Agricultural Research Centre of Finland, Institute of Crop and Soil Science, Jokioinen, Finland
Field experiments were performed in Southern Finland on three soil types: fine sand (1989-1991), clay (1989) and mull (1990-1991). The following soil mechanical treatments were applied to autumn ploughed land: soil loosening by ridge preparation (ridge distance 45 cm), rotary harrowing (to a depth of 20 cm, clay 15 cm), and soil compaction track by track by a tractor weighing 3 Mg (1 or 3 passes, wheel width 33 cm) before seed bed preparation. One plot was untreated. These treatments were set up in April (on clay in May) under moist soil conditions. Sprinkler irrigation (one application of 30 mm) was applied to clay and fine sand when soil moisture in top soil had decreased to around 50% of plant-available water capacity. PVC cylinders (r = 15 cm, h = 60 cm) were fixed in the experimental areas during the growing periods. At harvest, these cylinders were removed for specific analysis of tap and fibrous roots of carrot. Length and width of fibrous roots were quantified by image analysis in the USA. The impacts of soil loosening and partial compaction were determined by measuring soil physical parameters to a depth of 25 cm in mineral soils, and to greater depths in organic soil. Dry bulk densities of the plough layers increased with increasing tractor passes by 8%, 10% and 13% for fine sand, mull and clay soils, respectively. The lowest dry soil bulk density in the plough layer was obtained by rotary harrowing to a depth of 20 cm. Comparison of gamma ray transmission and gravimetric analysis indicated that dry soil bulk density was slightly lower when determined by gravimetric analysis. Increased soil bulk densities were reflected by increased water retention capacity (matric suction ≤ 10 kPa) and greater penetrometer resistance. Relatively similar increases in bulk density increased the penetrometer resistance much less in mull than in fine sand. In contrast, greater bulk densities in the mull soil affected soil air composition adversely by decreasing the O2 content to 10% when the subsoil had high wetness. In other soils, the lowest soil oxygen contents of 16-18% were recorded in early summer (compacted clay) and during periods of vigorous plant growth (fine sand) when soil water contents were high. Even though the highest degree of soil compactness (D) in a plough layer approached 93 (gravimetric) in all soils, only clay soil was compacted to a soil macro-porosity below 10% (pore diameter > 30 μm). Soil compaction promoted crop establishment and early growth as compared with loose soil beds. Optimum soil compactness for carrot yield (D = 82) was observed only in clay field where excess loosening or compaction affected yield quantity adversely at different stages of growth. During biomass accumulation, excessive penetrometer resistances limited tap root growth in compacted fine sand without irrigation. Water applications promoted shoot growth, but did not affect final shoot and tap root yield. Among the three soil types tested in this study, compaction of mull soil had the least effect on carrot growth and external quality. This paper presents evidence that the internal quality of carrots is only slightly affected by changes in soil physical properties, while the adverse effects of soil compaction on carrot external quality (short, deformed and conical tap roots with greater maximum diameters) are clear. Even though compacted clay soil greatly limited the biomass accumulations in the tap root, which had a high crude fibre content, the carotene (10 mg/100 g carrots) and sugar contents (5%) reached acceptable levels. The lowest carotene contents (4 mg/100 g carrots) were observed in loose mull, following a cool late summer in 1990. The effect of irrigation on carotene content varied from one year to another. High sugar and carotene contents appeared to respond to the high below-ground absorption surface. The fibrous root system of carrots, consisting of mostly very fine roots (diameter 0.15 mm), had total lengths of 150 m in loose fine sand at a soil depth of 0-50 cm (rotary harrowed), 200 m and 300 m in fine sand and mull soils subjected to 3 passes by a tractor wheel. The maximum dry weight (60 μg), length (1.2 cm) and surface area (0.05 cm2) of the fibrous root system per soil volume (cm3) were observed in compacted or irrigated soil to a depth of 30 cm, and also in relation to tap root dry weight. This suggests a capacity of carrot plant for high below-ground absorption potential and optimal biochemial maturation of tap root tissue even when surface soils are compacted. This is supported by higher leaf area, as the early shoot growth was promoted by partial soil compaction. Soil compaction affected the soil physical properties and carrot external quality in agreement with previous studies. Carotene and sugar contents appeared to be unaffected or were slightly increased in riper and firmer carrots of compacted soils. This is consistent with the earlier information about the internal quality of carrot which is shown to be highly dependent on genetic factors and developmental stage of carrot. The present study emphasizes the surface area of carrot fibrous root system as a beneficial factor for maintaining high levels of carotene and sugar contents in tap roots after partial soil compaction.
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