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March/April
2002
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By Thomas J. Rice, Ph.D.,
Soil Science Department,
California Polytechnic State University, San Luis Obispo, CA
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Natural soils are comprised of soil particles that vary in size.
Soil scientists categorize particle-size groups, called “soil
separates,” as follows: sands (thecoarsest); silts; and clays
(the smallest/finest).
What is soil texture?
Soil texture refers to the weight proportion (relative proportion
by weight percentage of sand, silt, and clay) of the mineral soil
separates for particles less than two millimeters (mm) as determined
from a laboratory particle-size distribution. Texture is soil’s
most important physical property for grapegrowing, since it influences
water holding capacity, root growth, and overall vine vigor.
The terminology used to describe soil texture in the popular press
often does not adhere to standard definitions. Therefore, much confusion
and outright misinformation is spread about this vineyard soil property.
The purpose of this text is to present standardized definitions
for soil texture terms and to discuss the importance of soil texture
to vineyard management.
Soil separate sizes
The U.S. Department of Agriculture (USDA) has established limits
of variation for the soil separates and has assigned a name to each
size class (Table I). This system has been approved by the Soil
Science Society of America (SSSA), and is the one used in all published
USDA soil survey reports. Most of the pertinent USDA publications
regarding soil survey standards and soil texture terms can be accessed
from the USDA web page cited in the references section below. Other
standard soil science terminology can also be referenced by examining
the Glossary of Soil Science Terms linked to the SSSA web page (SSSA,
2001).
USDA soil textural classes
The texture classes are shown on the USDA textural triangle (Figure
I).
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 Subclasses
of sand are subdivided into coarse sand, sand, fine sand, and very fine
sand. Subclasses of loamy sands and sandy loams that are based on sand size
are named similarly. Detailed definitions for the many different subclasses
of sands, loamy sands, and sandy loams are found in the USDA soil survey
manual (Soil Survey Division Staff, 1993).
The textural triangle is used to resolve problems related to the detailed
word definitions. The eight subclasses in the sand and loamy sand groups
provide refinement greater than can be consistently determined by field
techniques. Only those textural distinctions that are significant both to
agricultural use and management, and which can be consistently made in the
field, are commonly applied.
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Groupings of soil texture classes
It is often convenient to speak of general texture groups. The general soil
texture groups, in three (sandy, loamy
or clayey) or five classes, are outlined in Table II. In some areas where
soils are high in silt, a fourth general class, silty soils, may be used
for silt and silt loam.
Determination of
soil texture
Apparent field texture is a tactile evaluation only, with no inference as
to laboratory test results. Field estimates of soil texture should be rechecked
with laboratory testing when detailed data are needed, and the field criteria
should be adjusted as necessary. Sand particles feel gritty and can be seen
individually with the naked eye. Silt particles cannot be seen individually
without magnification; whether dry or wet, they have a smooth feel on the
fingers.
Clay soils range from slightly sticky to very sticky when wet. This is because
soil texture and soil clay mineralogy are not directly related. Soils dominated
by swelling and cracking clays are more sticky and more plastic (referring
to the ability of wet soil to form a ribbon or putty ball when it is manipulated)
than soils that contain similar amounts of micaceous (high in muscovite
or biotite mica) or kaolinitic (non-swelling) clays.
Soil texture and soil organic matter content are also not directly related.
In fact, to determine soil texture using standard USDA methods, the organic
matter and all other soil aggregating agents (like calcium carbonate, silica,
and iron oxides) are chemically removed from the total soil sample prior
to the determination of soil texture. Therefore, standard soil texture methods
call for removal of these most chemically important soil components. This
fact makes it critical to look at the soil chemical properties like pH,
organic matter content, and plant available nutrients — in addition
to the soil texture — in order to make wise viticultural management
decisions.
Many analytical labs will determine the soil saturation percentage (such
as the amount of water, by weight, in a saturated soil sample) and make
indirect soil texture determinations from this value (Table III). Generally,
the higher the saturation percentage, the higher total soil clay and organic
matter contents. The saturation percentage is directly related to the total
soil porosity and total soil water-holding capacity and, therefore, is a
valuable number to use for irrigation system design. |
 Rock
fragments
Rock fragments are unattached pieces of rock, 2 mm or larger in diameter,
that are hard or strongly cemented. They are physically removed (by sieving)
from the soil separates in the laboratory determination of soil texture.
Then, volume and weight measurements are performed to determine their amounts
in soil.
Rock fragments are described by size, shape, and, in some cases, the type
of rock. If one size or range of sizes predominates in a soil, the textural
class is modified using additional information. For example, compound terms
like “fine pebbles,” “cobbles, 100 to 150 mm in diameter,”
“channers, 25 to 50 mm in length” may be used.
Gravel is a collection of pebbles that have diameters ranging from 2 to
75mm. The term is applied to the collection of pebbles in a soil layer with
no implication of geologic rock type. The terms “pebble” and “cobble”
are usually restricted to rounded or subrounded fragments; however, they
can be used to describe angular fragments if they are not flat.
