Soil Test Values Research Articles

Spatial Variability of Soil Test Phosphorus, Potassium, and pH of Ontario Soils

Grid soil sampling is typically used for establishing management zones for site‐specific application of nutrients. The geostatistical procedures used to estimate values between sample locations require samples to be taken close enough to together that they are correlated to one another. An evaluation of the scale of variability of soil test P (STP), soil test K (STK), and soil pH for Ontario soils was conducted using autocorrelation analysis of 23 Ontario farm fields, which were grid‐sampled using a 30‐m spacing. The results of the autocorrelation analysis indicated that 13 of the 23 farm fields would require a grid spacing of less than 30 m to adequately assess their spatial variability. For only one site was the commonly used 100‐m grid spacing adequate for the assessment of the spatial patterns of STP and STK. Further analysis using F tests compared the residuals from three gridding procedures (kriging, inverse distance, and nearest neighbor) using 60‐ and 90‐m grid data to that of the residuals using the field mean soil test value. In most cases, soil test variation maps based on 60‐ or 90‐m grid soil samples did not result in an increased ability to predict the soil test level at a given location in the field. It was concluded that a grid spacing of 30 m or less would be required to adequately assess the spatial variation of STP, STK, and soil pH. Sampling at this intensity would require approximately 11 times as many soil samples as the commonly used 100‐m grid, which is likely to make the cost prohibitive.

Increasing phosphorus supply in subsurface soil in northern Australia: Rationale for deep placement and the effects with various crops

Three field experiments involving wheat, lucerne or cotton were established at different sites in the semiarid cropping regions of northern Australia, to test whether the deep placement of P fertiliser improved P availability, compared to the conventional practice of placing the fertiliser beside or adjacent to the seed. At Mulga View, near St George in southern Queensland on a red Kandosol soil with a Colwell soil test value of 19 mg P kg soil−1 in the top 10 cm, there was no response to 10 kg P ha−1 applied in the 5–7 cm layer. However, increasing the depth of placement of 10 kg P ha−1 from 5–7 to 10–15 cm resulted in increased shoot growth and grain yield of spring wheat (Triticum aestivum) by 43 and 30%, respectively. A further grain yield increase of 43% to 3.2 t ha−1 resulted when the deep P rate was increased from 10 to 40 kg P ha−1. At Roma, in southern Queensland, on a grey/brown Vertosol with a Colwell soil test value of 15 mg P kg soil−1, there was no difference in the winter growth of lucerne (Medicago sativa) when P fertiliser had been applied at 5–7 cm depth at rates of 10 and 40 kg P ha−1. Shoot dry matter yields were around 2 t ha−1. However dry matter yields increased significantly to 2.6 and 3.7 t ha−1 when 10 and 40 kg P ha−1, respectively were applied at the 10–15 cm depth. The third experiment was carried out on a grey Vertosol at Kununurra in Western Australia. Significant increases in the yield of seed cotton (Gossypium hirsutum) occurred when 50 kg P ha−1 was applied at depth (10–15 and 25–30 cm), compared with the conventional placement at 7–10 cm, with maximum yield response to deep placement occurring with DAP, and the minimal response with MAP. The cotton was grown on raised beds and the crop was irrigated according to district practice. The response to deep P at all sites was attributed to the rapid drying of the soil surface layers, reducing the availability of soil or fertiliser P in these layers. The deep fertiliser P remained available during the growing season and alleviated the P deficiency that appears to be a feature of these soils when the surface layers become dry.

