Uptake Of Corn Research Articles

Nutrient uptake of corn grown using moldboard plow or no‐tillage soil management

Abstract Crop nutrition under different tillage systems continues to be an issue. Corn (Zea mays L.) growth and nutrient (N, P, K, Ca, Mg) uptake were monitored for two years under either no‐tillage or conventional (moldboard plow plus disc) tillage management. Two rates of applied N (84, 168 kg N ha‐1) were imposed as well. The crop was planted in mid‐May, and sampled about 60 days after planting and at crop maturity. Grain yields were generally increased by the higher N application rate and by tillage reduction. Phosphorus, calcium, and magnesium uptake were generally unaffected by treatment. Total nitrogen recovery by the crop was lowered by no‐tillage soil management at the 84 kg N ha‐1 application rate, but was equal across tillage treatments at the 168 kg N ha‐1 rate. Potassium acquisition by the crop was strongly related to the surface stratification of K in the no‐till soil environment.

Nitrogen Utilization of Corn Under Minimal Tillage and Moldboard Plow Tillage. I. Four‐Year Results Using Labeled N Fertilizer on an Atlantic Coastal Plain Soil1

AbstractCrop responses and N uptake of corn (Zea maysL.) grown with moldboard plow tillage (PT) and minimal tillage (MT) were measured in a 4‐yr field experiment on a Mattapex silt loam soil (finesilty, mixed, mesic, Aquic Hapludult) near Salisbury, MD (Atlantic Coastal Plain region). Labeled ammonium sulfate or ammonium nitrate (15N depleted) was applied annually to the entire area (74 m2) of each plot at five N rates ranging from 0 to 180 or 270 kg N ha−1. Corn dry matter yield, N content, and isotope composition were determined in the entire plant at the 11 to 12 leaf stage and at silage harvest, as well as in the corn grain at maturity. Below 135 kg N ha−1, dry matter yields, total N uptake, and fertilizer N (FN) uptake were greater with PT than with MT, but with 180 kg N ha1−1there was either no difference between the tillage systems or MT was greater than PT. Fertilizer N additions had no consistent or practically significant effect on soil N (SN) uptake. At 135 kg N ha−1, average crop FN recoveries were 62% for PT and 53% for MT, but this difference decreased with increasing N rate until both tillage systems averaged about 53% recovery at 180 kg N ha−1. Average SN uptake was greater with PT culture than with MT culture, averaging 76 kg N ha−1and 59 kg N ha−1, respectively. The crop N requirement of corn was about 17 kg N ha−1greater for MT corn than PT corn due to a 10% greater yield of total dry matter with MT. Corn grown with MT required about 68 kg more FN ha−1than PT, due to a lower uptake of SN and a greater crop N requirement. A lower uptake of N on MT plots during the second half of the growing season suggests (i) a greater net mineralization of N on PT plots and/or (ii) a greater loss of N via leaching and/or denitrification on MT plots. A nitrogen concentration of about 11 to 12 mg N g−1in the silage total dry matter, or about 15 mg N g−1in the grain, can be used as an approximate value for N sufficiency. However, these values should be considered broad guidelines since significant deviations were observed in individual years.

Allelopathic Effects of Annual Weed Residues on Growth and Nutrient Uptake of Corn and Soybeans1

AbstractAnnual weeds have shown allelopathic potential in recent years. Few studies clearly compare effects of weed residues on growth inhibition and nutrient uptake in corn (Zea maysL.) and soybeans [Glycine max(L.) Merr.]. Therefore, the residues (aboveground biomass) of common lambsquarters (Chenopodium albumL.), redroot pigweed (Amaranthus retroflexusL.), common ragweed (Ambrosia artemisifoliaL.), velvetleaf (Abutilon theophrastiMedic.), and yellow foxtail [Setaria glauca(L.) Beauv.] were evaluated for their allelopathic effects on growth and uptake of N, P, and K in corn and soybeans in controlled environments. Weed residues in silica sand produced more inhibitory effects than the mixture of soil (Typic Agriudoll) and sand. The growth inhibition was more (8 to 14%) when the residues were incorporated in all soils than when they were surface applied. All residues except common ragweed reduced corn (6 to 20%) and soybean (2 to 20%) dry matter production. The inhibition or stimulation of N, P, and K uptake in both crops was not consistent and depended on the residue source, residue placement, or soil texture. Addition of supplemental N and P to various residues provided no alleviating effect on crop growth. Addition of P markedly enhanced P uptake without affecting total dry weights of both test species when compared to the controls. Yellow foxtail and common ragweed residues increased K uptake (21 to 48%) in corn but not in soybeans. The weed residues demonstrated an allelopathic inhibition of growth independent of N and P nutrition, and suggest that the inhibitory effects are not related to nutrient uptake.

