INTERPRETATION OF SOIL
TESTS
(Please read in
conjunction with "Soil Testing and Plant Tissue Testing".) INTRODUCTION There
are many analytical laboratories that provide a chemical soil testing
service. Some
are woefully inadequate testing only for phosphorus, potassium, pH and
salinity and ignoring all other nutrients. Others
can be very comprehensive but the results badly presented. This makes it
difficult for the farmer in particular to make any sense out of his own soil
test and puts his decisions regarding fertiliser firmly in the minds of the
"experts" who are also often the manufacturers/sellers of
particular types of fertiliser. A
standard soil testing procedure and a standard method of reporting is badly
needed. Also
a reasoned interpretation of soil test results can only be made if other
information about the soil is made available. In determining the fertility
status of the soil consideration must be given to the three major components
of soil fertility: - The physical characteristics of the soil - The biological status of the soil - The chemical status of the soil All
these major factors interact with each other. The unfortunate trend of the
last 70 years has been to use fertilisers to feed the plant rather than to
feed the soil. A
fertile soil will grow excellent healthy crops with good disease and pest
resistance. Trying to feed crops directly may give some spectacular results
but almost inevitably results in imbalances that result in unsound growth and
increased post harvest problems. This places the farmer on the treadmill of
continued and increasing reliance on chemical fertilisers, fungicides,
pesticides and animal health remedies. This
brief /paper will attempt to promote the practice of fertilisers for soil
fertility rather than direct crop growth. Also
in the interpretation of soil test results an attempt will be made to allow
the reader to recognize patterns and relationships between the various
readings and to apply general principles to recommend fertiliser combinations
that enhance soil fertility. At the very best many of the measurements made
during a soil analysis is the chemist’s attempt to try and mimic the
action of a plant root. This is looking at a detailed landscape through heavily
frosted glass - vague impressions may be gained. Therefore it is ludicrous as
promoted by some learned soil authorities to apply complex formulae to come
up with a precise level of N.P.K., or whatever, for a particular season or a
particular crop. This
outline is based 6n recognizing major factors or patterns that influence soil
fertility when looking at soil test results. A bit like being able to get a
fairly good impression of a car by looking at it rather than by reading the
designers calculations and specifications. Although this paper looks mainly
at soil tests plant tissue tests can be infinitely more valuable, especially
when used in conjunction with soil tests. This is particularly so where soils
have been abused or degraded in any way. SOIL, PHYSICAL
CHARACTERISTICS (See also attached information) The
physical characteristics of a soil that influence soil fertility and plant
growth are; SOIL TEXTURE This
describes a soil in terms of the major particle classes i.e. sand, silt and
clay and occasionally gravel. The
percentage and types of sand, clay and gravel will influence: - Structure - Compaction - Infiltration - Drainage - Water Storage - Mineral Reserves - Nutrient Storage (cation and anion exchange) All
these factors will be enhanced by the presence of organic matter. SOIL STRUCTURE Structure
describes the way in which the soil is formed into aggregates, which may
range from minute grains through to large blocks that do no readily divide
into smaller units. Structure
greatly influences the: - Porosity - Water Storage - Infiltration - Drainage - Root Penetration - Aeration - Stability Structure
is influenced by: - Texture - Organic Matter - Soil Biological Activity - Drainage - Soil Chemistry - Farming Practices B SOIL CHEMISTRY Soil
chemistry relates to; - The level of individual elements - The effects of pH - Reactions between the elements - The moisture content - Aeration - The influence of organic substances and living organisms Assuming
that moisture and aeration factors are not over-riding the main chemical
factors affecting soil fertility and crop production are; - Soil pH - Cation and anion exchange capacity - The level of soluble salts in the soil - The level and form of individual elements SOIL pH This is a measure of acidity or
alkalinity 5.8 Neutral is pH 7.0. Pasture grows best
at pH 7.5 A pH of 5.0 is 10 times more acid
than pH 6.0 A pH of 4.0 is 100 times more acid
than pH 6.0 The
main effect of an acid pH is that it decreases the availability of many
nutrients eg. Ca. Mg. K. Cu. Zn. Fe. Co. Mo. Also
a low pH can increase the availability of aluminum to toxic levels in some
soils. A low pH will suppress biological activity and prevent organic matter
from breaking down. PH increases can be slow to achieve - normally done with
finely ground limestone (calcium carbonate), which neutralises acidity by
mopping up hydrogen ions - the cause of acidity (will talk about lime later). CATION EXCHANGE CAPACITY (CEC) Clay