Correlation Between Soil P and Wheat Shoot P Contents in a Network of Hungarian Long‐Term Field Trials

The shoot weight and P concentration of winter wheat (Triticum aestivum L.) in the tillering stage, and the H2OP, bicarbonate P, Bray and Kurtz No. 1 P, and Mehlich‐3 P contents of the soil were determined in the 28‐yr‐old Hungarian network of uniform National Long‐Term Field Trials (NLFT). The network included four P fertilization rates at nine locations, representing the various agroecological and soil conditions of the country. Although there were great differences in the amounts of P extracted with the different methods, on average there was a three‐ to fivefold increase in exchangeable P within each P test method, as related to the P application rate. By using soil P test methods that were less dependent on soil properties (H2O, bicarbonate) a 1.5‐ to 2.0‐fold difference was found between the sites, whereas this difference rose to 2.2‐ to 2.5‐fold for methods more dependent on soil properties (Mehlich‐3 and Bray and Kurtz No. 1). The site (i.e., different agroecological conditions) had a 2.0‐fold effect on wheat shoot weight at the tillering stage. Soil P status, on the other hand, resulted in 2.6‐fold differences in wheat shoot weight. The P concentration of winter wheat shoots was influenced to a much greater extent by the P status (2.1‐fold differences) than by the site (1.3‐fold differences). A quadratic correlation was found between the P concentrations and weights of wheat shoots (r2=0.40). When shoot P concentrations were >0.35%, there was no further increase in shoot weights. The correlation between wheat shoot weight and bicarbonate P was also quadratic (r2=0.29). Wheat shoot weight increased with soil test values, up to a 20 mg kg−1 bicarbonate‐P level. There was a logarithmic correlation between bicarbonate‐P and shoot P contents (r2=0.61). The lower limit of good P supplies (indicated by 0.35% shoot P concentration at tillering) was usually reached when the bicarbonate‐P value was around 15–20 mg kg−1. The Mehlich‐3, H2O, and bicarbonate methods proved to be independent of soil reaction status. A stronger linear correlation was found between the bicarbonate and Mehlich‐3 P contents (r=0.92) than between the bicarbonate and H2O P contents (r=0.82). When comparing the bicarbonate and Bray and Kurtz No. 1 methods, acidic (r=0.93) and calcareous soils (r=0.83) formed two separate groups. The Bray and Kurtz No. 1 solvent extracted more P from acidic than from calcareous soils. The correlation between the bicarbonate soil test P and the wheat shoot P indicated that both methods were suitable for evaluating soil and wheat P status under Hungarian agroecological conditions.

Integrated phosphorus nutrition system for blackgram - ragi sequence

Studies on the effect of phosphatic fertilizer alone and in combination with organics and inoculants on black gram-ragi sequence were conducted at the Agricultural Engineering College and Research Institute Farm, Kumulur, Trichirappalli during 1999. 2000 and 2000-2001. The results revealed that there was a positive response for the application of phosphorous and the highest seed and haulm yields were recorded with 100 per cent P20s based on soil test value with pressmud and Phosphobacteria. Application of P fertilizer alone showed a depletion of available P with the advancement of stage of cropping but in combination with pressmud a positive balance was observed. Among the organics, the application of pressmud performed better than FYM. The residual effect of P was found to be higher while conjoint incorporation of inorganic P with pressmud and phosphobacteria than applying P alone.

Levels and Changes of Soil Phosphorus in Subtropical Beef Cattle Pastures

Long-term pasture management is believed to change soil chemical properties, but little is known about whether pasture management, such as fertilizer application, grazing, or haying can initiate such change in sandy and well-drained subtropical beef pastures. The objective of this study was to investigate the long term effect of pasture management (grazing + haying, GZ + HY) on soil phosphorus (P) dynamics (levels and changes) in subtropical beef cattle pastures with bahiagrass (BG, Paspalum notatum) and rhizoma peanut (RP, Arachis glabrata) with (WP) or without (WNP) P fertilization in Brooksville, FL from 1988 to 2000. Soil P dynamics in Subtropical Agricultural Research Station (STARS) was significantly affected by P fertilization (p ≤ 0.001) and pasture management (p ≤ 0.0001). The soil P levels across years from the fertilized fields of 119.0 ± 4.9 mg kg−1 was significantly higher than those pasture fields with no P fertilization (62.8 ± 7.8 mg kg−1). However, during the past 12 years, there was no P build up despite of the annual application of P-containing fertilizers in addition to the daily in-field loading of animal waste bi-products like fecals and urine. The average soil test values for P in STARS had declined by about 28.3%. The soil test values of P in BG-GZ was about 23% higher than that of BG-GZ + HY, suggesting that GZ followed by HY could have lowered levels of soil P. Soil testing program in the station should continue to measure the amount of soil P that is proportional to what is available to BG and RP, and also continue looking at alternative soil P tests that are better predictors of the loss and/or build up of total and dissolved P to soil and water systems.

