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a Dep. of Agron., Purdue Univ., West Lafayette, IN 47907-1150
b Dep. of Plant Agric., Univ. of Guelph, Guelph, ON, Canada N1G 2W1
Corresponding author (tvyn{at}purdue.edu)
Received for publication January 3, 2000.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CT, conventional tillage • NT, no tillage • ZT, zone tillage
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Potassium stratification associated with NT is of concern because adequate K nutrition for corn is not only dependent on the available K concentrations in the bulk soil, but also on the availability of K in soil volumes where roots are actively growing during periods of rapid uptake. Most K uptake occurs before pollination, and uptake corresponds closely to corn vegetative growth. For example, Hanway (1962) reported that 38% of the total K uptake by corn for the whole growing season occurred 38 to 52 d following planting. Any deficiencies in K availability in soil volumes that are actively exploited by corn roots during the rapid dry matter accumulation phase of corn growth before pollination can result in inadequate K nutritional status and may result in reduced yields (Heckman and Kamprath, 1992). Potassium stratification may increase the likelihood of inadequate K nutrition because K is concentrated close to the surface where soil may be too dry for optimal root function. During years with extended early season dry periods, this can lead to low K concentrations in corn tissue and loss of yield potential.
MacKay et al. (1987) suggested that placing K deeper in the soil profile in stratified NT fields should minimize the likelihood of inadequate K nutrition for corn. Randall and Hoeft (1988), after reviewing a number of K placement studies for corn production, concluded that significant corn yield increases can be obtained when a given rate of K fertilizer is subsurface-banded rather than surface-broadcast, especially on soils that have inadequate K fertility or during dry years. Subsequent K fertility research for NT corn also has indicated a yield advantage from banding K fertilizer using traditional planter-band placement (i.e., 5 cm beside and 5 cm below seeding depth), especially under conditions where initial K fertility is low or surface soil moisture is depleted because of either low rainfall or low residue levels (Yibirin et al., 1993). In addition, deeper placement of K (15 cm beneath the row) was reported to increase NT corn yield, especially during years with less than normal June rainfall (Bordoli and Mallarino, 1998). In ridge-till systems, deep placement of K in the center of ridges also has been reported to increase corn yields on fields with a history of K deficiency (Rehm, 1995).
In environments where corn yields under NT are less than those under conventional tillage (CT), modifying the in-row seedbed characteristics of the NT system by reducing the quantity of residue (Kaspar et al., 1990) or strip-tilling zones 15 to 25 cm wide by 10 to 15 cm deep (Vyn and Raimbault, 1992) can result in yields similar to those obtained in the CT system. Opoku et al. (1997) demonstrated that aggressive fall zone tillage (ZT) on clay-textured soils following wheat (Triticum aestivum L.) resulted in corn yields that were greater than those in NT and similar to those in CT; corn yield gains with ZT were correlated with reduced in-row residue and improved seedbed soil physical properties relative to NT. Reduced corn yield potential associated with NT following wheat also has been reported by other authors (Lund et al., 1993; Schreiber, 1992). However, if K stratification is limiting corn yields in the NT system, then limitations may also occur in the ZT system because 70% of the field area is not tilled and remains stratified. Little information is available regarding the consistency of corn yield response to ZT, especially in high-residue environments (such as those following wheat) and when soil K concentrations are stratified because of a NT cropping history.
Uncertainty over the most appropriate K fertility and tillage system combination to use for corn in fields with a history of continuous no-till production and evidence of soil K stratification prompted the present study. The research objectives were: (i) to evaluate corn response to fall K fertilizer rates in combinations of specific tillage and K placement systems and (ii) to determine corn response to starter-banded K fertilizer when alternate tillage and fall-applied K treatments had been imposed after long-term NT.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The soils were classified as a Listowel silt loam (medium, mixed weakly to moderately calcareous Typic Hapludalf) at Kirkton and a Toledo silt loam to silty clay loam (medium, mixed weakly to moderately calcareous Typic Humaquept) at Belmont. Description of soil textural properties (percentage of sand, silt, clay, and organic C) and fertility (pH, available P, soil-test K, and plant available Mg) are presented in Table 1.