Silage Corn Uptake of Sludge‐Applied Zinc and Cadmium as Affected by Soil pH

AbstractLiming is often recommended to control metal uptake in plants. In western Washington, where silage corn (Zea mays L.) is often fed to dairy cattle, studies were conducted to determine the effect of liming on the uptake of Zn, and especially Cd from land‐applied sewage sludge. Two soils, a Sultan silt loam and a Puyallup fine sandy loam were amended annually for 2 y with anaerobically digested sewage sludge, and seeded to silage corn. Half of the plots were limed, which increased soil pH from 4.6 to 6.5 for both soils. In general, liming reduced Zn uptake in most corn tissues, whereas Cd uptake was generally unaffected. Concentration of Zn and Cd in corn leaves from both soils increased significantly with increased sludge rate. Liming significantly lowered leaf Zn, although leaf Cd was not significantly decreased by liming. In fact, at higher sludge application rates, leaf Cd was greater from limed than unlimed plots for the Puyallup soil in 1976. Kernel Zn and Cd concentrations for both soils generally increased with sludge rate. For the Sultan soil, kernel Cd concentration at silage harvest was similar, regardless of lime treatment, whereas Cd values for the limed Puyallup soil increased significantly over those from unlimed soil in 1976. For both soils, Cd and Zn concentrations followed the order: leaves > stover > cobs > kernels. Leaf and DTPA‐extractable Zn/Cd ratios from limed plots were lower than those from unlimed plots. These results suggest that liming to pH 6.5, as recommended in a number of sludge application guides, would not reduce Cd uptake by corn in the soils studied.

Influence of Ca, pH and humic acid on Cd uptake

Solution culture experiments were conducted to examine the effect of naturally occurring compounents of soil solutions such as Ca-ion, H-ion and organic acids on the Cd uptake of corn and snap beans. An increase in the Ca-ion concentration of solution cultures depressed the translocation of Cd from roots to tops of snap beans and corn but had no apparent deffect on the absorption of Cd by roots. Suppression of Cd translation by Ca was less marked for the corn than for the beans. No change in Cd absorption or translocation in corn was noted for solution pH values ranging from 4 to 7. Addition of humic acid to the solution decreased the Cd activity in soolution and the subsequent absorption of Cd by corn roots, indicating that Cd-ion activity in solution directly affectes Cd uptake. The addition of humic acid had no effect on Cd translation in corn plants.

Heavy Applications of Liquid‐Digested Sludge on Three Ultisols: II. Effects on Mineral Uptake and Crop Yield

AbstractA study was made of the nutrient uptake of corn (Zea mays L.), grain sorghum (Sorghum bicolor (L.) Moench), and soybeans (Glycine max L.) grown on land to which liquid‐digested sludge (sludge) containing 2.6% solids had been applied as a source of plant nutrients. The sludge was applied over a 6‐year period to give a total of 0, 87, 174, 241, 288, and 335 metric tons/ha dry matter. Corn did not contain Pb in the leaves or grain but the leaves contained up to 2.0 ppm Cd and 0.23 ppm Hg when high levels of sludge were applied. Cadmium and Hg were not detected in the corn grain. No measurable amount of Cd and Pb were found in the leaves or grain of grain sorghum, however Hg accumulated in the leaves up to 0.06 ppm. Soybeans took up Cd and Hg in the leaves but none accumulated in the seed. Some soybean seedlings became chlorotic and died at the 288 and 335 metric tons of dry matter/ha treatments. These seedlings contained 963 and 1,188 ppm Zn and 484 and 725 ppm Mn, respectively, probably causing the soybean plant injury. Soybean leaves taken from the 335 metric tons/ha treatment at the R5 stage contained 619 ppm Zn and 220 ppm Mn, and corn leaves taken at the hard‐dough stage contained 707 ppm Zn and 458 ppm Mn, but neither showed any detrimental foliar symptoms.The yield and plant data analyses indicated that annual applications of sludge from Pensacola, Fla., equivalent to 28 metric tons dry matter/ha would be an acceptable rate of disposal by land‐spreading for corn, grain sorghum, and soybeans grown on three Paleudults: Orangeburg, Troup, and Lucy soils.

