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INTERPRETATION OF SOIL TESTS
(Please read in conjunction with "Soil Testing and Plant Tissue Testing".)
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;
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:
- Water Storage
- Mineral Reserves
- Nutrient Storage (cation and anion exchange)
All these factors will be enhanced by the presence of organic matter.
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:
- Water Storage
- Root Penetration
Structure is influenced by:
- Organic Matter
- Soil Biological Activity
- 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
- 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
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 -
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;
Calcium 60 - 75%
Magnesium 7 - 20%
Potassium 2 - 5%
Sodium 0.5 - 5%
Trace Elements 5%
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:
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 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;
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;
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.
Should be moderate to high. Can be depressed where there are very high levels of calcium.
Generally in the form of sulphates, can be reasonable levels in acid soils due to acid/sulphur reactions.
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.
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)
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)
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
Start program of reactive rock phosphate application - seek manufacturers recommendation.
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.
A homemade lick will ensure trace element availability until increasing pH will naturally make more available.
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.
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:
The most suitable method and product will depend on
1) Soil pH
2) Type of farming
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.
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.