Performance of a non‐nuclear resonant frequency capacitance probe. III. error analysis

Abstract

All techniques used to measure soil water content have errors associated with them. These errors are further propagated when soil volumetric water contents are numerically integrated over a number of depth intervals to estimate the soil water storage for the entire profile. Previous studies had field‐tested the performance of a commercial capacitance probe (Troxler Sentry 200‐AP). The probe was calibrated and the measured data on crop water use were compared with that reported previously in literature using other instruments. However, the errors associated with the field estimates of soil water storage and crop water use measured with the Sentry 200‐AP capacitance probe were not evaluated. The objective of this study was to analyze these associated errors. Equations were derived to evaluate the errors incurred in calculating the volumetric water content using the fitted field calibration curve for the Sentry 200‐AP capacitance probe and in the estimates of the crop water use. This study suggests that the error in making in situ measurements of soil water content in heterogeneous soil systems using the Sentry 200‐AP capacitance probe depends on the degree of saturation at the point of measurement in the soil profile. The Sentry 200‐AP capacitance probe produces errors of >5% in measuring soil volumetric water contents of >35% in heterogeneous soil systems.     

Performance of a non‐nuclear resonant frequency capacitance probe. II. Measurement of dryland crop water use

Abstract

Resonant frequency capacitance techniques have been recently developed as a safe and reliable method for measuring water content of various materials. A previous study with a commercial capacitance probe (Troxler Sentry 200‐AP) showed it to be a safe, reliable, and a rapid method of in situ measurement of soil water content in the field provided it is calibrated for individual soils. Further testing of this resonant frequency capacitance probe was done to evaluate the performance of the probe by comparing results of field measured crop water use to those reported in similar studies using other methods of determining in situ soil water content in the field. These tests were done in two field experiments which were conducted during the summer of 1994 using corn and sorghum as the indicator crops. The experimental field was the same used to calibrate the capacitance probe in the previous study. Treatments consisted of three plant populations of corn and four plant populations in two cultivars of grain sorghum. The crop water use for corn and sorghum averaged over all treatments were 452 and 424 mm, respectively. The measured crop water use values for corn and sorghum were comparable to the crop water use values reported in several similar previous studies using different in situ soil water measuring instruments.     

The effect of lime and fertilizers on four lignite overburdens under non‐drained conditions: I. Yield and nutrient concentration of common bermudagrass.

Abstract

Common bermudagrass (Cynodon dactylon (L.) Pers.) was grown on four different overburdens from the potential lignite mining area of Louisiana. A pot study was conducted in the greenhouse to evaluate the fertility status of the overburdens for future recommendations of fertilizers and soil amendments during the revegetation of the reconstructed soil.

Fertilizer at the rate of 0, 1/2>x, 1x, 2x and 4x the soil test recommended rate of N, P and K for common bermudagrass were applied to the materials. Lime was applied to the Cow Bayou formation material to raise the pH to 6.5. Bermudagrass forage was harvested every six weeks over a 24 week period to evaluate dry matter yield and mineral composition.

Yield and nutrient uptake (N, P, K, Ca, Mg, S, Fe, Mn, Zn, and Cu) were significantly (α = 1%) affected by increasing rates Df fertilizer in all overburden materials except P uptake in the Cow Bayou material. In general, yield and nutrient uptake increased with each addition of fertilizer. Total yield was not significantly (α = 1%) different when the recommended rate of N, P and K fertilizer was applied in all overburdens. However, total yield was highest in the Prairie Terrace and Dolet Hills overburdens when 4x this rate was applied. Nutrient concentration was not significantly affected by fertilizer application except for N in the Naborton material, Ca in Prairie Terrace and Naborton, S in Dolet Hills and Zn in Prairie Terrace and Naborton (α = 5%).     

