Aluminum inhibition of NADH‐linked electron transfer by corn root plasma membranes

Vesicles enriched in right‐side‐out plasma membranes were isolated from corn roots by a modified two‐phase partition method. Using reduced nicotinamide adenine dinucleotide as electron donor, isolated vesicles were capable of reducing both ferricyanide and oxygen. At pH 6.0, the presence of Al inhibited both reduction processes to a similar extent. This result indicates that these two reduction processes share at least one common step that is sensitive to Al. Inhibition was not associated with a change in the structure of the membrane domain, as revealed by fluorescence polarization of membrane incorporated probes. These results are the first indications that the electron transfer processes of plasma membranes are sensitive to the presence of Al.     

Effects of root‐zone acidity on utilization of nitrate and ammonium in tobacco plants

Abstract

Tobacco (Nicotiana tabacum L., cv. ‘Coker 319') plants were grown for 28 days in flowing nutrient culture containing either 1.0 mM NO₃ ^‐ or 1.0 mM NH₄ ⁺ as the nitrogen source in a complete nutrient solution. Acidities of the solutions were controlled at pH 6.0 or 4.0 for each nitrogen source. Plants were sampled at intervals of 6 to 8 days for determination of dry matter and nitrogen accumulation. Specific rates of NO₃ ^‐ or NH₄ ⁺ uptake (rate of uptake per unit root mass) were calculated from these data. Net photosynthetic rates per unit leaf area were measured on attached leaves by infrared gas analysis. When NO^‐ was the sole nitrogen source, root growth and nitrogen uptake rate were unaffected by pH of the solution, and photosynthetic activity of leaves and accumulation of dry matter and nitrogen in the whole plant were similar. When NH₄ ⁺ was the nitrogen source, photosynthetic rate of leaves and accumulation of dry matter and nitrogen in the whole plant were not statistically different from NO₃ ^‐ ‐fed plants when acidity of the solution was controlled at pH 6.0. When acidity for NH₄ ⁺ ‐fed plants was increased to pH 4.0, however, specific rate of NH₄ ⁺ uptake decreased by about 50% within the first 6 days of treatment. The effect of acidity on root function was associated with a decreased rate of accumulation of nitrogen in shoots that was accompanied by a rapid cessation of leaf development between days 6 and 13. The decline in leaf growth rate of NH₄ ⁺ ‐fed plants at pH 4.0 was followed by reductions in photosynthetic rate per unit leaf area. These responses of NH₄ ⁺ ‐fed plants to increased root‐zone acidity are characteristic of the sequence of responses that occur during onset of nitrogen stress.     

Effects of tillage,nitrogen placement,and wheel compaction on denitrification rates in the corn cycle of a corn‐oats rotation

Denitrification rates under various tillage systems were determined in the corn (Zea mays L.) cycle of a corn‐oats (Avena sativa L.) rotation. Denitrification was measured directly with an in‐situ soil cover method which supplied the soil with acetylene (C₂H₂) and evacuated the nitrous oxide (N₂O) produced. Denitrification rates were measured in both a field or non‐wheel track (NWT) area and in a compacted wheel track (WT) area for the no‐till (NT), chisel plow (CH), moldboard plow (MP) tillage systems after nitrogen (N) was applied by broadcast/incorporation with 112 kg N/ha as ammonium nitrate. Nitrogen was also applied to the NT treatment by injection with modified anhydrous ammonia knives prior to planting. Most of the cumulative N loss occurred over a 22 day period following heavy rainfall in June. Denitrification was greatest on days after rainfall events for the NT systems. Cumulative N loss was estimated at 25, 16, and 11 kg N/ha from May 29‐September 8 for NT, CH, and MP treatments, respectively, for the broadcast/incorporated N application. Mean denitrification rates from WT areas were about 1.6 times greater than the NWT areas.     

