The art of soil testing _______________________________
Taking soil tests to ascertain nutrient levels for subsequent crop production is a very small but essential cost which can pay back large dividends.
Not only will a good soil test identify which elements the fertiliser budget should focus on applying to the crop and what quantities are required, but it will also identify which elements are not required. Applying unnecessary or excessive nutrients to the soil is not only a waste of money, but they can also be bad for the environment when these are leached into ground water or lost as run-off into streams. Too much of one element can also negatively affect overall crop yield, making crops more susceptible to attack from pathogens, and affect the quality of crop grown itself, as well as the shelf life of certain fruits and vegetables.
When taking soil tests off a particular field, you need to identify if there are any soil type, aspect or contour differences within the area you are sampling. If the field is flat and uniform with a known similar fertiliser history, and previous yields across the field have been similar, then a random walk across the field taking 10-20 plugs should suffice. If however there are ponding areas, or if there are distinct soil type or contour differences, then these should be sampled separately, or not be included in the sample should they only be a small fraction of the area the sample is to represent, as including these anomalous areas in the bulk sample will skew the results and lead to incorrect diagnosis of what fertiliser nutrients are needed.
If the paddock has previously been in pasture, areas around water troughs, gateways, shelter belts, hedges or trees should be avoided as these are areas where livestock will have congregated and will be higher in fertility than other parts of the paddock due to more dung and urine being deposited there. If the paddock has been recently grazed with cattle in particular, you should wait for at least 2-3 weeks after grazing to identify urine patches and avoid including these in the sample as this will lift the nitrogen and potassium levels in particular. Sheep and deer urine patches are not as big and concentrated so will not skew the sample as much as cattle urine patches.
When to take samples is another important consideration. Soil samples should not be taken if fertiliser has been applied within the previous three months as this could contaminate sample. The moisture content of the soil is also important and tests should not be taken in drought conditions or if the soils have been