N Uptake and Corn Yield as Affected by Applications of Nitrapyrin with Anhydrous Ammonia1

AbstractThe use of nitrapyrin as a nitrification inhibitor of ammonium fertilizers applied to corn (Zea mays L.) has gained widespread interest. Even though initial nitrification rates are usually decreased by nitrapyrin, its use does not always result in increased grain yield. To evaluate the effects of nitrapyrin applied with anhydrous ammonia on yield and N uptake of corn grown on poorly drained soils, field studies were conducted in Illinois at Urbana, Brownstown, and DeKalb. The soil at Urbana and DeKalb was Typic Haplaquoll and Mollic Albaqualf at Brownstown. Anhydrous ammonia was applied with and without nitrapyrin in the fall and spring. Nitrapyrin and N rates ranged from 0 to 1.12 and 0 to 268 kg/ha, respectively.The addition of nitrapyrin had varied effects on N concentration in the plant tissue and on grain and total dry matter yield; however, these effects were not consistent among N rates, locations, or years. At Urbana, the addition of nitrapyrin increased the N concentration in the plant tissue at the fifth‐leaf growth stage in 1975 by as much as 7% and in the ear leaf at silk in 1976 by as much as 6%, when comparing within N rates and seasons of application. The N concentration in the grain and stover was reduced by as much as 8 and 27%, respectively, by the addition of nitrapyrin at low rates of N. At DeKalb in 1975, nitrapyrin did not affect N in the plant tissue. In 1976, applications of nitrapyrin with fall‐applied N increased N in the ear leaf up to 20% over fall‐applied N without nitrapyrin, and increased it up to the concentration found for spring‐N applications with or without nitrapyrin. At Brownstown in 1975, plants did not respond either to applied N or nitrapyrin. In 1976, plants responded to nitrapyrin, but adverse weather resulted in data that did not follow expected trends.Grain and stover‐yield increases could not be attributed to applied nitrapyrin when N was applied at levels required for maximum economical yield. Soil moisture during both years was not favorable, however, for N losses that would occur normally through leaching and denitrification.

EFFECTS OF MANURES, AMMONIUM NITRATE AND S-COATED UREA ON YIELD AND UPTAKE OF N BY CORN AND ON SUBSEQUENT INORGANIC N LEVELS IN SOILS IN SOUTHERN QUEBEC

Effects of added N in the form of ammonium nitrate, S-coated urea, solid cow manure, liquid hog manure, and liquid hog manure plus straw on yield and N uptake of corn (Zea mays L.) were determined in 1971. Residual effects of the fertilizers were measured in 1972. Levels of added N were 150 kg/ha, on three soils varying in texture. In 1971, highest yield and N uptake was found using ammonium nitrate, followed by S-coated urea. The manures were not different than the control, and the average values with liquid manure plus straw were generally lowest. Ammonium nitrate treatments had the lowest mineral N levels in the soil profile in September. Thus, ammonium nitrate was considered to have the lowest pollution potential, assuming that fall-accumulated inorganic soil N was a hazard for pollution of groundwater.

Evaluation of phosphorus in forest soils: Comparison of phosphorus uptake, extraction method and soil properties

Phosphorus in soils from plantation of red pine (Pinus resinosa Ait.) was determined using six extractants: 0.002N H2SO4 (pH 3.0); 0.025N HCl+ +0.03N NH4F; 0.5N NaHCO3 (pH 8.5);N NH4OAc (pH 4.8); anion exchange resin (Dower −2, Cl-form); H2O. Correlations of extractable P with Al- and Al-+Fe-P indicated that these fractions are the dominant forms of inorganic P in most of the soils. Uptake of P by corn and Monterey pine seedlings grown in greenhouse culture was correlated with soil P extracted by the different methods. The most successful of the extractants for predicting P uptake was resin extractable P; the simple correlation coefficients were 0.811 and 0.609 for pine and corn respectively. P uptake by pine correlated significantly with 0.002N H2SO4 P (r=0.679),N NH4OAc P (r=0.443), H2O P (r=0.549) and Al-+Fe-P (r=0.532) while P uptake by corn correlated with 0.002N H2SO4 P (r=0.579), H2O P (r=0.477) and organic P (r=0.460). Per cent P in pine seedling tops correlated significantly with 0.002N H2SO4, resin andN NH4 OAc extractable P. Multiple regressions which included silt+clay and organic P improved correlations of some soil tests with P uptake in corn and pine seedlings respectively.