Iron‐stress response of inoculated and non‐inoculated roots of an iron inefficient soybean cultivar in a split‐root system

The objective of this study was to establish whether the iron‐stress responses observed in T203 soybean (Fe‐inefficient) with active nodules are products of the nodules or of the entire root system. A split‐root system was used in which half the roots of each plant were inoculated and actively fixing nitrogen and the other half were not. Soybean cultivar T203 is normally Fe‐inefficient and does not exhibit the Fe‐stress responses, however an iron‐stress response did occur during active N₂ fixation in earlier studies. This implies that the active nodules influenced the plant's ability to respond to Fe‐deficiency stress. In this study, the Fe‐stress response (H⁺ and reductant release) observed in T203 soybean was limited to the inoculated side of the split‐root system. The severe Fe chlorosis which developed in these plants was overcome in a manner similar to Fe‐efficient cultivars undergoing normal Fe‐stress response and the T203 plants completely regreened. Exudation of H⁺ ions was similar in both the presence and absence of Fe, and was generally limited to inoculated roots. Reductant release was nearly nonexistent from the non‐inoculated roots and was greater for the Fe‐stressed (‐Fe) plants than for non‐stressed (+Fe) plants. Thus, the response observed, which alleviated Fe chlorosis, appeared to be associated with N₂ fixation of the active nodules.     

Monthly variation of forage and soil minerals in central Florida. I. macro‐minerals

Abstract

A two‐year study was conducted to determine the macromineral status of cattle grazed forages, mostly bahiagrass, and soils in central Florida. Soil and forage samples were collected every month for two years. Month differences (P < 0.01) were observed in all forage macrominerals and in crude protein (CP) for both years. No month effect (P > 0.05) was observed in IVOMD level during year 1. Year effects (P < 0.05) were observed in calcium (Ca), sodium (Na) and CP. Concentrations below the critical level were observed in all macrominerals studied. Higher forage macro‐mineral concentrations were found during spring‐summer months. In general, higher (P < 0.05) soil aluminum (Al), Ca, magnesium (Mg), phosphorus (P), and organic matter (OM) were observed during fall‐winter months, while Na was higher in winter. Soil Ca and Mg were adequate and potassium (K), Na and P were deficient. Year 2 showed higher (P < 0.05) soil macromineral concentrations. Correlation coefficients (r >|0.5|, P < 0.05) were present between forage K and forage CP (r = 0.557) and between forage P and forage CP (r = 0.554). Low correlations were found between soil and forage macrominerals. Percentages of total forage samples with macromineral and CP concentrations below critical levels (in parentheses) and suggestive of deficiency were as follow: in forage, Ca (0.30 ppm), 21%; Mg (0.18 ppm), 34%; K (0.60 ppm), 47%; Na (0.06 ppm), 89%; P (0.25 ppm), 85%; and CP (7%), 18%.     

Agronomic effectiveness of phosphate rock and superphosphate for aluminum‐tolerant and non‐tolerant sorghum cultivars

Abstract

About 35% of soils in Venezuela are acid and low in available phosphorus (P). To solve this problem farmers lime and apply phosphate fertilizers to the soils, but both lime and fertilizers are expensive. A good alternative to overcome soil acidity is the use of aluminum (Al)‐tolerant cultivars. The objective of this study was to test the hypothesis, by use of a pot experiment, that sorghum cultivars tolerant to Al toxicity are able to use P from phosphate rock more efficiently than are susceptible cultivars. Three sorghum (Sorghum bicolor L. Moench) cultivars, Chaguaramas III (Ch), AI‐tolerant, Decalb D59 (D59), and Pioneer 8225 (Pi), both Al‐susceptible, were grown in the greenhouse for 20 and 35 days in two acid soils fertilized with 0 and 100 mg P kg^‐1 as triple superphosphate (SP) and Riecito phosphate rock (PR). Santa Maria soil was very low in available P (2 mg kg^‐1) and highly saturated in Al saturation (64.5%) and Pao soil was higher in available P (20 mg kg^‐1) and low in Al saturation (6.5%). Chaguaramas dry matter production, P uptake and root length was higher in Santa Maria soil as compared with Pi and D59 when grown with both SP and PR fertilization. Chaguaramas response to PR in Pao soil was not as good as in Santa Maria soil. The results of our experiment suggest that Al‐tolerant Ch is able to utilize P from PR more efficiently in soils like Santa Maria than Al‐susceptible cultivare Pi and D59.     