Effects of low root temperature on sap flow rate,soluble carbohydrates,nitrate contents and on cytokinin and gibberellin levels in root xylem exudate of sand‐grown tomato

Tomato (Lycopersicon esculuntum Mill.) grown in open fields in dry land areas or in non‐controlled greenhouses are subjected to substantial daily changes in root temperature. In the field, root‐zone temperatures fluctuate both diurnally and during the growing season. The purpose of this study was to monitor root‐zone temperature effects on tomato initial growth, transpiration, sap flow rate, leaf and air temperatures differences, nitrate accumulation, total nitrogen, and soluble carbohydrates in the shoot and roots as well as levels of endogenous cytokinins and gibberellins in xylem exudate. Tomato seedlings were grown in three growth cabinets with variable control of root temperatures. Three day/night root temperature regimes (12/12, 16/8 and 20/20°C) were employed. Low day root temperatures of 12 and 16°C reduced shoot dry weight by 47 and 26%, root dry weight by 36 and 14%, shoot nitrate by 79 and 50%, root nitrate by 49 and 16%, levels of cytokinins in root xylem exudate by 27 and 13% and gibberellins by 65 and 23%, in relation to the respective values of 20°C day root temperature. Soluble carbohydrates in the shoot and roots were increased significantly (18 and 111%) by 12°C root temperature. The main effects of low root temperatures on shoot growth stem from slow upward transport of plant hormones and nitrate rather than reduction in their rate of biosynthesis or entry to the root, respectively.     

Effects of iron nutritional status and time of day on concentrations of phytosiderophores and nico‐tianamine in different root and shoot zones of barley

The diurnal pattern in concentrations of phytosiderophores (PS) and its precursor nicotianamine (NA) was studied in different root and shoot zones of iron (Fe)‐sufficient and Fe‐deficient barley (Hordeum vulgare L. cv. Europa) grown in nutrient solution. Roots were separated into apical (0–3 cm) and basal zones (>3 cm) and shoots into young (3 cm basal zones of youngest two leaves) and old (remaining zones of youngest two leaves and oldest leaf) parts. The main PS in barley was identified as epi‐hydroxymugineic acid (epi‐HMA). Regardless of the sampling zone and time of day, epi‐HMA concentrations were several times higher in Fe‐deficient than in Fe‐sufficient plants and several times higher in the roots than in the shoots. In roots and shoots, epi‐HMA concentrations were always higher in the younger compared with the older zones. In both root zones of Fe‐deficient plants, an inverse diurnal rhythm occurred in epi‐HMA concentrations and in its release by the roots. In contrast, such a rhythm was absent in roots of Fe‐sufficient plants and in the shoots regardless of the Fe nutritional status. Nicotianamine concentrations in roots were not affected by the Fe nutritional status in apical zones but slightly enhanced under Fe deficiency in basal zones. In contrast to roots, NA concentrations in both shoot parts were lower in Fe‐deficient than in Fe‐sufficient plants. Regardless of the Fe nutritional status in roots and shoots, NA concentrations were higher in young than in old parts and no consistent diurnal variations were observed. The results suggest that PS are also synthesized in the shoot, although at much lower rates than in roots. As with roots, PS synthesis in the shoot is enhanced under Fe deficiency and is mainly localized in young growing tissue. The distinct diurnal rhythm in PS release in roots is apparently not regulated by variation in the rate of PS synthesis during the day.     

Effects of sulfur nutrition on photosynthesis in cadmium‐treated barley seedlings

The effects of sulfur (S) nutrition at 0.1 or 1 mM S on cadmium (Cd) toxicity measured by photosynthesis in barley (Hordeum vulgare L. cv. UC 476) seedlings were studied. Eight‐day‐old seedlings were treated with 25 μM Cd by adding cadmium chloride (CdCl₂) to the nutrient solution. Then photosynthetic carboxylation efficiency (ACi curve) and stomatal conductance of the primary and second leaves were measured at four and eight days after Cd treatment. Fluorescence parameters were measured every 24 h for eight days after two days of Cd treatment. At 20 days, plant growth parameters were measured and dry biomass determined. The results showed that ACi was significantly reduced by Cd, but more in the low (0.1 mM) S than in the high (1 mM) S‐treated plants. Stomatal conductance of plants was also decreased by Cd, but more in the low S‐treated plants. Low S‐treated plants exposed to Cd showed an increase in Fo and Fq, and a decrease in Fv/Fm and T_1/2, indicating photoinhibitory damage to PSII. Analysis of the growth parameters showed that Cd decreased plant size and biomass, but the reduction was more severe in the low S‐treated plants. These results support the hypothesis that S is a critical nutritional factor in plants which is important for the reduction of Cd toxicity.     