particles and humus carry small negative electrical charges (like static
electricity or small magnets). Some elements have small positive charges and
these are called cations. These
include - Hydrogen H Calcium Ca Magnesium Mg Potassium K Sodium Na Zinc Zn Copper Cu Iron Fe Manganese Mn Aluminum Al Others The
cation exchange capacity is an estimate of the amount of nutrients that are
stored in such a way that plant roots can gain access to them. A
subsidiary set of figures, the base saturation percentages estimates
the percentage of the cation exchange capacity that is taken up by the
following;
A
good cation capacity is dependent on having the right sort of clays and active
organic matter (humus). If a soil has good CEC but low pH the exchange
capacity will be clogged with hydrogen rather than Ca. Mg. K. and trace
elements. ANION EXCHANGE CAPACITY The
soil also contains sites that have positive charges and these attract and
store negatively charged anions - the most common of which are: Phosphate P04 Nitrate N03 Sulphate SO Chloride C14 The
cation exchange system is more dominant than the anion exchange and of less
importance as most of the anions reside in the soil as salts or salts in
solution in the soil moisture. SOLUBLE SALTS (Salinity) High
levels of salts in the soil moisture can adversely affect plant growth by
preventing osmosis and by forcing the plant to accept a poorly balanced
nutrient uptake. Salinity
is more a problem of low rainfall areas where there is insufficient downward
movement of salts out of the soil profile, total soluble salts is measured by
electrical conductivity or by calculation as total soluble salt
concentration. INDIVIDUAL ELEMENTS Individual
elements are generally assessed on an available' basis rather than the older
methods of assessing the total levels of an element. In trying to determine
what level of an element is available to plants the soil test uses different
"extractions" to try and imitate what the plant may be able to
extract from the exchange system in the soil. Results
are given in either parts per million (or other metric units e.g. mg/kg or
ug/gm) which measures 'available' levels in terms of the soil mass or as
milli equivalents per 100 gms (or meq %) which looks at 'available3 elements
in terms of the exchange sites available. An
approximate conversion between meq and ppm 'available' levels are as follows; meq/lOOgms ppm(mg/kg) K x 390 Ca x 200 Mg x 120 Na x 230 Al x 90 The
cation exchange capacity is equal to the total of all 'available' cations. Of
equal importance as the levels of individual elements are the relative levels
between the elements. The
following diagram summarises the two major effects, stimulation and antagonism
that can affect the availability or uptake of elements by plants.
This
chart shows the effect of various plant nutrients on each other. The solid lines
show that one element suppresses another in the direction of the arrowhead.
For some pairs of elements, both are suppressed when excessive amounts occur,
Similarly. For example, high applications of zinc will not "cure"
zinc deficiency if there is an excessive level of calcium. In such a
circumstance, there would also be symptoms of potash, boron and iron
deficiency. It
is for this reason that the application of soluble nutrients, especially
trace elements must be carried out carefully. There are many opinions as to
the desirable levels of elements in a soil and where there is good local
knowledge and benchmarks developed by a laboratory for particular soil types
these should be used as the main guide. As
an example SWEP analytical laboratories give the following available
benchmarks for soils in the Trafalgar region of Victoria.
Other
examples are given in the accompanying information. Other
aspects of soil test information are as follows; AVAILABLE NITROGEN In
pasture this is always changing due to weather, stage of growth, presence of
legumes etc it might be high one month - low the next. Available nitrogen
tends to be low in colder weather and when very wet: i.e. can be low early
Spring when a little extra N is often useful, e.g. foliar urea 6-12kg/Ha. It
is best to encourage legume growth for natural, low cost nitrogen. AVAILABLE PHOSPHORUS Should
be moderate to high. Can be depressed where there are very high levels of
calcium. AVAILABLE SULPHUR Generally
in the form of sulphates, can be reasonable levels in acid soils due to
acid/sulphur reactions. TRACE ELEMENTS All
should be ideally in the medium-high range. Increasing
pH generally results in significant increases in trace element availability.
Increased pH will reduce the availability of potentially toxic aluminum. ORGANIC MATTER If
there are very good levels it can be largely inactive due to acidity which is
also preventing it acting as a store for nutrients in the soil. Ask for test
to include; - Total organic matter - Reserve organic matter - Active organic matter (humus) (organic matter % = organic carbon % x 1.8) Ca/Mg RATIO See
above. EXCHANGE SODIUM %
(or sodium absorption ratio) To
determine sodium hazard. This should be low (excess sodium causes the loss of
soil structure by dispersing clays - Can be countered by use of gypsum) RECOMMENDATIONS pH
The only way to permanently lift pH is by the use of agricultural lime.
