Knowing your phosphorus requirement

The Phosphorus Regulations became effective on 1 January 2007. Farmers must not apply any chemical phosphorus fertiliser unless there is a requirement. This can only be established through a soil test.

Area to sample: The size of the area from which one sample can be taken varies, but must not be more than four hectares. Generally one sample should be collected from each field. Within one field, areas which are not uniform for crop growth, and areas which have been cropped or fertilised differently, must be sampled separately.

Time of sampling: Sampling every fourth year is satisfactory as a basis for phosphorus fertiliser recommendations. A field must not be sampled for phosphorus until at least three months after the last application of any fertiliser (organic or chemical) containing phosphorus. Results from a previously taken soil sample taken is valid for a four year period from the date sampled.

Depth of sampling: Grassland must be sampled to a depth of 75 mm and arable land to a depth of 150 mm.

Method of sampling: A soil sample must be made up by bulking at least 25 sub-samples taken from the area to be sampled. The sub-sampling points must be selected systematically to give an even distribution over the whole sampling area. This distribution should be achieved by following the pattern of a letter “W” and taking sub-samples at regularly spaced intervals. Taking sub-samples from headlands, dung and urine patches, areas where stock gather or other unusual features must be avoided.

Who can test the sample? Analysis must be carried out by a soil-testing laboratory competent to analyse soils for phosphorus. Each analysis, provided by the competent laboratory, will require a UKAS accreditation or (National equivalent) statement.

With good management of livestock manures most farms can eliminate chemical phosphorus with no loss to productivity due to the levels of phosphorus present in the soil. If you are considering applying any chemical fertiliser containing phosphorus this season, sample now before slurry is applied to establish the requirement. You must soil sample to decide whether not to spread chemical phosphorus.