Effects of β-amylolysis on the resistant starch formation of debranched corn starches

Retrograded amylose is resistant to digestion by amylolytic enzymes, which is known as resistant starch type III (RS3). In this study we investigated the effect of β-amylase hydrolysis on the formation and physicochemical properties of RS3 from debranched corn starches. Three types of corn starch (Hylon VII, Hylon V, and common corn) were first gelatinized and then hydrolyzed using β-amylase to varying degrees. The resultant hydrolyzed starch was debranched with isoamylase and then exposed to temperature cycling to promote RS formation. A broad endotherm from approximately 45 to 120 °C and a small endotherm above 150 °C were noted for all retrograded starches. All three corn starches had increased RS contents after moderate β-amylolysis, with Hylon V having the highest RS content at 70.7% after 4 h of β-amylolysis. The results suggest that RS3 formation is affected by the starch composition as well as the starch structure and can be increased by moderate β-amylolysis.     

Effect of wastewater irrigation on mineral com‐position of corn and sorghum plants in a pot experiment

Effect of wastewater irrigation was investigated on mineral composition of corn and sorghum plants in a pot experiment. The ranges for the concentration of different minerals in corn plants were 0.67–0.89% calcium (Ca), 0.38–0.58% magnesium (Mg), 0.09–1.29% sodium (Na), 0.81–1.87% nitrogen (N), 1.81–2.27% potassium (K), 0.12–0.16% phosphorus (P), 190–257 mg/kg iron (Fe), 3.5–5.6 mg/kg copper (Cu), 37.1–44.5 mg/kg manganese (Mn), 21.6–33.6 mg/kg zinc (Zn), 1.40–1.84 mg/kg molydbenum (Mo), 11.0–45.7 mg/kg lead (Pb), and 2.5–10.8 mg/kg nickel (Ni). Whereas for sorghum plants, the ranges were: 0.56–0.68% Ca, 0.19–0.32% Mg, 0.02–0.27% Na, 0.69–1.53% N, 1.40–1.89% K, 0.10–0.14% P, 190–320 mg/kg Fe, 3.8–6.0 mg/kg Cu, 29.2–37.6 mg/kg Mn, 21.1–29.9 mg/kg Zn, 2.2–3.7 mg/kg Mo, 12.3–59.0 mg/kg Pb, and 2.5–15.2 mg/kg Ni. Heavy metals such as cobalt (Co) and cadmium (Cd) were below detection limits at mg/kg levels. The concentrations of Ca, N, K, P, Cu, and Mn in corn plants were in the deficient range except for Mg, Fe, Zn, and Al. The concentrations of Ca, N, P, K, Cu, Mn, Mg, and Zn in sorghum plants were in the deficient range except for Fe and aluminum (Al). The analysis of regression indicated a strong interaction between Pb, Ni, Ca, and Fe in corn and sorghum plants. In conclusion, waste water irrigation did not increase mineral concentrations of either macro‐ and micro‐elements or heavy trace metals in corn and sorghum plants to hazardous limits according to the established standards and could be used safely for crop irrigation.     

Effects of supplemental‐nitrogen on the quality of rice proteins

A study was conducted on the effect of supplemental nitrogen (N) (20 hg/ha) applied as a foliar spray or to the soil on seed production, protein percentage, and protein fractions of rice. Plants were grown in a greenhouse over two different periods of time, i.e., August 1988 to January 1989 (Period I), and December 1988 to April 1989 (Period II). Nitrogen was applied to the leaves 10 and 20 days after anthesis (DAA), and to the soil at anthesis and at 15 DAA. Average temperature was 28.7°C during Period I and 32°C during Period II, corresponding to 18.7 and 22.0 growing degree‐day/day (GDD/day), respectively. The difference in GDD/day reduced the plant cycle from 130 days during Period I to 109 days during Period II. Plants grown during Period II had larger numbers of spikelets, a higher percentage of “full grown grains”;, and higher grain weight. Although percentage crude protein was about the same for the two periods, prolamin content was increased and the albumin+globulin fraction was decreased during Period II, but with no difference in glutelin content. The increase in number of spikelets, percent full grains, and grain weight appeared to result in a greater energy demand for plants grown during Period II. This may explain the increase in prolamins, since prolamin synthesis requires less energy than globulin or albumin synthesis. There was a simultaneous decrease in albumin and globulin synthesis during Period II. The content of glutelins, which represent the major reserve proteins in rice grains, was constant during both periods.     

