Tillage and Potassium Placement Effects on Potassium Use Efficiency in a Corn-Soybean Rotation

Illinois Fertilizer Conference Proceedings
January 29-31, 1996

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Tillage and Potassium Placement Effects on Potassium Use Efficiency in a Corn-Soybean Rotation

S.A. Ebelhar and E.C. Varsa¹

Introduction

Potassium (K) fertilizer placement and mixing within the soil Ap horizon can influence its availability for plant uptake. Other factors affecting uptake are root growth of plants, diffusion rate of K to roots, K fixation, environmental conditions and tillage. Previous work by Kovar and Barber (1987) at Purdue suggested the possibility for optimizing potassium uptake and utilization by corn based on optimal placement and soil mixing of K fertilizers. Fertilizing too large of a soil volume (i.e. broadcasting and mixing in the Ap horizon) could result in the excessive fixation of K in soils which fix K. Fertilizing too small of a soil volume may create a situation where the plant roots, which occupy only about one percent of the soil volume, do not occupy enough of the fertilized volume for optimal fertilizer utilization.

Farmers in Southern Illinois need to utilize no-tillage and reduced tillage management practices on highly erodible land. This limits their ability to mix fertilizer throughout the plow layer. For this reason, farmers need to know how to best fertilize no-till areas to enhance optimum K uptake by plants. By comparing different K placement methods we can determine if stratification, low K utilization and reduced yields would warrant the periodic tillage of these soils.

The objectives of this study are to:

Determine the placement method which optimizes potassium utilization for corn and soybeans under no-till and reduced tillage systems.
Determine effects of using K in starter fertilizer.
Determine effects of potassium rate and placement on potassium uptake and yields of com and soybean.
Evaluate monthly sampling of selected plots at each location to measure variance among samples and seasonal effects on soil test K values.
Determine soil K stratification after 3 years of K placements.
To include a final report at the conclusion of this project to address each of the objectives stated above

Materials and Methods

A field study was initiated in 1994 at the Dixon Springs Agricultural Center (DSAC) and at the SIU Belleville Research Center (BRC). No-till (NT) and chisel tillage (CT) treatments were used in a corn:soybean rotation. Prior to study initiation, soil samples were taken from the top 8" of soil at each location. The averages are shown in the table below. Lime, at a rate of 4 tons per acre was spring applied to the DSAC site before tillage was performed in 1994. A finely ground lime (54% passed #60 mesh sieve) was chosen for quick reaction.

Soil test levels (0-8" samples) at DSAC and BRC at start of study.

Location	1994 Crop	pH	P1	K	Ca	Mg	CEC
			lb/acre	lb/acre	lb/acre	lb/acre	meq/100g
DSAC	Corn	4.9	30	192	545	114	4.6
	Soybeans	4.6	26	250	177	125	4.1
BRC	Corn	7.1	56	194	3286	351	10.3
	Soybeans	6.9	74	249	3604	463	11.8

1995 Study. Chisel tillage was performed at DSAC on 6 April and at BRC on 14 June. The delay at BRC was due to over 17 inches of precipitation received during May (Figure 1). Tillage systems were blocked (whole plots) within each replication. A factorial arrangement of K rates and K placements plus a zero K check (subplots) were randomly placed within each tillage block in a split-split plot design. K rates consisted of 60, 120 and 180 lbs K₂0 per acre. K placement methods consisted of surface broadcast (BC), 8-10" surface band (BD) over intended row, surface dribble (DR) 6" to side of intended row, and starter (ST). The starter treatment placed 30 lbs of K₂0 2" to the side and 2" below the seed and was applied at planting. The remainder of the K rate with the starter treatment was surface broadcast. These treatments were applied to both corn and soybeans at DSAC and BRC with 4 replications per crop per location.

The K treatments were applied in solution form by dissolving potash (0-0-60) in water. The treatments were applied after chiseling but before secondary tillage to provide some mixing of the fertilizer with the tillage treatment. In 1995 a power tiller was used to incorporate the fertilizers. This power tiller worked very well at incorporating the bands vertically with little spread horizontally. Corn (Pioneer 3394 variety) was planted on 26 May at DSAC and 19 June at BRC at a seeding rate of 26,000 seed acre. Soybeans (Pioneer 9451 variety) were planted on 31 May at DSAC and 19 June at BRC at a planting rate of 10 seeds per foot in 30" rows (175,000 seeds per acre). Phosphorus fertilizer was broadcast across all plots prior to planting. Nitrogen was sidedressed on corn at a rate of 150 lbs N/acre at 4 weeks after planting.

