Effect of soil pH and zinc on rice cultivars in Missouri.
Rice zinc deficiency occurs in Southeast Missouri. When present, the deficiency is usually observed shortly after the permanent flood is established at the first tiller growth stage. Low levels of zinc in rice plants can cause loss of turgidity of the leaves, basal chlorosis of the leaves, delay of plant development, "bronzing" of leaves, and in some cases death of the rice seedlings (Wells et al., 1993). Zinc tissue concentrations generally range from 25 to 150 mg Zn [kg.sup.-1], with deficiency symptoms appearing whenever the tissue concentrations are less than 20 mg [kg.sup.-1] (Tisdale et al., 1985).
The availability of zinc in rice soils decreases as soil pH increases (Ntamatungiro et al., 1999; Barbosa et al., 1992). Because well water used to irrigate rice in Southeast Missouri is usually high in calcium carbonate, flooding rice fields often has a liming effect, which increases soil pH. Tracy and Hefner (1991) found that the average concentration of Ca in irrigation wells in Butler County, Missouri was 249 part per million. They calculated that applying 1,234 [m.sup.3] (1 acre foot) of irrigation water was equivalent to applying 1.9 tonnes of calcium carbonate equivalents per hectare.
Soluble soil Zn concentrations vary greatly in the aqueous phase (2 to 75 [mu]g [L.sup.-1]), with the specific concentration dependent upon the native amount of soil Zn, the pH, the quantity and type of organic materials, and the character of the adsorbing oxide and clay surfaces (Mills and Jones, 1996). Approximately 65% of the total Zn in the aqueous phase is organically complexed. Sajwan and Lindsay (1986) demonstrated that Zn applications to flood irrigated rice increased the DTPA (diethylenetriaminepentaacetic acid) extractable levels of Zn and decreased the levels of DTPA extractable Mn and Fe. The DTPA concentrations for Zn, Mn, and Fe were proportional to the corresponding metal uptake by nce. Sajwan and Lindsay suggested the elevated levels of Mn might effectively reduce the plant uptake of Zn.
In Arkansas, Slaton et al. (1999) reported that recent field studies suggest that rice yield response to Zn fertilization occur less frequently today than 25 years ago. This may be due to improvements in rice cultivars. On one site, Slaton et al. (1999) found a significant effect on grain yield from the type of Zn fertilizer applied.
The purpose of this investigation was to determine whether elevated soil pH is antagonistic towards Zn uptake in Missouri rice. The secondary objective was to determine the efficacy of zinc fertilization with soil and foliar Zn applications on rice cultivars.
Materials and Methods
A rice study was conducted on a field at the Missouri Rice Research Farm (36[degrees]N, 90[degrees]W) in Dunklin County, Missouri in 1998 and 1999. Rice was planted on a Crowley series (fine, montmorillonitic, thermic Typic Albaqualf). The soil has a silt loam eluvial horizon which overlies a thick silty clay loam argillic horizon. This is a typical soil for producing drill seeded rice in Southeast Missouri (Garrett et al. 1978). The soil [pH.sub.water] of the surface horizon was 6.2. There was no record of zinc fertilizer ever being applied on the site. The field was conventionally tilled.
The experimental design was a split plot design with three replications. Rice was planted with a grain drill in late May in 1998 and 1999. Main plot treatments were three high yielding, medium season rice cultivars available to Missouri rice producers in 1997 (Minor and Stafford, 1998). The cultivars were Cypress, Kaybonnet, and Drew. Each subplot was 3.1m wide and 6.2 m long. Subplot treatments were six combinations of lime and Zn fertilization levels. Lime and soil Zn ([ZnSO.sub.4]) treatments were broadcast by hand on subplots two weeks before planting. The two lime treatment levels were 0 (control) and 4.48 tonnes [CaCO.sub.3] [ha.sup.-1]. In 1998, pelletized lime was applied. The value of this lime was 544 effective neutralizing material (ENM) per tonnes. In 1999, calcite agricultural lime (431 ENM per tonne) was applied. Values for lime quality were determined at the University of Missouri Experiment Station Chemical Laboratory. The three zinc treatment levels were 0, 22.4 kg [ZnSO.sub.4] [ha.sup.-1], and 5.6 kg ZnEDTA [ha.sup.-1]. The [ZnSO.sub.4] was dissolved in water for each subplot and evenly applied with a watering can. The ZnEDTA was foliar applied in 187 L [ha.sup.-1] water with a [CO.sub.2] backpack sprayer before rice was flooded at first tiller growth stage.
