Interpreting your soil test results

Introduction

Crop Nutrition Laboratory Services (Cropnuts) perform three main soil tests

  1. Basic Soil Analysis
  2. Complete Soil Analysis
  3. 1:2 volume extract Analysis

We highly recommend that at the beginning of any project, and thereafter on an annual basis you perform a basic or complete soil analysis to get a good picture on the base saturation and fertility of the soil, and what amendments are required to balance the soils.  The 1:2 volume extract analysis should be performed quarterly , and gives you a snap shot of the plant available nutrients in the soil with a recommendation on how to fertigate to feed the crop.  Interpretation of the 1:2 volume extract is dealt with in another paper.

Soil Texture Analysis

In addition to the above tests it is a good idea to do a Soil Texture Analysis to understand your soils better.  Soil texture does not change very much – this test only needs to be performed once.

Texture refers to the size of the particles that make up the soil.  Sand, being the larger size of particles feels gritty, silt being moderate in size has a smooth or floury texture. Clay being the smallest size feels sticky.

NAME PARTICLE DIAMETER 
(USDA Classification)
Clay below 0.002 mm
Silt 0.002 to 0.05 mm
Very fine sand 0.05 to 0.10 mm
Fine sand 0.10 to 0.25 mm
Medium Sand 0.25 to 0.5 mm
Coarse sand 0.5 to 1.0 mm
Very coarse sand 1.0 to 2.0 mm
Gravel 2.0 to 75.0 mm
   


The soil texture determines crop suitability and helps us to approximate the soils response to environmental and management conditions.  It affects the water and nutrient holding ability of the soils.  Sandy soils for example hold less water and nutrients and are leached more easily – so it is better to apply small doses of water/fertilizer more often.

Soil pH

This is measured using a 1:2 water extraction.  The soil pH is a measure of the acidity or alkalinity of the soil.  It is most important as it influences the chemical and physioloogical processes in the soil and the availability of nutrients to the plants.  Very high and very low soil pH's will lock up nutrients making them unavailable to the plants, or volatise them causing wastage and possible environmental contamination.  The soil pH also affects the biological activity in the soil, which is important for nutrient availability and plant health.
Level Peat Loam Sand
Very Low 
(acid)
4.0 5.0 5.0
Low 4.5-5.0 5.1-5.5 5.1-5.8
Medium 5.1-5.5 5.6-6.5 5.9-6.8
High 5.6-6.0 6.6-7.0 6.9-7.5
Very high 
(Alkaline)
>6.0 >7.0 >7.5
       

The figure below summarises the effect of soil pH on nutrient availability (Truog 1948).  At very low pH aluminium can be very toxic and it is imperative to lime the soil to prevent this.

Soil EC

The soil electrical conductivity (EC) is a measure of the amount of salts, or salinity in the soil.  It is an important indicator of soil health.   It affects crop yields, crop suitability, plant nutrient availability and microbial activity in the soil, as well as the soil structure.  Excess salt hinder plant growth by affecting the soil/water balance.  EC can also be correlated to the concentration of nutrients in the soil.  An optimum EC is <800uS/cm.

Factors that affect the EC that cannot be changed include soil minerals, soil texture and climate.  High EC is common in arid and semi arid zone with low rainfall, and where high EC irrigation water has been used, or there has been an over application of fertiliser or manure.  Salts move with water and can also accumulate at the bottom of slopes and in depressions.  Soils with a high CEC, for eg clays, tend to have higher EC's.  Soil with restrictive pans also build up high EC because the water cannot move through the soil profile and leach out the salt.

Soil EC is affected by cropping, irrigation, land use and application of manure, compost and fertiliser.  When managing salinity on irrigated land it is important to have your irrigation water tested.  Irrigating with amounts of water that are too low to leach the salts, allows the salts to accumulate in the root zone, increasing the EC.  Irrigating with high salinity water also accumulates salts in the root zone.

