Interpretation of soil test reports requires specialized knowledge of local conditions and crops.
How to interpret your soil test
If you use your soil to earn your living, soil tests should be a routine part of your management. A soil test gives you a snapshot of some of the nutrients in your soil and helps you decide which ones to add to make your soil more productive. However, it is not a magic formula, and test results need to be considered together with plant tissue tests, and your farm’s cropping, pasture and fertiliser history.
If you sell produce off the farm (including milk), you need to test soil annually because crop removal rapidly depletes the soil of nutrients. If you graze animals you need to test every 2–3 years to ensure nutrients are in balance.
This publication contains information to help you understand the most significant results, but it is important that you back up this broad interpretation with advice from your horticulturist or agronomist.
Preferred level pH (CaCl2):
Soil acidity is measured on a pH scale from 0 (most acid) to 14 (most alkaline), with 7 as neutral, that is, neither acid nor alkaline. The scale is logarithmic, that is, going down the scale from pH 7 (neutral), each number is 10 times more acid than the one before it. For example:
- soil with a pH of 6 is ten times more acid than soil with a pH of 7 (neutral);
- soil with a pH of 5 is one hundred times more acid than soil with a pH of 7.
The term CaCl2 after the pH figure signifies that the pH was measured in a solution of calcium chloride, a test preferred by most soil scientists. When soil pH is measured in a solution of CaCl2, the pH is 0.5–0.8 lower than if measured in water.
Cation exchange capacity (CEC)
Preferred level: above 10
This is a measure of the ability of the soil to hold the nutrients calcium, magnesium and potassium. Good fertile soils with high clay content and moderate to high organic matter levels usually have a cation exchange capacity of 10 or higher. (Note: a ‘cation’ is a positively charged ion.)
The major cations are calcium, magnesium, potassium, sodium and aluminium. These are held in the soil by organic matter and clay. The preferred percentages and the suggested quantities of exchangeable cations in the soil are given in Table 1.
|Table 1. Preferred percentages of exchangeable cations as a proportion of CEC, and suggested quantity values|
|Cation||Preferred percentage (%)||Suggested quantity (meq/100 g)|
Note: meq/100 g or meq% is the same as cmol/kg.
If your soil test report does not provide percentages, you can calculate them yourself by dividing the quantity of each cation (the meq figure) by the CEC figure, and multiplying the result by 100.
- Sometimes the level of hydrogen cations is reported, but this should not be added to your total CEC.
- If any other cations are reported, such as manganese, this may indicate a toxicity problem.
- High levels of aluminium are toxic to some plants, and this situation is usually associated with more acidic soils.
- High sodium levels can indicate sodicity problems (i.e. soil structure problems), or salinity problems.
When your soil test report gives quantities in parts per million (ppm), you can use the following conversions to obtain meq figures:
|Cation||Number to divide by|
|Calcium||Divide by 200|
|Magnesium||Divide by 120|
|Potassium||Divide by 390|
|Sodium||Divide by 230|
|Aluminium||Divide by 90|
Preferred level: above 3
The calcium/magnesium ratio is found by dividing the quantity of calcium (meq/100 g) by the quantity of magnesium (meq/100 g). If the figure is below 2, it is more difficult for plants to take up potassium, and there can be problems with soil structure breaking down due to dispersion.
If you use dolomite (2 parts calcium to 1 part magnesium) regularly, your soil’s calcium:magnesium ratio will fall because too much magnesium is applied compared with calcium. Calcium can be added in the form of gypsum or lime. High calcium:magnesium ratios up to 20:1 have not been shown to adversely affect plant yields.
There are two different tests for phosphorus in NSW: Bray and Colwell. Since they give very different results, it is important to know which one is used in your report.
Bray phosphorus levels vary with land use:
- 15–20 mg/kg for dryland pastures
- 25–30 mg/kg for irrigated and improved pastures
- 30–50 mg/kg for tree crops
- 50+ mg/kg for vegetables.
Note: ‘mg/kg’ is the same as ‘parts per million (ppm)’.
Colwell levels vary from 20 to 100 mg/kg depending on soil texture.
The Bray test tends to be more suitable for the North Coast’s acid soils. Because phosphorus tends to tie up with aluminium and iron and become unavailable to plants in acid soils, it is important to keep your pH at around 5 if your soil is to benefit from phosphorus.
Preferred level: none calibrated
Nitrate levels fluctuate widely, depending on the season or rainfall. No levels have been calibrated for the North Coast, but agronomists generally like to see a level of 10 mg/kg or more in pasture soils, and a level greater than 20 mg/kg in horticultural crops.
Preferred level: below 0.15 dS/m (EC1:5)
Electrical conductivity is a measure of salts in the soil. A productive soil’s conductivity should be below 0.15 dS/m (decisiemens per metre).
Plants vary in their reaction to salt stress, from ‘sensitive’ to ‘tolerant’, and the degree of reaction is less in clay soils than in sandy soils. For this reason, soils affected by salt should also have a saturation conductivity test (ECse). However, these results should not be compared with EC1:5 figures.
Salinity problems can be caused by too much fertiliser, salty irrigation water or saline ground water. Salts can be leached out with rainfall or low salinity irrigation water without affecting soil pH. Because of its high rainfall, the North Coast generally does not have a great problem with soil salinity except in some low, poorly draining soils close to tidal rivers.
Preferred level: varies according to crop
The extraction procedures for trace elements can vary between laboratories, resulting in different figures. A general guideline for the preferred level of trace elements in the soil is given in Table 2:
|Table 2. Preferred level of trace elements in soil|
|Trace element||Preferred level in soil (mg/kg)|
Soil test results that fall outside these ranges deserve a closer look.Iron, manganese and zinc are all readily available in the North Coast’s acid soils. Zinc is fixed by iron on red basaltic soils, and since boron leaches easily, deficiencies are quite common on the North Coast, particularly in horticultural crops.
Molybdenum is less available in acid soils on the North Coast, so it needs to be added to the soil, particularly for pastures and vegetable crops.
If you suspect your soil has a trace element deficiency, have a plant tissue test done.
Preferred level: above 2%
Organic carbon is a measure of the organic matter in the soil. It includes undecomposed plant litter, soil organisms and humus. Soil organic carbon stores important nutrients, stabilises soil structure and feeds soil microbes. If soil organic carbon is declining over time, then consider practices such as green manure crops, minimum tillage, mulching or strategic grazing.
Other soil properties
Laboratory tests are important but they will not alert you to soil compaction, structure decline, erosion or subsoil problems. These types of degradation are much harder to fix than a nutrient deficiency, and recognising the problem early can make a big difference. When collecting your soil samples, note the following:
- condition of the soil surface;
- depth of the topsoil;
- structure of the soil;
- penetration of plant roots.
A soil health card is a simple guide to this type of soil examination.