Nota CaN Soil PeNetratioN reSiStaNCe aNd Bulk deNSity Be determiNed iN a SiNgle uNdiSturBed SamPle ?

Soil quality indicators such as penetration resistance (Pr) and bulk density (Bd) are traditionally determined in a single undisturbed soil sample. the aim of this study was to assess the effect of Pr measurements of undisturbed samples on the determination of Bd in the same sample of two soils differing in clay contents. to this end, samples were collected from the 0.00-0.10 and 0.10-0.20 m layers of two soils of clayey and very clayey texture. Volumetric rings were used to collect a total of 120 undisturbed soil samples from each soil layer that were divided into two subsets containing 60 units each. one sample set, designated “perforated samples”, was used to determine Pr and Bd in the same undisturbed sample; the other, named “intact samples”, was used to determine Bd only. Bulk density values for perforated and intact samples were compared by analysis of variance, using a completely randomized experimental design. means were compared by the t-test at 5 %. the Bd values for the clayey soil were similar in perforated and intact samples from the two layers. However, Bd of the very clayey soil was lower in the perforated than in the intact samples at both depths. therefore, Pr and Bd in clayey soils can be accurately determined in the same undisturbed sample whereas in very clayey soils, different samples are required for this purpose. keywords: soil physics, latossolo Vermelho, clayey texture, very clayey texture. Received for publication on September 12, 2014 and approved on January 22, 2015. DOI: 10.1590/01000683rbcs20140593

In most cases, bulk density is determined after measuring soil penetration resistance in the same undisturbed sample.Particularly when analysing predominantly clayey soils, some particles of the undisturbed sample remain on the metal rod of the penetrometer after measurements.This circumstance is a result of peculiar surface properties of the soil clay fraction.As is well-known, clay particles are highly plastic and sticky, especially when wet (Brady and Weil, 2007), which is typically the case when undisturbed soil samples are subjected to measurements of penetration resistance.As a result, determining bulk density in an undisturbed sample previously used to measure penetration resistance can lead to underestimated values depending on the amount of particles adhered to the penetrometer in the previous operation because bulk density is calculated as the ratio of dry soil mass to total sample volume.Therefore, a reduced amount of soil will lead to an underestimated bulk density value.
Based on the foregoing, the aim of this study was to assess the effect of PR measurements of undisturbed samples on the determination of BD in the same sample of two soils differing in clay contents.

material aNd metHodS
This study analysed samples of soil classified as Latossolo Vermelho (Oxisol) based on the criteria of the Brazilian Soil Classification System (Embrapa, 2013).Samples were collected from two experimental areas with different soil textural class.Both were located in the municipality of Jaboticabal, Sao Paulo, Brazil (21° 14' 05" S, 48° 17' 09'' W; average height 615 m a.s.l.) and had been used for sugarcane cultivation with mechanized harvesting for more than 15 years.The soils were analysed for textural class in disturbed samples from the 0.00-0.10 and 0.10-0.20 m layers.A total of 20 samples per layer and area were collected at randomly selected points.The soils were classified as clayey and very clayey reSumo: É Possível Determinar a resistênCia Do solo à Penetração e a DensiDaDe Do solo na mesma amostra inDeformaDa?
Undisturbed soil samples were collected in October 2011 from the 0.00-0.10 and 0.10-0.20 m layers from in-between the sugarcane rows (interrow) after the first harvest.A total of 120 volumetric rings (height 0.05 m, diameter 0.05 m) were used, as well as an Uhland sampler to collect the same number of samples from each soil layer.Therefore, a total of 240 samples per experimental area was collected.Sampling points were randomly distributed in an area of ca.0.5 ha.
The samples from each layer were split into two subsets comprising 60 samples each.One set, designated "perforated samples", was used to determine both bulk density (BD) and soil penetration resistance (PR) in each undisturbed sample; the other, designated "intact samples", was used to determine BD only.
The moisture content of all perforated samples was standardized at 100 hPa on a pressing table, after which the samples were used to measure PR by using a static electronic laboratory penetrometer, as suggested by Tormena et al. (1998).Penetration resistance was measured at the geometric centre of each sample, using a penetration rate of 1.0 cm min -1 along the sample height (5 cm).The measurements corresponding to the top and bottom of each sample were discarded, so only those at the three central points were considered in calculations.The sampling frequency of PR recorded at 0.68 s intervals resulted in a total of 265 values per sample, which were averaged.Any soil particles of an undisturbed sample sticking to the metal rod of the penetrometer were removed with a spatula and returned to the sample.Once PR was measured in all perforated samples, BD was determined according to Grossman and Reinsch (2002).In intact samples, however, only the latter parameter was determined.
The bulk density of perforated and intact samples was subjected to analysis of variance, using a completely randomized design with 60 replications.Means were compared by the t-test at 5 %.All statistical analysis were performed using software SAS (SAS, 2003).

reSultS aNd diSCuSSioN
The average soil penetration resistance of the 60 perforated undisturbed samples was 2.90 MPa for the 0.00-0.10m soil layer and 2.88 MPa for the 0.10-0.20 m layer in the clayey soil; and 3.38 and 3.26 MPa, respectively, in the very clayey soil.
In the clayey soil (contents of 441 ± 20 g kg -1 clay in the 0.00-0.10m and 437 ± 13 g kg -1 clay in the 0.10-0.20 m layer), bulk density (BD) was similar in the perforated and intact samples (Table 2).Therefore, PR and BD can be determined in a single undisturbed sample of this soil.
However, in the very clayey soil, BD differed between perforated and intact samples in both layers.Thus, the BD values measured in samples previously perforated to measure PR were lower than those determined in intact samples (Table 2), used exclusively to determine BD.The difference resulted from the loss of soil particles sticking to the ,* and **: not significant, significant at the 5 and 1 %, respectively.LSD: least significant difference.Means followed by different letters in a column were different by the t-test at 5 %.metal rod of the penetrometer and the consequent reduction in dry soil mass measured after ovendrying the perforated samples.Even if most of the particles were removed from the penetrometer and returned to the sample for measurement, a fraction remained that could only be removed with a moist cloth and not be returned to the sample.As a result, BD was underestimated in the very clayey soil (contents of 680 ± 06 g kg -1 clay in the 0.00-0.10m layer and 687 ± 07 g kg -1 clay in the 0.10-0.20 m layer), when determined in samples previously used to measure PR.Obviously, due to the high clay content of this soil caused, a greater amount of particles stuck to the penetrometer by effect of the high plasticity and stickiness of the clay fraction (Brady and Weil, 2007).
The time needed to collect undisturbed soil samples in the field justifies the use of a single sample to accurately determine BD and PR.However, if the clay content of the soil is very high, the use of a single undisturbed soil sample for this purpose can cause an underestimation of BD values as a result of the loss of clay particles sticking to the penetrometer during PR measurements.Therefore, BD estimates of very clayey soils are inconsistent with field observations and can induce misleading conclusions for different agricultural systems.In such soils, PR and BD should therefore be determined in separate samples.

CoNCluSioN
Accurate measurements of soil penetration resistance and bulk density in a single undisturbed sample are feasible in clayey soil, but not in very clayey soil, for which the two soil properties should be determined in different samples.