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There is more to soil testing than analysing the soil's nutrient status. The process incorporates the sampling procedure, soil analysis and interpretation of the results leading to a sound recommendation.
Accurate soil tests allow landholders to maximise the health of their soils and make sound decisions about fertiliser management to ensure crops and pastures are as productive as possible.
Before collecting soil samples consult a local agronomist to discuss the need for additional tests such as deep soil nitrogen tests. In most soils the nutrients are concentrated in the top 10cm of soil, so ensure samples are consistently taken to this depth.
So before you start digging around your paddocks, check out our 7 Steps to carry out a successful soil test:
Pick the right time to collect samples. Changes in soil moisture, plant growth stage and decomposition of organic matter all affect soil nutrient levels. For example, available nutrients can be low in soil samples collected during spring as nutrients are still in the plant and are not returned to the soil until after decomposition. Check with your local agronomist as to the best time to collect soil samples in your area. Most comparisons are based on midsummer (January to March) sampling when the soil is dry.
Plan regular tests to build better profiles. Because many factors influence soil test results, soil analysis for one season is not conclusive. Subtle differences in soil type can impact significantly on the availability and exchange of nutrients between the soil and plants so it is important to test soil regularly.Testing the soil at the same time each year improves the comparison of results between years and builds a clear profile of soil health over time.It is also important to send samples to the same laboratory for testing as results between laboratories cannot be compared easily.Collect enough samples to make up a representative picture of a paddock. It is better to over-sample, as this will provide a more accurate picture of the soil and will help reduce unnecessary fertiliser application.
Ensure samples are representative. Sampling often limits the success of soil testing. One hectare of soil to a depth of 10cm contains about 1300t of soil. A 10g subsample sent to a laboratory represents only one part in 1300 million. So ensure your samples are representative. To increase test result accuracy avoid sampling soil near fences, trees, troughs, headlands, dams, stock tracks and clumps of manure, fertiliser dumps, fertiliser bands from the previous year, burnt heaps, areas of abnormally good or poor growth or poorly drained areas. Also avoid collecting samples from areas where fertiliser, gypsum or lime have been applied during the preceding three months. Wet soil can alter test results due to microbial activity and mineralisation.
Remember to account for variability. Variability of soil is often overlooked. Most soils in Western Australia (WA) are not uniform and comprise different soil types and slopes. Even individual paddocks often have variations in soil surface depth, soil type and nutrient levels, which can be significant over relatively short distances. Many soil types can be found in a single paddock. This, combined with management practices, can lead to varying nutrient levels within and across paddocks. Even if the paddock has a uniform single soil type, stock can spread nutrients unevenly through urine and dung. Management can concentrate or spread nutrients through clearing, burning, grazing or hay production. Where soil differences within a paddock are obvious and areas can be treated differently, take separate samples from each area. Where there is more than one soil type, take about 20 cores from each major soil type. Ensure each soil type is sampled and labelled separately to allow for individual analysis. To increase productivity on larger properties it is worthwhile classifying the land and soil types and ensuring samples are only collected from within a specific land and soil type.
Set a Pattern with Sampling sites. Take samples from across a paddock using a dedicated soil sampling tube or 'pogo'. Take at least five (preferably more) samples per hectare, covering the entire area. Keep in mind that a hectare is 100m by 100m and to take five samples diagonally will involve taking samples about 30m apart in a zig-zag pattern. If the paddock is predominantly of one soil type take at least 40 cores, each to a depth of 10cm. For each soil type, bulk all samples together, thoroughly mix and take a 500g subsample to be sent to the laboratory with clear labels. Note in your records the pattern that you used to collect samples. Following the same pattern in future years will provide a clearer picture of soil fertility trends.
Handle Samples with care. Collect cores in a clean plastic bag and label clearly. Do not use second-hand containers or touch soil samples with bare hands as this will contaminate the sample and affect the test results. Air dry samples by leaving the top of the bag open to the air if there is a delay between sampling and posting. Send samples to the laboratory early in the week if possible to avoid postal delays over weekends. Prepared soil sampling kits are available from most rural supply stores. If using an off-the shelf kit read the instructions carefully as they may have specific instructions.
Get help Interpreting the results. A number of laboratories are available to test and analyse your soil samples. Some services offer recommendations relating to the test results. Contact your local agricultural consultant, agronomist or rural supply store for the contact details of available soil testing and support services in your region.
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Relted post: How to take a soil sample.
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