Organic Matter %

Organic matter is key to soil fertility, and a 'bank account' to the intelligent farmer.  Most productive agricultural soils have an OM% of 3-6%.  Organic matter is made up of four main components:-

  1. Living organisms (microbes and macrobes)
  2. Fresh crop residue and roots
  3. Decomposing organic matter (active organic matter, detritus)
  4. Stable soil organic  matter - humus.

The first three types of organic matter affect nutrient recycling and availability to the plant and plant health.  Decomposing organic matter releases nutrients in a plant available form, and provides the food needed for living organisms in the soil - enhanced microbial diversity in the soil will help suppress pests and diseases. 

There are numerous benefits to having stable organic matter in the soil, these can be grouped in 3 categories:-

Physical Benefits of Stable Organic Matter

  • Enhances aggregate stability, improving water infiltration and soil aeration, reducing run off and soil erosion
  • improves water holding capacity - organic matter behave like a sponge and can hold up to 90% of its weight in water in a form that is readily available to the plant.  In contrast clay can hold a grat deal of water but much of it is unavailable to the plants.
  • Organic matter reduces the stickiness of clay soils making them easier to work with
  • Organic matter reduces surface crusting, facilitating seedbed preparation and improving seed germination.

Chemical Benefits of Stable Organic Matter

  • Increases the soils CEC, and its ability to hold onto and supply over time essential plant nutrients eg calcium, magnesium, potassium and trace elements.  Humic acid has a CEC of 450, fulvic acid has a CEC of 1400.... Even a small amount of stable humus will have an enormous effect on the soil CEC.
  • Improves the ability of the soil to resist pH change - ie improves the buffering capacity of the soil
  • Enhance the rate at which the soil mineral decompose and become available to the plants.
  • Increases the rate at which pesticides and herbicides are decomposed in the soil.

Biological Benefits of Stable Organic Matter

  • Provides food for the living organisms in the soil and a skeleton for them to live on.
  • Promotes enhanced soil micorbial biodiversity that can activity suppress diseases and pests.
  • Enhances pore space and aeration through the action of soil microorganisms - this helps to increase filtration and reduce run off.

Improving your organic Matter

Inherent factors that affect soil organic matter such as climate and soil structure cannot be changed.  However there are a number of good agricultural practises that can maintain or improve soil organic matter such as: - 

  • reducing or eliminating tillage, every time you work the soil it gets aerated, creating a flush of microbial activity that speeds up organic matter decomposition
  • Soil test and fertilise properly - proper fertilisation encourages growth of plants, developing increased root and top growth, increased root growth can help build or maintain OM% even if you are removing the top growth.
  • reduce erosion using appropriate measures - the OM is held in the topsoil and if this is washed away you have lost it forever.
  • use of cropping systems that incorporate cover crops, manure/compost application and diverse rotations with high residue crops that can be worked back into the soil
  • avoiding soil compaction, and waterlogging and maintaining the soil at the correct pH to enhance microbial activity
  • use of perenial forages with extensive fibrous root systems.

% Nitrogen (N)

% Nitrogen is measured using the Kjeldahl technique.  This determines both  the organic and inorganic nitrogen availability in the soil.  This is used as a guideline for your N fertilisation.  A good level of nitrogen in the soil is 0.3-0.4%. The availabiliity and uptake of nitrogen from the soil is a very complex process and is affected by many parameters, these include soil moisture levels, microbial activity, soil aeration, EC, pH and soil structure.  By optimising your soil health, you will optimise your nitrogen availability.  

Applying excess nitrogen is not advised, as this results in very soft growth that is susceptible to pest and diseases.  Also excess nitrogen can be leached out of the soil and end up in waterways, creating an environmental problem.

C:N Ratio

The carbon to nitrogen ratio (c:N) ratio, is a measure of the mass of carbon to the mass of nitrogen in the soil.  This is an important indicator for soil health, microbial activity and nitrogen availability.  An optimum C:N ratio in the soil is about 15 to 20.  Soil micro-organisms need a C:N ratio of 24:1 in order to grow and thrive, and a C:N ratio of 8:1 to exist.    A low C:N ratio (<15) means that the microbes will consume the organic matter and leave any excess nitrogen in the soil (mineralization).  A high C:N ratio (>25) means that the microbes take nitrogen out of the soil (immobilisation) so that it is temporarily unavailable to the plants.

