S. A. Khan, R. L. Mulvaney, T. R. Ellsworth, K. Ferrie, T. J.
Smith, J. S. Strock, and R. M. Vanden Heuvel 1
There are inherent differences among soils in their capacity to supply plant-available
N, which can also be markedly affected by management and cropping practices.
The implication is that fertilizer practices should account for differences in
soil N availability, and ideally should be implemented on a site-specific basis.
This approach has become feasible with the development of the Illinois soil N
test (ISNT).
Although not generally recognized, the soil rather than fertilizer is usually
the major source for N uptake by corn (Zea mays L.). This is apparent,
for example, from on-farm N-response studies, since there is often a limited
difference, or none at all, between fertilized and unfertilized (check) plot
yields (e.g., IAEA, 1970; Blackmer et al., 1989; Brown, 1996; Khan et al., 2001;
Mulvaney et al., 2001, 2006; Lory and Scharf, 2003). Such evidence is substantiated
by numerous 15N-tracer investigations reported since the 1970s, showing that
the corn crop largely utilizes soil rather than fertilizer N (e.g., IAEA, 1970;
Blackmer and Sanchez, 1988; Jokela and Randall, 1997; Omay et al., 1998; Stevens
et al., 2005). Clearly, the efficient use of N fertilizers requires a reliable
estimate of the soil’s capacity to supply plant-available N through mineralization.
Since the development in 2001 of the Illinois soil N test (ISNT) for estimating
alkali-labile organic N (Khan et al., 2001), a growing body of evidence indicates
that soil N-supplying power can be quantified in a way that significantly relates
to crop N requirement.
Recent results from N-response evaluations have generally verified the ISNT as a useful tool for manure (Klapwyck and Ketterings, 2006; Klapwyck et al., 2006) and fertilizer (Ruffo et al., 2006) N management, although misinterpretations have sometimes occurred (Barker et al., 2006) because other factors were neglected that affect crop N uptake or soil N availability, such as plant population, soil acidity, or a deficiency of P or K. Particular attention must be given to corn planting rates (Mulvaney et al., 2006), which affect not only crop N uptake, but also the quantity of carbonaceous residues returned, and thus the tie-up of plant-available N through immobilization during microbial decomposition. Higher plant populations necessarily affect soil test calibrations (Bray, 1948; Melsted and Peck, 1973), and have long been recognized as an important means of increasing yield on highly productive soils (Dungan et al., 1958).
The work reported was designed to explore the interaction of soil N-supplying power (ISNT) and plant population for fertilizer N management within the context of economic profitability. The specific objectives were:
Historical yield data were utilized in comparing fertilized (NB) and unfertilized (NA) Morrow Plots under continuous corn (1876-present), a corn-oats (Avena sativa L.)(1876-1966) or corn-soybean (Glycine max L. Merr.)(1967-present) rotation, or a corn-oats-hay rotation (1876-present). Beginning in 1955, the fertilized plots have received urea (150-200 lb N acre-1) when cropped to corn, with the use of limestone and commercial fertilizer as needed to maintain a desired soil test level of pH, P, and K.
Nitrogen-response experiments were conducted during 2005 and 2006 on University of Illinois cropland managed by the Department of Agricultural Engineering or the Department of Animal Sciences. The soil type in each case was a Drummer silty clay loam, but there was a difference in management, such that two site-years were under a nonmanured corn-soybean rotation, and a third was cropped to continuous corn with annual manuring. The experimental design provided three replications of eight N rates (0-210 lb acre-1 in 30-lb increments), utilizing compact triangular plots in 2005 with equidistant spacing of 20,000, 24,000, or 40,000 plants acre-1 (Pioneer 33J24), or rectangular plots in 2006 that were planted (Stine 9803RRYGPL) in 30-inch rows (16,000, 20,000, or 40,000 plants acre-1) or by hand with equidistant spacing (20,000, 30,000, or 40,000 plants acre-1). Sprinkler irrigation was employed during dry weather in 2005, along with insecticide applications to control silk clipping by rootworm beetles. At physiological maturity, grain yield was determined by hand-harvesting the central one-third of each plot, with correction to 15.5% moisture content.
Soil sampling for the ISNT was done in mid-April prior to planting, by collecting a 5-core composite (0-12 and 12-24 inches) from the central part of each plot where yield data would be collected. Following drying (40°C) and grinding (< 2 mm), the ISNT was performed in duplicate as described in a technical note (15N Analysis Service, 2004).
Optimum yield was determined by fitting N-rate and corresponding yield data to a quadratic or linear plateau model by nonlinear regression, from which optimum N rate was calculated assuming a corn:N price ratio of 0.1.
Data for 2006 were subsequently segregated into two groups, using the optimum N rates and yields determined by incremental estimation for each replicate. Group 1 included all plots with £ 20,000 plants acre-1, whereas the population for group 2 was ³ 30,000 plants acre-1. A stepwise regression was performed in each case, so as to examine the relationship of yield to plant population and fertilizer N rate, either alone or in combination with ISNT data.
Soil Productivity as Related to Plant Population and N Fertilization
Management practices have long been recognized to have a major impact on soil productivity. This is clearly evident from the Morrow Plots, where a single soil type (Flanagan silt loam) has been annually cropped for 130 years with different rotations and inputs. Historically, yields have consistently been lowest when corn was grown continuously without nutrient inputs and highest for a corn-oats-hay rotation with manure, limestone, and rock phosphate. Major changes in management occurred in the 1950s and 1960s, involving (i) return of aboveground residues following grain cropping, (ii) introduction of commercial NPK fertilization to selected subplots on the western half of the experimental area, (iii) substitution of soybean for oats in a 2-year rotation with corn, and (iv) a progressive increase from 8,000 to 28,000 plants acre-1 for NPK and manured subplots.