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The experimental design was a randomized complete block split-split plot with four replications. Tillage system was the whole-plot treatment, fall K rate was the split-plot treatment, and starter K rate was the split split-plot treatment. The split split-plot dimensions were 21 m long by 3 m (4 corn rows) wide.
Fall K was applied as muriate of potash [KCl] (0–0–60) at rates of 0, 42, and 84 kg K ha-1. Fall K was applied in a manner that was specific to each of the tillage practices. Therefore, the various tillage systems not only describe the nature of soil disturbance, but also the method of fall K application. The tillage systems, including the method of K application, are as follows:
Corn was planted using a John Deere (Moline, IL) Model 7000 row crop planter in 1996 and 1997 and a John Deere Model 1760 row crop planter in 1998. The corn planters were equipped with unit-mounted tined row cleaners, a single 5-cm fluted coulter positioned directly in front of the seed openers, and a NT fertilizer coulter. The NT fertilizer coulter was positioned to deliver starter fertilizer in a band 5 cm beside and below seeds. The same coulter arrangement was used for planting corn in all three tillage systems.
Spring K was applied as part of a starter fertilizer blend including urea [(NH2)2CO] (46–0–0), monoammonium phosphate [NH4H2PO4] (11–52–0), and muriate of potash (0–0–60). The low K application rate was either 8 (1996) or 0 (1997 and 1998) kg K ha-1. The high K application rate was either 50 (1996) or 42 (1997 and 1998) kg K ha-1. Each of the two starter fertilizer formulations applied 30 and 13 kg ha-1 N and P, respectively.
Approximately 10 d before corn planting, a glyphosate burndown was applied to the NT and ZT plots at a rate of 1.1 kg a.i. ha-1. Corn (cv. Pioneer 3752) was planted in 76-cm wide rows at a seeding rate of 74000 seeds ha-1. The planting dates were 30 May 1996, 30 May 1997, and 8 May 1998 at Kirkton and 30 May 1997 and 8 May 1998 at Belmont.
Weed control consisted of a pre-emergence broadcast application of cyanazine {2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropionitrile} at 2.0 kg a.i. ha-1 and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 2.6 kg a.i. ha-1. Dicamba (3,6-dichloro-2-methoxybenzoic acid) was applied postemergence, whenever necessary, at a rate of 0.3 kg a.i. ha-1 to control broadleaf weed escapes. Additional N was sidedress-applied when corn was at the seven- to nine-leaf stage as urea ammonium nitrate solution (28% N) at 150 kg N ha-1.
Soil-test K was determined in the fall before conducting tillage or applying K fertilizer. The sampling of each split plot consisted of a composite of at least 10 cores collected randomly from three depths (0–10, 10–20, and 20–30 cm). Samples were thoroughly mixed, and soil-test K was extracted using ammonium acetate [NH4(C2H3O2)].
Corn ear-leaf K and aboveground biomass were measured shortly after (within 7 d) of achieving 50% silking. Corn biomass at silking was measured by harvesting 15 consecutive plants from the center two rows; plants were dried in forced-air ovens for at least 3 d at 80°C. Row length of the harvest area was recorded and used to calculate total dry matter on an area basis. Ear-leaf K concentration was determined by removing the ear leaves from the same 15 plants used to estimate silking dry matter. After drying, the ear leaves were ground to pass through a 1.0 mm screen and digested in 1.0 M HCL for determination of K concentration using atomic absorption.
Corn grain yields were determined by hand-harvesting two adjacent 5-m lengths of the center two rows; yields were adjusted to a moisture content of 155 g kg-1. Plant population at harvest and percent of lodged corn plants were estimated from the same sampling area as that for grain yield. Because covariate analysis indicated that corn yield responses to K rate and placement were not affected by plant population differences among treatments, all yield data presented are actual yields (i.e., not adjusted for population).
