Soil analysis: key to nutrient management planning

Interpretation of soil P, K & 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 & grass
0	Very low	highly likely	highly likely
1	Low	highly likely	probable
2	Moderate	likely	unlikely
3	High	possible	nil
4	High	unlikely	nil
5	High	nil	nil

Soil P, K & 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 discussed above.

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

The Scottish Agricultural College laboratory uses different extractants to those used in England and Wales and a descriptive rather than a numeric scale.

DESCRIPTION	PHOSPHORUS		POTASSIUM		MAGNESIUM
	Modified Morgans extraction
Very low	0-10		0-40		0-20
Low	10-25		40-75		20-60
Moderate	26-75		76-200		61-200
High	76-200		201-1000		201-1000
Excessively High	201-		1001-		1001-

Relationship between Defra and SAC scales

Defra Index	SAC description
0	Very low
1	Low
2	Moderate
3-7	High
8-9	Excessively high