Maya region is particularly promising given the highly calcareous nature of the soils derived from the carbonate geology of the region, as calcium ions and soil alkalinity render phosphorous, iron, and other metallic ions insoluble. This paper reports the results of soil chemical analysis of modern and ancient residential structures at the archaeological park of Aguateca, Guatemala. An ethnoarchaeological study of the guards’ living quarters and archaeological data from rapidly abandoned Classic-period residences provided opportunities to refine an understanding of the relationship between human activities and soil chemical signatures. Both cases exhibited good correlations of high phosphorous concentrations in soils with food processing, consumption, and disposal. High levels of heavy metals in the modern structures probably derived from the filing of machetes and the disposal of flashlight batteries, whereas the use of mineral pigments and craft activities appear to have contributed to the concentrations of these elements in archaeological contexts.

Evaluation of Phosphorus Availability for Paddy Rice Using Eight Chemical Soil Tests Under Oxidized and Reduced Soil Conditions

Phosphorus (P) soil test methods have had limited success in prediction of lowland rice (Oryza Sativa L.) response to P fertilization. Therefore, the relative efficiency of Olsen, Colwell, Bray1, Soltanpour, Mehlich 3, Morgan, Sr-citrate, and Resin to predict P availability in waterlogged soils was evaluated by incubating 23 calcareous soils under oxidized and reduced soil conditions and P response of rice was assessed in a greenhouse experiment. Anion exchange resin extracted the highest amounts of P from both waterlogged and air-dried soils followed by Colwell, Morgan, Olsen, Sr-citrate, Mehlich 3, Soltanpour, and Bray 1. Reducing soil conditions increased the amount of P extracted by all soil test methods. The highest correlation coefficient was observed between Olsen and Colwell extractable P under both oxidized and reduced soil conditions. Olsen method gave the best correlation coefficients with rice growth parameters (dry matter yield and P uptake), followed by Colwell and Soltanpour under aerobic and Soltanpour under anaerobic conditions. Furthermore, the relationship between top dry weight and/or P uptake and P extracted by Olsen, Colwell, Soltanpour, Sr-citrate, and Resin were improved when organic matter (OM), pH, clay content (Clay), electrical conductivity (ECe), and calcium carbonate equivalent (CCE) were included in the multiple-regression equations. The results reported in the present study suggest that Olsen test method appears to be a more appropriate extractant for predicting P availability for rice under oxidized and reduced soil conditions. The relatively high variability in physical and chemical characteristics of the calcareous soils of southern Iran, due to variations in topography and geology justify more correlation and field calibration studies to improve P availability evaluation.

The potential of NIR spectroscopy to predict nitrogen mineralization in rice soils

Prediction of nitrogen (N) mineralization is important for specifying the optimum rate of N fertilizer for flooded rice at the time of sowing. To develop a predictive test, soils (0–0.1 m) were sampled from 22 farms throughout the rice-growing region of southern Australia over a 4-year period. Near infrared reflectance (NIR) spectra of the soils were compared with sixteen biological and chemical soil tests for the prediction of N-uptake by rice plants from these soils in the field and glasshouse. The aim of the study was to develop a soil-NIR calibration as an accurate, rapid and economical mineralization test. Nitrogen uptake by field-grown and glasshouse-grown plants was poorly correlated (r = 0.30), even though significant NIR calibrations were developed with both. Since N uptake by rice in the field was affected by varying weather and management, the field calibration is probably spurious. The calibration of soil NIR spectra with N uptake by glasshouse plants was satisfactory, with a standard error (SE) of 13 kg ha−1 over a range of 11 – 95 kg ha−1, and a correlation between calculated and measured N uptake (r = 0.87, P<0.001). An even better soil-NIR calibration was found with N-mineralization after 21 days of anaerobic incubation (SE 16 mg kg−1, range 52–175 mg kg−1). Analysis of the soil spectra showed that similar wavelengths were correlated with both plant-N uptake and mineralization. NIR spectroscopy shows considerable potential to predict soil N mineralization, and may assist future fertiliser decision support.

Characterization of drinking water treatment residuals for use as a soil substitute.

The beneficial use of drinking water treatment residuals (WTRs) as a potential source of topsoil for land reclamation was evaluated. Seventeen WTRs were characterized for use as soil substitutes by comparing chemical and physical properties and plant nutrients of the WTRs with soil. A tomato (Lycopersicon esculentum) bioassay was performed to determine the ability of soil chemical tests to measure WTR phosphorus (P) adequacy. The WTR chemical and physical properties were typically adequate for crop growth. None of the WTRs were considered unsuitable as soil substitutes based on plant nutrients, with the exception of P. Tomato vegetative yield and tissue P were poor either because of phytotoxic nitrite-nitrogen (NO2-N) (> 10 mg/kg) generated during the bioassay or because of WTR P deficiency. Limited data suggest that WTRs with NO2-N less than 10 mg/kg and Olsen P greater than 50 mg/kg, water soluble P greater than 580 micrograms/L, or Mehlich III P greater than 54 mg/kg support growth but still produce inadequate tissue P in tomatoes.