Nonuniform leaching of nitrate and other solutes in a furrow‐irrigated,sludge amended field

Abstract

Nitrate leaching losses were estimated using soil core samples from three different locations in a furrow irrigated, N fertilized and sludge amended cotton field. These losses were controlled by irrigation efficiency, as well as sources and quantities of N applied. Statistical comparisons of sample locations and N treatments revealed N treatment to be less significant than the field sampling location. However, sludge amended soils had significantly higher levels of nitrates in the root zone and consequently suffered higher nitrate leaching losses. A NO₃‐N profile (30–210 cm) balance indicated that about forty percent (40%) of available NO₃‐N was leached below the root zone (0–150 cm) in the upper two‐thirds of the field plots during the pre‐plant irrigation. Whereas, the lower one‐third of the field did not experience significant nitrate losses below the root zone. A one‐dimensional finite difference layered model, was used to estimate the depth of moisture penetration at the field (furrow) locations following pre‐plant irritation. It was concluded that the lower one‐third of the plots received less than 50% of mean plot application (30 cm) water during pre‐plant irrigation.     

The colonization of host-root and soil by Rhizobia—I. Species and strain differences in the field

Growing-season populations of rhizobia associated with annual host-plant roots and nearby soil were examined in a field soil showing a nodulation problem in the second year after establishment. Rhizobium lupini reached higher populations at a faster rate than R. trifolii. A sharp drop in the population of R. trifolii associated with subterranean clover roots early in the growing season was followed by a recovery to high numbers. No such phenomenon occurred with R. lupini. The numbers of rhizobia under patches of non-nodulated plants in second-year stands were very low, usually <5/g soil, whereas the numbers under healthy plants in problem stands were similar to those under established stands. Differences in the colonization of both root and soil by R. trifolii in the first year were reflected in the second-year nodulation.     

The effect of lime and fertilizers on four lignite overburdens under non‐drained conditions: II. Weathering.

Abstract

Four lignite overburdens from a potential lignite mining area of Louisiana were used to evaluate the effect of rates of fertilizer, lime, plant growth and simulated poor drainage conditions on the weathering of these materials. Common bermudagrass (Cynodon dactylon) grown on each overburden material in pots lined with polyethylene bags received 0, 1/2>x, 1x, 2x and 4x the recommended rate of fertilizer which was a combination of N, P and K. Lime was applied to the overburdens based on soil test lime requirement to increase the pH to 6.5.

Enrichment (fertilizers, non‐drained conditions and lime) had a significant effect on the weathering of the overburdens. The pH, EC, organic C, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, Na and Al were significantly affected by one or more of these parameters. Visual observations indicated that fertilizers stimulated root production, which effectively increased the weathering of the overburdens. Lime, applied as CaO, did not increase the pH of the Cow Bayou material substantially, suggesting the presence of pyrite, which was confirmed by X‐ray diffraction analysis. As pyrite oxidized in the Cow Bayou material, soluble salts accumulated on the surface of the material and on the lower parts of the plant. Gypsum and jarosite precipitated as the solution ionic strength of this material increased due to the weathering of the pyrite.

The Prairie Terrace overburden was selected as the best potential topsoil since it had more favorable physical and chemical characteristics which would promote successful reclamation. Reduced materials such as Cow Bayou and Naborton had undesirable physical and chemical properties. Changes in the chemical properties of these materials would be difficult to predict and soil amendments such as lime and gypsum would be needed in significant quantities for successful reclamation. Dolet Hills, a sandy material, had a less desirable texture and would require frequent irrigation and fertilization if it were used as a topsoil substitute.     

Growth and mineral nutrition of cattails inhabiting a thermally‐graded South Carolina reservoir: I. Growth and the macronutrients 1

Growth and macronutrient uptake in two species of cattail, Typha latifolia and T. domingensis, were determined during an annual growth cycle in Par Pond, a thermally‐graded nuclear reactor cooling reservoir in South Carolina. Shoots of the same age class were sampled periodically from the initiation of growth in early spring to senescence in late autumn. Although differences between thermal regimes were mostly statistically non‐significant, growth and macronutrient uptake, especially of N, P, and K, tended to be enhanced by elevated water temperatures. The pattern for growth and N, P, and K uptake in decreasing order were: “Warm”; > “Intermediate”; > “Ambient”; regimes. The non‐significant statistics were attributed to large between‐stand and between‐temperature regime variations. Both Typha species exhibited sharp declines in N, P, and K concentrations in the shoots, and some decline in the rhizomes, early in the growing season, followed by either a more gradual decline or leveling off of concentrations until senescence. Calcium in shoots of both species increased with season, whereas Mg remained fairly constant. Thermal effluents can alter the growth form of Typha, as confirmed by the elimination of T. domingensis in the “Warm”; regime, and can affect their mineral nutrition.     

