Monica Siler, Heidi Windler, Donn Beighley and *Michael Aide
Southeast Missouri State University
David Dunn and Gene Stevens
University Missouri Delta Center, Portageville, Missouri
Corn (Zea mays L.) grown after rice (Oryza sativa L.) in the Lower Mississippi River Valley frequently experiences yield reductions, greater lodging problems, and overall reduced vigor. No clear consensus exists concerning the reasons for corn's poor performance following rice; however, the soil's physical status created by the cultivation of rice is likely an important aspect. Recently, numerous research efforts have been directed towards understanding the function of soil organic matter (SOM) and the soil structure in promoting the root zone (Baldock and Nelson, 2000; Kay and Angers, 2000, Burke et al., 1998; Cambardella and Elliot, 1993; Franzluebbers and Arshad, 1997).
Soil structure may be defined as the arrangement and organization of the primary soil particles (Hillel, 1980). The primary soil particles may be present in various quantities, sizes, shapes, and orientations and their incorporation into aggregations may be complex, producing natural aggregates that differ in size, shape, strength, and may vary over distance and time (Hillel, 1980). These aggregates may be strengthened by SOM and other cementing agents (Beare et al., 1994ab; Bowman et al., 1999).
The value of soil structure revolves around the creation of soil porosity, resulting in gas exchange, water holding capacity, and space for root development. Each of these attributes are critical to the support of highly effective agricultural systems. Soil structure is profoundly affected by climate, biological activity, crop rotations, and management practices (Al-Darby and Lowery, 1987; Bruce et al., 1990; Hill, 1990; Merrill et al., 1996). Agricultural practices that may reduce benefits afforded by the soil structure include tillage operations and erosion prone cropping systems.
Gas exchange in soils is important for supplying oxygen for root respiration and transferring CO2 to the atmosphere. Soil structure provides pore space for this gas exchange; however, the total porosity is only one factor in the overall efficiency of the gas exchange process. A certain percentage of the total pore space is filled with water (fwater), leaving the remainder of the pore space occupied by gases (fgas), with the relationship expressed as fwater + fgas = 1. Thus, as the soil moisture content changes fgas changes accordingly, altering gaseous diffusion. Oxygen diffusion in air is 104 times more effective than oxygen diffusion in water (Kramer and Boyer, 1995). Secondly, the connectivity of the air-filled porosity must be assessed. If soil pores are connected to other soil pores, the soil is able to sustain gaseous diffusion, whereas isolated pores generally provide vanishingly small soil gaseous diffusion rates. The gas composition includes N2, H2O, O2, CO2, and trace gases, with O2 and CO2 levels determined by the intensity of root and microbial respiration. The CO2 of the bulk atmosphere is approximately 0.0034%, whereas the CO2 content of the soil atmosphere may reach concentrations that approach 0.3%, levels that could become plant toxic (Hillel, 1980). Oxygen consumption in the soil environment may exceed the supply of O2 from the atmosphere (0.21%), resulting in O2 depletion and the onset of anaerobic conditions. Hillel (1980) reviews the soil physics of gaseous diffusion in the soil environment. Root surfaces generally require 0.4 mg O2 x cm-2 x min-1 for normal physiological functioning, whereas 0.2 mg O2 x cm-2 x min-1 is considered limiting (Hillel, 1980).
The root system of a plant provides many important physiologic functions essential to the proper growth, development, and functioning of a plant. These functions include: anchorage, hormonal synthesis, nutrient and water uptake, and carbon storage (Kramer and Boyer, 1995). Many soil and climatic factors influence root function, with soil water extremes, soil acidity and reduced aeration being the most prevalent and important. Reduced aeration affects hormonal synthesis in the root system, resulting in reduced production of exportable cytokinins and gibberellins and enhanced production of abscissic acid (Kramer and Boyer, 1996). The nutrient uptake of P, K, and other elements is restricted because of a reduced respiratory rate (Tisdale et al., 1985). Typically, plants that experience reduced oxygenation of their root systems are associated with poorly drained and fine-textured soils in which a large percentage of the pore space contains water and the air-filled pores present a torturous pathway for oxygen diffusion. Typical symptoms include a reduced and shallow root system, anoxic conditions in the stele, stunting, reduced nutrient uptake, a proliferation of adventitious roots near the soil surface, thin culms, and reduced crop yields (Kramer and Boyer, 1996).