Soil samples of the study area were collected from the 0 to 15 cm depth before planting in 1998 and one month after harvest each year. Soil analysis consisted of pH (water), neutralizable acidity, exchangeable cations (Ca, Mg, K. Na), loss on ignition (LOI), Bray 1 extractable phosphorus, and DTPA extraction and atomic absorption determination of Zn, Fe, Mn, and Cu.
Nitrogen fertilizer was applied at first tiller growth stage immediately prior to flood at the rates of 101 kg N [ha.sup.-1] as urea on Cypress rice and 84 kg N [ha.sup.-1] on Drew and Kaybonnet. All plots received 34 kg N [ha.sup.-1] urea applications at 1.3 cm internode elongation and a second application one week later. Proponil (3,4-dichloropropionanilide) was broadcast at a rate of 4 kg a.i. [ha.sup.-1] for grass control before the flood was established.
Twenty five new growth rice leaves were collected from each subplot at mid tillering and 1.3 cm internode elongation growth stages in 1998 and 1999. The elemental concentrations of Ca, S, Mg, Al, P, K, Zn, B, Fe, Mn, Cu, Na in the leaves were determined by inductively coupled plasma emission spectroscopy after acid digestion of dried and ground plant tissues. Field measurements involved tiller counts, the number of panicles per row, the number of spikelets per panicle, seed weight, dry matter accumulation, moisture content and yield. The rice was harvested with a plot combine. Grain was collected from 1.5 m wide by 5.2 m long areas from each subplot. Yields were adjusted to 13% seed moisture content. Statistical analyses of the data were preformed with SAS (1990) using General Linear Modeling procedures. Fisher's Protected Least Significant Difference (LSD) was calculated at the 0.05 probability level for making treatment mean comparisons.
Results and Discussion
Soil pH was increased in subplots by lime treatments. In 1999 the untreated check was pH 6.1 as compared to 7.3 for lime treatments (Table 1). This was due to two years of lime application on the same subplots. Soil pH had a significant effect on the extractable soil Zn following two years of applying [ZnSO.sub.4] (Table 1). Extractable soil Zn in plots that received [ZnSO.sub.4] fertilizer and no lime was 63% greater than Zn levels in plots with [ZnSO.sub.4] and lime. However, the DTPA extractable Zn was greater than 0.4 mg Zn [kg.sup.-1] in all plots, suggesting that Zn levels were sufficient for rice production. The extractable Mn values generally reflect the changes in pH associated with the lime treatments. Manganese levels decreased as pH increased. Iron values were not effected. There were no evidence for a Zn x Mn or Zn x Fe interactions, as suggested by Sajwan and Lindsay (1986).
The elemental concentrations of S, P, K, Mg, Ca, Na, B, Cu, and Al were typical for normally developing rice (Mills and Jones, 1996). No significant differences in these elements were evident within or among cultivars. In 1998, the concentration of Zn was not effected by cultivars (Table 2). In 1999, a significant interaction between cultivars and zinc fertilizer sources for plant Zn concentration was found. Tissue Zn concentrations associated with soil Zn applications for Drew and Kaybonnet cultivars were significantly greater than for plants in foliar Zn and untreated check plots. There were no differences found in plant Zn uptake in Cypress plots. All Zn concentrations were above the deficiency threshold of 20 mg Zn [kg.sup.-1], especially for the 1999 trial.
Growth and Yield
Cultivar, Zn, and lime treatments did not significantly effect tillering, panicle development, or seed weight. The only significant differences in rice yield were due to cultivar selections.
After two growing seasons, soil pH in plots with lime treatments was elevated to 7.2. Extractable soil Zn levels were lower in limed plots that received soil applied [ZnSO.sub.4] fertilizer than unlimed plots with [ZnSO.sub.4]. This indicates that elevated soil pH may be antagonistic towards extractable Zn levels. However, liming did not significantly reduce Zn plant uptake or the growth and development of rice on this soil. We attribute this lack of response to the high native levels of Zn released in the soil. High tissue levels of Mn may have inhibited the luxury uptake of Zn. Certainly the high Mn levels did not induce a Zn deficiency.
