Factors affecting soil analysis

Nutrient values can vary as a result of a number of factors which are discussed below. Further study is needed to define the scale and cause of such variation.

Variation can exist over very short distances (less than 1 metre) and the number of cores that are needed to ensure that sampling is representative of an area will vary according to the scale of this variation. When sampling to obtain an average value for a field this is not so important because all the cores taken are bulked into a single sample; for a typically uniform field 25 cores are sufficient. However, local variation is important when grid sampling because each grid point is viewed as a separate sample. Normally, because the extent of the variation is unknown, at least 16 cores need to be taken on 1 metre grid-spacing around each grid point to provide a single bulked sample for analysis. If it is planned to produce a fully interpolated 'map' of the variability of phosphorus or potassium within the field, then the grid points should ideally be not more than 50 metres apart. However cost considerations usually limit the number of sample points to one per hectare i.e. 100 metres apart.

Where considerable differences occur within a field, a single sample can be misleading because the averaging will disguise different treatment requirements. If different areas are known to vary they should be sampled separately. Knowledge of field amalgamations, soil type changes observed when ploughing or cultivating, visual crop growth difference across the field or any other specific information on field variation should be used to ensure that the area is sampled to best effect. Alternatively grid sampling may provide a more detailed picture of in-field variation, albeit at extra cost.

Variations in nutrient values have been observed at different times of the year and there is evidence to suggest that soil P, K and Mg values may be higher in the early spring than in the autumn as a result of chemical weathering over winter, biological activity and lack of uptake by growing crops.

If soils are dry at the time of sampling the analytical results can be affected and may appear a little lower for pH, P and possibly K. Movement and uptake of all nutrients will of course be restricted in very dry soils but this is a transient problem and does not reflect the normal availability of nutrients in the soil. At present it is not possible to quantify this effect in order to improve interpretation of analytical results.

During periods of rapid growth crop uptake, especially of potassium, can be large and may deplete available soil nutrient levels for a short period until the nutrient status returns to equilibrium. This could affect results for some soils if samples are taken at such times.

Nutrients are returned to the soil in crop residues but will not be determined by analysis until the plant material is broken down. It has been suggested that sampling close to harvest may result in an under-estimation of true soil nutrient status.

Frequently there may be a gradient in nutrient level down the soil profile, usually declining with depth, reflecting the accumulation of nutrients in the plough layer (see section: 'Effects of different cultivation systems'). This is accentuated in minimal cultivation systems where phosphate and potash may be concentrated in the top 5 to 8 cm (2-3 inches). With continuous direct drilling there may be a large difference between the top 5 cm (2 inches) and 8 to 15 cm (3- 6 inch) zones. For permanent grass where there is no soil disturbance, a consistent sampling depth of 7.5 cm (3 inches) is particularly important.

Applications of fertilisers and manures obviously have a major impact on measurement of soil nutrients. It is difficult to lay down rules as to how soon sampling should be undertaken after application. General guidelines are:

• 8 weeks after P, K, Mg fertiliser application.
• 12 weeks after slurry or manure application.
• 12 months after lime spreading.

Ploughing and cultivating help distribute nutrients from fertilisers and manures, and lime, throughout the depth of cultivation, but this takes time.