Phosphorous

The phosphorous (P) level in ppm is measured using the Mehlich 3 method if soil pH is <7.4, and the Olsen method if soil pH is >7.4. An ideal level in the soil is approximately 30-70 ppm.  In plants the concentration of phosphorous ranges from 0.1-0.5%.  It is used in plants for :- Energy transfer reactions, Development of reproductive structures, Crop maturity,  Root growth, Protein synthesis.

The uptake of phosphorous is affected by many things.  At low pH it binds with aluminium and iron and at high pH it binds with calcium or magnesium becoming unavailable to the plants.  The optimum pH for availability is 6.5.  Phosphates do not move easily through the soil and any condition that affects the root volume will also affect the phosphorous uptake. In addition microbial activity greatly enhances P uptake.  The availability of other nutrients will sometimes affect P uptake, for eg zinc deficiency is often lonked with P deficiency

Organic matter is very important for increasing the P availability in soils by:- organic matter complexes with organic phosphates  which increases the phosphate uptake by plants, organic anions displace the phosphate anions that have been sorbed onto soil particles, humus coats aluminium and iron oxides reducing P-lock up, organic matter is a source of P through mineralization.

Potassium

Potassium (K) is an essential plant nutrient, required in large amounts for proper growth a reproduction of plants. Potassium is considered second only in nitrogen in importance to plants.  It is known as the 'quality nutrient'.  Ideally it should be present at a concentration >120 ppm.  Potassium is extracted using the Mehlich III procedure, which measures the potassium adsorbed to the soil colloid and the potassium available in the soil solution, this is the plant availbale potassium.

Potassium regulates the opening and lcosing of the stomata, and therefore regulates the CO2 uptake required for photosynthesis.  It also plays a major role in the reulation of water in plants (osmo-regulation), both the uptake of water through the plant roots and its loss through the stomata are controlled by potassium.  Potassium is known to improve drought resistence.  Potassium triggers enzymes and is essential for production of ATP, which is an imprtant energy source for many chemical processes in plant tissue.  Potassium is essential for the production of proteins and starches.

The availability of potassium in the soil is affected by moisture levels, more moisture makes more K available, low soil temperatures reduce the uptake of potassium, oxygen is essential for the uptake of K, so it can become deficient in waterlogged and compacted soils, high CEC soil can hold more K, and of course soils that are deficient in potassium will produce potassium deficient plants.  Potassium deficiencies are often observed in no - till systems.  It is important to develop a good root system for plants to be able to 'mine' the potassium tha they require.  Potassium is mobile in plants and can be moved from the older to the younger leaves.

Low potassium levels in plants causes chlorosis, stunting, early leaf drop, poor resistence to temperature change and drought, weak unhealthy roots, uneven ripening of fruit, and poor resistence to pests and diseases.

Calcium

Calcium (Ca), is an essential plant nutrient.  The calcium levels in the soil are measured using the Mehlich III method, which measures the calcium adsorbed onto the soil colloid and the calcium in the soil solution.  It should be present in the soils at levels of 430-540 ppm.  When a deficit of calcium is indicated in soils ph<6.2, the corrective treatment is to apply high calcium lime.  If both calcium and magnesium are low, the corrective treatment is to apply dolomitic limestone.

Calcium has many roles in the plant, and is known as the 'post harvest' nutrient.  Calcium is important in th emetabolic uptake of other nutrients, it promotes proper cell elongation.  Calcium is an essential part of the cell wall, forming calcium pectate compounds which strengthen and stabilise cell wall and bind them together.  It helps protect plants from diseases, by building stronger cell walls that are better able to with stand attack from fungi and bacteria.  Calcium helps protect the plants from heat stress by improvong stomata function and participiating in induction of heat shock proteins.  Calcium affects fruit quality.

