Analysis

There are several reasons why different results may be obtained for apparently identical samples analysed by different laboratories. The most obvious reason is that different extractants may have been used. Variations in results can also occur if identical protocols are not followed - techniques of soil drying, grinding and sieving, reagent and equipment temperature, extractant concentrations, extraction shaking, stirring and filtration, and extraction time can all affect the result. Results are usually reported in mg/l but where a laboratory measures soil by weight instead of volume, the units will be as mg/kg. This can give rise to differences on some soils - particularly those with high levels of organic matter. The Professional Agricultural Analysis Group (PAAG) was established in the UK in 2009 to operate a ring test of participating laboratories to ensure commonality among those in the Group, who all use the official English, Welsh and Northern Irish methods for soil analysis identified in The Fertiliser Manual (RB209). The Group also publishes an annual Report on the overall nutrient status of the soils which have been analysed by members.

If different laboratories are used for analyses these factors must be taken into account.

Soil P, K and Mg concentrations (mg/l) and Defra Index scale

Note that the Index is split in half for potassium only and described as 2- (or lower Index 2) and 2+ (or upper Index 2). In the past, Index 2 was not divided in half for potassium but some soil reports used + and - signs to denote the extreme top and bottom 10% of each band; laboratories should no longer be using this convention.

PHOSPHORUS		POTASSIUM		MAGNESIUM
Olsen extraction		Ammonium nitrate extraction
Olsen P		exchangeable K		exchangeable Mg
Index	mg/l	Index	mg/l	Index	mg/l
0	0-9	0	0-60	0	0-25
1	10-15	1	61-120	1	26-50
2	16-25	2-	121-180	2	51-100
		2+	181-240
3	26-45	3	241-400	3	101-175
4	46-70	4	401-600	4	176-250
5	71-100	5	601-900	5	251-350
6	101-140	6	901-1500	6	351-600
7	141-200	7	1501-2400	7	601-1000
8	201-280	8	2401-3600	8	1001-1500
9	over 280	9	over 3600	9	over 1500

Besides providing a basis to decide fertiliser quantities, soil analysis should also be used to monitor changes in fertility especially where there are uncertainties in the amounts of nutrient removed (e.g. with forage crops) and in the amounts of nutrients applied (e.g. with manures and slurries). For this purpose it is desirable to use the mg/l values not the Index. However differences of less than 5 mg/l Olsen P, 25 mg/l K and 10 mg/l Mg should be ignored unless part of a sustained trend. Where accurate nutrient balance information is used in conjunction with regular soil analysis, it is important to recognise the possibilities of variation as outlined above.

Soil P, K and Mg concentrations (mg/l) and SAC descriptive scale

The SAC laboratory in Scotland uses different extractants to those used in England, Wales and N Ireland, and a descriptive rather than a numeric scale.

SAC Status	PHOSPHORUS	POTASSIUM	MAGNESIUM
	Modified Morgans extraction
	mg/l	mg/l	mg/l
Very low (VL)	0.0-1.7	0-39	0-19
Low (L)	1.8-4.4	40-75	20-60
Moderate - (M-)	4.5-9.4	76-140	61-200
Moderate + (M+)	9.5-13.4	141-200	61-200
High (H)	13.5-30.0	201-400	201-1000
Very high (VH)	>30.0	>400	>1000

Relationship between Defra and SAC scales

Defra Index	SAC description
0	Very low
1	Low
2	Moderate
3	High
>3	Very high