Bulk density determinations. Comparison between modified excavation method and core method with different diameter.

Accurate determinations of the bulk density are usually obtained trough the excavation method, which is especially useful when applied to the top soil. A previous paper showed that these results are very sensitive to meteorological short term conditions and suggested to eliminate the upper 5 cm of soil before this determination. The use of the core method is less cumbersome more practical and gives much less variable results at least when taken under 5 cm depth; the samples are subjected, however, to alteration due to compression depending on the moisture, clay content and core diameter, so that a correction was proposed. A verification was needed. In two different soils at three times in a year, after removing the 5 cm top layer, moisture and bulk density were determined by excavation and replicated twice. At the same time two core samples were taken from a plane at the depth of 10 cm; one of the sampling brass cylinders had 5 cm and the other 8 cm inner diameter. After further digging two more cores were taken at 60-65 cm. The moisture determinations showed (a) the lack of homogeneity in the upper soil layer (about 5-10 cm) and (b) the danger of some water extrusion when using 5 cm diameter cores. The evaluation of the bulk density at each location in 3 different seasons and different methods (excavation and cylinders having 2 different diameters) gave very variable results. As a rule the rb vary inversely with soil moisture. The removal of the 5 cm top layer before the excavation is usually sufficient to suppress the effects of short and mid term meteorological events an shallow tillage, except in cases of prolonged severe drought. Main tillage effects are still evident in the Ap horizon. The cylinder sampling attenuates seasonal variation of rb as well as the effect of meteorological events and tillage. The differences between 5 and 8 cm diameter disappear at a depth of 67.5 cm or when both moisture and clay content are high. The farther from the ploughing time the determinations are made, the closer are the results from all methods. In this soils rich in expanding-type clay it is possible to use a general equation sensitive to factors acting at the soil surface to find estimates of the rb in the upper 5 cm soil layer. At least when moisture is not much lower than the plastic limit, it is possible to evaluate the rb expected from excavation in the layer 5-20 cm (surface effects suppressed) by simplified formulas based on core moisture, cylinder diameter, cumulated rainfall since the ploughing date