The Effect of Oil Pollution of Soil on Germination, Growth and Nutrient Uptake of Corn

AbstractThe effect of crude oil pollution of soil on the growth of plants and uptake of nutrients was investigated by growing corn (Zea mays L.) on a soil polluted by crude petroleum. The levels of the crude oil application varied from 0 to 10.6% by weight of soil. Three corn crops were raised in succession, each for a period of 6 weeks, in the same soil. The yields and plant contents of N, P, K, Ca, Fe, and Mn were determined. The soil was analyzed for organic C, total and available N, extractable P, and exchangeable K, Ca, Fe, and Mn after each cropping. Germination and yields were drastically reduced as the level of pollution increased. At 4.2% crude oil pollution level, the average reductions were 50% and 92% in germination and yield, respectively. The amount of organic C, total N, and exchangeable K, Fe, and Mn increased in the soil with level of crude oil addition, while extractable P, NO3‐N, and exchangeable Ca were reduced. The poor growth was attributed to suffocation of the plants caused by exclusion of air by the oil or exhaustion of oxygen by increased microbial activity, interference with plant‐soil‐water relationships, and toxicity from sulfides and excess available Mn produced during the decomposition of the hydrocarbons.

Nitrate Uptake Effectiveness of Four Plant Species

AbstractLittle information is available on the characteristics of plant roots that determine their effectiveness in nitrate uptake. The effectiveness of nitrate uptake of corn (Zea mays L.), soybeans (Glycine max L.), sorghum (Sorghum bicolor L.), and bromegrass (Bromus inermus L.) intact roots were investigated in nutrient solution culture. The maximum uptake rate per centimeter of root for corn occurred at 10 mM, for sorghum at 2.4 mM, and for bromegrass at 0.8 mM. Increasing the nitrate level above 1.0 mM did not increase the growth rate during the first 3 weeks for any of these species.The minimum level to which the plant roots reduced the nitrate concentration was 1.7, 2.7, 2.4, and 1.4 µM for forage sorghum, grain sorghum, soybeans, and bromegrass, respectively. Three cultivars of corn were compared. Two reduced the nitrate level to 2 µM and the third to 4 µM.The results of this research indicate that the roots of the species investigated absorbed nitrate of maximum rates from relatively low nitrate concentrations provided the concentration was maintained. Also, the degree of reduction in nitrate level where nitrate in solution was not maintained indicated that these plant roots had the absorptive capacity to reduce solution nitrate to concentrations of 4 µM or less.

Influence of Water Management Through Irrigation and a Subsurface Asphalt Layer on Seasonal Growth and Nutrient Uptake of Corn1

AbstractDuring the past 5 years improved water management of droughty sandy soils has been attained by constructing, at a depth of 60 cm in the soil profile, a continuous layer of asphalt about 0.3 cm thick. Plots (60.9 × 60.9 m) on a Typic Quartzipsamment (Lakeland fine sand) were treated in 1967 and used in 1970 and 1971 to measure seasonal growth and nutrient uptake of corn (Zeamays L.) in response to the following water management treatments: check, irrigation, asphalt layer, and irrigation with asphalt layer. Responses to fertilizer rate and plant population were also obtained.The asphalt layer increased corn fodder yields above the check and irrigation treatments during the first 11 weeks when moisture stress was moderate. At this time an 11‐day drought caused a severe water stress on the check and asphalt layer treatments resulting in grain yields of 5140 and 5020 kg/ha, respectively, whereas the irrigated and irrigated with asphalt layer treatments produced 7590 and 8164 kg/ha, respectively. The aphalt layer reduced the need for supplemental water and maintained or improved the N and K contents of corn fodder during periods of moderate stress. Fertilizer rates did not affect corn grain yield significantly. The lowest rate, 224‐73‐187 kg/ha of N‐P‐K, produced 6378 kg/ha. The highest yield, 681.5 kg/ha of grain, was obtained for the plant population density of 47,000 plant/ha as compared to 5994 kg/ha and 6552 kg/ha for 32,000 and 81,000 plants/ha, respectively.The seasonal response of corn to water management showed that the asphalt layer system was effective in increasing water use efficiency in well‐drained sandy soils.

