Agriculture Reference
In-Depth Information
Texture Analysis and Calibration for Texture
Texture refers to the “feel” of a soil and depends on the size distribution of the min-
eral particles. Texture analysis is especially important for obtaining estimates of readily
available water, as discussed in chapter 4. In a traditional soil survey carried out by an
experienced surveyor, texture is usually assessed by hand-texturing using the procedure
outlined in table 2.1. The results can be confirmed by particle-size analysis of selected
samples in a laboratory (see the following). However, a laboratory analysis of texture is
mandatory for the calibration of an EM survey or any other means of sensing.
Calibration of an EM38 or similar instrument for texture is based on a
particle-size analysis of soil samples (the sample set) that represent the full range
of texture variation in the vineyard. The samples are collected from specific points
so that their EM reading and location are known. Ideally, soil salinity, as measured
by the electrical conductivity (
EC
) of a saturated paste, should be less than 0.6
deciSiemens (dS) per meter (dS/m), because salinity higher than this swamps the
effect of any other variable on the EM signal. For particle-size analysis, a known
weight of dry soil is broken into its constituent particles by mechanical and chemi-
cal treatment. Gravel and stones larger than 2 mm diameter are excluded, with the
remaining material called “fine earth.” Although the distribution of particle sizes
in the fine earth is continuous, for practical purposes the distribution is divided
into classes. Not all countries use the same size-class divisions, but two of the most
widely accepted classification schemes are described in the following.
Table 2.1
A Simplified Guide to Field Texturing
Texture
class
Sample preparation
Distinguishing features
Take enough soil to fit into the
palm of your hand, slowly
moisten it with water and
knead it between finger and
thumb. Continue kneading
and wetting until the soil
“bolus” just sticks to your
fingers and all aggregates are
broken down (may take 1-2
minutes). Assess the texture
class by feel, sound, and the
length of the soil ribbon that
can be squeezed between
finger and thumb
Coarse
sand
Particlesarelargeenoughtobeseenandthey
grate together during kneading; bolus
breaks apart
a
Fine sand
Particlesmaybediiculttoseeandfeelbut
can be heard grating or “squeaking” when
the bolus is kneaded close to the ear; bolus
lacks cohesion
Silt
Confers a smooth silkiness to the bolus, which
is coherent but not very sticky (particles
cannot be felt)
Clay
The bolus is hard to knead initially but
gradually becomes very sticky; forms an
intact ribbon up to 50 mm long for a light
clay and 75+ mm for a heavy clay
b
a
Humified organic matter makes a sandy texture feel smoother and more cohesive.
b
The type of clay mineral affects stickiness. When thoroughly wet, montmorillonite clay is sticky, but
kaolinite clay is not (see “Retention of Nutrients by Clay Minerals and Oxides,” chapter 3).
