Soil Index System

In order to simplify advice tables, it is normal to classify soil available levels of nutrients and trace elements into classes. The class is referred to as the Soil Index. At Johnstown Castle, soil analysis levels are classified into Index 1 – 4. The exact interpretation of the Soil Index varies somewhat with the element and the crop but the definitions in Table1 apply in most circumstances.

Table 1: The Soil Index System
Soil Index	Index Description	Response to Fertilisers
1	Very Low	Definite
2	Low	Likely
3	Medium	Unlikely/Tenuous
4	Sufficient/Excess	None

Nitrogen (N)

There is, as yet, no useful Irish laboratory test for N in soils.  Therefore, the nutrient N advice for grassland systems (grazing and conservation) depends mainly on land use and farming system, and particularly on the stocking rate. 

For crops requiring cultivation, N fertilizer advice is determined by the soil N supply status which relates to the previous cropping history and is categorised into an index system. Account is also taken of the requirement of the crop and the likely crop yield. 

Tables 2 and 3 show how the N Index takes into account the past farm management history and reflects the likely rate of release of N from the soil.

In continuous tillage it is usually only necessary to consider the last crop grown to estimate N Index (Table 2).  However, where long leys or permanent pasture occur in the rotation, it is necessary to consider the field history for longer than one year (Table 3). 

Table 2:  N Index for tillage crops that follow short leys (1-4 years) or tillage crops
Previous crop
Index 1	Index 2	Index 3	Index 4
Cereals, Maize	Sugar beet, Fodder beet, Potatoes, Mangels, Kale, Peas, Beans, Oil seed rape.
	Swedes removed.	Swedes grazed in situ.
	Leys (1 – 4 years) grazed or cut and grazed.
Vegetables receiving less than 200 kg N/ha.	Vegetables receiving more than 200 kg N/ha.

 

 

 

Table 3:  N Index for tillage crops that follow permanent pasture
Previous crop
Index 1	Index 2	Index 3	Index 4
The 5th tillage crop following permanent pasture. For subsequent tillage crops use the continuous tillage table.	The 3rd or 4th tillage crop following permanent pasture. If original permanent pasture was cut only, use index 1.	The 1st or 2nd tillage crop following permanent pasture (see also Index 4). If original permanent pasture was cut only, use index 2.	The 1st or 2nd tillage crop following very good permanent pasture which was grazed only. .

Phosphorus (P)

The P Index depends on the level of available P in soil.  This is determined by measuring the amount of the element that is extracted by Morgan’s solution.  The ranges are shown in Table 4.   The ranges for grassland crops are different from other crops as many tillage and vegetable crops requires higher P levels for optimum production.

Phosphorus tends to be leached or washed out of peats each winter.  This was particularly important in deep peats where the P could be lost to the drainage water instead of being trapped by the mineral layer underneath as happens in shallow peats. To minimise possible losses of nutrients to the environment, the Nitrates Directive (SI 113 of 2022) requires that the fertilisation rates for soils which have more than 20% organic matter shall not exceed the amounts permitted for Index 3 soils. The effect of this is that a peat soils at P Index 1 or 2 is fertilised as if it were at soil P Index 3.

 

Table 4:  The P Index System
Soil P Index	Soil P Ranges (mg/l)
	Grassland Crops	Other Crops
1	0.0 – 3.04	0.0 – 3.04
2	3.05 – 5.04	3.05 – 6.04
3	5.05 – 8.0	6.05 – 10.0
4	Above 8.01	Above 10.01

 

 

Potassium (K) and Magnesium (Mg)

The Index system for K and Mg are given in Tables 5 and 6.

Table 5: The K Index System
Soil K Ranges (mg/I)
Soil K Index	Mineral Soil	Peat *
1	0- 50	0-100
2	51- 100	101-175
3	101- 150	176-250
4	Above 150	Above 250
* To be defined as a peat, there must be no mineral soil in the upper 10cm throughout the sampled area.

Table 6: The Mg Ranges (mg/I)
Soil Mg Inde x	Soil Mg Ranges (mg/I)
1	0- 25
2	26- 50
3	51- 100
4	Above 100

Trace Elements Cobalt( Co), Maganese( Mn), Copper( Cu), Zinc( Zn) ,Iodine (I) and Boron( B)

The Index system for Co, Mn Cu, Zn, I and B are given in Tables 7 to 12. 

Table 7: The Co Index System
Soil Co Index	Soil Co Ranges (mg/kg)
1	0 – 3.0
2	3.1- 5.0
3	5.1 – 10.0
4	Above 10

Table 8: The Er-Mn Index System
Soil Er-Mn Index	Soil Er-Mn Ranges(mg/l)1
11	< 90
2	90-120
3	>120
1Extrantant for Er-Mn : 0.5m EDTA-pH7.0

Table 9: The Cu Index System1
Soil Cu Index	Soil Mg Ranges (mg/I)
11	< 1.00
2	1.01- 1.50
3	1.51-3.00
4	>3.00
1 Extractant for Cu 0.5m EDTA- pH 7.0

Table 10: The Zn Index System
Soil Zn Index	Soil Mg Ranges (mg/I)
11	< 1.00
2	1.01- 1.502
3	1.51- 3.00
4	> 3.00
1 Extractant for Zn: 0.5 m EDTA – pH 7.0 2 At pH greater than 7.0, Zn deficiency may be severe when the soil contains less than 1.50 mg/I Zn

Table 11: The Iodine Index System
Soil I Index	Soil I Ranges (mg/kg)
1	0 – 5
2,3,4	Above 5

Boron (B)

The B Index (Table 12) applies only to boron-responsive crops, i.e. swedes, turnips, oil-seed rape, sugar beet, fodder beet, mangolds, all horticultural brassicae, carrots and celery. Boron is toxic to many crops at soil concentrations greater than 3.0 mg/l. Potatoes and legumes are particularly sensitive to high soil B 

Table 12: The B Index System
Soil B Index	Soil B Ranges (mg/kg)
1	<  0.5
2	0.5 – 1.0
3	1.1 – 1.5
4	1.6 – 2.0