Cumulative and Residual Effects of Repeated Sewage Sludge Applications: Forage Productivity and Soil Quality Implications in South Florida, USA (9 pp)

Background, Aim and Scope The cow-calf (Bos taurus) industry in subtropical United States and other parts of the world depends almost totally on grazed pastures. Establishment of complete, uniform stand of bahiagrass (BG) in a short time period is important economically. Failure to obtain a good BG stand early means increased encroachment of weeds and the loss of not only the initial investment costs, but production and its cash value. Forage production often requires significant inputs of lime, N fertilizer, and less frequently of P and K fertilizers. Domestic sewage sludge or biosolids, composted urban plant debris, waste lime, phosphogypsum, and dredged materials are examples of materials that can be used for fertilizing and liming pastures. Perennial grass can be a good choice for repeated applications of sewage sludge. Although sewage sludge supply some essential plant nutrients and provide soil property-enhancing organic matter, land-application programs still generate some concerns because of possible health and environmental risks involved. The objectives of this study were to evaluate the cumulative and residual effects of repeated applications of sewage sludge on (i) bahiagrass (BG, Paspalum notatum Flugge) production over years with (1997–2000) and without (2001–2002) sewage sludge applications during a 5-yr period, and (ii) on nutrients status of soil that received annual application of sewage sludge from 1997 to 2000 compared with test values of soils in 2002 (with no sewage sludge application) in South Florida.

Soil Sampling Techniques for Alabama, USA Grain Fields

Characterizing the spatial variability of nutrients facilitates precision soil sampling. Questions exist regarding the best technique for directed soil sampling based on a priori knowledge of soil and crop patterns. The objective of this study was to evaluate zone delineation techniques for Alabama grain fields to determine which method best minimized the soil test variability. Site one (25.8 ha) and site three (20.0 ha) were located in the Tennessee Valley region, and site two (24.2 ha) was located in the Coastal Plain region of Alabama. Tennessee Valley soils ranged from well drained Rhodic and Typic Paleudults to somewhat poorly drained Aquic Paleudults and Fluventic Dystrudepts. Coastal Plain soils ranged from coarse-loamy Rhodic Kandiudults to loamy Arenic Kandiudults. Soils were sampled by grid soil sampling methods (grid sizes of 0.40 ha and 1 ha) consisting of: (1) twenty composited cores collected randomly throughout each grid (grid-cell sampling) and, (2) six composited cores collected randomly from a ∼3×3m area at the center of each grid (grid-point sampling). Zones were established from (1) an Order 1 Soil Survey, (2) corn (Zea mays L.) yield maps, and (3) airborne remote sensing images. All soil properties were moderately to strongly spatially dependent as per semivariogram analyses. Differences in grid-point and grid-cell soil test values suggested grid-point sampling does not accurately represent grid values. Zones created by soil survey, yield data, and remote sensing images displayed lower coefficient of variations (%CV) for soil test values than overall field values, suggesting these techniques group soil test variability. However, few differences were observed between the three zone delineation techniques. Results suggest directed sampling using zone delineation techniques outlined in this paper would result in more efficient soil sampling for these Alabama grain fields.