Corn data were analyzed using an analysis of variance appropriate for a randomized complete block split split-plot design. Significant effects of fall K rate were determined using the appropriate linear and nonlinear (quadratic) contrasts. The significance of spring K rate was determined using the appropriate t-tests. Differences among tillage systems were identified using a protected LSD test at the 0.05 level of probability.
A regression model was developed to relate the yield response associated with increasing starter K rate by 42 kg K ha-1 with initial soil-test K within the various tillage systems and depth increments sampled. Soil-test K for the 0- to 15-cm depth was estimated by computing a weighted average for soil-test K concentrations in the 0- to 10- and 10- to 20-cm depth intervals where the 0- to 10-cm depth interval was weighted two times higher than the 10- to 20-cm depth interval. Yield response to starter K (at a fall K rate of 0 kg K ha-1) was regressed with the soil-test K concentrations on a subplot basis. The yield response to starter K was the difference between corn grain yields of adjacent split split-plots receiving the high and low rates, respectively, of starter K. The regression analysis was conducted using data combined over all five site-years; this resulted in 20 data points for each tillage system. The regression model was a quadratic curve, which became a nonresponse (horizontal) line when the curve reached it's minimum; the model is described as follows:
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Potassium stratification was evident at both the Belmont and Kirkton sites during 1996–1997 and 1997–1998 with initial soil-test K in the surface 10 cm about 1.5 to 2.5 times higher than concentrations present at 20 to 30 cm (Table 2). However, the Kirkton 1995–1996 site had relatively uniform soil-test K concentrations throughout the 0- to 30-cm layer.
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Midseason Corn Potassium Nutrition
Because the method of fall K application was different for each tillage system, corn plant responses (such as ear-leaf K, midseason biomass, and yield) to tillage may not only be due to the degree of soil loosening associated with tillage, but also may be the result of K fertilizer placement in fall. Therefore, interpretation and discussion of corn responses to tillage systems in the present study is not attributed solely to soil loosening associated with tillage, but rather to the combined effects of soil loosening and K placement (whenever tillage comparisons are based on those treatments where fall K was applied).
Analysis of variance indicated that the majority of the variation in ear-leaf K concentrations observed at Kirkton could be explained by the main effects of tillage system, fall K rate, and spring K rate (Table 3). When interactions with year occurred (i.e., spring K x year and fall K x spring K x year), the cause was mainly due to differences in the magnitude of the increase in ear-leaf K attributed to the higher spring K rate among years (Table 4).
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Ear-leaf K concentrations at Belmont were significantly affected by spring K rate but not by fall K rate (Table 3). The high starter rate raised ear-leaf K concentrations by an average of 0.8 g kg-1 in 1997 (Table 5).
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The higher ear-leaf K concentrations when K fertilizer was applied at Kirkton (Tables 4 and 6) suggests K availability was limiting at this site. Ear-leaf K concentrations, especially where no K fertilizer was applied, were often <12 g kg-1, and therefore, below the critical level in Ontario (OMAFRA, 1997). At Belmont, corn ear-leaf concentrations responded less to K fertilization, and concentrations with no K fertilizer exceeded 16 g kg-1; K fertility was less limiting for corn at Belmont than at Kirkton.
Midseason Corn Growth
Fall K application did not significantly affect the quantity of aboveground biomass accumulated by silking in each of the tillage systems at either site (Table 3). However, the high starter rate increased corn biomass at Kirkton by 13% in the CT system in 1996 and the ZT system in 1998 (Table 7).