Accumulative Effects of Manure and N on Continuous Corn and Clay Soil. II. Chemical Changes in Soil1

AbstractContinuous corn (Zea mays L.) was grown on a Panton clay soil (Typic Ochraqualf). Four rates of manure (0, 22, 44, and 66 tons/ha) and five rates of N (0, 56, 112, 168, and 224 kg/ha) were combined in a factorial arrangement and applied each year to the same plots. Also, 120 kg/ha of P and 90 kg/ha of K were applied for 4 years to all plots. Maintenance of structure and fertility had been difficult in this soil. It had not been intensively cultivated even though it lay in large, level fields.The objectives were to study the effects of the treatments on growth, yield, and nutrient uptake of corn and on the physical and chemical properties of the soil over a period of 5 years (six crops). Results of the first objective have been previously published (7) and effects chemical properties will be reported here.Cultivation and cropping for 5 years on the check plots reduced organic matter; (by N) 8.7% and OM (by 17.7%. An annual application of 44 tons/ha of fresh dairy cattle manure was needed to maintain soil OM. CEC was increased by manure treatments as would be expected from increase in soil OM.Soil test results (pH 4.8, NH4OAc extractions) showed that P from manure was more available than fertilizer P; 119 kg/ha of P from manure increased soil test P about the same as 480 kg/ha of P from fertilizer. However, K, either from manure or fertilizer, was readily fixed by the soil (60 to 75% of the applied). More than 414 kg/ha of K was needed to bring K potential in soil to the −3,500 calorie level suggested as adequate for standard free energy of exchange of K for Ca and Mg (13).Manure application up to 44 tons/ha did not contain enough Ca or Mg to maintain initial levels in the soil. Manure and N treatments had no measurable effects on Fe and Na levels in the soil. Averaged over all treatments, exchangeable Na decreased from 123 to 85 kg/ha and Mn from 45 to 20 kg/ha during the 5 years (1966 to 1971). Treatments of N did not affect rate of breakdown of OM nor decrease pH. The highest rate of N, compared to check plots, slightly increased Mn extracted (by 6 kg/ha). The highest rate of manure decreased extractable Mn (8 kg/ha) and AI (5 kg/ha).These data showed the value of fresh manure as a soil amendment in addition to its nutrient content. Land applications of manure should be encouraged as a method of disposal but its agronomic value emphasized to encourage use with a minimum of loss.

Accumulative Effects of Manure and N on Continuous Corn and Clay Soil. I. Growth, Yield, and Nutrient Uptake of Corn1

AbstractFour rates of manure and five rates of N were applied yearly in a factorial arrangement to a Panton clay soil (Typic Ochraqualf). Applications of 120 kg/ha of P and 90 kg/ha of K were made each year to all plots. The plots were planted to corn (Zea mays L.). The objectives of the study were to study the effects of continuous corn and manure and N treatments on the physical and chemical properties of the soil and on the growth, yield, and mineral composition of the corn plants over a period of at least 5 years. The results for the latter objective are reported in this paper.During years of normal or less than normal rainfall (1965–67), manure application significantly increased yields of corn grain and stover. Averaged over all treatments of N, plots receiving 66 tons/ha of manure yielded 2.04 tons/ha more than no‐manure plots. This response is especially significant in that basic applications of P and K had been made and starter P used at planting each year. For these years, the first increment of N (56 kg/ha) increased yields on unmanured plots. Higher applications of N reduced yields. Where manure was applied, there was no significant response to N.During relatively wet years (1968–69), manure had beneficial effect on corn growth and yield. The highest, or 66 ton/ha, manure application appeared to be detrimental, particularly where N had been applied. Nitrogen, on the other hand, significantly increased growth rate and yield of corn grain and stover at all levels of manure. For example, plots receiving 224 kg/ha of N yielded 5.51 tons/ha more than no‐N plots.Manure treatments increased percentage K by as much as 0.30% in the corn ear leaves but decreased Ca and Mg. Manure had little effect on percentage N and P. In 1969, a wet year, manure reduced N from 2.72 to 2.44% when averaged over all treatments of N.Chemical analysis of small plants showed the same trends as did analysis of ear leaves. Percentages of AI and Fe were high. But Al varied proportionally to applied N and inversely to manure. For all treatments of N, manure decreased Al from 139 to 85 ppm.Manured plots were slightly but consistently higher in soil moisture. Small differences of about 1% were measured when the soil was near saturation (43% moisture). Differences of 2.5% were measured 1 week later when the soil was near 30% moisture.

