2.10 Soil Tests

Soil Tests for nutrients, salinity and disease

Soil and plant tests provide a useful tool on which to base nutrient decisions. There reliability is highly dependent on the use of a good sampling method and for plant test the treatment of the sample after collection.

pH

Two standard pH tests; pH(water) measure d in a 1:5 soil/water suspension or pH(calcium Chloride) measured in a 1:5 soil/0.01 M calcium chloride suspension. pH (calcium chloride) is normally 0.5 - 1.2 units lower than pH(water).

Both can be done in the paddock or in the laboratory. For accurate results use new paddock test kits as the chemicals go off with storage.

Figure 1:
 pH(water)

pH extremes result in nutrient lock-up or toxic levels being available and reduced biological activity.

Nitrogen (N)

Deep nitrogen testing is becoming the more popular assessment of soil nitrogen status than 0- 10 cm soil test. It can be carried out before seeding or during the season to determine the level of crop available nitrogen. This laboratory test requires soil sample taken from the 0- 60 cm.. Results are given in terms of nitrate (mg/kg) and ammonium (mg/kg) and total available nitrogen (kg/ha).

Figure 2
nitrogen testing

Organic carbon

Laboratory test requires a soil sample from the top 0 - 10 cms of the profile. Organic carbon trends rather than individual results are an indicator of the soils total organic carbon pool; only a portion of this will be available. Results are expressed as a percentage of organic carbon in a unit of soil.

Figure 3:
Organic carbon testing

Phosphorus (P)

In South Australia the most popular laboratory test for phosphorus uses the Cowell method. Results are quoted in mg P/kg soil. As a general rule of thumb applications of 3kg P/ha will raise the soil P levels by 1mg/kg.

Figure 4:
Phosphorus  testing

Potassium (K)

Potassium is rarely an issue in cropping soils, however it is usually measured in a standard 0 - 10cm soil test. It is of more concern in pasture and vegetable crops. Results are in mg K/kg of soil.

Figure 5:
Potassium testing

Sulphur

Soil sulphur (top soil) is generally a poor indicator of sulphur fertiliser requirements and deep soil testing is now the recommended method. Currently a S level of less than 40mg/kg in the 0-60cm zone of soil is considered to be low.

Figure 6:
Sulphur testing

Cation Exchange Capacity (CEC)

Clay particles and soil organic matter have large surface areas with negatively charged sites, which can attract and hold positively charged atoms called cations. There are two groups of cations:

Basic cations 

• calcium (Ca)

• potassium (K)

• magnesium (Mg)

• sodium (Na)

this is the main group and are important in controlling plant nutrition.

Acidic cations 

• hydrogen (H)

• aluminium (Al) 

• manganese (Mn)

Aluminium and manganese may become significant exchangeable cations when the soil pHwater is less than 5.5 (pHca 4.7). These can be toxic to plant roots and soil micro-flora if present in significant levels.

At soil pHwater above 5.8 (pHca 5.0) the basic cations dominate a soil's exchange capacity.

The cation exchange capacity (CEC) is the sum of individual cations. The greater the number of cations (charged sites), the greater is the nutrient retention ability of the soil and the better its capacity to supply nutrients to plant roots. Values of CEC (expressed as cmol(+)/kg) range from 1 cmol(+)/kg (coarse sand) to 75 cmol(+)/kg (organic soils).

A level of more than 15 indicates high inherent soil fertility and less than 5 low inherent soil fertility.

Caution: The presence of salts, gypsum or lime can lead to an over estimation of the CEC.

Testing is recommended every 3 - 4 years in a particular paddock so that significant changes can be detected and corrective action taken.

Desirable levels of soil cations for sandy loam - clay loam soils.

Figure 7:
Desirable soil pH and cations in clay loam - sandy loam soils

Exchangeable cations can be looked at two ways: sufficiency levels, ie. is there enough for plant growth and balances between -expressed as a ratio or %.

If a ratio of calcium:magnesium is less than 2 structural problems may be observed in the plant.

The amount of and ratio between cations will vary significantly down the soil profile as different soil layers will have different characteristics. Interpretation of surface soil tests need to be done with care as clay layers etc underneath will often have quite different cation characteristics

Most recent information and research suggest that most plants and particularly dryland crops and pastures are robust in their cation requirements having a wide tolerance of cation ratios, with no optimum ratio being demonstrated in the field. Approximately 300 experiments have been conducted looking at cation ratios and only one has shown evidence that these are necessary. There may, however be instances where specific plant species have requirements that cannot be met by a specific soil. All the information suggests that if the amount of each exchangeable cation is sufficient, ratios are usually not important.

Cation ratios do get out of balance, but in these cases a deficiency of some kind is usually obvious.

Salinity

Measured as the electrical conductivity of a 1:5 soil water suspension. Results are expressed in decisiemens/meter. This is multiplied by a factor for soil texture and becomes and estimated electrical conductivity for a saturated paste ECe

EC values of less than 0.16 dS/m are generally regarded as harmless to plants.

Figure 8:
Salinity

Chloride

Measured in a 1:5 soil solution and measured in mg/kg. Critical levels for salinity are below. Above these figures damage may occur depending on soil drainage and plant tolerance.

Figure 9:
Chloride

Aluminium and Boron

Exchangeable Aluminium is also used in the assessment of lime requirements where aluminium sensitive species such as barley are grown. Where extractable aluminium is greater than 4 mg/kg sensitive plants will be affected and liming is required.

Boron deficiencies can occur if extractable boron concentrations are less than 0.5 mg/kg for most crops. Boron toxicity may occur with sensitive crops if the level is greater than 5 mg/kg. For cereals grain analysis is the most accurate indicator of boron toxicity.

Trace elements

are best monitored using plant tests.

Soil borne diseases

These can significantly impact on root growth and plant ability to take up nutrients. The Cereal Root Disease Testing Service provided by SARDI can detect the level of five soil borne diseases from one soil sample.

• Take-all

• Rhizoctonia

• Cereal cyst nematode

• Pratylencus neglectus

• Pratylencus thornei

Dry soil samples are collected before sowing and sent to the laboratory for disease and/or nutrient analysis.

2.10 Soil Tests

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