Abstract
The residual effect of repeated P fertilizer applications was studied in a material of 30 silty clay soil samples collected from an 11-year field experiment in which a total of 0, 154, 309, 541 or 696 kg P/ha had been applied in annual doses. Half of the experiment had been limed twice with CaCO3 (10 tons/ha). In a pot experiment, six yields of Italian ryegrass were grown in soils taken from each plot, and the P uptake by the grass was determined. Soil P was extracted with water (Pw) and 0.5 M ammonium acetate-0.5 M acetic acid at pH 4.65 (PAAAC)- Reversibly adsorbed P (Pi) was extracted by a new method in which P desorbing from the soil was collected in strips of filter paper impregnated with iron hydroxide. P uptake by pot-grown grass from soils fertilized with increasing rates of P in the field corresponded to 30, 72, 100 and 112 kg larger quantities of P per hectare, compared to the soil not receiving P in the field experiment. The apparent utilization of residual fertilizer P ranged from 16 % to 25 %. The reserve of potentially desorbable P in soil had been affected much more by the fertilizer applications than had P uptake by crops in the field. The ability of the three extraction methods (Pw, Pi, PAAAC) to predict P uptake by pot-grown ryegrass was discussed. The Pi method appeared to be well suited for assessment of potentially available P reserves both in limed and unlimed soils.
Highlights
Since soil testing was started in Finland in the 19505, the average of soluble P, extracted with an ammonium acetate solution at pH 4.65 (Paaac) (Vuorinen and Mäkitie 1955), has increased from 5.4 mg/dm3 in1955—60 to 12.5 mg/dm3 in 1988 (Kähäri et ai. 1987, Kähäri 1989), indicating accumulation of soluble residual P in soils
The pot experiment showed that the different rates of P fertilization applied during the previous field experiment had a considerable residual effect and had obviously greatly influenced the pool of soluble P of the soil, as was suggested by the results of the three extraction methods (P w, P, Paaac)P uptake by the pot-grown ryegrass corresponded to the range of 110 to 220 kg P/ha in a plough layer consisting of 2 500 000 kg of soil, revealing the considerable reserve of potentially desorbable P in the soil
The cereal crops grown in the field experiment had taken up a total of 155 to 171 kg P/ha during the 11 experimental years (Yli-Halla 1989 b), and had responded only slightly to the wide range of plant-available P reserves in the plots even though the experimental soil was only average in P status, compared with the means of Pw, P, and presented in other studies (e.g. Hartikainen 1982, Kähäri 1987, Yli-Halla 1989 a)
Summary
Since soil testing was started in Finland in the 19505, the average of soluble P, extracted with an ammonium acetate solution at pH 4.65 (Paaac) (Vuorinen and Mäkitie 1955), has increased from 5.4 mg/dm in1955—60 to 12.5 mg/dm in 1988 (Kähäri et ai. 1987, Kähäri 1989), indicating accumulation of soluble residual P in soils. Differing from fertilization recommendations, the actual quantity i of potentially desorbable P (capacity factor) cannot be determined by methods such as AAAc or water extractions, which primarily reflect the concentration of P in the soil solution (intensity factor), because the relationship between the intensity and capacity factors of P is regulated, for example, by clay content and the contents of poorly crystalline Al-oxide and Fe-oxide. Neither can the reserves of potentially solubleP be estimated to be equal to the quantity of fertilizer P remaining in the soil, i.e. the difference between P fertilization and uptake by the crop, because the solubility of residual P decreases over time. This phenomenon was recently exemplified in an. Intensive pot experiments have been used to estimate the residual effect of P fertilization (Novais and Kamprath 1978, Steffens 1987)
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