Soil Sampling Depth Research Articles

Sulphur cycling between terrestrial agroecosystem and atmosphere

Central gas station of the natural gas borehole system Podravina is located near the village Molve. It delivers more than a quarter of total energy used in Croatia to its consumers. Over the years, adapting technology to increasingly demanding and rigorous standards in environmental protection has become paramount. Yet, despite all the industry has undertaken to address the risk of harmful substances entering the food chain, a multidisciplinary research team of independent scientists monitors the content of specific substances in all components of the ecosystem. This paper presents measurements of total sulphur contents in soil surface [(0 to 3) cm] and subsurface [(3 to 8) cm] layers (study period: autumn 2006 - spring 2010) and in plants (study period: spring 2000 - spring 2010), and the concentration of gaseous sulphur compounds in the air. Concentrations of hydrogen sulphide (H2S) and mercaptans (RSH) were measured from the summer of 2002 until the autumn of 2010, while concentrations of sulphur dioxide (SO2) were measured from the spring of 2008 until the autumn of 2010. The paper also shows total annual atmospheric sulphur (S-SO4) deposition at Bilogora measuring station (study period: 2001 - 2010). Average monthly concentrations of H2S in air varied between 0.2 μg m-3 and 2.0 μg m-3, RSH between 0.1 μg m-3 and 24.5 μg m-3, and SO2 between 0.4 μg m-3 and 2.8 μg m-3 depending on the location and the season of sampling. Mean values of total sulphur in soil and in Plantago lanceolata plant ranged between 610 mg kg-1 and 1,599 mg kg-1 and between 3,614 mg kg-1 and 4,342 mg kg-1, respectively, depending on the soil type, location, and sampling depth. Average values of total sulphur mass ratio for all examined single soil samples (n=80) were 1,080 mg kg-1 for both studied layers, and 4,108 mg kg-1 for all analysed plant samples (n=85). Average total annual atmospheric sulphur deposition at Bilogora measuring station was 6.3 kg of S-SO4 per hectare.

Estimation of distribution coefficient of uranium around a uranium mining site

Distribution coefficient (Kd) of uranium and its daughter products are very important for migration study around uranium mining sites. Since the distribution coefficient depends very much on the soil and groundwater chemistry, generation of site specific Kd is very important. In the literature there is a large variation of Kd values of uranium. For realistic prediction of contaminant migration, literature Kd value is not very effective. So site specific experimental Kd values are required. The present study emphasizes on the estimation of site specific distribution coefficient for uranium around a uranium mining site. The soil and groundwater parameters which affect the Kd value of uranium have also been estimated. Soil and groundwater samples from nine locations around Turamdih uranium mining site were collected and chemically characterized for various parameters. The distribution coefficient of uranium in top and one meter depth soil samples from above locations were estimated using laboratory batch method. The distribution coefficient of uranium varies from 69 ± 4 to 5524 ± 285 l/kg. No significant difference in uranium Kd values was observed for top and one meter depth soil samples. In the top and one meter depth soil samples uranium Kd values vary from 129 ± 8 to 5524 ± 285 and 69 ± 4 to 3862 ± 195 l/kg respectively. For the estimation of distribution coefficient of uranium different parameters like equilibration time, solid to solution ratio, method of tracer addition to solution, solid-solution separation method etc. have been optimized. The distribution coefficient of uranium determined in the present study will be used for the migration study of uranium around uranium mining sites.

Correlation between Spectrophotometric Absorbance of Soil Solution and Contents of Variable Carbon and Nitrogen in Larch Plantation Forests

