What is measured

pH, phosphate, potash and magnesium

The "standard soil analysis package" measures soil acidity (pH) and estimates the plant-available concentrations of the major nutrients, phosphorus (P), potassium (K) and magnesium (Mg), in the soil.

Soil acidity is measured as pH (the concentration of H⁺ ions) with a logarithmic scale running from pH 1 (very acid), through pH 7 (neutral) to pH 14 (very alkaline). The range found when soil sampling is likely to be between pH 4.5-8.5.

The total P and K content of a soil can be measured exactly but has little relevance to crop yield because only a relatively small proportion of the total P and K in soils is available to the plant (see diagram). Soil analysis in the laboratory therefore uses chemical extractants to provide an estimate of the nutrient which would be available under field conditions. The results provide the best practical guide for determining P, K and Mg in the readily plant-available reserve 'pool' shown in the diagram. The methods of soil analysis used in the UK have been developed over many years and have been correlated to crop response, and therefore to fertiliser recommendations, on a wide range of soils in numerous field experiments. Other measurements of nutrient content or ratios may be made on soils, but unless there is dependable correlation with yields, they are of limited practical value.

Available sodium (Na) can also be measured for potentially responsive crops such as sugar beet.

Nutrient reserves in soil

Trace elements

Of the elements required in small amounts by crops - the 'trace' elements or 'micronutrients' - some can be measured effectively in soil, namely boron (B), chloride (Cl), copper (Cu), molybdenum (Mo), cobalt (Co) and zinc (Zn).

Other elements which are not effectively assessed by soil measurements and need to be measured in herbage or in the animal include manganese (Mn), iodine (I) and selenium (Se).

Heavy metals

In the UK there are statutory soil limits for 7 potentially toxic elements (PTEs) to ensure compliance with EU legislation when sewage sludge is to be applied. These are the 'heavy metals' cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), nickel (Ni) and zinc (Zn). The total (acid extractable) content of each of these in soil is measured. The regulations also link the level of these elements to pH which affects their availability to plants. The limits for PTEs are defined in the Sludge (Use in Agriculture) Regulations 1989 and given in the Code of Practice for the Agricultural Use of Sewage Sludge. The Code shows soil limits for additional elements: molybdenum (Mo), selenium (Se), arsenic (As) and fluorine (F).

Nitrogen and sulphur

Measuring available soil nitrogen (N) and sulphur (S) can be useful but is more complex than for other nutrients; these are not covered in this leaflet.