Soil Sampling - GPS or
conventional?
September 2009, Precision Farming Articles - Willington Crop
Services
As harvest comes to an end and fields are clear of crops, so thoughts turn to the next seasons needs for soil sampling and one query may be the cost versus benefits of low cost soil sampling in conventional manner or more intense GPS soil sampling at higher cost.
Conventional soil sampling has involved walking or driving each field in a W pattern collected 25 cores from along the W to 15cm depth, and these cores amalgamated into a single sample for laboratory analysis. Hence for each field there is then a single result for pH, P, K and Mg. And these single results are used with cropping and soil type to make recommendations on the amounts of lime and fertilizer needed for the field for nutrient management.
GPS soil sampling has now been used by some farmers for over 15 years as a more detailed alternative to conventional soil sampling. Each field is logged onto a digital display and a grid pattern is superimposed onto the field outline to line up with working field edges or tramlines. Each grid area is often 1ha in size and cores are collected from within the grid area and these cores are amalgamated into a single soil sample for laboratory analysis. So for a field of 16ha, 16 sets of soil samples would be collected and analysed giving 16 results each for pH, P, K and Mg. Hence any areas of fields with less nutrients or more nutrients can be identified with GPS nutrient mapping.
As an example of the sorts of soil nutrient data the same fields were sampled by the 2 sorts of soil sampling, conventional w and GPS, and results are shown below:
| EXAMPLE |
Soil Nutrient Results mg/l |
Fertilizer for 1yr sugar beet + 1 yr wheat (kg/ha) | ||
| FIELD | Conventional soil sampling | GPS soil sampling | Conventional soil sampling | GPS soil sampling |
| pH |
7.99 | 7.92 to 8.08 | 0 | 0 |
| P |
INDEX
3 (34.6) |
INDEX
1 to 4 (14.2-54.8) |
220 P205 | 425 P205 |
| K |
INDEX
2- (180.3) |
INDEX
1 to 3 (95.8-289.9) |
2145 K2O | 1547 K20 |
| Mg |
INDEX
2 (70.9) |
INDEX
1 to 3 (44.1-142.1) |
0 | 0 |
What do these differences in soil nutrients and fertilizer requirements mean?
- For P, more needed with GPS soil sampling results, an extra 205kg P2O5/ha. By applying the conventional soil sampling requirements for P, parts of the field with index 1 would be getting 150kg P2O5 /ha less than needed for crop growth, therefore potentially restricting crop yields in these areas of field.
- For K, less K2O is needed, 502kg K2O less so preventing over fertilizing some areas of field that are already higher in K than field average value.
GPS soil sampling will always give a more detailed picture of soil fertility within a field to allow more detailed nutrient management using GPS nutrient mapping results. But good conventional soil sampling also gives useful nutrient management information.
