EFFECT OF SOIL COMPACTNESS ON THE GROWTH AND QUALITY OF CARROT

Field experiments were performed in Southern Finland on three soil typ es: fine sand (1989-1991), clay (1989) and mull (1990-1991). The follo wing soil mechanical treatments were applied to autumn ploughed land: soil loosening by ridge preparation (ridge distance 45 cm), rotary har rowing (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) b efore seed bed preparation. One plot was untreated. These treatments w ere set up in April (on clay in May) under moist soil conditions. Spri nkler irrigation (one application of 30 mm) was applied to clay and fi ne 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) w ere fixed in the experimental areas during the growing periods. At har vest, these cylinders were removed for specific analysis of tap and fi brous roots of carrot. Length and width of fibrous roots were quantifi ed by image analysis in the USA. The impacts of soil loosening and par tial 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 increasi ng tractor passes by 8%, 10% and 13% for fine sand, mull and clay soil s, respectively. The lowest dry soil bulk density in the plough layer was obtained by rotary harrowing to a depth of 20 cm. Comparison of ga mma ray transmission and gravimetric analysis indicated that dry soil bulk density was slightly lower when determined by gravimetric analysi s. Increased soil bulk densities were reflected by increased water ret ention capacity (matric suction less than or equal to 10 kPa) and grea ter penetrometer resistance. Relatively similar increases in bulk dens ity increased the penetrometer resistance much less in mull than in fi ne sand. In contrast, greater bulk densities in the mull soil affected soil air composition adversely by decreasing the O-2, content to 10% when the subsoil had high wetness. In other soils, the lowest soil oxy gen contents of 16-18% were recorded in early summer (compacted clay) and during periods of vigorous plant growth (fine sand) when soil wate r contents were high. Even though the highest degree of soil compactne ss (D) in a plough layer approached 93 (gravimetric) in all soils, onl y clay soil was compacted to a soil macroporosity below 10% (pore diam eter > 30 mu m). 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 lo osening or compaction affected yield quantity adversely at different s tages of growth. During biomass accumulation, excessive penetrometer r esistances limited tap root growth in compacted fine sand without irri gation. Water applications promoted shoot growth, but did not affect f inal shoot and tap root yield. Among the three soil types tested in th is study, compaction of mull soil had the least effect on carrot growt h and external quality.