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Biomass accumulated by silking was weakly correlated with ear-leaf K concentrations at Kirkton in 1997 (r = 0.38, P = 0.0008, and n = 72) and at Kirkton in 1998 (r = 0.24, P = 0.05, and n = 72). These correlations suggest an association between higher K nutrition levels (as indicated by ear-leaf K concentrations) and the quantity of biomass accumulated by silking at Kirkton in 1997 and 1998. However, significant correlations could not be identified at the other three sites.
End-of-Season Populations and Grain Yields
Harvest corn plant populations at Belmont and Kirkton were significantly affected by tillage but not by K treatments (Table 3). The tillage effect at Kirkton was predominantly due to lower NT populations during 1996 when populations were 67700 plants ha-1 in NT and more than 71200 plants ha-1 in ZT and CT systems (data not presented). Population differences at Kirkton during other years were fewer than 1500 plants ha-1. Tillage effects on harvest population at Belmont (Table 5) were similar to those at Kirkton in 1996. Although the tillage x spring K interaction was significant at Belmont, plant population differences between spring K rates did not differ by more than 2000 plants ha-1. Percent of lodged plants at harvest were not affected by K treatments at either site (data not shown).
Potassium fertilization was associated with higher corn grain yields only on fields that had soil-test K concentrations <120 mg kg-1 (Tables 5, 7, and 8). On the one site with high soil-test K concentrations (Belmont, 1996–1997), K fertilization did not increase corn yields (Table 5).
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At Kirkton, tillage system, fall K rate, and spring K rate all had significant effects on corn grain yields (Table 3). However, yield responses to tillage system and spring K application were not consistent over years; interactions of tillage x year and tillage x spring K x year were significant. The latter interaction was predominantly due to: (i) greater yield increases with spring K application in the NT and ZT systems in 1998 compared with either 1996 or 1997 and (ii) relatively small yield responses to spring K in the CT system, especially in 1998 (Table 7). When yield responses to spring K were significant, the higher spring K rate resulted in higher corn yields. However, significant yield responses to starter K did not occur each year in either the ZT or CT systems; this factor also contributed to the significant tillage x spring K x year interaction.
Tillage system and spring K effects on grain yield response at Kirkton, which were consistent over years, included observations that: (i) significant yield increases occurred when the high spring K rate was applied in the NT system (Table 8) and (ii) yields in ZT were never greater than those obtained in the NT and CT systems.
When soil-test K concentrations were <100 mg kg-1 (Kirkton sites), fall-applied K also increased NT and ZT yields but not to the same extent as spring-applied K (Table 8). Grain yield response to fall K application was affected by both tillage system and spring K rate (Table 3). When spring K was not applied, a small (0.3 Mg ha-1) linear response to increasing fall K rate from 0 to 84 kg K ha-1 occurred in the ZT (P = 0.10) and CT (P = 0.05) systems. Fall-applied K also increased NT corn yields when the low rate of spring K was applied; however, the yield response was maximized at the intermediate rate of fall K.
Although deep-banding 84 kg K ha-1 in ZT increased corn yields by 0.3 Mg ha-1, the yield increases associated with deep-banding K in the present study probably were not great enough to recover application costs. Bordoli and Mallarino (1998) reported similar conclusions with regard to yield response and economics for deep-banding K for NT corn in Iowa.
When the higher rate of spring K was applied, CT corn yields also increased (0.5 Mg ha-1) linearly with increasing fall K rate (Table 8). However, increasing the fall K rate did not affect corn yields in the ZT and NT systems at the high starter rate. In fact, applying 42 or 50 kg K ha-1 as part of the starter blend maximized NT and ZT yields. In contrast, the highest CT yield was produced (response of 0.7 Mg ha-1 over where no K was applied) when both highest fall and spring K rates were applied.
At Kirkton, ear-leaf K concentrations at silking were positively correlated with grain yield (r = 0.41, P = 0.0004, and n = 72 in 1996; r = 0.62, P = 0.0001, and n = 72 in 1997; and r = 0.39, P = 0.0008, and n = 72 in 1998). These significant correlations suggest that inadequate K nutrition was potentially limiting corn grain yields at Kirkton. Similar correlations did not occur at Belmont.