Whole plant corn samples (from check plots and 120 lb K₂0 rates) were sampled between the 6 and 8 leaf growth stage and whole soybean plants were sampled at about 12 inches of top growth from selected plots. These plants were weighed in the field, transported to the lab, dried and weighed again in order to determine a wet weight, dry weight and % moisture. After drying, plants were ground and analyzed for nutrient concentrations (%). Nutrient contents were determined by multiplying nutrient concentrations (in decimal form) by dry weights. Corn ear-leaves were sampled from each plot at mid-silking and uppermost fully developed trifoliate leaves of soybeans were sampled at early bloom for nutrient analysis. Grain yields, grain moisture, and final plant stands were measured after physiological maturity.

Since the start of the study in 1994, soil samples have been taken each month from the 120 lbs K₂0/acre plants along with the check plots for each crop at each location. These samples are analyzed for pH, P, K, Ca, and Mg levels.

Results and Discussion

Because much of the interactions between K rates, K placement and tillage are not significant, only main effects will be included in the discussion.

BRC Corn Results

K rate effects. There was a small but significant increase in corn grain yields as K rates increased (Table 1). These differences are probably not economical. Grain moisture, plant stands and ear-leaf nitrogen were unaffected by K rate or the effects were small. Increasing K rates significantly increased ear-leaf K concentrations while decreasing Mg levels. There is an antagonistic relationship between K uptake and Mg uptake. The Mg levels were approaching a critical level especially with high K rates. No visual Mg deficiency symptoms appeared in the field. The zero K treated corn had leaf K levels below the sufficiency range of 1.8 to 3%.

K placement effects. K placement had no significant effect on corn yields, plant stands, and ear-leaf nitrogen (Table 2). Grain moisture differences existed but were small. There were no differences between placement methods for ear-leaf K and Mg concentrations, but all placement methods were significantly different from the checks. In the case of leaf K, the check had a lower concentration (by 0.3%) than the various placement methods. For the Mg levels, the checks were significantly higher.

Whole plant data. There were no significant differences between placement methods for whole plant wet weight, dry weights, N content and K contents (Table 3). N and K contents are determined by multiplying the dry weight by the respective concentration (%). The starter K treatment had lower plant moisture contents, although the wet weights were the highest of any treatment. Aside from the check treatment, the broadcast treatment had the lowest K concentrations. This is an indication that the broadcast treatment was not as efficient in getting K into the plant early. By silking there was no difference in the leaf K concentrations.

Starter nitrogen. In 1995 a starter nitrogen plus potassium treatment was added to compare with the starter K treatment without N. At BRC, the nitrogen in the starter increased grain yields by 6-7 bu/acre (Table 4). There were also significant increases in whole plant wet weights, dry weights, %N, N content and K content. The increased K content is merely a function of increased dry weight since the K concentrations are the same. By silking, there were no differences measured by the ear-leaf concentrations of N or K.

DSAC Corn Results

K rate effects. As at BRC, increasing K rates at DSAC significantly increased yields, and ear-leaf K while decreasing ear-leaf Mg concentrations (Table 5). In this case all of the Mg levels were below what's normally considered a critical range for ear-leaf Mg. However, there were no visible Mg deficiency symptoms, and grain yields were quite good for the type of growing conditions found in 1995 (wet early, dry late in the season with very high nighttime temperatures during silking and grain fill). The leaf N and K levels are within the sufficiency range for corn even with the checks.

K placement effects. The effects of placement method for K had no significant effect on grain yield, plant stand, or ear-leaf N, K and Mg concentrations (Table 6). There were grain moisture differences, but they were very small and difficult to explain.

Whole plant data. The starter and dribble treatments both had significantly higher whole plant wet weights and dry weights than the other treatments (Table 7). The starter treatment also had higher N and K concentrations and higher N and K contents than most of the other treatments. Clearly the starter and the dribble treatments got off to a better start.