Based on 100 samples from 4 soil depths,the spectrophotometric absorbances of soil solutions at 14 wavelengths(250,254,260,265,272,280,285,300,340,350,365,400,436 and 465nm) and soil carbons(dissolved organic carbon DOC and soil organic carbon SOC)and nitrogens(dissolved soil nitrogen DON and soil organic nitrogen SON) were concurrently measured.We are aiming to answer the question that absorbance at which wavelength is most correlated with the soil carbon and nitrogen,and whether or not differences exist in these relations between top-and sub-soils.The following conclusions were obtained.Firstly,although absorbance values of surface and deep soil solutions significantly decreased with wavelength,the decrease in top soil was much steeper than that in sub-soils.The lower region of the studied wavelengths was more powerful to distinguish the difference between surface and deep soils,accordingly.Comparing with deep layers,more robust correlations between soil solution absorbance and SOC,DON and SON were found in surface soil layer(0-20cm).However,the difference in the DOC and absorbance relations between surface and deep soils was much weaker.Secondly,the absorbance values of 8 wavelengths(250-300nm) were significantly auto-correlated,indicating that each of them could function the same in characterizing soil solution.Using 254nm as a representative of them,the regression analysis between absorbance(254,340,365,400,436 and 465nm) and soil carbon,nitrogen have been carried out by using the entire pooled data.The best regressions(R2=0.53-0.59) for SOC,DON,SON were found in 254nm,and sharp decreases in R2 value with wavelength increases were found.No such large change was found in DOC,and R2 from 0.25 to 0.33 were found in wavelengths of 254,340,365 and 400nm.Our findings indicate that the spectrophotometric absorbance of soil solution(particularly from 250nm to 300nm) can be used as indicators for monitoring soil carbon and nitrogen changes,and attention should be paid to the soil sampling depths and variation in different soil carbon and nitrogen parameters.

Effects of tillage and nitrogen fertilizers on CH4 and CO2 emissions and soil organic carbon in paddy fields of central China.

Quantifying carbon (C) sequestration in paddy soils is necessary to help better understand the effect of agricultural practices on the C cycle. The objective of the present study was to assess the effects of tillage practices [conventional tillage (CT) and no-tillage (NT)] and the application of nitrogen (N) fertilizer (0 and 210 kg N ha−1) on fluxes of CH4 and CO2, and soil organic C (SOC) sequestration during the 2009 and 2010 rice growing seasons in central China. Application of N fertilizer significantly increased CH4 emissions by 13%–66% and SOC by 21%–94% irrespective of soil sampling depths, but had no effect on CO2 emissions in either year. Tillage significantly affected CH4 and CO2 emissions, where NT significantly decreased CH4 emissions by 10%–36% but increased CO2 emissions by 22%–40% in both years. The effects of tillage on the SOC varied with the depth of soil sampling. NT significantly increased the SOC by 7%–48% in the 0–5 cm layer compared with CT. However, there was no significant difference in the SOC between NT and CT across the entire 0–20 cm layer. Hence, our results suggest that the potential of SOC sequestration in NT paddy fields may be overestimated in central China if only surface soil samples are considered.

Sample preparation and selection for qualitative and quantitative analyses of soil organic carbon with mid-infrared reflectance spectroscopy

Mid-infrared spectroscopy (MIRS) is a well-established analytical tool for qualitative and quantitative analysis of soil samples. However, effects of soil sample grinding procedures on the prediction accuracy of MIR models and on qualitative spectral information have not been well investigated and, in consequence, not standardized up to now. Further, the effects of soil sample selection on the accuracy of MIR prediction models has not been quantified yet. This study investigated these effects by using 180 well-characterized soil samples that were ground for different times (0, 2 or 4 minutes) and then used for MIR measurements. To study the impact of sample preparation, soil spectra were subjected to principal component analyses (PCA), multiple regression and partial least square (PLS) analysis. The results indicate that the prediction accuracy of MIR models for soil organic carbon (SOC) and pH and the qualitative spectral information were better overall for lightly ground (2 minutes) soil samples compared with intensively (4 minutes) or unground soil samples. Whereas the grinding procedure did not show any effect on spectra of clay minerals, spectral information for quartz and for SOC was modified. Even though it is difficult to recommend a global standardized soil sample grinding procedure for MIR measurements because of different mill types available within laboratories, we highly recommend using an internally standardized grinding procedure. Moreover, we show that neither land use nor soil sampling depth influences the prediction of the SOC content. However, sand and clay content substantially affect the score vectors used by the PLS algorithm to predict the SOC content. Thus, we recommend using soil samples similar in texture for more precise SOC calibration models for MIR spectroscopy.