The Effect of Linuron on Ion Uptake in Corn, Soybean, and Crabgrass

An increase in Ca and SO4 uptake from nutrient solution in juvenile corn (Zea mays L. var. N. J. 9), soybean (Glycine max (L.) Merr., var. Kent) and large crabgrass (Digitaria sanguinalis (L.) Scop.) was obtained with 3-(3,4-dichloroyhenyl)-l-methoxy-l-methyl-urea (linuron) treatment. The uptake of K, Mg, NO3, and PO4 also was followed and varied with the ion and species.

Relative Efficiency of Several Zinc Sources for Corn (Zea mays L.)1

AbstractCorn was grown on two soils in greenhouse experiments to determine the relative efficiency of various Zn sources at different levels of lime and P applications. Plant deficiency symptoms, growth, (oven‐dry weight of corn tops), height, Zn content, and Zn uptake were evaluated.Corn growth was increased by Zn fertilization on both soils. No differences were found among the Zn sources on the Red Bay soil where only a slight response to Zn was obtained. Increases in growth up to 154% were obtained from Zn fertilization on the Norfolk soil. ZnSO4 was slightly more effective than ZnO or Zn chelate and organic (polyflavonoid) Zn was the least effective source. ZnSO4 and ZnO were equally efficient or superior to chelate and organic Zn in increasing the Zn content and uptake of corn on the basis of amounts used.Liming resulted in decreased corn growth, plant Zn content, and Zn uptake, especially when no Zn was applied. High P fertilization decreased the Zn content of corn but tended to increase growth when applied with Zn.

On the Mechanism of Phosphorus‐Zinc Interaction in Corn Nutrition

AbstractStudies were carried out in the greenhouse and field to elaborate the mechanism of P‐Zn interaction in the nutrition of corn (Zea mays L.). From this work depressive action of P on Zn uptake of corn appears to be largely physiological in nature, expressed at root surfaces and/or in root cells, and is not chemical inactivation of Zn by P in soil. Translocation of Zn from roots to tops is inhibited by elevated P concentration, with resulting sharp reduction in Zn concentration of nodal and internodal tissues. No clearly definable P/Zn ratio in tissue was found above which yield restriction could be predicted. Corn seems to tolerate high concentrations of P in its tissues provided some modest quantity of Zn is present.Other elements counteract somewhat the damaging effects of P. Concurrently placed N promotes Zn uptake at the same time that it benefits P utilization. Increased level of native or applied K reduces the depressive effects of P on Zn.

Corn Response to Time and Rate of Phosphorus Applications

AbstractA greenhouse experiment was conducted to study the effects of rate and time of P applications on the growth and P uptake of corn. Different initial rates of P were applied and supplementary P was applied to make all treatments equal to the highest initial rate of 105 pounds P per acre. These supplementary applications were made to one set of each initial treatment at weekly intervals for 7 weeks after planting.In most cases the yield was lower if the supplementary P was not applied within 4 weeks after planting. The higher the initial rate of P the longer the supplementary application could be delayed without yield reduction. In most instances the total P uptake was greatest when the supplemental P was applied 5 or more weeks after planting.A critical period no doubt exists in the development of corn before which P must be available if it is to increase plant growth. If P becomes available after this critical period it will be absorbed by the plant and will be reflected in a higher percentage P composition.

The Effect of Soil Water Movement vs. Phosphate Diffusion On Growth and Phosphorus Content of Corn and Soybeans

AbstractThe effect of soil water movement vs. phosphate diffusion on plant growth and composition was studied by soil‐sand dilution and continuous movement of the soil solution by recirculation. Yield, percent P, and P uptake of corn and soybeans remained approximately constant at soil‐sand dilutions up to 1:1.1 by volume, but decreased at greater dilutions in both the recirculated and noncirculated (normal watering) systems. Recirculation increased yield and percent P of corn and soybeans. These increases were attributed partly to the sampling of a larger soil volume for replenishment of the soil solution. The diffusion process alone was unable to renew P at the root surface as fast as the observed uptake rate. Soil water movement probably accounts for a much greater transfer of P to the root surface than diffusion.