Starter nitrogen. As at BRC, plants exposed to the starter treatment at DSAC with N got off to a better start than without N. Whole plant wet weight, dry weight, moisture, N concentrations, and whole plant N and K contents were higher with N in the starter than without (Table 8). However, at silking there were no differences in leaf N or K concentrations and at harvest the corn treated with N in the starter yielded less than that without the N. The only plausible explanation for this might be that the starter+N made it to silking and grain fill sooner than the others and ran out of moisture at a critical time or that it simply put too much of the water into vegetative growth and ran out faster during the dry months of July-September (Figure 1).

BRC Soybean Results

Note: Some of the trifoliate tissue analysis is not finished at this writing and cannot be included.

K rate effects. Grain yields and moisture increased significantly with increasing K rates to a maximum at 120 lb K20/acre (Table 9). Above the 120 lb rate yields declined slightly.

K placement effects. K placement had no significant effect on either grain yields or moisture (Table 10). The highest absolute yield occurred with the starter treatment, as occurred with corn at the same location.

Whole plant data. Whole plant wet and dry weights were unaffected by K placement (Table 11). Moisture contents, K concentration, and K contents were higher with K placement than check plots but Mg concentrations and contents were lower. The broadcast and banded treatments had the highest K concentrations and contents.

Starter nitrogen. The N added to the starter increased early season growth but failed to provide any large yield benefit. Adding N to the starter treatment increased whole plant wet weights, dry weights, N contents and K contents (Table 12). There was a small but nonsignificant increase in yields and whole plant N concentrations with starter+N. K concentrations were unaffected by addition of N to the starter.

DSAC Soybean Results

K rate effects. Increasing K rates had a negative effect at DSAC in 1995 (Table 13). The highest yield occurred at the 60 lb K20 rate and declined at higher rates. The check had the highest economical yield. There appears to be a salt injury occurring due to the high rates of K. K rates had no effects on grain moisture.

K placement effects. In this situation, the starter seemed to have the most negative effect on grain yields (Table 14). This may be another indication of salt-type injury to soybeans. Grain moisture was relatively unaffected by K placement.

Whole plant data. Addition of K fertilizer increased whole plant wet weights, dry weights, moisture, K concentrations, N contents and K contents (Table 15). This indicated that K increased early season growth at the 120 lb rate, but somewhere the effects of too much K took over to produce lower yields. This probably occurred when the drought began in July. Perhaps the leaf data may show something.

Starter nitrogen. There was a very slight but nonsignificant increase in yields with starter+N treatments but little of no benefit to early season growth (Table 16).

Monthly Soil Samples

Monthly soil sampling at BRC reveals some interesting trends (Figure 2). In June of 1994, soil test K levels for the check plots were 250 lb when averaged across both corn and soybean areas. By September that level had fallen to under 170. This would be attributed to plant uptake and possible some fixation by the soil. The broadcast (BDCT) treatment of 120 lb K₂0 increased the soil test level approximately 30 lb by the June sampling and continued to be higher than the check throughout the growing season. After September 1994, soil test levels began to increase, corresponding to about the time of leaf drop by the crops and at a time when soil moisture was rising. The soil test levels remained high throughout the winter months until April, when there was a dip in the soil test level. Interestingly, April was a very dry month at BRC and K fixation may have occurred. After April soil tests began to rise, even with the check plots. This shows the seasonality of soil testing for K and is probably a result of K release from crop residues breaking down or from fixed K being released. Sampling has continued but tests are not back yet from the lab or have not been summarized. Soil tests for K at DSAC (Figure 3) showed similar trends throughout 1994 but have behaved somewhat differently in 1995. Starting in January, soil tests dropped and then rose steadily until June where they seemed to have peaked right after planting. We are currently going back to the weather data for each area to determine how the soil test levels may be related to soil moisture and/or soil temperatures just prior to sampling.

Summary

Increasing K rates seemed to increase early plant growth and yields for corn more so than soybeans. Corn usually requires more K which would explain its responsiveness to K. The placement method of K has not been that critical, even for no-till for these first two seasons. Soil sample data (Figures 2 and 3) indicate that soil test levels are seasonal in nature and the June-July peaks in 1995 are lower than 1994, an indication that there may be a net loss of K from the system through grain removal or K fixation. Certainly, this study needs to continued into the next year and probably beyond to look at long-term effects of nutrient placement.