Calibração de métodos de extração de potássio em solos cultivados sob sistema plantio direto

As recomendações de adubação no Rio Grande do Sul e em Santa Catarina são baseadas nos estudos realizados em culturas cultivadas no sistema convencional de preparo do solo e nos conhecimentos acumulados ao longo dos anos. Nesse período, ocorreram mudanças no sistema de cultivo, na profundidade de amostragem do solo e no aumento de rendimento das culturas. Isso pode alterar os teores críticos de K, as faixas de fertilidade e as quantidades de fertilizante para as culturas. Este trabalho teve por objetivo calibrar os teores de K disponível, extraído pelos métodos Mehlich-1, Mehlich-3 e resina de troca iônica, para as culturas de soja, trigo e milho cultivadas no sistema plantio direto. Para isso, utilizaram-se amostras de terra de 17 experimentos e resultados de rendimento de grãos de 48 avaliações das culturas de soja, trigo e milho cultivadas em sistema plantio direto, com doses de K, no Rio Grande do Sul; a partir das funções de produção, determinaram-se a curva de calibração e as faixas de fertilidade. Os resultados indicaram que as respostas das plantas à adubação com K foram maiores, nesta sequência: milho, trigo e soja; os teores críticos de K com a solução de Mehlich-1, Mehlich-3 e resina foram de 89,5; 91,6; e 83,8 mg kg-1 de K na camada de 0-20 cm de profundidade e de 117,2; 116,1; e 102,5 mg kg-1 de K na camada de 0-10 cm de profundidade, respectivamente, e são maiores do que os recomendados pela CFSRS/SC (2004) para as culturas soja, trigo e milho cultivadas no sistema plantio direto.

Utilizing space-based GPS technology to determine hydrological properties of soils

Competition for limited water resources is one of the most critical issues being faced by irrigated agriculture in the United States. Site-specific irrigation applies irrigation water to match the needs of individual management zones within a field, significantly reducing water consumption, runoff, and nutrient leaching in ground water. Remote sensing for real-time and continuous soil moisture measurements at specific depths is essential for success of site-specific irrigation system. The overall objective of this study was to investigate the feasibility of utilizing a GPS-based sensor technology to determine site-specific information such as the soil moisture condition by recording the GPS signal reflected from the earth's surface. A modified GPS Delay Mapping Receiver (DMR) tracks and measures the direct, line-of-sight, Right-Hand-Circularly Polarized signal of a GPS satellite. It also simultaneously measures the delayed, earth-reflected, near-specular, Left-Hand-Circularly Polarized GPS signal. These measurements can be used to estimate the surface scattering coefficient and path delays between the direct and reflected GPS signals. Over land, scattering coefficients can be used to estimate changes in soil moisture contents. Our results showed that the space-based technology has a great potential for determining soil volumetric moisture contents in the pursuit of site-specific irrigation management. There were strong correlations between the GPS reflectivity measurements and soil moisture contents. The GPS reflectivity increased as the soil moisture contents increased. Careful analysis of the test data showed very conclusively that the sensitivity of L-Band signal (1.575 GHz) to soil moisture contents changed with soil type and sampling depth. The sensitivity decreased with sampling depth in light soils and increased in heavy soils.

Spatial Variability of Soil Chemical and Biological Properties in Florida Citrus Production

Soil and yield variability is a common problem in Florida citrus production; however, only limited progress has been made in charactering and identifying major yield-limiting properties. This study characterized the variability in soil chemical and biological properties. To achieve this, a citrus grove was divided into five productivity zones based on tree canopy volume (0–1.84, 1.84–3.67, 3.67–5.51, 5.51–7.34, and 7.34–9.18 m3 m−1 row for very poor, poor, medium, good, and very good, respectively) using geographic information system software. Six random soil samples were collected from each productivity zone at four depths (0–15, 15–30, 30–45, and 45–60 cm) to analyze various soil properties. Soil organic matter, Mehlich I extractable P, K, Ca, and Mg, and oxalate-extractable Fe and Al varied greatly along the productivity gradient, and their differences among zones were most prominent in the lower soil depths. These soil properties had a common pattern of spatial distribution (range 150–200 m) that matched the variation in fruit yield. The results of discriminant function and partial least squares regression analyses revealed that the productivity within the grove differed in the 0- to 60-cm soil depth, signifying that greater root zone depths could better explain the differential productivity of citrus groves than the 0- to 15-cm layer, which is the depth used presently for fertilizer recommendations. Collectively, our results suggest that soil sampling depths for citrus should be 0 to 60 cm and sampling should be done based on the variogram range of easily measured soil properties.