Response of corn to plowed‐down and disked‐in Zn as zinc sulfate

Abstract

A field investigation was conducted to compare the efficacy of plowed‐down and disked‐in Zn as ZnSO₄.H₂O in correcting Zn deficiency of corn (Zea mays L.). The soil, Buchanan fine sandy loam, was nearneutral in pH and contained 0.7 ppm of EDTA‐extractable Zn and 1.4 ppm of dilute HCl‐H₂SO₄ extractable P. Application of 6.72 kg Zn/ha as ZnSO₄.H₂O corrected Zn deficiency of corn plants on the soil. Corn grain yields and Zn concentrations in tissue samples indicated that the plowed‐down and disked‐in Zn were about equally effective in correcting Zn deficiency where the level of Zn application was 6.72 kg/ha.     

Effect of aluminum on soybean nodulation and nodule activity in a vertical split‐root system

Soybean plants (Glycine max L. cv Santa Rosa) grown hydroponically in nutrient solutions had reduced nodule mass and numbers in the presence of aluminum (Al). Reduced nodule number was attributed mainly to hydrogen (H) ion toxicity, whereas Al had a stronger effect on nodule growth. Using a vertical split‐root system with Al exclusively in the lower (hydroponic) layer also resulted in a significant reduction of nodulation and nodule growth in the surface compartment (vermiculite). This indirect effect could be attributed mainly to Al rather than H. Subsurface Al had no apparent effect on shoot growth or root growth of the upper compartment, but significantly limited root growth in the lower compartment where it was applied. The indirect effect of Al on nodulation could be a reflection of the abnormal root growth in the lower compartment. Split‐root experiments with a high Al soil, however, produced different effects. High Al in soil used exclusively in the lower compartment did not reduce nodule numbers or mass in the upper compartment despite being more harmful than the Al solutions to nodulation and growth of plants when used in a single compartment. Growth of roots in the subsurface compartment was also much less affected by the high soil Al compared with the Al‐containing nutrient solutions. Nodule activity, as estimated by xylem sap ureide levels, was only reduced after direct exposure of nodules to Al. A pronounced increase in the ratio of asparagine/glutamine occurred in all Al treatments where nodulation was reduced, and in some cases, there was an increase in total amino acid concentration of the xylem sap.     

Distribution of cations in leaves of salt‐tolerant and salt‐sensitive lines of sunflower under saline conditions

Two lines of sunflower (Helianthrus annuus L.), a salt‐tolerant Euroflor and a salt‐sensitive SMH‐24, were grown for three weeks in sand culture containing 0 or 150 mol/m³ NaCl in full strength Hoagland nutrient solution. Distribution of cations in the leaves of varying ages was determined. The older leaves of SMH‐24 contained more sodium (Na) in the laminae than the younger leaves at the salt treatment, whereas laminae of leaves of varying ages of Euroflor maintained Na concentration almost uniform. Distribution of potassium (K), calcium (Ca), and magnesium (Mg) in the laminae was strongly age‐dependent in both lines, i.e., the older leaves contained greater concentrations of these cations than did the younger leaves. The lines did not differ in concentrations of the three cations. The older leaves of SMH‐24 had significantly lower K/Na ratios than those of Euroflor, but the lines did not differ in lamina Ca/Na ratios. It is concluded that distribution of K, Ca, and Mg in the leaf laminae is age‐dependent. Salt‐tolerance in sunflower is related to exclusion of Na in the leaf laminae and to maintenance of almost uniform concentrations of this ion in leaves of all ages.     

Effects of media on early‐season sweet potato transplant production in heated beds

‘Georgia Jet’ sweet potato transplants were produced in heated beds to determine the effects of aged pine sawdust and builders’ sand (traditional media) and fresh and aged pine bark, and fresh pine sawdust (alternative media), on early‐season transplant production and media characteristics. At the first harvest, higher transplant numbers were obtained with the fresh pine sawdust or pine bark (mean 1,053 transplants/m²) than with the other media (mean 619 transplants/m²). Transplant fresh weights at the first harvest responded in a similar manner. The total early‐season number of transplants obtained with fresh pine bark (1,455 transplants/m²) was also greater than the total number of transplants obtained with the traditional media and aged pine bark (mean 951 transplants/m²), but not greater than the total number of transplants obtained with fresh pine sawdust. Mean lengths and weights of transplants obtained with the alternative media were at least as great as the values obtained with transplants grown in the traditional media. Highest media pH values were obtained with builders’ sand; the pH values of the fresh pine media were less acidic than the pH values of the aged pine media. Lower volumetric water contents and greater fluctuations in temperature were obtained with builders’ sand than with the pine media. Media had no effect on the percentage of intact roots at the end of the experiment.     

