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Soil Index System

Advice on nutrient and trace element application rates clearly must depend on the quantity of the element in the soil that is available to the crop. Apart from N, this is determined by soil analysis. For most elements, the soil is extracted with a suitable reagent and the amount extracted is deemed to be or related to the amount available to the plant. For elements that are extracted, the analysis unit is milligrams per litre of soil (mg/l). For elements that are digested in strong reagents, the analysis units are mg/kg. These include cobalt, total manganese, sulphur and iodine.

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 IndexIndex DescriptionResponse 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, the available soil N can be deduced from the previous cropping and manurial history, and the type of soil. Thus, N fertiliser advice is determined by the soil N supply status. This depends in turn on the previous cropping history. The supply status is categorised into an index system for grass establishment and tillage crops. Account is also taken of previous applications of chemical and organic manures, 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). Previous applications of animal manures must also be taken into account.

Table 2: N Index for tillage crops that short leys or tillage. This table can also be used for grass establishment.
Previous crop
Index 1Index 2Index 3Index 4
Cereals, Maize
(no animal manures)
Sugar beet, Fodder beet, Potatoes, Mangels, Kale, Peas, Beans, Oilseed Rape    
  Swedes removed Swedes grazed in situ  
  Leys (1-4 years) grazed or cut and grazed    
  Any crop receiving dressings of organic fertiliser    
Vegetables receiving less than 200 kg N/ha Vegetables receiving more than 200 kg N/ha    
Table 3: N Index for pasture establishment or tillage crops that follow long leys (5 years or more) or permanent pasture
Previous crop
Index 1 Index 2Index 3Index 4
Any crop sown as the 5th tillage crop following long leys or permanent pasture. Any crop sown as the 3rd or 4th tillage crop following long leys or permanent pasture.
 If original long ley or permanent pasture was cut only use Index 1.
Any crop sown as the 1st or 2nd tillage crop following long leys or permanent pasture (see also Index 4). If original long ley or permanent pasture was cut only use Index 2. Any crop sown as the 1st or 2nd tillage crop following very good long leys or 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.

In the past, higher ranges were used for peat than for mineral soils for each Index level. This resulted in higher agronomic P advice rates for peats than for mineral soils. This was unsustainable from an environmental point of view, as P 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 Good Agricultural Practice for Protection of Waters Regulations 2010 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 fertilized as if it were at soil P Index 3.

Table 4: The P index system
 Soil P Ranges (mg/I)
Soil P IndexGrassland cropsOther crops
1 0.0 - 3.0  0.0 - 3.0 
2 3.1 - 5.0 3.1 - 6.0
3 5.1 - 8.0 6.1 - 10
4 Above 8.0 Above 10.0

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 IndexMineral SoilPeat *
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
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 IndexSoil 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 IndexSoil 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 IndexSoil 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 IndexSoil 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 IndexSoil 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 IndexSoil B Ranges (mg/kg)
1 <  0.5
2 0.5 - 1.0
3 1.1 - 1.5
4 1.6 - 2.0