EXCLOSURE EFFECTS ON RECOVERY OF SELECTED SOIL PROPERTIES IN A MIXED BROADLEAF FOREST RECREATION SITE

ABSTRACTHuman‐caused trampling that results from excessive recreational use has caused damage to soil and vegetation in forest ecosystems in the Belgrad Forest of Istanbul. The objectives of this study were to examine effects of exclosure on selected soil properties and to determine the recovery time required for soil characteristics in a broadleaf forest recreation site.Litter biomass and topsoil (0–15 cm) were sampled in the forest, exclosure and recreational sites, and soil samples were analysed for saturation capacity, permeability, bulk density, total porosity, organic matter, root biomass, electrical conductivity and soil pH.Results showed that saturation capacity, permeability, total porosity and organic matter increased whereas bulk density decreased significantly in the topsoil under the exclosure, and all these soil properties in the topsoil of the exclosure were greater than those of recreational site. When effects of main factors were compared, averaging over sampling year and soil sampling depth, soils from the exclosure had significantly greater saturation capacity, permeability, total porosity, organic matter and litter biomass and lower bulk density values than the soils from recreational site.Six years of exclosure was effective in improving most of the soil properties in the topsoil. When topsoil and subsoil are considered together, it is obvious that a longer time period is needed for soil recovery in the forest recreational sites. Copyright © 2011 John Wiley & Sons, Ltd.

Soil bulk density pedotransfer functions of the humus horizon in arable soils

Soil bulk density (D b) is an important indicator of soil quality, site productivity and soil compaction and is widely used as an input variable for various models. However, D b is often not determined during actual soil surveys, and many attempts have been made to predict it from other more easily and routinely measured variables. The aim of this study was to develop a predictive model for D b estimation for the humus horizon of arable soils. In addition, we hypothesised that the mixed model approach would enable more accurate predictions than the multiple regression model. Estonian National Soil Monitoring data (1983–2008) were used for D b modelling. The dataset contains 17,293 entries for the humus horizon of Estonian arable soils. Mixed model methodology and linear multiple regression analysis were used to develop prediction models. Soil sampling depth, soil organic carbon and water content had the greatest effects on D b estimation. D b is determined through interactions of explanatory variables, and their contribution to D b variation in mixed model differs from variable contribution indicated by the simple regression model. We found that the regression model overestimates the importance of SOC. The mixed model prediction was an improvement compared to multiple regression models (MSE = 0.009 compared to MSE = 0.014). In addition, the predicted values in the mixed model showed good agreement with observed values, while the multiple regression model underestimated D b values above 1.6 g cm −3 and overestimated D b values below 1.2 g cm −3. We conclude that the mixed model approach enables higher prediction accuracy than multiple regression analysis. We propose a mixed model with 20 variables as a prediction model for the humus horizon of Estonian arable soils. The methodology is also applicable for other soil conditions.

Agroforestry Systems and Environmental Quality: Introduction

Investments in agroforestry research during the past three decades-albeit modest-have yielded significant gains in understanding the role of trees on farmlands, and the ecological and economic advantages of integrated farming systems. While early research focused mostly on farm or local levels, broader-level ecosystem services of agroforestry systems (AFS) have raised high expectations in recent years. The nine papers included in this special collection deal with three of such environmental benefits of AFS: water-quality enhancement, carbon sequestration, and soil improvement. These benefits are based on the perceived ability of (i) vegetative buffer strips (VBS) to reduce surface transport of agrochemical pollutants, (ii) large volumes of aboveground and belowground biomass of trees to store high amounts of C deeper in the soil profile, and (iii) trees to enhance soil productivity through biological nitrogen fixation, efficient nutrient cycling, and deep capture of nutrients. The papers included have, in general, substantiated these premises and provided new insights. For example, the riparian VBS are reported to increase the reservoir life, in addition to reducing transport of agrochemicals; the variations in C storage in different soil-fraction sizes suggest that microaggregate (250-53 μm) dynamics in the soil could be a good indicator of its C-storage potential; and the use of vector analysis technique is recommended in AFS to avoid consequences of inaccurate and overuse of fertilizers. The papers also identified significant knowledge gaps in these areas. A common theme across all three environmental quality issues covered is that more and varied research datasets across a broad spectrum of conditions need to be generated and integrated with powerful statistical tools to ensure wide applicability of the results. Furthermore, appropriate management practices that are acceptable to the targeted land users and agroforestry practitioners need to be designed to exploit these environmental benefits. The relative newness of research in environmental quality of AFS will pose some additional challenges as well. These include the lack of allometric equations for tree-biomass determination, absence of standardized norms on soil sampling depth, and limitations of fixed-effect models arising from issues such as pseudo-replication and repeated measures that are common in studies on preexisting field plots. Overall, this special collection is a timely effort in highlighting the promise of AFS in addressing some of the environmental quality issues, and the challenges in realizing that potential.