The purpose of this research project is to determine if the soil fertility and the physical status of the soil are impacted by rice and if these soil changes are detrimental to the growth of corn. As a corollary, this project hopes to propose tillage methods that optimize the growth and yield potential of corn cultured in soil after rice cultivation.
Materials and Methods
Experiments were conducted to determine: (1) if soil hipping (raised beds) was a better tillage option than no-till planting and (2) to assess corn growth on land that had been previously planted to: (i) soybeans (Glycine max. L), (ii) drill-seeded rice, and (iii) water-seeded rice.
Experiment One: Raised Beds (Hipping) versus No-till Planting (Flat)
An experiment evaluating corn growth on hipped and flat seedbeds was conducted at the Missouri Rice Research and Demonstration Farm (near Glennonville, Missouri) on a somewhat poorly drained Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualfs) during 2000 and 2001. Soil testing demonstrated that the soil had a satisfactory soil fertility level during 2000 and was somewhat P and K deficient during 2001 (Table 1). The experimental design consisted of replicated blocks for the hipped (raised) and flat beds with individual plots established after stand development. Raised and flat beds had a row spacing of 36 inches (0.914 m) and were planted with hybrid corn to establish a population of approximately 1.4 plants / row-foot (20,300 plants / acre) in 2000 and 1.7 plants / row-foot (26,000 plants / acre) in 2001. Nitrogen fertilizer (Urea) was applied according to soil test (120 lbs N x acre-1) at planting in 2000 and at emergence in 2001. Irrigation was flood, followed by draining, with scheduling conducted climate records.
After planting, the bulk density was determined using the ring method and the saturated hydraulic conductivity was determined using enplaned steel cylinders (Carter, 1993). Measurements were taken in triplicate for the raised bed, the flat bed, and the underlying subsoil. Additionally, the distribution of the soil aggregates was estimated using dry sieving. In this procedure, bulk soil samples were dried at 110EC and gently sieved through sieves (5, 20, and 60 Mesh) to roughly assess if the tillage program altered the distribution of the aggregates (DeFreitas et al., 1996).
Tissue testing in 2000 was performed five times [22 May (fully emerged leaves from 20 plants), 6 June (fully emerged leaves from 20 plants), 27 June (uppermost fully developed leaf from 10 plants), 17 July (leaf one node above first developing ear from 10 plants)]. Tissue testing in 2001 consisted of a mid-July sampling involving leaf sampling, one node above the developing ear, from 10 plants. Nitrogen was determined using semi-micro Kjeldahl, whereas S, P, K, Ca, Mg, Na, Al, Fe, B, Mn, Cu and Zn were determined by inductive-coupled plasma-emission spectroscopy after ignition at 500 - C in a controlled temperature furnace, followed by uptake in 5% HNO3. Roots were similarly collected (6 June and 17 July) and washed repeatedly in distilled water before tissue testing.
Total plant biomass was assessed twice during the 2000 growing season and once during the 2001 growing season. Four (2000) and eight (2001) visually representative plants were selected for analysis from each plot. The plant and its allotted soil volume were excavated and transported to the laboratory. The soil was carefully washed from the root system after soaking of the soil-root mass in large tubs of water for several days. Roots, stems (culms), leaves, tassels, and ears were separated and dried at 70 - C for several days. Plant dry weight (mass) was obtained by weighing.