Calcium is moves into the roots by passive movement through water moving into the transpiration stream, and therefore uptake is directly related to the transpiration rate.  It is only mobile in the xylem, and as such only moves upwards.  It is therefore important to make sure that calcium is constantly available during plant growth.  It cannot be move down to the root tips, so it is also important for calcium to be incorprated throughout the soilk profile.  Conditions of high humidity, cold weather, low soil moisture and low transpiration rates may result in calcium deficiency.  Since calcium is so imobbile in plants deficiencies will be seen first in the younger leaves and in the fruits.  Symptoms include curling and scorching of younger leaves and blossom end rot, or bitter pith in fruits.

Several factors in your soil analysis will help determine the availability of calcium from the soil.  At higher pH calcium tends to be more available.  It forms insoluble compounds with phosphorous, so high phosphorous levels will reduce uptake.  A high EC/salinity will reduce the amount of water moving into the plants and can induce a calcium deficiency.  Soil's with a high CEC will be better able to hold onto more calcium, therefore there is a higher calcium availability.  Calcium competes with other positively charges ions in the soil structure for eg potassium, magnesium and sodium, and an excess of any of these ions might decrease uptake of calcium by the plants.

Calcium is very important for stabilising soil structure.  Sodium and potassium ions, adsorbed to the soil surface, in excess, cause the soil to crack when dry and swell when wet.  Calcium can replace these adsorbed ions in the soil surface and stabililse the structure.  In addition to this calcium is most important for creating a beneficial environment for soil microbes.

Magnesium

Magnesium (Mg), is measured using the Mehlich III method, which measure the extractable magnesium, either adsorbed onto the soil colloid or in the soil solution. A level of about 25-45 ppm in the soil is recommended.  When magnesium is low in soil with a ph<6.2, the corrective treatment is to apply dolomitic limestone.  When the soil ph is >6.2, fertilisers containing magnesium are recommended.

Magnesium  is an essential plant nutrient, it has a wide range of roles in the plant, its key role is in the production of chlorophyll, which is needed for photosynthesis,  it is responsible for making the leaves appear green.  Magnesium deficiency can be a signification factor in limiting crop production.  

Magnesium is present in the soil in three factions. 1) magnesium in the soil solution - this is in equilibrium with the exchangeable magnesium and is readily available to the plants, 2) exchangeable magnesium - this is held by clay particles and organic matter and is in equilibirum with the soil solution, this is the more important fraction for plant available magnesium, 3) non exchangeable magnesium - this  is a constituent of the primary minerals in the soil and is not available to the plant, but is slowly released by weathering.

Magnesium is taken into the plant as Mg2+ by passive uptake driven by the transpiration stream, and by diffusion from the soil solution. Magnesium uptake is affected by low soil temperatures, dry soil conditions, and competitiion from other cations eg ammonium and potassium.  Magnesium is mobile in the plant and can move form the older to the younger leaves, resulting in chlorosis of the older leaves.  Magnesium deficiencies due to soil conditions can be addressed by applying Magnesium sulphate as a foliar feed.

Magnesium deficiency can result in a significant reduction in yield and also leads to much higher disease suceptibility.

Sulphur

Once considered a secondary nutrient sulphur is now considered an essential macro-nutrient.  The test results indicate the ppm of sulphur that is easily eaxtracted form teh soil by the Mehlick III method.  This method mainly extracts the sulphate form of sulphur.  Sulpur should be available in the soil at 12-25 ppm.  Below this plants show signs of suplur deficiency.  Some of the sulphur in the soils is in the organic matter and is made available to the plants through mineralization by soil microbes.  This is affected by temperature, moisture, pH and aeration.  Sulphur deficiency is often linked to nitrogen availability.  The in-organic form of sulphur, the sulphate (SO4-2) ion is very mobile in soils and readily leached out, so often one finds that it is lower on the topsoil than the subsoil.

Sulphur is often deficient in light sandy soils with low OM%, and in places with high rain fall.  Sulphur is not mobile in the plant and defiencies start with yellowing of the younger leaves, although symptoms vary dramatically between plant species and the best way to find out if sulphur is a limiting factor in your crop production is via tissue sampling. 