The effects of the aforementioned management changes are documented by Fig. 1, which shows corn yield data averaged for periods of constant management. When NPK fertilization was initiated in 1955, yield response was greatest where continuous corn had seriously depleted soil nutrient availability, and was least where soil fertility was less depleted by a corn-oats-hay rotation. In contrast, subsequent population increases were most effective in the latter case, and have led to the highest yields.
The data in Fig. 1 are consistent with the concept that N fertilization is crucial for soils of limited fertility, while an adequate plant population is important for highly productive soils. There are obvious economic and environmental incentives for identifying areas that differ in soil N-supplying power, so as to optimize management through site-specific N fertilization and variable-rate planting. This strategy has become feasible with the development of the ISNT, but test values must be interpreted relative to other factors that can limit mineralization or crop N uptake, such as residue C input, soil acidity, or a P or K deficiency. As would be expected if the ISNT reflects productivity differences in the Morrow Plots, test values were highest for the corn-oats-hay rotation (252 mg kg-1), intermediate for the corn-soybean rotation (187 mg kg-1), and lowest for continuous corn (158 mg kg-1).
Plant Population and Fertilizer N Response
Modern corn production relies on high populations of high-yielding varieties, and on soils managed for high productivity. The benefit is evident not only from the Morrow Plots, but from numerous other reports in the scientific literature (e.g., Dungan et al., 1958; Cardwell, 1982). High plant populations create more pressure on soil nutrient supply, and thereby increase fertilizer requirements. This has often been found to hold in N-response studies with corn in Illinois and elsewhere, spanning several decades (e.g., Lang et al., 1956; Illinois Agronomy Handbook, 1967; Mulvaney et al., 2006).
The present study provides further evidence from small-plot and field-scale experiments in Illinois and Minnesota that higher plant populations increase the yield response by corn to N fertilization. Some of this evidence is reported by Fig. 2, for N-response experiments involving three sites under continuous corn or a corn-soybean rotation in two growing seasons, with 96 plots per site. When corn followed soybean, optimum N rates tended to increase with plant population and were higher for the 2006 site, which tested lower by the ISNT than did the 2005 site. This relationship was not observed for continuous corn, probably because of difficulties with irrigation that prolonged the duration of moisture stress, thereby limiting mineralization. Despite this difficulty, there was a significant yield benefit from higher populations, as is consistent with both sites under a corn-soybean rotation.
Potential of Soil-based N Management for High Yields
Figure 2 clearly demonstrates that productivity can be increased substantially with high plant populations; however, consideration must be given to several plant and soil factors before implementing this strategy. A hybrid with high yield potential and standability at high planting densities is a necessary prerequisite, as is a highly productive soil managed to maintain adequate pH, P, and K. Soil N-supplying power must also be adequate, and can be assessed with the ISNT.
The sites studied in our work were adequate in pH, P, and K, so a relationship should have existed between plant population and N demand. This is confirmed by the statistical results summarized for low (group 1) and high (group 2) populations by Table 1, which quantifies the effect on yield of fertilizer N with or without the use of ISNT data. With the piece-wise linear regression employed, a significant relationship was observed between yield and plant population with high planting densities (R2 = 0.79***), whereas this relationship was insignificant at low planting rates (R2 = 0.37), no doubt because N supply exceeded plant productivity. The same limitation accounts for the lack of statistical significance reported in Table 1 for low populations, when yield was related to fertilizer N rates, either alone or in combination with the corresponding plot-scale ISNT values. With high populations, this relationship improved dramatically for fertilizer N, and became statistically significant (P < 0.05) with the addition of ISNT measurements. The latter finding emphasizes the importance of soil testing in high-yield management, so as to optimize pH and the supply of N, P, and K. This will be especially critical for soils that test high by the ISNT, not only as a means of identifying sites that can support a higher plant population, but also to avoid excessive N fertilization that can promote lodging.
Economic Benefit of High Plant Populations
The current trend toward high planting rates is closely linked to hybrid corn improvement, and has been widely advocated by the seed industry as a cost-effective means to enhance profitability. The effectiveness of this strategy for productive soils is verified by Fig. 3, which shows that net profit benefited considerably as planting rates increased from 16,000 to 40,000 plants acre-1.
For high corn yields, N fertilization is crucial for poor soils (ISNT < 150 mg kg-1), while good soils (ISNT > 220 mg kg-1) need an adequate plant population. Profitability in the latter case can be increased with plant populations in the vicinity of 35,000 plants acre-1, provided that plant growth is not subject to a soil or management limitation. As plant populations are increased, there will be a greater need for soil N testing to optimize fertilization and thereby enhance fertilizer N uptake efficiency.
Appreciation is expressed to J. M. Lang and B. D. Martin for technical assistance with the N-response experiments reported.
Figure 3. Effect of plant population on net profitability of corn production.
1S. A. Khan is a Research Specialist, R. L. Mulvaney is a Professor, and T. R. Ellsworth is an Associate Professor, Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, IL; K. Ferrie is an Agronomist, Crop-Tech Consulting, Heyworth, IL; T. J. Smith is an Agronomist, Cropsmith, Monticello, IL; J. S. Strock is an Associate Professor, University of Minnesota, Lamberton, MN; R. M. Vanden Heuvel is an Agronomist, VH Consulting, Hudson, WI.
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