Words like granite, limestone, and shale refer to a rock type, not a rock
fragment. Composition of the fragments can be described as follows: “granite
pebbles,” “limestone channers” and “shale gravels.”
In USDA soil survey reports, the adjective describing rock fragments in
soils is used as the first part of the textural class name according to
the following conventions: |
- Less than 15% by volume — No mention of rock fragments is used.
- 15% to 35% by volume — The dominant kind of rock fragment is
used (such as “gravelly loam,” or “cobbly clay loam”).
- 35% to 60% by volume — The word “very” precedes the
name of the dominant kind of rock fragments (such as “very cobbly
loam,” or “very cobbly sandy loam”).
- More than 60% by volume — Add the word “extremely”
in front of the coarse fragment name (such as “extremely gravelly
loam,” or “extremely cobbly clay loam”).
- If there are too few soil separates present to determine the soil
textural class (less than 10% by volume), terms such as “gravel,”
“cobbles,” “stones,” or “boulders” are
used, as appropriate, without mention of the textural class.
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 Soils
generally contain rock fragments smaller or larger than those identified
by the adjective term. The “rock fragment” adjective applies to
the most predominant rock fragment size found within any soil layer. For
example, on a volume basis, a stony loam with 20% stones may also contain
10% gravel-sized pebbles, but “gravelly” is not mentioned in the
name. In this case, stones are the most abundant rock fragment size and
“stony” is used as the adjective.
More precise estimates of the quantity of rock fragments than are provided
by the defined classes are needed for some purposes as potential erosion
prediction and determination of soil water holding capacity. If more precise
information is needed for specific land use purposes, estimates of percentages
of each size class or combination of size classes are included in the description:
such as “very cobbly loam;” “30% cobbles and 15% gravel”
or “silt loam; about 10% gravel.”
If rock fragments are significant in soil vineyard management, they are
the basis for designing and describing new soil map units which are identified
on vineyard soil maps. In contrast, bedrock exposed at the earth’s
surface is not soil and is separately identified on soil maps.
The volume occupied by individual rock fragments can be seen in the field,
and aggregate volume percentage can be estimated. Rock fragment volume percentage
may be converted to a weight percentage, using USDA conversion tables or
direct laboratory measurements.
Influence of soil texture on other soil
properties
As discussed above, individual soil mineral particle diameters range over
six orders of magnitude, from boulders (>600 mm) to clays (less than
10–9 mm) which can only be seen with an electron microscope. The soil
textural classification established by the USDA is used for agricultural
soils. The size ranges for these separates are not purely arbitrary, but
reflect major changes in how the particles behave and in the physical properties
they impart to soils (Table V).
Texture is an important soil characteristic because it will, in part, determine
water intake rates (infiltration); water movement through soil (hydraulic
conductivity); soil water holding capacity; the ease of tilling the soil;
and the amount of aeration (which is vital to root growth). Texture will
also influence soil fertility.
For instance, a coarse sandy soil is easy to till, has plenty of aeration
to stimulate root growth, and is easily irrigated. However, this same sandy
soil will rapidly dry out after irrigation due to its low water holding
capacity. Water soluble plant nutrients (like nitrates and potassium) will
be rapidly leached below the vine root zone by percolating waters.
In contrast, moist clay soils (over 35% clay) are composed of very small
particles that fit tightly together with fewer large interconnected pores.
Clay soils should be irrigated less frequently than sands, but with higher
amounts of water and over longer periods of time. Most clay soils have greater
cation exchange capacities (CEC) and will adsorb higher amounts of water-soluble
plant nutrients (especially potassium, calcium, and magnesium). Wet clay
soils are also difficult to till due to their relative stickiness and inability
to support the weight of a tractor.
Summary
For most intensive vineyard management operations, a detailed soil map should
be prepared to supplement the more general USDA soil survey reports. The
soil textures for each soil layer should be identified for the dominant
soils using the standard USDA terms defined here.
When describing soil textures within a vineyard, it is important to not
only identify the soil textural classes but also the soil parent material
rock types and the amounts of rock fragments within the soil profiles. Related
soil properties like saturation percentage should also be determined to
aid in irrigation system design.
Soil texture information should also be supplemented with data regarding
soil organic matter content and soil chemical properties, such as pH and
plant essential nutrient concentrations. Soil organic matter and humus contents
(highly decomposed organic matter) will modify the general effects of soil
mineral texture (Table V) by enhancing soil structure formation, increasing
soil water holding capacity, and increasing CEC. The complex interrelationships
among all these soil properties should be considered when vineyard management
decisions are made.
References
Soil Science Society of America,
“Glossary of soil science terms.”
Soil Survey Division, Natural Resources Conservation Service, United States
Department of Agriculture “USDA
soil survey standards.”
Soil Survey Division Staff. 1993. “Soil survey manual.” Soil Conservation
Service. U.S. Department of Agriculture Handbook 18.
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