The Zinc Fertility of Saskatchewan Soils
Singh, Jai Pal
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Field, growth chamber, and laboratory studies were conducted to investigate the zinc fertility of Saskatchewan soils. These studies included an examination of Zn spatial variability, a survey of DTPA-extractable Zn levels of soils from across the province, a variety of experiments on Zn fertilizers, and investigations on the mechanisms of P X Zn interaction in wheat (Triticum aestivum L.) and beans (Phaseolus vulgaris L.). In addition, the distribution and plant availability of native and applied Zn in various soil Zn fractions were investigated in three soils that varied widely in physical and chemical properties. DTPA-extractable micronutrients (Zn, Cu, Mn and Fe) in fields of level topography showed a high degree of variability in samples taken along transects and geostatistical analyses (semivariograms) indicated that the extractable micronutrients were randomly distributed. In contrast, DTPA-extratable Zn and tissue Zn concentration in undulating topography showd spatial structure with a range of 25 m in all except one of the transects considered. DTPA-extractable Zn and organic C levels were highly correlated and had similar spatial structure. Current criteria for predicting Zn deficiency in Saskatchewan soils are based on DTPA-extractable Zn values. DTPA-extractable Zn levels in 12% of 1200 samples taken across Saskatchewan contained <0.5 mg Zn kg-1 soil and would be classified as potentially Zn deficient. However, 23 field trials in 1982, 1983 and 1984 with spring wheat (Triticum aestivum L.),barley (Hordeum vulgare L.), lentils (Lens esculenta Moench.), peas (Pisum sativum L.), alfalfa (Medicago sativa L.), corn (Zea mays L.), and flax (Linum usitatissimum L.) produced only one significant response to Zn fertilization. The relative effectiveness of ZnSo4, ZnEDTA, low-yield ammonium-based lignosulphonate (ZnLY) and high-yield ammoniurn-based lignosulphonate (ZnHY) for bean production was tested in growth chamber and incubation experiments. ZnLY was more effective than ZnEDTA and ZnHY in correcting Zn deficiency of bean plants. While biomass production was best with znS04, ZnLY was more effective in increasing Zn content of the foliage. Field and growth chamber studies were carried out to elucidate the mechanisms of P X Zn interaction in the nutrition of wheat and beans, respectively. In the field, residual P levels from 80 or 160 kg P ha-1 applied in 1979 to a Sutherland soil (Dark Brown Chernozemic) increased wheat yield significantly. In the presence of P, soil and foliage applied Zn in 1984 resulted in significant increase in grain yield and Zn uptake into grain. High levels of P in the soil increased tissue P concentration but Zn concentration was decreased significantly. DTPA-extractable soil Zn levels in the non-Zn amended treatment were independent of P application rate. A close relationship was obtained between Zn levels in the above ground plant parts and vesicular-arbuscular mycorrrhizal (VAM) infection. VAM infection of wheat roots was significantly lower in high P treatments than in the control. In the growth chamber, the P X Zn interaction in roots and above ground parts of bean was studied in the Sutherland (Dark Brown Chernozemic) , Carrot River (Dark Gray Chernozemic) and Meota (Black Chernozemic) soils. The P X Zn interaction on dry matter yield of above ground parts was significant for the Sutherland and Carrot River soils only. Applied Zn increased yield only when P was applied. Application of P reduced Zn concentration in tops below the critical levels at 1/10 bloom growth stage in the Sutherland and Carrot River soils. In the 0 applied Zn treatment, a response was obtained with P application but translocation of Zn from roots to tops was inhibited at both the 80 and 160 mg applied P kg-1. Thus both Zn dilution in the plant and reduced translocation of Zn from roots to tops accounted for the P X Zn interaction observed. Phosphorus absorption by roots and accumulation of P in above ground plant parts was markedly reduced by Zn application. However, in the absence of applied Zn, P levels in bean tissue never reached toxic levels. Native and applied Zn in three soils (used for P X Zn interaction experiment with beans) were separated into water soluble plus exchangeable (EX), specifically adsorbed (ExAD), organically bound (OM), Mn-oxide bound (MnOX), Fe-oxide bound (FeOX) and residual (RES) fractions. The major proportion of native Zn in all soils was recovered in the RES and FeOX fractions . The EX, ExAD, OM and MnOX fractions accounted for only a small portion of native Zn. In contrast, most of the applied Zn in the Meota and Carrot River soils was recovered in the ExAD fraction, whereas in the Sutherland soil in the OM and FeOX fractions. The amount and percent distribution of applied Zn in the OM, FeOX and MnOX fractions of all soils varied directly with the organic C and Fe- and Mn-oxide contents, respectively. The distribution of applied Zn in various fractions was independent of applied P. The distribution of applied Zn in various Zn fractions in combination with Zn plant uptake indicated that the EX and ExAD fractions were the most potentially plant available pools.