DEVELOPMENT OF A SOIL N TEST FOR FERTILIZER REQUIREMENTS FOR WHEAT

Optimal fertilizer nitrogen (N) rates result in economic yield levels and reduced pollution. A soil test for determining optimal fertilizer N rates for wheat has not been developed for Quebec, Canada, or many other parts of the world. Therefore, the objectives were to determine: 1) the relationship among soil nitrate (NO− 3)- N, soil ammonium (NH + 4)- N and N fertilizer on wheat yields; and 2) the soil sampling times and depths most highly correlated with yield response to soil NO− 3-N and NH + 4-N. In a three year research work, wet and dried soil samples of 0- to 30- and 30- to 60-cm depths from 20 wheat fields that received four rates of N fertilizer at seeding and postseeding (plants 15 cm tall) were analyzed for NH + 4-N and NO− 3 -N using a quick-test (N-Trak) and a standard laboratory method. Wheat yield response to N fertilizer was limited, but strong to soil NO− 3-N.

Bacterial diversity dynamics in a long-term petroleum-contaminated soil

Bacterial diversity dynamics were investigated in the soil samples in different distances and depths from/at a long-term petroleum-contaminated site. Microbial activity in the soil samples showed ATP values closely correlated with organic matter content (OC) and total petroleum hydrocarbon (TPH). Bacterial community diversity (H) and evenness (J) using PCR-DGGE (polymerase chain reaction-denaturing gradient gel electrophoresis) and PCR-T-RFLP (terminal restriction fragment length polymorphism) results showed positive correlation with concentration of TPH or OC, but tmoA (toluene monooxygenase gene)-based bacterial H and J using a PCR-T-RFLP result did not. No significant difference of H and J values in the bacterial and the tmoA communities was observed. The bacterial community structure characterized by PCR-DGGE and PCR-T-RFLP techniques showed similarity according to soil sampling distance rather than soil sampling depth. Canonical correspondence analysis demonstrated that OC including TPH had the most significant effect on the bacterial community diversity at the long-term petroleum-contaminated site.

Soil Sample Depth in Pasture Soils for Environmental Soil Phosphorus Testing

Phosphorus (P) in runoff from agricultural land is a major contributor to eutrophication of surface waters. This study investigated the relationship between dissolved P in surface runoff and soil-test P measured at different sample depths (0–2 and 0–10 cm). Soil at these depths was collected from 136 sites in southeast New South Wales, Australia, under pastoral agriculture, covering a wide range of soil types and land-use intensity, from native pasture to intensive dairying. Bicarbonate-extractable soil P concentrations at these two depths were curvilinearly related over a very wide range of P (r2 = 0.91). Small-plot rainfall simulations were conducted at 14 of these locations, for which the relationship between the sampling depths was even closer (r2 = 0.95). After dividing the soils into two groups based on parent material (basalt and nonbasalt), linear relationships were found between extractable soil P and runoff dissolved reactive P, with more of the variance being accounted for with the sedimentary soils (r2 = 0.89, 0–10 cm; 0.91, 0–2 cm) compared with basalt soils (r2 = 0.63; 0–10 cm; 0.57, 0–2 cm). The results suggest that agronomic soil P testing in pastoral soils (typically 0–10 cm depth) is sufficient for estimating the potential for losses of P in runoff and that there is no need to collect shallow soil samples especially for this purpose.

