Plant-availability of soil and fertilizer zinc in cultivated soils of Finland

The Zn status of cultivated soils of Finland was investigated by chemical analyses and bioassays. The effect on ryegrass of different Zn fertilizers and Zn rates was studied in pot experiments and their effect on barley and timothy in field experiments. In an uncontaminated surface soil material of 72 mineral soils and 34 organogenic soils, total Zn (Zntot) was 10.3-202 mg kg-1(median 66 mg kg-1). In mineral soils, Zntot correlated positively with clay content (r = 0.81***) and in organogenic soils negatively with organic C (r = -0.53***). Zinc bound by organic matter and sesquioxides was sequentially extracted by 0.1 M K4P2O7 (Znpy) and 0.05 M oxalate at pH 2.9 (Znox), respectively. The sum Znpy + Znox, a measure of secondary Zn potentially available to plants, was 2 - 88% of Zntot and was the lowest in clay (median 5%) and highest in peat soils (median 49%). Water-soluble and exchangeable Zn consisted of0.3 - 37% (median 3%) of Zntot, the percentage being higher in acid soils, particularly in peat soils. Zinc was also extracted by 0.5 M ammonium acetate - 0,5 M acetic acid - 0.02 M Na2-EDTA at pH 4.65 (ZnAC), the method used in soil testing in Finland. The quantities of ZnAC (median 2.9 mg dm-3, range 0.6 - 29.9 mg dm-3) averaged 50% and 75% of Znpy + Znox in mineral and organogenic soils, respectively, and correlated closely with Znpy. In soil profiles, ZnAC was with few exceptions higher in the plough layer (0 - 20 cm) than in the subsoil (30 - 100 cm). In an intensive pot experiment on 107 surface soils, four crops of ryegrass took up 2 - 68% (median 26%)of Znpy + Znox. The plant-available Zn reserves were not exhausted even though in a few peat soils the Zn supply to grass decreased over time. Variation of Zn uptake was quite accurately explained by ZnAC but increasing pH had a negative impact on Zn uptake. Application of Zn (10 mg dm-3 of soil as ZnSO4 * 7 H2O) did not give rise to yield increases. In mineral soils, increase of plant Zn concentration correlated negatively with soil pH while ZnAC was of secondary importance. In those organogenic soils in which the reserves of native Zn were the most effectively utilized, plant Zn concentration also responded most strongly to applied Zn. In two 2-year field experiments, Zn application did not increase timothy or barley yields. Zinc concentration of timothy increased from 30 mg kg-1 to 33 and 36 mg kg-1 when 3 or 6 kg Zn ha-1 was applied, respectively. The efficiency of ZnSO4 * 7 H2O alone did not differ from that of a fertilizer where ZnSO4 * 7H20 was granulated with gypsum. Zinc concentration of barley grains increased by foliar sprays of Na2Zn-EDTA but only a marginal response to soil-applied Zn (4.8 or 5.4 kg ha-1 over three years) was detected in three 3-year experiments. High applications of Zn to soil (15 or 30 kg ha-1 as ZnSO4 * 7H2O) were required to increase Zn concentration of barley markedly. In order to prevent undue accumulation of fertilizer Zn in soil, it is proposed that Zn fertilizer recommendations for field crops should be based on both soil pH and ZnAC. In slightly acid and neutral soils, even if poor in Zn, response of plant Zn concentration to applied Zn remains small while there is a high response in strongly acid soils