Dryland corn response to tillage and nitrogen fertilization. I. Growth‐yield‐N relationships

Abstract

Crop response to fertilizer nitrogen (N) is dependent upon tillage management. This study was conducted to determine how tillage rotation influences non‐irrigated crop growth, N uptake and yield. The effects of tillage rotation, N rate and N timing schedule on early season dry matter production and N uptake, ear leaf N concentration at silking, and yield of corn [Zea mays (L.) Pioneer 3378] were investigated at Painter, VA, on an Altavista loam (fine‐loamy, mixed, thermic Aquic Hapludult). In 1986, maximum yields achieved in the 6‐year continuous no till (NT) [5.82 Mg/ha] and first year no till (AT) [5.64 Mg/ha] were significantly greater than that of the 6‐year continuous conventional till (CT) [3.67 Mg/ha], but no yield differences were obtained in the drier 1987 season. A higher rate of N fertilizer was required to obtain maximum yield in the first year no till (168 kg N/ha) than in the NT (112 kg N/ha) during 1986. Early 1986 N uptake and growth response with and without N at planting increased in the order CT < AT = NT and AT < CT < NT, respectively, indicating greatest immobilization of soil N occurred in the newly established no till soil. Lack of differences in critical ear leaf N values developed for NT and CT in each year imply that plant norms developed for one tillage system may accurately assess N status of corn grown under different tillage practices.     

Performance characteristics in the determination of soil‐water pH and soil‐SMP buffer pH using a flow‐through junction combination electrode

Abstract

A laboratory robot has been used to assess the performance of a commercially available flow‐through junction pH electrode. The electrode was tested for accuracy and precision in the determination of soil‐water pH (pH_w) and the Shoemaker, McLean, and Pratt buffer solution pH (pH_SMP). The significance of the pH_SMP determination lies in subsequent calculations of soil lime requirement (LR). Response times and hysteresis effects were compared under different operating conditions.     

Phosphorus in the Pampean semi‐arid region of Argentina. I. bray and anionic exchange resin ex‐tractable phosphorus

Abstract

There is often a lack response of wheat to phosphorus (P) fertilization in the Pampean semi‐arid area of Argentina. Consequently, the fate of fertilizer P in an entic Haplustoll soil was investigated under three production systems commonly practiced in that region. The following procedures were conducted: i) comparison and linear regression of the values of P obtained by Bray I (Pbray) versus’ the anionic exchange resin (P_aer) procedure (P_aer = 2.35 + 0.68 Pbray), ii) addition of different amounts of KH₂PO₄ to the Ap horizon, and after incubation at 25°C for 6 months, extraction of the Paer in order to establish an index of fertilizer availability F₁ which was: F₁ = ‐ 0.13, and iii) determination by X‐ray diffractometry of the generated brushita (DCPD) in the incubated samples to which P was added. The decrease in labile P could explain the lack of wheat response to the P fertilization in certain years.     

Pressurized Hot Water and DTPA‐Sorbitol as Viable Alternatives for Soil Boron Extraction. I. Boron‐Treated Soil Incubation and Efficiency of Extraction

Abstract

Serious challenges associated with hot water extraction, the standard extraction method for water‐soluble boron (B), limit its use in commercial soil‐testing laboratories. Several alternatives to make B testing more practical have been proposed and studied; none of the alternatives have readily replaced the hot water method. Two relatively new, promising B extraction methods are pressurized hot water and DTPA‐Sorbitol. Very little reported work compares B extraction values obtained from the standard hot water extraction method and these two alternative methods. This study was conducted to complete an initial step in validating new procedures—extracting the designated nutrient from fertilized, incubated soils by using standard and alternative extraction methods and comparing the resulting values. The three extraction methods were used to extract B from samples of calcareous sand and silt loam soils and limed, loamy fine sand, all which had been treated with 10 levels of B (0–8 mg kg^?1) and incubated for 7 and 28 days. The amount of B extracted increased as the rate of B application increased with all three soil‐extraction methods. High correlations (r 0.977–0.999) were observed between extractable B and rate of B application with all three procedures. Correlations between the amount of extractable B using hot water extraction and the value obtained with an alternative extraction method were similar for both methods (r=0.89). Hot water generally extracted the least and pressurized hot water the most B regardless of soil type, rate of application, or duration of incubation. This study suggests the more easily used methods of pressurized hot water and DTPA‐Sorbitol could be recommended as replacements to the cumbersome hot water extraction.     