Sulphur may also be added to the soil in order to correct high pH problems, in the form of gypsum (calcium sulphate), or elemental sulpur.  Sulphates may be added to the soil in the form of ammonium sulphate, magnesium sulphate, potassium sulphate and of course organic matter.

Oil crops, legumes, forages, and some vegetable crops require sulphur in large amounts.  It is essential in the production of amino acids and proteins, chlorophyll production and oil synthesis, and active in the metabolism of nitrogen.

Iron

The ppm of iron idicated in your soil test result is the Mehlich III extractable iron.  At levels of < 30ppm it becomes deficient in the soil at levels >300 ppm it becomes antagonistic to other ions. Iron deficiency is a limiting factor of plant growth.  It is generally present in high quantites but it is very often unavailable to the plant.  Iron is a nutrient that is highly involved in the chlorophyll reaction, helping to contribute to the greening of the plant. Iron is not easily translocated throug plant tissue and deficiency shows up as yellowing of the younger leaves.  It should be available to the plant throughout the growing season. 

Plants take up iron in the Fe2+ and Fe3+ form, using various uptake mechanisms: 1) via the chelation mechanism, where the plants release siderophores that bind with the iron and increase its solubility, this process involves bacteria, plant toots can release protons and reductants that lower the pH in the root zone making the iron more soluble.  New roots and root hairs are more active in iron intake so it is important to maintain a healthy root system.

The type of nitrogen fertilsiation is important for iron uptake - ammonium nitrate can reduce the pH around the root zone and facilitate iron uptake whereas nitrate nitrogen enhances the relaese of OH- molecules around the root zone, lowering the pH and reducing iron solubility.

Iron deficincies are often induced by compacted, waterlogged soils with low oxygen levels, low pH, high pH, low soil temperatures and the presence of antagonistic ions (phosphorous, calcium and competitive microelements).  Iron defiencies can be corrected by foliar sprays - but it is important to get to the root cause of the problem.

More information on iron nutrient can be found here 

Manganese

The ppm of manganese idicated in your soil test result is the Mehlich III extractable manganese.  Like iron manganese should be available in the soil >30ppm and <300 ppm. Manganese, like so many trace elements is involved in a  multitude of reactions within the plant.  Manganese availability will impact on disease resistance, nitrogen utilisation, photosynthesis, water utilisation and cold tolerance to name just a few.  Manganese is highly reactive to the soil conditions and loose, aerated high pH soils substatially reduce the availbility of manganese.  The application of gyphosates can also great reduce manganese uptake in plants.  Manganese deficiencies can be identified by leaf analysis and corrected with foliar sprays.

At very low pH manganese becomes very soluble in the soil solution and can become toxic to plants.

Copper 

The ppm of copper idicated in your soil test result is the Mehlich III extractable copper. Copper should be present in the soil at >1.5 and <10 ppm.  Copper plays many roles in plants but the two most important arease are nitorgen utilisation and lignum formation.   Copper improves the plants utilization of nitrogen and the formation of protiens.  As nitorgen is increased in a crop fertility program - so should the copper be increased.  A lack of copper affects the flavour and storability of fruits.

High levels of phosphorous and potassium can induce copper defiencies.  Copper is the most immobile micronutrient, and plants short in copper generally have a below average root mass due to nutrient deficiencies, compaction, waterlogging or root pruning. Very high levels of OM lock up copper, as do high levels of Zinc.  Low levels of N affect copper uptake.  Deficiencies manifest as yellowing and necrosis of younger leaves and excess tillering in cereals.  Deficiencies can be identified by leaf analysis and corrected by foliar sprays.

Boron

Boron should be present in the soil at >0.8 and <2.00 ppm taken up by plant roots in a passive process moving with the water in plant tissue and accumulating in the leaves.  Therefore the rate of transpiration plays an important role in boron uptake.   Boron deficiency symptoms include, limited budding, bud break, distorted shoot growth, short internodes, increased branching, flower bud drop, and inhibition of fruit and seed development.  Most of the boron in the soil is adsorbed to organic matter.