Liquid lime can give good short term responses because of a short-term
availability of calcium but it will not affect/change pH. If
magnesium is also low a long-term program of lime/dolomite is desirable. This
could be as much as lOT/Ha over 6-8 years. Ca. Mg. K To
improve the short term levels of these I suggest: 20L/Ha Bubbs liquid lime liquid spray 8kg/Ha magnesium sulphate 40kg/Ha potassium sulphate - solid PHOSPHORUS Start
program of reactive rock phosphate application - seek manufacturers
recommendation. SOIL ACTIVATION Foundation
Fish @ 15L/Ha Spring and Autumn. Obviously
there may have to be compromises or reduced applications of all the above for
economic/financial reasons. LICK A
homemade lick will ensure trace element availability until increasing pH will
naturally make more available. Suggest: 25kg dolomite 1kg rock salt 500gm copper sulphate 200gm zinc sulphate 3Ogm cobalt sulphate Check
with Vet about molybdenum (sodium molybdate) to add to this. Mix well dry and then add 2 litres
cider vinegar and 2 litres molasses and mix again. Alternative extra - 2kg seaweed meal
or 1 litre liquid Kelp. FURTHER NOTES Liquid
Lime formations can give short-term responses mainly from increased calcium
availability. Once
pH levels support a good population of earthworms the rate of lime
incorporation into pasture soils will increase dramatically. At
all stages of pH correction with lime the use of Foundation Fish/Kelp to
promote biological activity will improve the rate of pH improvement IMPROVING MINERAL & TRACE ELEMENT LEVELS A. MINERALS ALREADY IN THE SOIL Often
minerals and trace elements will exist in soils but are unavailable to plants
due to: 1. Being locked with silt and sand particles 2. Being locked in inactive organic matter 3. Being locked in the soil complex by strong
electrical/chemical bonds Much
of this reserve of minerals and trace elements can be made more available by: 1. pH correction (lime/dolomite) 2. Increased biological activity Increased
biological can be improved by: 1. pH correction 2. Direct stimulation (Foundation Fish/ King Kelp) 3. Reducing the use of very soluble chemical fertilisers
and broad spectrum pesticides, herbicides, etc. which can reduce
biological activity Where
soils are very acid the trace element deficiencies are best corrected by
foliar applications of the deficient elements or by direct intake by stock
via a simple mineral lick supplement. B. CORRECTING TRUE DEFICIENCIES True
deficiencies are best corrected by the application of mineral' fertilisers
which are slow release and will provide a long-term reserve in the soil. Some
examples are: LIME - Calcium DOLOMITE - Calcium and Magnesium CALCINED MAGNESITE - Magnesium GYPSUM - Calcium and Sulphur ROCK DUSTS - Broad spectrum of minerals and trace elements POTASSIUM SULPHATE - Potassium Specific
trace element deficiencies can be addressed by: Foliar applications Stock licks Soil applications The
most suitable method and product will depend on 1) Soil pH 2) Type of farming MONITORING Regular Monitoring is Essential Regular
monitoring by Soil Test and Leaf Tissue Test will indicate how corrective
measures are progressing. Regular
monitoring will also help pick up any trends in nutrient availability and
allow corrective measures before a real problem arises. One
Soil Test and at least one Leaf Test should be carried out each year. OBSERVATION Of
equal or greater importance are the farmer’s own observations of crop health,
stock health and behavior, root depth, earthworm numbers etc. Farmer’s
notes and observations are invaluable in helping to interpret Soil and Leaf
Test results. SUNDRY MINOR NOTES -
High levels of soluble potassium can suppress the uptake of magnesium and
copper - a big cause of sickness in cattle -
Very high calcium levels can suppress phosphorus uptake -
Molybdenum is required for copper uptake and for the N fixing bacteria that
live in association with legumes -
If the Ca; Mg ratio falls below three to one there can be adverse structural
consequences with a loss of infiltration and drainage -
Soils with low pH often have very high organic matter levels - usually as
unrotted root residues and manure and thatching of the turf. Active organic
matter (humus) in these soils can be very low -
Humus acts as a natural chelator - allows less fertiliser to be used -
Humus can be very old as it is very stable except if cultivated when wet or
the organic cycle is over-stimulated e.g. by the use of large amounts of urea -
Muriate of potash and sulphate of ammonia kills worms - as do many other
fertilisers if they lie in a dry form on the soil surface. |
Author Unknown