Application of phosphorus (P) and potassium (K) are required to attain high quality yields in corn (Zea mays L.). The application of these fertilizers represents a multi-billion dollar investment for US farmers. Thus, improving management practices that optimize returns on fertilizer investment can be of major importance. In recent years, with the adoption of conservation tillage systems, such as no-till (NT) and strip-till (ST), there has been a renewed interest on the efficient placement of P and K fertilizers. Broadcast application of P and K under NT results in higher content of these nutrients in the surface compared to the subsurface layer of the soil (vertical stratification) while deep band applications in ST result in enrichment of these nutrients in the band and depletion between bands (horizontal stratification) (Buah et al., 2000; Holanda et al., 1998; Rehm, et al., 2002).
Corn roots takes up P and K from the soil mostly by diffusion (Barber, 1995). In order for this process to occur, the root system needs to be active and there needs to be water available in the soil for these nutrients to diffuse to root surfaces. Stratification forces P and K uptake to be more dependent on the ability of plant roots to exploit the volume of soil with elevated concentrations of these nutrients and on the characteristics of that portion of the soil. Under vertical stratification in rainfed conditions, inadequate soil water supply can diminish P and K availability at the soil surface layer. On the other hand, deep band placement of P and K can take advantage of potentially higher water content present at lower depths. However, without sufficient rainfall, subsurface soils could eventually become dry and low rainfall might re-wet the surface soil layer, but may not be sufficient to penetrate to the subsurface soil layer. In addition, the ability of the crop to take up nutrients from the subsurface can be reduced because the fibrous root system of corn is more prolific in the surface compared to the subsurface soil layer, and soil oxygen levels needed for nutrient uptake decline rapidly with increased soil depth. In light of all these factors, the question then becomes: What placement technique of P and K constitutes the best management practice for conservation tillage rainfed-corn?
Despite the importance of all the factors mentioned above for adequate P and K nutrition, seldom are P, K, and water status monitored at the different rooting depths of the soil through the growing season; nor is this information coupled with the corresponding root data necessary to adequately quantify plant P and K availability. The amount of root surface area and soil water availability determine by in large how much of the applied P and K fertilizers could be taken up by corn. Unfortunately, these factors that determine P and K availability have not been properly identified in fields under conservation tillage. This lack of knowledge and the need to improve nutrient use efficiency by truly understanding when, where, and under what field conditions P and K are being taken up by corn, constitutes the basis of this research.
This study was started in the spring of 2007 at the Crop Sciences Research & Education Center in Champaign on a field that remained untilled after the previous corn crop. The study site is mostly on a Flanagan silt loam soil (Fine, smectitic, mesic Aquic Argiudolls) and small portions of the field are in a Drummer silty clay loam soil (Fine-silty, mixed, superactive, mesic Typic Endoaquolls). At the start of the experiment the soil had the following characteristics: CEC of 14.2 cmol (+) kg-1, 3.6% organic matter, a pH of 5.7, P concentration of 16.2 mg kg-1, K concentration of 185 mg kg-1,Ca concentration of 1564 mg kg-1, and Mg concentrations 253 mg kg-1. The field was divided to establish a corn-soybean rotation system with both crops present each year. Annual applications of all possible combinations of four P levels (0, 25, 50, 75 lb P2O5 acre-1) and four K levels (0, 45, 90, 180 lb K2O acre-1) were applied either by broadcast or deep band under the row (30 inches apart and 6 inches below the soil surface). The treatments will remain in the same 20 x 75 ft. plot during successive years. The experimental design was a split-split-plot arrangement of treatments in a randomized complete block design with 3 replications. Tillage/placement (no-till with broadcast application (NT-BC), no-till with deep placement application (NT-DP), and strip-till with deep placement application (ST-DP)) was the main plot, P treatment was the subplot and K treatments was the sub-subplot. Application of P and K treatments in NT-DP was done with a low disturbance knife to minimize tillage effects. Fertilizer was applied early in the spring of 2008. Urea ammonium nitrate at the rate of 180 lb N acre-1 was applied on the corn plots with a sprayer prior to planting. Corn hybrid Pioneer 34N40 was planted on May 22, 2008.