Trends in some soil test data over a 14‐year period in New Zealand

To establish long‐term trends in soil fertility in New Zealand pasture lands, data for Olsen phosphate (Olsen P), pH, and QuickTest (QT) calcium (Ca), magnesium (Mg), and potassium (K) from 246 000 soil samples submitted to a commercial soil sampling laboratory from 1988 to 2001 were analysed. Each record was categorised according to year, land use (dairy, sheep/beef including deer), soil “group” (sedimentary, volcanic, pumice, peats, rest), and regions. Olsen P values were, on average, higher on dairy farms than sheep/beef farms and lower on sedimentary soils than other soils (P > 0.05). Sedimentary and volcanic soils had higher QT K than pumice and peat soils (P > 0.05). Regional differences were largely explained by differences between land use and soil group distribution. However, given this distribution, average Olsen P values were higher in the Waikato, Bay of Plenty/Taupo and Taranaki regions, the east coast of both islands generally had higher average pH, K, and Mg soil test values, and the West Coast region had lower average soil test values than most other regions. Over the 14‐year period, Olsen P followed an asymptotic increase, reaching the asymptote between 1994 and 1997 depending on land use and soil group. For the period 1997–2001, 30–65% of samples from dairy farms had Olsen P levels above the range required for near‐maximum pasture production. QT Mg decreased consistently by about 0.35 QT units year−1 when QT Mg was 25. The database provides a tool for looking at soil test trends over time on a national basis to assess the sustainability of soil fertility management practices on pastoral farms.

Efficacy of Grid and Zone Soil Sampling Approaches for Site-Specific Assessment of Phosphorus, Potassium, pH, and Organic Matter

Within-field variability of plant-available nutrients often results in different fertilizer requirements across a field. There is uncertainty concerning the efficacy of alternative sampling strategies suitable for site-specific management. This study compared various soil sampling approaches for P, K, pH, and organic matter (OM) in eight agricultural fields. Soil samples were collected using an intensive 0.2-ha grid-point procedure, and were used to compare less intensive sampling approaches. The approaches were based on 1.2–1.6-ha grid cells (Grid), soil series of digitized soil survey maps (SSM), soil series of detailed soil survey (1:12,000 scale) maps, elevation zones, and management zones based on various information layers (ZS). The approaches varied in reducing the within-unit soil-test variability and maximizing mean soil-test values across sampling units, but none was superior across all fields and nutrients. All approaches were less efficient for P and K than for pH or OM. The Grid and ZS approach were the most effective across all nutrients and fields. However, the Grid approach was more effective for P, the Grid and ZS approaches were better for K and pH, and the SSM and ZS approaches were better for OM. The ZS approach often resulted in fewer sampling zones than the Grid approach, which implies lower soil testing costs for producers, but required more knowledge and subjective judgement than a Grid approach to adapt it to field-specific conditions.

An Environmental Threshold for Degree of Phosphorus Saturation in Sandy Soils

There is critical need for a practical indicator to assess the potential for phosphorus (P) movement from a given site to surface waters, either via surface runoff or subsurface drainage. The degree of phosphorus saturation (DPS), which relates a measure of P already adsorbed by a soil to its P adsorption capacity, could be a good indicator of that soil's P release capability. Our primary objective was to find a suitable analytical protocol for determining DPS and to examine the possibility of defining a threshold DPS value for Florida's sandy soils. Four farmer-owned dairy sprayfields were selected within the Suwannee River basin and soil profiles were randomly obtained from each site, as well as from adjacent unimpacted sites. The soil samples were divided either by horizon or depth, and DPS was determined for each soil sample using ammonium-oxalate (DPSOx), Mehlich-1 (DPSM1), and Mehlich-3 (DPSM3) extracts. All methods of DPS calculations were linearly related to one another (r2 > 0.94). Relationships between water-soluble P and DPS indicate that the respective change points are: DPSOx = 20%, DPSM1 = 20%, and DPSM3 = 16%. These relationships include samples from Ap, E, and Bt horizons, and various combinations thereof, suggesting that DPS values can be used as predictors of P loss from a soil irrespective of the depth of the soil within a profile. Taking into consideration the change points, confidence intervals, agronomic soil test values, and DPS values from other studies, we suggest replacing Mehlich-1 P values in the Florida P Index with the three DPS categories (DPSM1 = <30, 30–60, and >60%) to assign different P loss ratings in the P Index.