The effect of cultivars on Zn uptake was evident in 1999. Tissue tests showed that Drew and Kaybonnet cultivars were more efficient than Cypress in taking up soil applied [ZnSO.sub.4] This may be because these cultivars have better root systems for taking up nutrients including nitrogen. Missouri preflood nitrogen recommendations for Drew and Kaybonnet are 84 kg N [ha.sup.-1] compared to recommended rate of 101 kg N [ha.sup.-1] for Cypress. We did not observe the Zn deficiency symptoms in rice in this study that are sometimes found in farmer's fields. If soil pH values had increased to near 8.2, Zn deficiency would have been more likely. Based on the results of this test, we recommend that if a field has a history of Zn deficiency that a farmer soil apply [ZnSO.sub.4] fertilizer and plant one of the newer cultivars.
Table 1 Mean effect of lime and zinc applications on soil pH, Fe, Mn and Zn concentrations in 1999 Applications pH Fe Mn Zn Units mg [kg.sup.-1] No Lime No zinc 6.1 108 42.0 1.21 Soil zinc 6.0 104 35.7 8.91 Foliar zinc 6.1 107 37.3 3.01 Lime No zinc 7.2 102 30.3 1.35 Soil zinc 7.1 97 30.3 3.25 Foliar zinc 7.2 102 30.3 2.05 [LSD.sub.05] 0.3 3 5.9 1.85 C.V.% 8 5 30.5 19.8 Table 2 Mean effect of lime and zinc applications on rice plant nutrient concentrations at internode elongation growth stage and yield in 1998 Cultivar Zinc Fe Mn Application mg [kg.sup.-1] mg [kg.sup.-1] Cypress No Lime No zinc 150 1266 Soil zinc 138 1240 Foliar zinc 180 1341 Lime No zinc 159 1247 Soil zinc 147 1105 Foliar zinc 184 1289 Kaybonnet No Lime No zinc 232 1772 Soil zinc 183 1698 Foliar zinc 140 1629 Lime No zinc 165 1827 Soil zinc 183 1569 Foliar zinc 119 1178 Drew No Lime No zinc 132 1392 Soil zinc 176 1758 Foliar zinc 152 1343 Lime No zinc 179 1571 Soil zinc 119 1199 Foliar zinc 150 1371 [LSD.sub..05] ns (a) ns C.V.% 32.5 20.0 Cultivar Zn Rice Yield mg [kg.sup.-1] kg [ha.sup.-1] Cypress No Lime 44 5242 34 5393 24 5292 Lime 21 5645 36 6098 27 5544 Kaybonnet No Lime 36 7560 28 6754 30 7510 Lime 31 6754 36 6905 32 6552 Drew No Lime 37 6804 48 7358 33 6955 Lime 30 7056 25 6602 41 7106 [LSD.sub..05] ns ns C.V.% 55.1 11.6 (a) Means were not significantly different at the 0.05 level. Table 3 Mean effect of lime and zinc applications on rice plant nutrient concentrations at internode elongation growth stage and yield in 1999 Cultivar Zinc Application Fe Mn mg [kg.sup.-1] mg [kg.sup.-1] Cypress No Lime No zinc 212 1104 Soil zinc 256 1107 Foliar zinc 235 1261 Lime No zinc 256 1269 Soil zinc 274 1242 Foliar zinc 208 1182 Kaybonnet No Lime No zinc 223 1167 Soil zinc 280 1231 Foliar zinc 222 1132 Lime No zinc 208 977 Soil zinc 184 1250 Foliar zinc 271 1181 Drew No Lime No zinc 304 1126 Soil zinc 197 1384 Folair zinc 310 1374 Lime No zinc 244 1468 Soil zinc 237 1269 Folair zinc 195 1131 [LSD.sub..05] (comparisons within cultivars) ns (a) ns C.V.% 29.0 16.0 Cultivar Zn Rice Yield mg [kg.sup.-1] kg [ha.sup.-1] Cypress No Lime 54 7610 49 6703 58 7157 Lime 55 7006 58 7358 46 7308 Kaybonnet No Lime 48 7358 65 7358 41 7711 Lime 46 7661 61 6552 53 6955 Drew No Lime 44 7711 63 7006 53 7610 Lime 52 7157 66 6955 54 7358 [LSD.sub..05] (comparisons within cultivars) 16 ns C.V.% 20.0 6.4 (a) Means were not significantly different at the 0.05 level. Analysis of variance showed a significant interaction between cultivar and zinc fertilizer for plant zinc concentration.
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