Soil analysis: key to nutrient management planning
Sampling
Soil analysis data are only as good as the sample taken. A sample normally comprises around 1 kg of soil which is taken to represent an entire area or field, which contains around 2,000 tonnes of soil per hectare to a plough depth of 20 cm (8 inches). It is therefore imperative to obtain as representative a sample as possible or the results will not reflect the nutrient status accurately.
Effects of different cultivation systems
Regular ploughing/cultivation: Nutrients are mixed into a fairly homogeneous soil layer 20-25 cm deep (depending on ploughing depth). Nutrient concentrations in a core taken to 15 cm should be the same as those in the whole mixed layer.
Regular minimum cultivation to 5 cm: Applied phosphate and potash will tend to accumulate in this shallow mixed layer leading to stratification of nutrient concentration in the top of the soil profile. Nutrient concentrations in a sample to 15 cm depth will not be equivalent to those in the traditional mixed layer, which is normally to the depth of ploughing. The samples will not be comparable and will tend to over-estimate the supply of phosphate and potash available to the crop.
Occasional ploughing after several years of minimum cultivation: The soil layer that is mixed by minimum cultivation (and relatively rich in nutrients) will be buried by ploughing to a depth that is not accessed during normal soil sampling. Samples taken after ploughing can underestimate the supply of phosphate and potash available to the crop in these systems.
An illustration of the potential for a soil sample to show an over-high value when taken to the standard
depth in a field where minimum cultivation has been practiced. (
download illustration)
Rules of sampling
1
Use 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 suitable tool and packaging that will not contaminate the sample. Galvanised sampling tools are unacceptable for trace element analysis.
4
Sample to a consistent depth. Normal depth is 15 cm (6 inches) for arable soils, 7.5 cm (3 inches) 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 stones 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 a minimum of 16 cores is 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 true 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 each time 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.