The lack of significant yield increases in the NT and ZT systems with the highest K application rates (84 + 42 kg K ha-1 in fall and spring, respectively) relative to high starter K alone (Tables 5 and 8), suggest that the higher K rates in these systems were not yield limiting. However, significant yield increases with the highest K application rate occurred in the CT system at Kirkton, and thus no conclusions can be made concerning corn response to even higher rates of fall K in the CT system for that soil.
In the NT system with zero fall K, the magnitude of the yield increase associated with applying the higher spring K rate was inversely related to the initial soil-test K concentration in the surface 20 cm. A regression model consisting of a quadratic curve (which becomes a horizontal line at its minimal point) explained approximately 50% of the variability in the size of the NT yield increase associated with starter K (Table 9). The NT yield response curves to starter K reach their minimal value within 0.10 Mg ha-1 of zero. For the standard soil fertility sampling depth (surface 15 cm), the minimal starter K yield response occurs at a soil-test K level of 160 mg kg-1, and the break-even concentration occurs at 112 mg kg-1 (Fig. 1). The break-even concentration is defined as the point on the quadratic curve where the yield response was 0.2 Mg ha-1 (an assumed minimum yield response where the value of the corn grain is equivalent to the cost of applying 42 kg K ha-1 as part of the starter fertilizer blend). Similar relationships could not be identified for the ZT and CT systems or for NT when yield response was regressed against the soil-test K concentrations below 20 cm.
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Four of the five sites received significant rainfall during the 4-wk period before silking (Fig. 2), which coincides with the onset of the rapid growth phase and period of maximum K uptake. This 4-wk period occurred at both sites from 13 July to 3 August in 1996 and 1997 and between 22 June and 13 July in 1998. The rainfall received during the 4-wk period increased the liklihood of favorable surface soil conditions for root function and K uptake. The latter, plus the possibility that root density is greater near the surface in NT systems, may have enhanced availability of near-surface placed K in NT systems in the present study. It is interesting to note that the Kirkton site in 1997 was relatively dry during the 4-wk period before silking and had ear-leaf K concentrations (Table 4) and yield response to spring K (Table 7) that were among the lowest of the five sites in the present study. The Kirkton site in 1997 started to receive normal levels (Canada Atmos. Environ. Serv., 1993) of precipitation after silking.
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When yields were combined over years at Kirkton, interpretation of corn yield response to tillage was complicated by the interactions with K treatments. When K was not applied, NT yields were 7% less than those obtained in the CT system while ZT yields were 6% less (Table 8). However, adding 42 kg K ha-1 as part of the starter blend as the only source of K fertilizer increased NT and ZT yields such that all three systems produced statistically similar yields. Even so, substantial yield differences among tillage systems did occur at the highest rates of K application (84 kg K ha-1 in fall + high K starter) where CT corn yields were 5% greater than NT corn yields and 9% greater than those of ZT. The latter suggested that corn in CT systems (or perhaps just corn grown in the 1st yr of plowing after a period of continuous NT) may continue to yield higher than NT corn on similar silt loam soils when K is not limiting. It also suggests, under the soil fertility and environmental conditions prevalent in this study, that deep-banding K in conjunction with fall ZT will not necessarily produce yields greater than NT when K is either surface-broadcast or when >40 kg K ha-1 was applied as part of the starter fertilizer blend.
On sites with medium soil-test K, CT was often associated with both higher ear-leaf K concentrations (Table 6) and higher yield (Table 8) than NT with 0 K or at the maximum fall-plus-spring K rate applied. This suggests that moldboard plowing of long-term NT fields may improve K uptake by corn when fields are below the critical K levels, regardless of the degree of stratification in soil-test K. The apparent limitation on K uptake in NT corn produced in the season immediately following K fertilizer application could not be overcome solely by increasing the rate of K fertilizer.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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