Since toxic levels of boron in the soil are only very slightly higher than deficiency levels it isimportant to keep a close eye on boron levels in both the soil solution and the irrigation water, and in cases of high boron in irrigation water treat the soil by peroidically flushing.

Common boron fertilisers are borax, boric acid and solubor.

Zinc

The ppm of zinc idicated in your soil test result is the Mehlich III extractable zinc.  Zinc should be present in the soil at >2 and <20 ppm. Zinc is one of the eight essential micronutrients.  It is needed in small amounts but is crucial for plant development and is a common cause of yield reduction.  It is a key constituent in many proteins and enzymes and plays an important role in growth hormone production and internode elongation, as well as being highly involved in the reproductive side.  Plants do a better job of taking phosphorous up when zinc is present.  High soil pH's and high phosphorous levels limit the availability of zinc dramatically.  Where manure is applied there is seldom a zinc deficiency.

Zinc is not very mobile inplant tissue when deficient and deficiencies tend to manifest early on in the growth cycle and symptoms appear first in the middle leaves.  It is important to send leaf samples for tissue analysis if zinc deficiency is expected - yields can be reduced by a significant 20% before any visual symptoms of zinc deficiency can occur. Zinc deficiencies can be rectified by foliar sprays, with zinc sulphates or zinc chelates.  Zinc deficiencies are the most common micronutrient deficiencies in the world. 

Soil conditions that result in zinc deficiency are as follows:  1) low zinc levels in the soil, 2) low organic matter content, 3) too high organic matter content (peat soils), 4) restricted root growth, 5) high soil pH, 6) calcareous or limed soils, 7) low soil temperature, 8) anaerobic waterlogged soils and 9) high phosphorous levels.

Sodium

The amount of sodium indicated in your soil test results is the Mehlich III, extractable sodium, including the sodium adsorbed to teh soil colloid and that which is dissolved in teh soil solution.  Sodium should be kept below <40 ppm.  It is not considered a plant nutrient and high levels can cause problems with salinity, soil structure and uptake of other nutrients.  High levels of sodium can severely affect crop performance and need to be addressed.  Often the application of gypsum is recommended to displace the sodium form the soil colloid and enable it to be flushed through the soil profile.

CEC

The cation exchange capacity of the soil (the CEC), is the total capacity of the soil to hold onto exchangeable cations.  The CEC is an inherent soil characteristic and difficult to change significantly.  The CEC is an important soil property influencing soil structure stability, nutrient availability, soil pH, and the soils reaction to fertilisers and other soil amendments.  The CEC influences the soils ability to hold onto nutrients and provides a buffer against soil acidification.  Soils with a higher clay fraction tend to have higher CEC.  Sandy soils rely heavily on their OM% for the retention of nutrients in the topsoil.

Percentages and ratios

Once the exchangeable base levels are edtermined by Mehlich III in ppm, as above the ratios and percentages are calculated usinf the meq basis (electrical change basis).  

These numbers represent the existing balance between the base elements: calcium, magnesium, potassium, sodium, other bases and hydrogen.  The ratio of these bases plays a significant role in 

  • soil structure
  • soil fertility
  • soil pH
  • nutrient availability
  • micobial activity

A good balance in the soil is calcium 60-75%, magnesium 15%, potassium 5-7%, sodium <5%.

Soils with sodium >15% are termed sodic soils and need special attention in order to yield.

With your complete soil analysis you will get a recommendation for soil fertility correction.  This gives you the quantities of soil amendments required to adjust your bases in the soil for optimum soil fertility.  Sometimes the amounts of amendments required are very high, and could be costly.  In this case one can make a fertility plan, and divide the applications up over 2-5 years to build soil fertility. 

Dolomitic lime is indicated for low Magnesium and low pH levels.  Calcitic lime is added to increase pH and clacium levels.  Gypsum is added to increase calcium at pH>6.2.  Magnesium sulphate is added to increase magnesium at ph >6.2.

Ca:Mg Ratio

The Ca:Mg ratio is important for soil structure and fertility.  A high level of magnesium compared to calcium can dramatically reduce water infiltration into and through the soil.  


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