Preface

Summary. In relatively few years, the Australian Soil and Plant Analysis Council Inc. (ASPAC) has conducted 2 inter-laboratory proficiency programs on plant material and 3 inter-laboratory proficiency programs on soils. The purpose of these performance-based programs is to enhance the quality of soil and plant analysis in Australasia, with guidance where necessary from the soil and plant expertise of ASPAC members. ASPAC’s inaugural ‘Accreditation Committee’ reviewed published standards and existing laboratory accreditation/proficiency programs in Australia and internationally before developing what is now in full operation. This historical perspective and the 12 principles that guide operations of ASPAC’s soil and plant proficiency programs are described, as are the numeric procedures used to determine satisfactory performance. Certificates are issued to successful laboratories on completion of each program. Moreover, these remain current until signed certificates from the next equivalent program are released. Wide variations in some data sets suggest there is considerable scope to improve laboratory accuracy, particularly for soil chemical tests. Some of these differences are sufficient to markedly affect the assessment of fertiliser requirements. The present ‘Accreditation Committee’, in addition to State Representatives, serve as ‘points-of-contact’ for laboratories that require assistance to overcome problems with analytical accuracy and precision. ASPAC encourages its member laboratories to seek and maintain NATA (National Association of Testing Authorities, Australia) accreditation, in addition to participating regularly in the performance-based proficiency programs run by ASPAC.

Australian Soil and Plant Analysis Council soil and plant quality assurance programs and the impact of variability of test results on clients

Summary. In relatively few years, the Australian Soil and Plant Analysis Council Inc. (ASPAC) has conducted 2 inter-laboratory proficiency programs on plant material and 3 inter-laboratory proficiency programs on soils. The purpose of these performance-based programs is to enhance the quality of soil and plant analysis in Australasia, with guidance where necessary from the soil and plant expertise of ASPAC members. ASPAC’s inaugural ‘Accreditation Committee’ reviewed published standards and existing laboratory accreditation/proficiency programs in Australia and internationally before developing what is now in full operation. This historical perspective and the 12 principles that guide operations of ASPAC’s soil and plant proficiency programs are described, as are the numeric procedures used to determine satisfactory performance. Certificates are issued to successful laboratories on completion of each program. Moreover, these remain current until signed certificates from the next equivalent program are released. Wide variations in some data sets suggest there is considerable scope to improve laboratory accuracy, particularly for soil chemical tests. Some of these differences are sufficient to markedly affect the assessment of fertiliser requirements. The present ‘Accreditation Committee’, in addition to State Representatives, serve as ‘points-of-contact’ for laboratories that require assistance to overcome problems with analytical accuracy and precision. ASPAC encourages its member laboratories to seek and maintain NATA (National Association of Testing Authorities, Australia) accreditation, in addition to participating regularly in the performance-based proficiency programs run by ASPAC.

Calibration of quefts, a model predicting nutrient uptake and yields from chemical soil fertility indices

The model quefts (QUantitative Evaluation of the Fertility of Tropical Soils) was calibrated using data from maize fertilizer trials in Kenya. quefts describes, in four steps, relations between (i) chemical soil test values, (ii) potential NPK supply from soils and fertilizer, (iii) actual NPK uptake, and (iv) maize grain yield, acknowledging interactions between the three macronutrients. All steps were calibrated separately, and yield a modified version of quefts. Major changes were the inclusion of ambient temperature and clay content in explaining potential nitrogen supply, and the replacement of the parabolic relation between potential supply and actual uptake by an exponential relation. The goodness of fit ( r 2) between measured and calculated yield was improved from 0.66 in the original version to 0.78 in the modified version of quefts and, when including a boundary condition for harvest index, to 0.88. A satisfactory validation was conducted with input data from fertilizer experiments in other parts of Kenya. Sensitivity analysis revealed that changing the parameters pH and organic N by 20% caused yield differences of at least 10%. The basic thinking and theoretical concepts underlying the original version of quefts still apply to the modified version. Agronomists in tropical environments are encouraged to collect the relatively few input data to further validate the two versions of the model. As a consequence, quefts can contribute to a more efficient procurement and use of mineral fertilizers at both regional and farm level.