Soil analysis: key to nutrient management planning |
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Sampling
Soil analysis data are only as good as the sample taken. A sample normally comprises around 1 kg (2 lbs) of soil which is taken to represent an entire area or field, which contains around 2,000 t of soil per hectare to a plough depth of 8 inches (20 cm). It is therefore imperative to obtain as representative a sample as possible or the results will not reflect the nutrient status accurately.
Rules of sampling
| 1 | Use of a suitable tool (cone auger, screw auger, corer etc) which facilitates and encourages the taking of more rather than fewer cores, of a uniform size and down to the full depth of sampling. |
| 2 | Use equipment and packaging that will not contaminate the sample. Galvanised sampling tools are unacceptable for trace element analysis. |
| 3 | Label samples clearly. |
| 4 | Sample to a consistent depth. Normal depth is 6 inches (15 cm) for arable soils, 3 inches (7.5 cm) for grassland. |
| 5 | Divide the field into areas which are as uniform as possible in soil type, past cropping, and manuring history and sample separately. Small areas of different soil e.g. wet, chalky, shallow, stony etc. should be excluded. |
| 6 | Avoid headlands, gateways, trees, mole hills, dung/urine patches, water troughs, areas where lime or manure has been dumped, old hedgerows/middens/ponds or any other irregular feature. |
| 7 | Discard stone and plant debris. |
| 8 | Take at least 25 cores from each area to be sampled and put them together to form a single representative sample. The numbers of cores should not be restricted simply because the container is full! Thoroughly mix all cores and take a sub-sample from this for despatch to the laboratory - this must be done carefully. |
| 9 | Ensure the sample represents the whole area. Sample on a W pattern over the field; for a regular shaped field this means 7 cores per leg of the "W". |
| 10 | In the case of grid sampling up to 16 cores are needed at each sampling point to obtain a representative sample. These should be taken in a regular pattern about 1 metre apart around the grid point. Grid points should be evenly spaced over the field and should not be more than 50 metres apart if a map is to be produced. |
| 11 | Sample at the same point in the rotation, before the crop which is most demanding or responsive to P and K. In descending order of importance these are: horticultural crops, vegetables, roots, pulses, spring sown combinable crops, winter sown combinable crops. For pH it is preferable to sample 12 months before a sensitive crop such as sugar beet or barley. |
| 12 | Sample at the same time of the year. |
| 13 | Avoid sampling under extremes of soil conditions e.g. waterlogged or very dry soil. |
| 14 | Do not sample within 8 weeks of fertilising, or within 12 weeks of manure or slurry application, for P, K and Mg analysis or sooner than 12 months after liming for pH analysis. |
| 15 | Maintain records and use the analytical results to develop nutrient management plans. |
| 16 | Where sampling to diagnose a crop problem take multi-cored samples (at least 16 cores per bulked sample) from areas of poor growth and separate samples from normal (good) areas. The relative values between good and poor will be more informative than the actual values of the problem area. |