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Transcript
Soil testing Soil property Field tests which provide information Off-site tests which provide information Colour Observation Colour charts Depth Tape measure or ruler to measure horizons in exposed profile or auger core Texture Field texture assessment Structure Observation Aggregate stability Test in water and gypsum Organic matter content Observation Colour Drainage & aeration Texture/structure Compaction Colour Water in a hole (time to drain away) Leaching Texture Horizon Colour Waterlogging Texture Horizon Colour Erodability Texture /Structure Organic matter content Aggregate stability Water holding capacity Estimate from texture RAW Water infiltration Infiltration rate Particle analysis Organic Carbon content Weigh and dry, reweigh Acidity or alkalinity pH colorimetric kit pH meter or Ca Cl2 test Fertility Parent material Texture Organic matter content Individual nutrient levels assayed Salinity Cation Exchange Capacity CEC Electrical Conductivity EC Nutrient availability pH Nutrient balance Susceptibility to compaction Structure Texture Presence of compaction Screwdriver test Bulk density Soil contamination, toxicity Bulk density Specific chemical tests Additional soil tests Carbonate testing As you saw in the first section on Soil classification, the Gradational group in the Northcote System is subdivided according to whether the soil is calcareous or not. Calcareous means that a form of calcium carbonate is present. This may come from some sort of limestone parent material or it may be injected into the rocks after they form and be present in the soil as nodules. The test for the presence of calcareous deposits is simple. A few mm of dilute acid (such as HCl Hydrochloric acid) is dripped onto a sample of the soil and if effervescence occurs (fizzing and bubbling) it indicates the presence of CaCO3, with CO2 being driven off as a gas. Remember that acid will burn. Use cautiously, wear gloves, protect eyes from splash, and place the soil sample in a saucer or similar container that will not itself react with the acid. Dispose of the tested sample in a safe place. Salinity When the Ec is low it means general nutrient levels are low. If the Ec is high then the soil may be saline, containing sodium chloride salt, or it may contain soluble salts other than sodium chloride. This can be the result of overfertilising. Sodicity Sodic soils have sodium ions attached to the clay particle. When the soils are wet this causes the particles to move away from each other, so the soil disperses. You have seen this in the aggregate stability test (see 1618C). Dispersion is caused by an interaction between the sodium ions and the particular type of clay. The sodium displaces other useful ions such as K+, Ca++, NH4+, Zn++, Cu++ etc and then destroys the soil structure. When the water evaporates, the individual particles of soil settle into a compacted impenetrable mass, causing waterlogging, poor infiltration and hard-setting crusts. Gypsum can help ameliorate these soils, but is not economical for large areas. Understanding off-site soil test results The following figures indicate generally acceptable levels for various soil tests. These figures are guidelines only and interpretations will vary depending upon soil type, soil history, sampling methods and requirements of particular plant species. Generally acceptable levels for various soil tests Off-site soil test pH (CaCl2 test) Organic Carbon Total Nitrogen N Available Sulphur S Phosphorus P Cation Exchange Capacity CEC Sodium levels Aluminium levels Calcium; Magnesium Ca:Mg Chloride levels and Electrical Conductivity EC Purpose pH between 5-7 ideal for most plants 7 = neutral Estimate of this indicates % Organic matter in soil Meaning of results PH can be adjusted to suit plant requirements %C x factor dependent on method used = % OM Aim for 2% OM Major nutrients required 200-300 kg N/ha is considered adequate 5-10 mg S/kg soil 20-35 mg P/kg soil is considered a medium level The higher the clay and/or Desirable ranges for % cations: organic matter content of the Ca 65-80% soil, the higher is the colloid Mg 10-15% level and the CEC. K 1-5 % Al < 5% Na < 5% If Na is more than 6% the soil becomes dispersive and should respond to gypsum. If Al is more than 5% toxicity starts to affect growth of sensitive plant species. Apply lime to raise pH above 5, the point where Al is no longer available as a cation. This ratio should be 2:1. Higher Magnesium levels can cause a dispersive effect similar to Sodium. These are measures of salinity. There is a risk of salinity if Cl levels > 300mg/kg soil, or if EC is more than 2 dS/m.