An environmental threshold for degree of phosphorus saturation in sandy soils.

There is critical need for a practical indicator to assess the potential for phosphorus (P) movement from a given site to surface waters, either via surface runoff or subsurface drainage. The degree of phosphorus saturation (DPS), which relates a measure of P already adsorbed by a soil to its P adsorption capacity, could be a good indicator of that soil's P release capability. Our primary objective was to find a suitable analytical protocol for determining DPS and to examine the possibility of defining a threshold DPS value for Florida's sandy soils. Four farmer-owned dairy sprayfields were selected within the Suwannee River basin and soil profiles were randomly obtained from each site, as well as from adjacent unimpacted sites. The soil samples were divided either by horizon or depth, and DPS was determined for each soil sample using ammonium-oxalate (DPS(Ox)), Mehlich-1 (DPS(M1)), and Mehlich-3 (DPS(M3)) extracts. All methods of DPS calculations were linearly related to one another (r2 > 0.94). Relationships between water-soluble P and DPS indicate that the respective change points are: DPS(Ox) = 20%, DPS(M1) = 20%, and DPS(M3) = 16%. These relationships include samples from Ap, E, and Bt horizons, and various combinations thereof, suggesting that DPS values can be used as predictors of P loss from a soil irrespective of the depth of the soil within a profile. Taking into consideration the change points, confidence intervals, agronomic soil test values, and DPS values from other studies, we suggest replacing Mehlich-1 P values in the Florida P Index with the three DPS categories (DPS(M1) = <30, 30-60, and >60%) to assign different P loss ratings in the P Index.

Soil testing for phosphorus: comparing the Mehlich 3 and Colwell procedures for soils of south-western Australia

Soil samples were collected from 14 long-term field experiments in south-western Australia to which several amounts of superphosphate or phosphate rock had been applied in a previous year. The samples were analysed for phosphorus (P) by the Colwell sodium bicarbonate procedure, presently used in Western Australia, and the Mehlich 3 procedure, being assessed as a new multi-element test for the region. For the Mehlich procedure, the concentration of total and inorganic P in the extract solution was measured. The soil test values were related to yields of crops and pasture measured later on in the year in which the soil samples were collected.The Mehlich 3 procedures (Mehlich 3 total and Mehlich 3 inorganic soil test P values) were similar, with the total values mostly being slightly larger. For soil treated with superphosphate, for each year of each experiment: (i) Mehlich 3 values were closely correlated with Colwell values; and (ii) the relationship between plant yield and soil test P (the soil P test calibration) was similar for the Colwell and Mehlich 3 procedures. However, for soil treated with phosphate rock, the Colwell procedure consistently produced lower soil test P values than the Mehlich 3 procedure, and the calibration relating plant yield to soil test P was different for the Colwell and Mehlich 3 procedures, indicating, for soils treated with phosphate rock, separate calibrations are required for the 2 procedures. We conclude that for soils of south-western Australia treated with superphosphate (most of the soils), the Mehlich 3 procedure can be used instead of the Colwell procedure to measure soil test P, providing support for the Mehlich 3 procedure to be developed as the multi-element soil test for the region.

Risk assessment of phosphorus loss from sugarcane soils — A tool to promote improved management of P fertiliser

Current strategies for phosphorus (P) fertiliser management in the Australian sugar industry do not account for the differences between different soils in their ability to sorb and release P. However, the off-site export of P from land under sugarcane has been shown to be a major factor contributing to elevated concentrations of P in stream waters draining catchments dominated by sugarcane production. This paper presents the results of a study conducted in the lower part of the catchment of the Herbert River, north Queensland, a major sugarcane growing region. Our approach was to combine a knowledge of P sorption by soil and riverine sediments with an assessment of the risk of P loss from lower Herbert sugarcane soils and knowledge of the requirements of sugarcane for P. The results provide a basis for future P fertiliser management by canegrowers which accounts for both production and environmental imperatives. They also point to an urgent need for experimentation, based on rundown of soil P fertility, to determine critical soil test values in soils of varying P sorption, and provide a useful regional framework for the design of such experimentation.