Effects of Fertilizer Nitrogen on Short‐Term Nitrogen Loss in Bypass Flow in a Vertisol

Bypass flow, the vertical flow of free water along the walls of macropores or preferential flow paths in the soil, can lead to movement of fertilizer nutrients beyond the reach of plants. Fertilizer type and the rate of application, as well as the amount, frequency, and intensity of rainfall, can influence the amount of fertilizer nitrogen (N) loss in leaching or bypass flow. The effect of fertilizer N form and rate of application on N recovery in bypass flow in a Kenyan Vertisol was determined. Calcium nitrate and ammonium sulfate, used to supply nitrate (NO₃ ^?)‐N and ammonium (NH₄ ⁺)‐N, respectively, were surface‐broadcast to 40‐cm‐long undisturbed soil columns at equivalent rates of 50, 100, and 200 kg N ha^?1. Using a rainfall simulator, two rainfall events (30 mm of water applied in 1 h) were applied to the soil columns, one before and the other after fertilizer application. Total N, NO₃ ^?‐N, and NH₄ ⁺‐N concentrations in the bypass flow were determined after the second rainfall event. The application of NH₄ ⁺‐N, regardless of the rate, had no effect on N recovery in the bypass flow. When nitrate N was applied, the amount of fertilizer N recovered in the bypass flow significantly increased with the rate of NO₃ ^?‐N application. Of the total N in the bypass flow, 24 to 48% was derived from the soil, the bulk of which was organic N. It is concluded that following the application of NO₃ ^?‐N, bypass flow is an important avenue of loss of both fertilizer and soil N from Vertisols.     

Effects of hydrogel incorporation in peat‐lite on tomato growth and water relations

Abstract

The effects of subirrigation and cylical (surface) irrigation on shoot growth and water stress of tomato grown in peat‐lite amended with 0, 4, 8, 12, and 16 kg Hydrogel/m were examined. While shoot growth showed a quadratic response to increasing Hydrogel rate with maximal growth occurring at the manufacturer's recommended rate (8 kg/m³), growth was greater with cyclical irrigation than with subirrigation. Subirrigation generally resulted in greater plant water stress than cyclical irrigation, but with ≤ 8 kg Hydrogel/m³ this differential irrigation effect diminished temporally. Initially, each kg Hydrogel/m³ increased peat‐lite water content by 9 percent but after 31 days, this value had increased to 27 percent and 16 percent with subirrigation and cyclical irrigation, respectively. The continued hydration resulted in a bulk volume increase sufficiently large with subirrigation to cause the Hydrogel‐amended peat‐lite to overflow the pots.     

Effects of polyolefin‐coated fertilizer on nutritional quality of spinach plants

Pot experiments were conducted in order to evaluate the effects of four different urea or ammonium containing polyolefin‐coated fertilizers (POCFs) on the nutritional quality of spinach (Spinacia olèracea L.) and to investigate the mechanisms of these effects in comparison with conventional, rapidly available fertilizer. Compared to the conventional fertilization method yield was decreased in all the four POCF treatments due to less available fertilizer nitrogen (N) and/or realized ammonium nutrition. However, application of POCFs decreased oxalate and nitrate contents and increased ascorbate concent in spinach. Decreased oxalate and nitrate contents were attributed to lower nitrate availability in the soil having caused by the controlled‐released characteristic of POCFs and/or ammonium nutrition. Increased ascorbate content was due to both decreased oxalate and decreased nitrogen contents of the spinach plants. It was concluded that band applications of urea or ammonium containing POCFs improved the nutritional quality of spinach due to realized ammonium nutrition and/or less amount of available fertilizer N.     