Development of a Soil N Test for Fertilizer Requirements for Corn Production in Quebec

Corn requires high nitrogen (N) fertilizer use, but no soil N test for fertilizer N requirement is yet available in Quebec. Objectives of this research were (1) to determine the effects of soil nitrate (NO3 −)-N, soil ammonium (NH4 +)-N, and N fertilizer rates on corn yields and (2) to determine soil sampling times and depths most highly correlated with yields and fertilizer N response under Quebec conditions. Soil samples were taken from 0- to 30-cm and 30- to 60-cm depths at seeding and postseeding (when corn height reached 20 cm) to determine soil NH4 + and NO3 − in 44 continuous corn sites fertilized with four rates of N in two replications using a quick test (N-Trak) and a laboratory method. The N-Trak method overestimated soil NO3 −-N in comparison with the laboratory method. Greater coefficients of determination were observed for soil NO3 −-N analyses at postseeding compared with seeding.

Ensemble Approach to Provide Uncertainty Estimates of Soil Bulk Density

Large scale environmental impact studies typically involve the use of simulation models and require a variety of inputs, some of which may need to be estimated in absence of adequate measured data. An important input, soil bulk density (Db) affects conditions for soil aeration, solute transport, and storage as well as the outcome of soil C stock calculations. Correct representation of Db in simulation studies is essential since any bias or uncertainty will propagate through a variety of processes and time steps. We used the U.S.‐wide NRCS National Soil Survey Characterization (NSSC) database of point measurements and the ‘k‐nearest neighbor’ (k‐NN) pattern recognition algorithm combined with random resampling without replacement to estimate Db and its uncertainty. Soil particle‐size distribution and organic C content were utilized as inputs and soil taxonomy classification, sample depth, and soil horizon notation were tested as optional grouping and limiting factors to the calibration (reference) data. We obtained an overall root‐mean‐squared error (RMSE) of 0.17 g cm−3 and mean error (ME) of 0.01 g cm−3 Grouping samples by taxonomic classification proved to be advantageous, while limiting samples by depth helped avoid depth‐specific bias in the estimations. Grouping samples by horizon notation did not yield significant improvement due to the great variability of bulk density within horizons of the same notation. Varying the proportion of data used in the resampled data subsets can be used to establish greater or lesser degree of confidence in mean estimates without affecting those mean estimates. Simulation based environmental impact and risk assessment studies can be direct beneficiaries of data with characterized uncertainty.

ALTERAÇÕES EM ATRIBUTOS QUÍMICOS DE SOLO CULTIVADO COM CANA-DE-AÇÚCAR E ADIÇÃO DE VINHAÇA

The research aimed to evaluate the effect of vinasse on the chemical properties of a eutrophic Alfissol cultivated with sugarcane (S. officinarum spp.), for 10 years, in two areas of the Usina São José do Pinheiro, located in the city of Laranjeiras, Sergipe State, Brazil. The experimental design was randomized blocks, with ten replications, which consisted of two areas (Area A - cultivated with sugarcane and fertirrigated with vinasse, for over 10 years; and Area B - cultivated with sugarcane without vinasse fertigation) and three soil sampling depths (treatments): 0-15 cm, 15-30 cm, and 30-45 cm. The experimental areas dimensions were 50 m x 20 m (1,000 m2) and 30 samples were collected from each site. After the analysis of pH, soil organic matter (SOM), and levels of macro and micronutrients from soil samples, it was observed that the addition of vinasse to sugarcane crops, for ten years, has altered the soil chemical properties, with increase of soil organic matter and macronutrient content and decrease of micronutrients availability. Based on the results obtained, it was concluded that the application of vinasse increases the fertility of soil cultivated with sugarcane. KEY-WORDS: Saccharum officinarum; organic matter; soil conditioning; macronutrients; micronutrients.

Consequences of soil sampling depth during the assessment of the effects of tillage and fertilization on soil quality: a common oversight

The objective of this work is to discuss the consequences of soil sampling depth during the assessment of the effects of tillage and fertilization on soil quality, within the context of a well-known recommendation: the imperative need to consider soil depths covering the entire plow layer during such assessment. It is necessary to (i) warn that this recommendation is most often forgotten and (ii) illustrate with real data the consequences of this common oversight. We carried out two field experiments in Derio (Spain): the first (2004) on a Dystric Cambisol and the second (2005-2007) on an Epidystric Cambisol. We hypothesized that conflicting conclusions regarding the effects of tillage and fertilization on soil quality can be drawn depending on the soil depth subjected to analysis. With respect to soil enzymes, it was found that inconsistencies in values with soil depth can be reduced when the results are expressed as specific activities.