CORRELATION BETWEEN SOIL P AND CORN LEAF P CONTENTS IN A NETWORK OF HUNGARIAN LONG-TERM FIELD TRIALS

Corn (Zea mays L.) leaf weight, leaf P concentrations at flowering stage, 0.01 M CaCl2-, Olsen-, LE-, and AL-soluble soil P contents were determined in a network of uniformed 27-year-old Hungarian long-term field trials (the so-called National Long-term Field Trials, NLFT) with four P fertilization rates on nine locations, representing various agro-ecological and soil conditions of the country. A 4–5 fold increase in soluble P contents was found in all soil P-tests, while the absolute values of dissolved P varied greatly (CaCl2-P: 0.1–3.7; Olsen-P: 3.7–47.7; and AL-P: 12.8–182.2 mg P kg−1). On the other hand, an average twofold difference occurred among the sites in case of soil P-test methods less dependent of soil properties (CaCl2, Olsen), and a fourfold difference in methods using acid solvents, more dependent of soil reaction status and CaCO3 content. On the average of all soils and all P levels, the amount of P dissolved by the different methods increased in the sequence of CaCl2<Olsen<LE<AL (1.5<20<44<74 mg P kg−1). The different agro-ecological conditions had a greater effect on corn leaf weights at flowering stage than soil-P status. Corn leaf P concentrations, however, were affected by both the P rates and the different sites, resp. There was no significant correlation between Olsen-P values and corn leaf weights. Corn leaf weight, however, increased jointly with soil test values, up to 10–15 mg/kg Olsen-P concentration. There was a weak quadratic correlation between corn leaf P concentrations and leaf weights (r=0.35* *). Above 0.25% leaf P concentrations, leaf weights did not increase any more. On calcareous soils, P-overfertilization could result in Zn deficiency induced by P. There was a moderate, logarithmic correlation between Olsen-P and leaf P contents (r=0.62* * *). The lower limit of good P supply,—indicated by 0.26% leaf P concentration at flowering stage—was usually reached when the Olsen-P value was around 10 mg/kg. Only the CaCl2- and Olsen-methods proved to be independent of soil reaction status. A strong, linear correlation was found between these two methods (r=0.80* * *). The behavior of acid LE- and AL- solvents, however, was different in acid and calcareous soils. Correlation between Olsen- and LE-, as well as between Olsen- and AL-methods could be found separately for acid and calcareous soils. The closeness of correlation within the calcareous soil group and within the acid soil group was similar (r=0.89* * * and 0.90* * * for the calcareous, and r=0.89* * * and 0.94* * * for the acid soils group). Soil and plant P analyses data proved to be useful tools in adapting the results of long-term field trials for improved, environmentally sound fertilizer recommendations.

APPLICATION OF MITSCHERLICH–BRAY EQUATION FOR FERTILIZER USE IN WHEAT

A field experiment was conducted for formulating fertilizer N P K requirements of wheat (cv. HD 2189) based on Mitscherlich–Bray equation in Vertic Ustropepts soil. There were 60 treatment combinations consisting of 5×4×3 levels of N, P2O5 and K2O fertilizers respectively. Soil available N P K were estimated and wheat yields recorded. The data were subjected to Mitscherlich–Bray equation: Where A=% theoretical maximum yield, y=actual yield obtained in q ha−1, b=native soil test value in kg ha−1, x=fertilizer nutrient applied in kg ha−1, c1 and c=constants i.e., efficiency of soil and fertilizer nutrients respectively. The validity of results were tested by conducting two field vertification trials on wheat in two soils (Typic Chromusterts and Vertic Ustropepts). The results showed that 85% theoretical maximum yield treatment recorded significantly highest wheat yields (5.8 and 5.6 t ha−1) and percent increase over control (286 and 522) in both the soils, respectively compared to all other treatments except 80% of theoretical maximum yield which was on par at both the locations. Considering the yields and economics, 80% theoretical maximum yield treatment was found to be superior to general recommended dose and as per soil test basis.