Separate soil samples were collected at the in-row and between-rows positions from the 0-2, 2-4, 4-7, and 7-20 inch soil depth increment after soybean harvest in 2007. From selected plots (P-K rates: 0-0, 75-0, 0-180, and 75-180 lb acre-1 of P2O5 and K2O, respectively) soil and root samples were collected at V12 and R2 corn development stages and shoot samples at V5, V9, R1, and R3 development stages. Three composite soil samples were collected from the 0-2, 2-4, 4-8, and 8-16 inch depth increments from the crop row and between the rows and kept separate for P and K analyses. Similar procedures were followed for root samples. A five-plant composite sample was harvested by cutting plants at the soil surface. Leaves were detached from the rest of the above ground plant materials to determine leaf area and the leaf and remaining plant materials were dried to determine leaf and total above ground dry matter accumulation. Volumetric water content was measured continuously using data loggers and soil water probes installed at the 0-2, 2-4, 4-6, 6-8, and 8-16 inch soil depth increment in plots with the P-K rate75-180 lb acre-1 P2O5-K2O. Yield component parameters were measured by hand harvesting 10 plants from the selected plots for intensive measurements. Grain yield was measured by machine harvest in the center of the middle two rows of each plot.
Soil P was determined by Bray and Kurtz P-1 test (Frank et al., 1998), soil K by the ammonium acetate extraction procedure (Warncke and Brown, 1998). Root samples were cleaned by elutriation and further cleaned by hand and analyzed for physical parameters by scanning with Win-RHIZO software and an Epson Expression 10000XL scanner (Régent Instruments Inc.). Tissue samples were analyzed for nutrient content following the official methods of analysis of AOAC International (Horwutz, 2000). Grain protein and oil contents were measured by near-infrared reflectance spectroscopy. Data was analyzed by the GLM procedure (SAS Institute, 2004). Results of significance are at p<0.05 unless otherwise indicated.
Precipitation during the growing season provided above average moisture conditions every month except for August (Table 1). While wet conditions early in the season resulted in delayed corn planting, May 22, the overall yield was 196 bushels per acre. Analysis of main effects showed a significant grain yield difference due to tillage placement and K rate but no difference due to P rate (Table 2). The ST-DP system produced the highest yield compared to the no-till systems. Since wet conditions prevailed during the early portion of the growing season, it is likely that strip-tillage created drier soil conditions for the roots of new seedlings and allow these plants to get well established early on in the season. Although there was significant difference in grain yield due to K rate, the differences were not meaningful since there was no statistical difference between the check and any of the K rates. However, the tillage-placement by K rate interaction revealed that the highest K rate in strip tillage significantly increased yield over the same rate in no-till systems and illustrates once again the advantage of strip tillage over no-till systems under early-season wet soil conditions.
This year due to late planting in much of the Midwest, drying cost of harvested grain was a concern for many farmers. In this study we found greater moisture content for the 90 lb K2O acre-1 rate than the check, and a general trend for greater grain moisture content with increasing K rate (Figure 1a). While not statistically different, a trend for slightly less grain moisture content was observed for the deep-placement treatments, especially with strip tillage, compared to the broadcast application (Figure 1b). Further examination of grain composition showed that starch content was 0.25 and 0.27% greater for NT-BC compared to NT-DP and ST-DP, respectively (Figure 2a). These data might indicate that tillage-placement influences starch content which in turn could impact grain moisture. Protein content decreased with increased P rates (Figure 2b) but increased with increasing K rates (Figure 2c).
Shoot dry matter accumulation (leaves and stalks) increased with P application of 75 lb acre-1 P2O5 over the check (no P applied) at the V5 development stage (data not shown). No differences were observed at later samplings. Shoot dry matter accumulation increased with the addition of 180 lb K2O acre-1 over the check plots (no K added) only later in the season when plants were at the R1 and R3 development stages. Growth component analyses are not presented since this information is being compiled presently.
Initial soil conditions in the spring of 2007 showed significant vertical stratification of both P and K levels with greater concentrations in the surface layer (Table 3). Initial conditions can be assumed to have been similar at the in-row and between-rows position since there was no history of previous band applications. While a direct comparison of concentrations between the measurements at V-12 and pre-treatments may be difficult because of the difference in the time of sampling (summer vs. early spring) and slight difference in sampling depth, the relative concentration ratios with soil depth between the two measurement times is relevant. As expected, P and K concentrations were similar for in-row and between-rows positions for the broadcast treatment. In the broadcast treatment, the ratio of P and K stratification between the first two depths of the soil (0-2:2-4) at V-12 was very similar to the ratio for the pre-treatment measurement. At V-12, concentrations of soil P and K for the broadcast treatment (averaged across row positions) were similar to those of the deep placement between-rows position. Conversely, concentrations for the in-row position for the deep-placement treatments differ substantially from those of the broadcast application (averaged across row positions). In the surface layer, a decline of 8 parts per million (ppm) for P and 40 ppm for K occurred due to deep placement compared to surface broadcast application. The deep placement treatments showed and increase of 27 and 128 ppm P and K, respectively over the broadcast treatment at the 4-8 inch increment. The increase in concentrations at the 4-8 inch depth increment coincides with the depth at which nutrients are placed in the deep placement treatments. This finding indicates that deep placement of nutrients, can help decrease surface concentrations. This is especially noteworthy for P. Lower P concentrations in the soil surface can be important in reducing the environmental hazards of P loading into streams during surface water runoff events.