The diagnosis and correction of potassium deficiency in New Zealand pastoral soils: a review

Field-trial data from a database comprising records of 804 potassium (K) fertiliser trials were used to define the production functions relating exchangeable soil K (quick test K (QTK) 0–75 mm) to the relative response to fertiliser K applications, for the major soil groups in New Zealand. For all soil groups for which there were sufficient data, the production functions were generally flat in the range QTK 5–10, and thus the estimated relative pasture production at QTK 5 and QTK 10 were similar. The critical QTK levels to achieve 97% maximum production were relatively well defined, being 6 (5–8) for sedimentary soils (brown and pallic) and brown soils, and 7 (5–10) for pumice soils. The data for the allophanic soils were unstable and the best estimate was 6 (5–10). For the remaining soils groups (podzols and raw soils, organic, recent and gley soils) for which there was much less data, the relationships were essentially flat over the range QTK 2–10. The probability of pasture responses to applied K increased as soil QTK decreased from 10. For the sedimentary and volcanic soils (including both allophanic and pumice) the probability was about 70–80% at soil QTK < 2. The comparable probabilities were 50–60% for the recent and gley soils, and 30–43% for the podzols and raw soils. A feature of the response functions was that some trials were not responsive to fertiliser K despite having low soil QTK. In most cases this could not be attributed to soil K reserves as measured by the soil TBK test (sodium tetra-phenol-boron extractable which measures exchangeable K plus plant-available but non-exchangeable K). Other possible reasons for this feature in the data are discussed, including uptake of K from below the soil sampling depth and the temporal effects of clover responses to applied K. Soil K buffer capacities—the amount of fertiliser K over and above maintenance required to increase soil QTK by 1 unit (ΔK)—ranged from 50 to > 150 kg K ha−1 (average 124) for sedimentary soils. For some soils (developed organic soils, gleyed soils and podzols), fertiliser K had very little effect on QTK (0–75 mm). It is not clear whether these differences are due to differences in leaching of K from the sampling depth, differences between soils in their ability to absorb and retain applied K or indeed the result of errors in the measurement of this parameter. Estimated maintenance K requirements (i.e. the amount of applied K required to maintain soil QTK levels) increased with increasing soil QTK from 4 to 10, from 0–150 kg K ha−1 yr−1 to 100–300 kg K ha−1 yr−1 in situations where losses of K were extreme due to the removal of all harvested clippings. Given the uncertainties in predicting K responses and the amount of fertiliser K required to correct K deficiency, practical suggestions are offered as to how best to diagnose and manage soil K deficiency. Areas for future research to improve the prediction of pasture responses to fertiliser K are also included.

Comparison of Selected Extractants for the Estimation of Available K in Soils of Edo State of Central Southern Nigeria

Comparison of five selected extractants for estimation of available K in soils of Edo State of Central Southern Nigeria was carried out in a greenhouse with maize as the test crop. The five extractants used were distilled water, 0.5 N NaHCO 3 (pH 8.5), 0.01 M CaCl 2 , 0.1 N HNO3 and 1 N NH 4 OAc (pH 7). Nine composite surface (0-15 cm depth) and subsurface (15-30 cm depth) soil samples were used in this study. The amount of extractable K by 1 N NH 4 OAc (pH 7) and 0.1 N HNO 3 showed the highest significant linear correlation with K uptake and dry matter yield with 'r' values of 0.910***, 0.895*** and 0.718***, 0.754*** (P = 0.001) respectively, when compared with distilled water, 0.5 N NaHCO 3 (pH 8.5) and 0.01 M CaCl 2 . The use of 1 N NH 4 OAc (pH 7) and 0.1N HNO 3 soil tests could be recommended because they appeared to have almost equal efficiency in terms of the ability of extracting available K from these soils. Keywords : available K; Central Southern Nigeria; Edo State soils; extractants DOI: 10.4038/tare.v10i0.1876 Tropical Agricultural Research & Extension Vol.10 2007 83-87