Short-term stability of soil test phosphorus in agricultural fields

The spatial and temporal stability of soil test values is important to the use of soil testing for site-specific soil fertility management. A study was conducted to evaluate the spatial and temporal stability of soil test phosphorus (P). Five sites ranging in size from 3.7 to 4.4 ha were soil sampled in the same locations in a 18.3 × 18.3-m grid either three or four times over a 2-yr period. Bray 1-P values were similar or decreased over time, while Olsen-P values at two of five sites decreased. One site showed no pattern and two sites had cyclic patterns where the spring sample values were greater than the fall. The spatial pattern of soil test values during a 2 yr rotation for Bray 1-P and Olsen-P was very stable. The changes in distribution in soil test P categories over a 2-yr period resulted in a shift to lower soil test categories. The decreases in soil test P were probably caused by plant P uptake in combination with no application of P fertilizer during the study. Key words: Soil test phosphorus, spatial stability, pH, temporal stability

Direct and residual phosphorus effects on soil test values and their relationships with grain yield and phosphorus uptake of upland rice on an ultisol

Establishing relationships between extractable phosphorus (P) in the soil and grain yield and P uptake in long-term field experiments are crucial for evaluating the efficacy of soil tests for determining the P requirements of crops. A field experiment was conducted for six years (1993–1998) to evaluate the response of four improved upland rice cultivars to direct and residual P. Fertilizer P was applied at 0, 45, 90, 135, and 180 kg P ha− 1 once in 1993 and the crops were grown during 1993–1998 without any fresh application of fertilizer P. The relationship between Bray 1 P and fertilizer P added in the initial P buildup phase were described by a linear regression equation (R=0.985). The prediction of decline in Bray 1 P with time, during the residual P phase was affected by P rate and well described by linear regression equations. The relationships between soil P test and P uptake were closer than between soil P and grain yield. The relationships between Bray 1 P and grain yield or total P uptake of the crop were significant in 1994 and 1995 seasons but not in the 1996 and 1998 seasons when the Bray 1 P values in all P treatments had fallen to low values. Results suggest that Bray 1 P indicated the availability of P to rice crops and thus can be used for determining the long-term fertilizer P requirements of upland rice in the Ultisol.

Soil and tissue tests to predict pasture yield responses to applications of potassium fertiliser in high-rainfall areas of south-western Australia

Dairy and beef pastures in the high (&gt;800 mm annual average) rainfall areas of south-western Australia, based on subterranean clover (Trifolium subterraneum) and annual ryegrass (Lolium rigidum), grow on acidic to neutral deep (&gt;40 cm) sands, up to 40 cm sand over loam or clay, or where loam or clay occur at the surface. Potassium deficiency is common, particularly for the sandy soils, requiring regular applications of fertiliser potassium for profitable pasture production. A large study was undertaken to assess 6 soil-test procedures, and tissue testing of dried herbage, as predictors of when fertiliser potassium was required for these pastures. The 100 field experiments, each conducted for 1 year, measured dried-herbage production separately for clover and ryegrass in response to applied fertiliser potassium (potassium chloride). Significant (P&lt;0.05) increases in yield to applied potassium (yield response) were obtained in 42 experiments for clover and 6 experiments for ryegrass, indicating that grass roots were more able to access potassium from the soil than clover roots. When percentage of the maximum (relative) yield was related to soil-test potassium values for the top 10 cm of soil, the best relationships were obtained for the exchangeable (1 mol/L NH4Cl) and Colwell (0.5 mol/L NaHCO3-extracted) soil-test procedures for potassium. Both procedures accounted for about 42% of the variation for clover, 15% for ryegrass, and 32% for clover + grass. The Colwell procedure for the top 10 cm of soil is now the standard soil-test method for potassium used in Western Australia. No increases in clover yields to applied potassium were obtained for Colwell potassium at &gt;100 mg/kg soil. There was always a clover-yield increase to applied potassium for Colwell potassium at &lt;30 mg/kg soil. Corresponding potassium concentrations for ryegrass were &gt;50 and &lt;30 mg/kg soil. At potassium concentrations 30–100 mg/kg soil for clover and 30–50 mg/kg soil for ryegrass, the Colwell procedure did not reliably predict yield response, because from nil to large yield responses to applied potassium occurred. The Colwell procedure appears to extract the most labile potassium in the soil, including soluble potassium in soil solution and potassium balancing negative charge sites on soil constituents. In some soils, Colwell potassium was low indicating deficiency, yet plant roots may have accessed potassum deeper in the soil profile. Where the Colwell procedure does not reliably predict soil potassium status, tissue testing may help. The relationship between relative yield and tissue-test potassium varied markedly for different harvests in each year of the experiments, and for different experiments. For clover, the concentration of potassium in dried herbage that was related to 90% of the maximum, potassium non-limiting yield (critical potassium) was at the concentration of about 15 g/kg dried herbage for plants up to 8 weeks old, and at &lt;10 g/kg dried herbage for plants older than 10–12 weeks. For ryegrass, there were insufficient data to provide reliable estimates of critical potassium.