Determination of total sulphur in soil using in‐ductively coupled plasma‐atomic emission spectrometry

Abstract

Total sulphur in soils was determined by ignition with a NaHCO₃/Ag₂O mixture, extraction with HC1 and measurement of sulphur by inductively coupled plasma‐atomic emission spectrometry. Significant spectral interference from other extracted elements (mainly iron and aluminium) was corrected using an off‐peak background reading. Recovery of sulphur added to soil as K₂SO₄ was complete and barium did not interfere when added in amounts equivalent to 2000 μg g^‐1 soil. The total sulphur values for a range of New Zealand pastoral soils determined by this method were on average 104% (SEM=1,4) of the values obtained by the NaOBr oxidation method of Tabatabai & Bremner (1). The method enables analysis of relatively large sample numbers.     

Effect of extractable zinc,phosphorus, and soil ph on zinc concentrations in leaves of field‐grown corn

Abstract

The variability in corn yield responses to applications of Zn fertilizer appears to be associated with several complex soil and climatic factors that affect the availability of endogenous soil Zn to the crop under specific conditions. Among the soil chemical properties that influence availability of endogenous Zn are soil pH, organic matter content, and extractable P. Over a period of several years, soil and plant analysis data were collected from 54 field experiments, field trials, and diagnostic visits to producer's fields. These data were subjected to multiple regression analysis, resulting in an equation: Zn_leaf = 37.14 + 1.513 Zn_st ‐4.04 pH_st ‐ 1.791 ln(P_st/100) where Zn_st, pH_st, and P_st were 0.1N HC1 extractable soil Zn (kg/ha), 1:1 soil‐water pH, and Bray's 1 extractable soil P (kg/ha), respectively. These factors accounted for 67% of variation in leaf Zn, which was a large portion of the variability in Zn_leaf considering that climatic conditions, management levels, and varietal differences were uncontrolled in most instances. Using the previously published critical level in the leaf opposite and below the ear as 17 μg Zn/g, these data can be used to set required soil test levels of Zn at different levels of extractable P and soil pH. Inadequate levels of extractable Zn would range from 2.5 (at pH 6.0, P = 70 kg/ha) to, 9.5 kg/ha (at pH 7.5, P = 420 kg/ha).     

Influence of nitrate‐nitrogen and ammonium‐nitrogen on calcium (45Ca2+) absorption in sorghum root tips

Abstract

Root‐tip, 1‐cm of Sorghum bicolor (L.) Moench cv SC283, SC574, GP‐10, and Funk G522DR were exposed to calcium (⁴⁵Ca²⁺) at pH 5.5 for 2‐hr in the presence of nitrate‐nitrogen (NO3^?‐N) or ammonium‐nitrogen (NH4+‐N). Nitrate (0.1 mM) induced significantly increased ⁴⁵Ca uptake in Funk G522DR, SC283, and GP‐10 while 0.01 mM NO₃ ^?‐N induced significantly increased ⁴⁵Ca'uptake in SC574, but ⁴⁵Ca absorption was significantly decreased at 1 mM NO₃—N. In the presence of the NH4⁺ ion, ⁴⁵Ca uptake was increased up to 8X that of the NH₄ ⁺‐N untreated roots. When ammonium chloride (NH₄CI) was used, the Cl^? tended to induce an increased ⁴⁵Ca uptake. Cultivar variation was present.     

Quantity‐intensity parameters of potassium in relation to uptake by guinea‐corn in representative soils of the ecological zones of Nigeria

Abstract

Quantity‐intensity (Q‐I) relation studies were often used to supplement information obtained from conventional soil tests for the estimation of potassium (K) needs of crops. With a view to ascertaining the reliability of the Q‐I relation parameters for comprehensive characterization of K dynamics in typical Nigerian soils, K values derived from Q‐I isotherms were related to neutral normal ammonium acetate (1 N NH₄OAc, pH 7.0) (exchangeable) K, other soil K forms [non‐exchangeable (K_ne), exchange (K_e), mineral K (K _m ), and solution K (K_s)] and the K uptake by Guinea‐corn (Sorghum bicolor, var. LS 187) subjected to weekly cuts in Neubauer cultivation vessels. Most of the soil K (about 98%) was in the form of soil minerals while less than 1% was plant available whereas about 1% was trapped within the interlattice layers of the clay minerals (as fixed K or K_ne). Mineral K (K_m) content was closely related to total K (K_t), but not to the other forms, K_ne, K_e, and K_s. A close relationship was noted between the two components of labile K (K_e and K_s). Except for % K saturation, the relationships between the K measurements with plant response were poor. The results of these investigations clearly demonstrate that the Q‐I relation could not adequately characterize the K dynamics of these tropical soils.