Since crops obtain P and K from the soil solution, measurement of volumetric water content (Θv) is important to help determine nutrient availability. Statistical analysis of soil water content showed no significant differenced due to tillage-placement and position with respect to the crop row, but significant differences occurred due to soil depth. Thus, data was averaged across all tillage-placement treatments and position with respect to the crop row for the 75-180 lb acre-1 P2O5-K2O rate treatment (Figure 3). This year soil water appeared to be sufficient for crop growth throughout the season, even during a drier-than normal August. The top 2 inches of the soil became dry at times, but sufficient water was present below the surface layer. Analysis of soil water content across the growing season for the top 2-inch depth increment showed greater water content for the between-rows position (Θv = 0.24) than the in-row position (Θv= 0.19). Less water at the in-row position was likely the effect of crop residue removal and tillage effect in the ST-DP treatment that allowed more water evaporation in the early season. Greater root development (see later discussion) at the in-row position could have also resulted in more water uptake.
Root length density (RLD), which is a measurement of root length per given volume of soil, was significantly influenced by tillage-placement, P rate, and soil depth increment (Figure 4). The NT-BC treatment produced greater RLD than the other tillage-placement treatments. The highest P rate helped reduced RLD compared to the check (no P added). Root growth is influenced in part by nutrient availability. Plants typically spend more energy in root development to help compensate for lower nutrient availability. It is possible that greater RLD in NT-BC as well as in the zero P rate is the result of lower P availability. As expected, the greatest root density was near the surface of the soil. There were few significant two- and three-way interactions, but the only meaningful interaction was tillage-placement by depth, which was explained by greater RLD for the 0-2 inch soil depth increment in the NT-BC treatment compared to the same depth for the NT-DP and ST-DP treatments. The soil depth by P rate interaction was not significant, indicating that there was no root proliferation in response to increased P availability in the 4-8 inch depth increment for the deep-placement treatments. Roots normally will proliferate in P enriched areas when the rest of the soil volume is depleted of the nutrient. In this case, there are still sufficiently high P levels in the soil not to allow such root response. Similar results to those in Figure 4 were observed for root surface area (RSA) (Data not shown). The in-row position samples measured had a RSA average of 64 cm2 compared to 51 cm2 for the between-rows position indicating that there is an overall greater potential for nutrient and water absorption by the root system in the row position. While RLD were not significantly different in-row or in-between rows positions, significant mean root diameter (MRD) differences of 0.33 mm in the row compared to 0.28 mm in the between- rows position were observed (p<0.001). Thus, similar to last year’s results, the greater RSA observed at the in-row position was related to larger overall root diameters and not to greater root lengths.
Barber, S.A. 1995. Soil nutrient bioavailability: A mechanistic approach. 2nd ed. Wiley & Sons, New York.
Buah, S.S.J., T.A. Polito, and R. Killorn. 2000. No-tillage soybean response to banded and broadcast and direct and residual fertilizer phosphorous and potassium applications. Agron. J. 92: 657-662.
Frank, K, D. Beegle, and J. Denning. 1998. Phosphorus. p. 21-29. In: J.R. Brown (ed.) Recommended chemical soil test procedures for the North Central Region. North Central Reg. Publ. 221(rev.). Univ. of Missouri, Columbia.
Hoeft, R.G., and T.R. Peck. 2002. Soil testing and fertility. In Illinois Agronomy Handbook, 23rd ed. College of Agriculture, Consumer and Environmental Sciences, Dept. of Crop Sciences, University of Illinois Extension, University of Illinois.
Holanda, F.S.R., D.B. Mengel, M.B. Paula, J.G. Carvaho, and J.C. Bertoni. 1998. Influence of crop rotations and tillage systems on phosphorous and potassium stratification and root distribution in soil profile. Commun. Soil Sci. Plant Anal. 29: 2383-2394.
Horwutz, W. (ed.) 2000. Official methods of analysis of AOAC International. 17th ed. Vol. 1. AOAC Int., Gaithersburg, MD.
Rehm, G.W., A.P. Mallarino, K. Reid, and J. Lamb. 2002. Soil sampling for variable rate fertilizer and lime application [on line]. Available at: http://www.extension.umn.edu/distribution/cropsystems/DC7647.html (Verified 21 Dec. 2007).
SAS Institute. 2004. SAS/STAT user’s guide. Version 9.1 ed. SAS Inst., Cary, NC.
Warncke, D., and J.R. Brown. 1998. Potassium and other basic cations. p. 31-33. In: J.R. Brown (ed.) Recommended chemical soil test procedures for the North Central Region. North Central Reg. Publ. 221(rev.). Univ. of Missouri, Columbia.