Experiment Two: Corn after Soybeans, Drill Seeded Rice, and Water Seeded Rice
Experiment two was conducted in a commercial field in 2000 where corn was planted after previous crops of soybeans, drill-seeded rice, and water-seeded rice. The soil type was a somewhat poorly drained Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualfs). Soil testing demonstrated that the soil had a satisfactory soil fertility level (Table 3). Fertilization consisted of 90 lbs of urea, 90 lbs of a (0-23-30) mixed fertilizer at planting and 200 lbs of liquid N after emergence. Planting dates for corn were 7 April (soybeans and drilled seeded rice plots) with a 29 inch row spacing for the soybean plot and an emergence count of 1.4 plant/row-foot and a 31 inch row spacing and an emergence rates of 1.3 plants / row-foot for the drill seeded rice plot. Corn was planted on 13 April for the water seeded rice plots, consisting of a 31 inch row spacing and an emergence rate of 1.3 plant / row-foot. Irrigation was furrow irrigation. All methods are similar to those described for the above experiment.
Soil Fertility and Physical Measurements (Hipped versus Flat)
Soil test results in 2000 suggest that the plots have reasonable soil fertility and soil fertility differences between the plots are minor. Phosphorus is somewhat P deficient (P value of 30 lbs P/ac is considered adequate for rice, whereas a value of 45 lbs P/ac is adequate for corn). Soil test results for 2001 indicate that P and K are somewhat deficient. The level of soil fertility was roughly equivalent throughout the study area and differences in soil fertility are not sufficient to influence or bias tillage treatments.
The soil bulk density is low, indicating a pore space of approximately 58% for the hipped and flat surface soil layers (Table 1). The bulk density of the subsoil is appreciably higher, indicating a more compact soil layer and a reduced total pore space. The pore space of the subsoil, if air-filled and consisting of connected pores, should be sufficient to support root development. However, the moisture content and the fine texture suggests that the soil pores are similarly small and frequently water saturated. The soil structure of the subsoil immediately below the seedbed is composed entirely of moderate, very fine to medium platy structures, suggesting a torturous pathway for water and air transmission. Additionally, the compact subsoil has the likelihood of restricting the developing root system of corn because of reduced soil temperatures, wetness, and physical hindrance of the elongating roots.
The aggregate size analysis indicates that the majority of the A horizon in the seed-bed is composed of very fine spherical aggregate and non-aggregated material, suggesting that the water culture of the earlier rice crop has "slaked" the natural aggregates and promoted the potential for compaction, filling of pores with silt, and reducing the diffusion of oxygen to the developing root system.
The saturated hydraulic conductivity for the hipped plots averages 1.05 x 10-4 cm x s-1, whereas the flat plots averaged 2.65 x 10-5 cm x s-1. The flat plots have a significantly reduced infiltration capacity, indicating that water flow and presumably air flow have been compromised by the lack of large connected pores. The saturated hydraulic conductivity of the subsoil is 2.39 x 10-6 cm x s-1, a very low value suggesting appreciable resistence to water and air flow.
Tissue Testing of Corn Planted in Hipped and Flat Beds following Rice
Tissue testing demonstrates that the nutrient levels for corn are appropriate and that soil fertility has not limited plant growth (Table 2). Nutrient levels decline with later sampling dates, a trend that is consistent with a normal corn growth pattern (Tisdale et al., 1985). Root tissues have smaller macronutrient concentrations than leaf tissues; however, Fe and Mn are appreciably more concentrated in root tissues, indicating root accumulation that is consistent for a soil with an acidic pH. Nutrient levels between the hipped and the flat tillage designs are equivalent, indicating that any yield differences between these two treatments are not likely attributable to soil fertility. Tissue testing in 2001 suggests that K is slightly low and B was low; however, differences in these nutrients between treatments were not significant (Table 3).
