Comparing GPS and Conventional Soil Sampling
9 September 2025Many farmers are adopting precision agricultural technologies such as variable rate lime and fertiliser spreading. Variable rate spreading relies on knowledge of the variation in soil nutrient content across a field, measured through GPS soil sampling. Precision spreading of inputs can save money through reducing over application of nutrients to certain parts of the field and can boost yields, crop quality, increase crop uniformity and reduce your impact on the environment.
Defining Conventional Sampling
Conventional soil analysis uses a pooled sample, where up to 25 individual soil cores are gathered across a field typically in a W pattern, before being mixed into a “pooled sample”, which is then analysed. This gives the average soil nutrient content of the field. However, this doesn’t reveal the variability within the field. This is a relatively cheap form of analysis and gives you a good idea of the nutrient status of the field. Therefore, it is a good option for farmers who haven’t done much soil testing in the past and is essentially an entry level form of soil sampling.
Table 1 - Benefits and disadvantages of conventional soil sampling.
| Soil Sampling Practice | Advantages | Disadvantages | |
|---|---|---|---|
| Conventional pooled sample | • Simple sampling procedure – can do it yourself. • Cheapest analysis – only one pooled sample per field. • Results are easily interpreted, and basic spreading equipment can be used to apply recommended fertiliser or lime. | • No measure of the variation within the field. • You may oversupply nutrients to some areas of the field – increased lime and fertiliser cost for no benefit. • You may undersupply nutrients to some areas – suboptimal plant nutrition. |
Defining GPS Sampling
GPS Soil sampling is becoming increasingly popular particularly among arable and dairy farmers. This is due to the increased yields producers are seeing as a result of improved crop nutrition, as well as savings on lime and fertiliser due to more accurate application of nutrients. GPS soil analysis splits the field down into quadrants, typically 1 ha blocks. The field can also be split into quadrants based on soil type. These quadrants are then sampled, and each sample is individually analysed giving an accurate nutrient content for the soil in that area within the field.
Figure 1 - Example of results from 1 field with GPS soil sampling.
The results from GPS soil sampling can be uploaded onto your tractor’s GPS systems PDA (Personal Digital Assistant) to enable exactly the required rate of fertiliser or lime to be spread in each part of the field.
GPS soil sampling is a more expensive method of analysis and typically requires professional sampling done by staff of the analysis company offering the service. In addition, to conduct variable rate spreading you may need to invest in upgrading your machinery with items such as GPS system and PDA for your tractor. Variable rate spreading is also typically charged at a higher rate by contractors in comparison to conventional spreading. However, these additional costs are either one off costs – such as the cost of mapping the farm’s soil types, or the cost of upgrading your kit.
These costs are also offset by the increased income from higher yields, improved crop quality and uniformity and from reduced fertiliser and lime use due to accurate applications. Research and farm trials have shown a saving in fertiliser use of around £30/ha. Reductions in use of fertiliser also have an environmental benefit, reducing risk of diffuse pollution and reducing your carbon footprint.
Table 2 - Benefits and disadvantages of GPS soil sampling.
| Type | Advantages | Disadvantages | |
|---|---|---|---|
| GPS soil sample | • Shows the variation in soil nutrition across a field. • Enables the use of variable rate spreading. • Can lead to savings in fertiliser and lime. • Can increase crop yield, quality and uniformity. • Reduced risk of diffuse pollution. • If paired with variable rate spreading, savings can be made on fertiliser and lime. • Reduced carbon footprint. | • More expensive method of soil sampling. • Will likely require expert input to conduct sampling and interpret results. • May require an initial investment to map farm soil types and invest in GPS system and variable rate spreading technology. |
Final Thoughts
The use of GPS Soil sampling when compared to the conventional pooled method can enable the use of variable rate spreading. This allows the farmer to address variability in nutrient status across the field, resulting in increased yield, improved crop quality, increased uniformity in the crop, reduced risk of nutrient run off and reduced fertiliser use due to not over applying nutrients.
Therefore, despite being more expensive per hectare as a sampling method, when compared to conventional soil analysis, savings and improved production can more than cover the additional cost.
If you’d like more information, or to speak with a specialist, contact the FAS Advice line for free at 0300 323 0161 or email advice@fas.scot
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