Sampling Techniques for Soil Fertility Evaluation: An Exercise in Understanding Variability

It is difficult to obtain a representative soil sample from fields with areas of very low to very high soil test values. This field exercise was developed to assist agricultural practitioners with understanding common sampling schemes and their limitations with highly variable soils. A 2.8-ha site with high variability in soil test P and K was selected for use at the University of Minnesota's Extension Field School. Instructors provided background site information including previous crops and yields, factors of soil formation, methodology used in obtaining a composite sample, a representative management zone sample, precision grid sample, and a soil survey map. Before the exercise, 36 soil samples were collected on a 30-m grid and analyzed for P, K, and pH. Each grid point was numbered and flagged. Participants were split into groups, assigned a sampling method, and conducted a site evaluation and determined which grid points to include in their sample(s). Each group was limited to choosing eight grid points to collect their sample. Although P deficiency was evident, approximately half of the groups recommended no fertilizer application due to inclusion of high testing areas in their samples. Overall the grid sampling method resulted in the highest fertilizer recommendation for the site. The management zone method recommended the least fertilizer overall but most of it was applied in low testing areas, and the composite method which often contained nonuniform areas within the sample resulted in fertilizer recommendations unrepresentative of the site.

Variation in Soybean Response to Early Season Foliar Fertilization among and within Fields

Foliar fertilization of crops can complement soil fertilization. Recent research showed that soybean [Glycine max (L.) Merr.] yield response to early season N–P–K foliar fertilization was inconsistent and difficult to predict. This study's objectives were to further evaluate soybean response to early season N–P–K fluid fertilizers with or without S or micronutrients and to assess the within‐field response variation. The mixtures 3–8–15 (N–P–K), 3–8–15–1 (N–P–K–S), 10–4–8, 10–4–8–1, and 10–4–8–1 plus B, Fe, and Zn were evaluated in 18 conventional small‐plot trials. The within‐field variation in response to a 3–8–15 fertilizer mixed with glyphosate [N‐(phosphonomethyl) glycine] herbicide was evaluated in eight strip trials harvested with combines equipped with yield monitors and differential global positioning systems receivers. A fertilizer rate of 28 L ha−1 was sprayed once at the V5 growth stage. There was a yield response to the 3–8–15 and 10–4–8–1 fertilizers in one small‐plot trial (260 kg ha−1) and to 3–8–15 fertilizer in another trial (360 kg ha−1). There was a small response to the 3–8–15 fertilizer across the 18 trials (93 kg ha−1). Soybean responded to 3–8–15 fertilizer only in one strip trial where the yield response was higher in acidic soil areas. No response variation was detected across contrasting soil test values or soil types within other trials. Soybean response to foliar fertilization across all production conditions will seldom offset fertilization costs. The probability of economic yield response could be increased if the fertilizer is mixed with a postemergence herbicide because application costs are reduced.