Biomass and Yields for Corn Following Rice (Hipped versus Flat)
The total plant biomass and the distribution of the biomass among the root system, stem, leaf, and ear in 2000 indicate differences because of the tillage treatments (Fig. 1). Total biomass, stem and root growth are appreciably greater in the hipped system. In 2001, the total biomass was greater for the hipped system (Fig. 2); however, the distribution of the biomass was roughly equivalent between the tillage systems. The leaf to root ratio for the hipped system (1.24 with a standard deviation of 0.08) was significantly smaller than leaf to root ratio for the flat system (2.0 with a standard deviation of 0.8). Average plant height (not measuring tassel length) was significantly greater for the hipped system (67 inches with a standard deviation of 7.5 inches) than the flat system (58 inches with a standard deviation of 4.7 inches). Yield estimates indicate that the hipped system in 2000 returned 5900 lbs/acre, whereas the flat system returned 4700 lbs/acre. Yield estimates in 2001 for the hipped treatment averaged (5670 lbs / acre) 103 bu/acre, whereas the flat treatment averaged only (3920 lbs / acre) 70 bu/acre (Fig. 3). Thus the yield potential of hipped system was appreciable for both years.
Results For Experiment #2 (Corn after Soybeans, Drill, and Water seeded Rice)
Soil test results suggest that the plots have reasonable soil fertility for rice culture and soil fertility differences between the plots are minor. Phosphorus is adequate for all systems (P value of 30 lbs P/ac is considered adequate for rice, whereas a value of 45 lbs P/ac is adequate for corn). The CEC and the percent base saturation are slightly greater than those found on the Missouri Rice Research and Demonstration Farm (Table 4).
Tissue Testing of Corn following Rice (Soybeans, Drill Seeded Rice, Water Seeded Rice)
Tissue testing demonstrates that the nutrient levels for the corn plant are appropriate, suggesting that the soil fertility has not hindered plant growth (Table 5). Nutrient levels decline with later sampling dates, a trend that is consistent with a normal growth pattern of an annual crop (Tisdale et al., 1985). Root tissues are generally lower in the macronutrients than the leaf tissues; however, selected micronutrients such as Fe and Mn are appreciably greater in root tissues, indicating root accumulation. Nutrient levels in the root systems and the leaves among the soybean, drill seed and water seeded designs are largely equivalent, indicating that any yield differences between these three treatments are not likely attributable to soil fertility.
Biomass and Yields for Corn (Soybeans, Drill Seeded Rice, Water Seeded Rice)
The total plant biomass and the distribution of the biomass among the root system, stem, leaves, and ears indicate differences because of the previous crop (Fig. 4). Total biomass, ear, stem, leaves, and root growth are appreciably greater in corn-after-soybean systems than either of the corn-after-rice systems. The corn after drill-seeded rice produced a greater total biomass and greater biomass among the root system, stem, leaves, and ear than the corn after water-seeded rice (Fig. 5). The leaf to root ratio for the corn after soybean system (1.76 with a standard deviation of 0.52) was similar to the leaf to root ratio for the corn after rice systems (1.8 with a standard deviation of 0.48). Average plant height (not measuring tassel length) for the corn after soybean system (68.5 inches with a standard deviation of 6.1 inches) was roughly equivalent to the corn-after-drill-seeded rice (71.3 inches with a standard deviation of 8.7 inches) and significantly greater than the corn-after-water seeded-rice (52.5 inches with a standard deviation of 3.0 inches).
Yield estimates indicate that the corn after soybean system returned 8571 lbs/acre and the corn-after-drill-seeded rice system returned 8686 lbs/acre, yields that are roughly equivalent (Fig. 5). The corn-after-water-seeded rice system returned 5828 lbs/acre, thus the yield potential of corn-after-water-seeded rice system was significantly impacted by the previous crop.
The following conclusions appear valid:
(1) Soil fertility did not appear to be a deciding factor in the development of corn between rotations involving corn-soybeans and corn-rice.
(1) Physical properties of the soil suggest that the total pore space is normal for a silt loam soil; however, the distribution of the pores, the average moisture content and the plate soil structure is such that the majority of pores are small and the movement of water and air is difficult. The lack of significant granular or blocky soil structures and the large percentage of small pores likely resulted from weakening and slaking of the soil structure during flooding of the previous rice crop.
(1) Hipping promoted corn growth and yield, suggesting that the tillage system provided a more suitable rooting environment and gas exchange.
(1) It is the consensus of the research team that oxygen diffusion from the atmosphere to the developing root system of corn is a limiting factor. Future research should address the augmentation of oxygen diffusion in soil.
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