Yield of iron‐sprayed and non‐sprayed strawberry cultivars grown on high ph calcareous soil

Abstract

Iron (Fe) deficiency chlorosis (FeDC) results in extensive reduction in yield of strawberry (Fragaria x ananassa Duch.) grown on high pH calcareous soils. Three cultivars differing in response to FeDC were grown on a high pH (8.2) calcareous soil (25.4% calcium carbonate equivalent in surface 20 cm) in the field (Choueifat, coastal area of Lebanon) to determine the effects of FeDC on fruit yield of cultivars sprayed with FeEDDHA [ferric ethylene‐diiminobis (2‐hydroxyphenyl) acetate]. The unsprayed plots were used as a control. No significant interaction (P<0.05) between cultivars x FeEDDHA spray treatment, and no significant differences (P<0.05) between one and two FeEDDHA spray(s)/week treatment was noted for visual FeDC, fruit number, and fruit yield. Sprayed cultivars once a week produced higher yields than unsprayed ones; overall increases were 33% (13% for ‘Motto’, 30% for ‘Chandler’, and 56% for ‘Douglas'). Even though only slight FeDC was noted on the ‘Motto’ cultivar receiving no Fe EDDHA spray, fruit yields were increased when sprayed with FeEDDHA. However, significant increases in yield for ‘Chandler’ and ‘Douglas’ cultivars with severe FeDC ratings were rioted when sprayed with FeEDDHA.     

Estimating non‐crystalline and crystalline aluminum and iron by selective dissolution in a riparian forest soil

Abstract

We compared sequential and separate extraction procedures for estimating non‐crystalline and crystalline aluminum (Al) and iron (Fe) in a floodplain forest soil. We used 0.2M acid ammonium oxalate (pH 3.0) to estimate non‐crystalline Al and Fe, dithionite‐citrate‐bicarbonate to estimate crystalline Fe, and 0.1M sodium hydroxide (NaOH) to estimate crystalline Al. Both separate and various combinations of sequential extractions were compared, as well as variations in soil:solution ratio, extraction time, extraction temperature, and the use of field‐moist versus dried soils. A sequential oxalate/NaOH extraction, using 0.4 g dry weight equivalent of field‐moist soil and a soil:solution ratio of 1:100, gave the best estimate of non‐crystalline Al and Fe and crystalline Al, while a separate 16 hour DCB extraction at room temperature, using 0.8 g dry weight equivalent of field‐moist soil and a soil:solution ratio of 1:50, gave the best estimate for non‐crystalline plus crystalline Fe. A sequential oxalate/NaOH extraction followed by a separate DCB extraction is a relatively simple procedure for estimating amounts of non‐crystalline and crystalline Al and Fe in large numbers of soil samples.     

Zinc adsorption by sterilized and non‐sterilized soil in the presence of citrate and catechol

Abstract

The effect of organic ligands on zinc (Zn) adsorption in the rhizosphere may be influenced by microbial activity depleting the concentration of the ligand over time. Zinc adsorption by sterile and non‐sterile lateritic soil was measured by shaking the soils with Zn solutions at a soil‐liquid ratio of 1: 5 for five periods of time ranging from 17–480 h. The concentrations of Zn and the ligands were determined after the selected times. The speciation of Zn was then estimated.

Changes in Zn adsorption with time in the presence of organic ligands were due to changes in pH and Zn complexation associated with the addition and breakdown of the organic ligands and with the technique of sterilization. Zinc adsorption was higher in non‐sterile than in sterile soil. Adsorption in the presence of citrate was lower than in its absence except in non‐sterile soil after 72 h. The adsorption in the presence of catechol was similar to that in its absence.     

Influences of ultra‐violet (UV)‐blue light radiation on the growth of cotton. I. Effect on iron nutrition and iron stress response

Cool white fluorescent (CWF) light reduces Fe³⁺ to Fe²⁺ while low pressure sodium (LPS) light does not. Cotton plants grown under CWF light are green, while those yrown under LPS light develop a chlorosis very similar to the chlorosis that develops when the plants are deficient in iron (Fe). It could be that CWF light (which has ultra violet) makes iron more available for plant use by maintaining more Fe²⁺ in the plant. Two of the factors commonly induced by Fe‐stress in dicotyledonous plants‐‐hydroyen ions and reductants released by the roots‐‐were measured as indicators of the Fe‐deficiency stress response mechanism in M8 cotton.

