Interpretation of soil P, K and Mg

Soil analysis provides an estimate of available P, K and Mg concentrations in soil to sampling depth - in practice this is equivalent to plough or cultivation depth because of the distribution of nutrients when the land is worked. Response experiments with different crop groups have provided the relationship between crop yield and soil nutrient concentration. Normally, yields increase with increasing nutrient concentration to a maximum, beyond which there is no further benefit from additional nutrient. Below this value, which will vary with crop species, there is a yield penalty. Whilst soil analysis is not a precise guide, the lower the value the greater the risk of poor performance. To aid interpretation of the different concentrations of individual nutrients, Index or descriptive scales are used. These scales provide a general indication of the likely crop response and therefore a guide to the need for additional nutrient supplementation, as shown in the table.

Crop response and soil analysis

Defra Index	SAC description	Yield response to added nutrient by
		vegetable crops	arable crops and grass
0	Very low	highly likely	highly likely
1	Low	highly likely	probable
2	Moderate	likely	unlikely
3	High	possible	nil
4	Very High	unlikely	nil
5	Very High	nil	nil

Frequency of sampling

Soil nutrient levels do not alter markedly over short periods of time unless major factors of supply or demand are introduced. There is therefore no point in incurring the additional cost of sampling more frequently than necessary, except perhaps as a check to ensure that earlier sampling has been carried out properly. Additional sampling may be justified when there is a major change to husbandry practice - for instance alteration of cropping or manure policy. General guidelines for sampling frequency are as follows:

		Sample every
	Permanent grass	7 years
	Intensively used grassland	3-4 years
	General arable cropping	3-5 years
	Arable/grass systems	3-5 years
	Field vegetables and horticulture	2-3 years

Interpretation of soil pH

The recommended target soil pH values for England, Wales and N Ireland and for Scotland are shown in the table below, for arable and grassland on mineral and peaty soils. pH values quoted from laboratories throughout the UK are numerically equivalent, even though the extractant is not always water. The Scottish SAC laboratory uses calcium chloride solution (0.01 M CaCl2) but pH values are converted to be equivalent to those obtained using water. The recommended values in the table are therefore applicable wherever the soil has been analysed.

	England, Wales and N Ireland1		Scotland2
	Optimum soil pH
	Mineral soils	Peaty soils	Mineral soils	Peaty soils
Continuous arable cropping	6.5*	5.8	6.0-6.2	5.7-5.9
Continuous grassland	6.0	5.3	6.0	5.3-5.5
1 For more detailed information see the Fertiliser Manual [RB209] 8th Edition, Section 1. 2 For more detailed information see the SAC Technical Note TN632 and SAC Technical Note TN633. * in arable rotations growing acid-sensitive crops such as sugar beet, maintaining soil pH between 6.5 and 7.0 is justified.

In-field measurements using pH indicator on some soils where free chalk or lime particles exist may give lower values than laboratory results for the same field. This is because grinding the soil for laboratory analysis pulverises any chalk/lime particles and the pH as measured is increased.

Acidity below pH 6.0 will reduce the availability of some nutrients, especially phosphorus.

Availability of trace elements is radically affected by pH and the need for trace elements should be assessed only after any required amendment of acidity has been undertaken and has had time to take effect.

Influence of soil pH on plant nutrient availability

The availability of different nutrients at the different pH bands is indicated by the width of the white bar: the wider the bar, the more available is the nutrient (redrawn for PDA from Truog, E. (1946). Soil reaction influence on availability of plant nutrients. Soil Science Society of America Proceedings 11, 305-308.).