Value of Soil Sampling

By Eddie Webb, B&T Soils & Environment Adviser, Teagasc Galway/Clare

Soil sampling is the most important stage in managing soil fertility. Soil test results are a reliable guide to assessing and monitoring soil nutrient levels so that informed decisions can be on lime, manure and fertiliser application. All participants in ACRES Tranche 1 must have valid soil sample results or soil sample exemption forms uploaded on the ACRES online system by 15^th May 2024. One analysis per every 5 hectares of land up to 40 hectares is required. The maximum area for individual samples is 5 hectares and only samples taken on or after 01/01/2022 will be considered valid for Tranche 1 applicants.

In preparation to soil sampling, it’s imperative that a soil sampling map is prepared showing defined soil sampling areas.  This will identify the fields/management blocks on the farm for nutrient applications. When taking the samples, a suitable soil corer for sampling to 10 cm deep is essential and a sample should be taken for every 2 to 4 hectares. Separate samples should be taken for areas that are different in soil type, previous cropping history, slope, drainage or persistently poor yields. Any unusual spots such as gateways, troughs, feeding points, dung or urine patches should be avoided. A representative soil sample can be collected by taking a W shaped pattern across the sampling area.

Nutrient advice is based on soil sampling to a depth 10 cm of the soil. It is critical that all samples are sampled from this top 10cm. In grassland soils, Phosphorous is immobile and tend to stay in top few centimetres of soil. Soil samples should be taken every 3-5 years as soil nutrient levels change slowly over time. These soil test results will provide the basis for nutrient advice until soils are resampled. The most appropriate time of year to take samples is from September to March.

A few key rules need to be followed betwwen application of lime, fertiliser, slurry or manures and taking soil samples. 

Lime
Allow at least 2 years after lime application before sampling a field for accurate soil pH reading. 

Fertiliser
Allow at least 3 months after the last application of feriliser P and K. Nitrogen and Sulphur compund fertilisers have no effect on test results.

Slurry & Manures
Allow at least 3 months after the last application of manure or slurry.

There is no point in sampling your soil unless you utilise the results. By applying the correct rate of fertiliser in the right field you will not only improve yield performance from the whole farm but will save money. When you get your soil sample results it is important to then develop a fertiliser programme specific to each field.

Potassium can be spread all year round (there is no restriction on its application), nonetheless there are a few points to consider. In spring, K applications should not exceed 90 kg k/ha in a single application. If you have a requirement above that it would be best to apply the balance in autumn. On rapidly growing swards which receive high levels of potassium, grass tetany can occur, as it prevents Mg uptake by the grass crop. Regardless, it would be best practice not to apply any fertiliser in high amounts in one application and split the application to get best results.

Soil pH – The most important deficiency to correct and the cheapest one! For grassland, target a soil pH of 6.2 to 6.5 (pH 5.5 on peaty soils). Apply lime according to soil test recommendations. A maximum of 7.5 t/ha (3 t/acre) should be applied in any one year. For recommendations greater than that, split the application and apply the balance 2 years later.

Phosphorus is necessary for physiology of the plant, including photosynthesis, root and tiller development. Like other nutrients P can get “locked–up” in the soil if the pH is low; another reason why correcting soil pH should be the first step in improving soil fertility. Many farms create a farm map showing which fields are low in phosphorous and potassium and this is very useful when deciding where to spread slurry. Speak to your advisor, local merchant or rep who are generally happy to help draw up a fertiliser plan to meet your needs. Put your plan in the tractor so when it comes to spreading fertiliser you know what fields need what products.