The plants were grown under LPS and CWF light in nutrient solutions containing either NO₃‐N or NH₄‐N as the source of nitrogen, and also in a fertilized alkaline soil. Leaf chlorophyll concentration varied significantly in plants grown under the two light sources as follows: CWF+Fe > LPS+Fe > CWF‐Fe ≥ LPS‐Fe. The leaf nitrate and root Fe concentrations were significantly greater and leaf Fe was generally lower in plants grown under LPS than CWF light. Hydrogen ions were extruded by Fe‐deficiency stressed roots grown under either LPS or CWF light, but “reductants”; were extruded only by the plants grown under CWF light. In tests demonstrating the ability of light to reduce Fe³⁺ to Fe²⁺ in solutions, enough ultra violet penetrated the chlorotic leaf of LPS yrown plants to reduce some Fe³⁺ in a beaker below, but no reduction was evident through a yreen CWF grown leaf.

The chlorosis that developed in these cotton plants appeared to be induced by a response to the source of liyht and not by the fertilizer added. It seems possible that ultra violet liyht could affect the reduction of Fe³⁺ to Fe²⁺ in leaves and thus control the availability of this iron to biological systems requiring iron in the plant.     

Tomato response to long‐term potassium and lime application on a sandy ultisol high in non‐exchangeable potassium

In a ten‐year study of potassium (K) and lime application to a Kalmia sandy loam (fine‐loamy, siliceous, thermic Typic Hapludult), a soil high in nonexchangeable K, corn (Zea mays L.) and soybean [Glycine max (L.) Herr.] have not responded to applied K. The objectives of this study were to determine if a high K‐requiring crop such as tomato (Lycocersicon esculentum Mill. cv. Redpak) would respond to KCl fertilizer rate or lime type (dolomitic, calcitic, and mixed) and rate on such a soil. Potassium was applied at 0, 56, and 112 kg K/ha every year for ten years. Lime was applied at 0, 2, and 9 Mg/ha in calcitic, mixed, and dolomitic forms twice in ten years (1970 and 1973). In 1980, the tenth year of the study, tomato fruit was harvested by hand once‐over to simulate machine harvest and divided into four maturity groups by color. Soil pH was higher with dolomitic than calcitic lime. Soil K saturation was not influenced by lime rate or type. Fruit yield and leaf phosphorus (P), calcium (Ca), and magnesium (Mg) concentrations increased with increasing lime rates. Leaf K, manganese (Mn), iron (Fe), boron (B), copper (Cu), zinc (Zn), barium (Ba), strontium (Sr), and aluminum (Al) concentrations decreased with increasing lime rate. Leaf Mn, Ba, and Sr concentrations were lower with dolomitic than with calcitic lime. Lime type had no effect on tomato yield. Wide ranges in basic cation saturation ratios had little effect on yield. Soil K saturation and leaf K, Zn, and Ba concentrations increased with increasing K rate. Soil Ca and leaf Ca, Mg, and Al concentrations decreased with increasing K rate. Applied K had no effect on total yield but onceover marketable yield increased linearly with increasing K rate. Marketable yield increased 14% with an increase in K rate from 0 to 56 kg/ha. Thus, fruit maturity was apparently hastened by K fertilization.     

Fertilizer phosphorus placement studies on calcareous and non‐calcareous chernozemic soils: Growth,p‐uptake and yield of flax

Abstract

We examined soil potassium (K) supply capacity as related to wheat response to K fertilization, using a cation exchange membrane (CEM) burial technique to measure potential K supply rate. A growth chamber experiment was conducted to determine soil and plant response relationships. Canadian Prairie Spring wheat (Triticum aestivum ’Biggar') was grown on three soils of different initial K fertility with three rates of added K. Wheat response to K fertilization was well related to the amount of exchangeable K and K supply rate in the soil. Soils with high initial K supply rate demonstrated an adequate K release rate which was associated with low response to added K fertilizer. A soil K supply rate less than 5 μg cm² hr^‐1 represented soil K supply power that is less than optimal for wheat nutrition.