Soil Index

The concentration of a nutrient that is available for uptake by a plant within a soil. A soil test (with difference methods being used by different organisations) determines how much of a nutrient can be extracted from a soil. The main nutrients of phosphorus, potassium and magnesium have indices that categorise availability from inadequate to excessive. England, Wales (Defra)and Northern Ireland (Daera) use Indices with 10 bands (0-9), whilst Scotland uses the SAC soil nutrient status, which is a descriptive scale of 6 categories (Very low; Low; Moderate-; Moderate+; High; Very High), whilst Ireland uses a (Teagasc) 4 scale index (1 = Very low; 2 = Low; 3 = Medium; 4 = Sufficient / Excess).
Soil index used by England, Wales and Northern Ireland:
Index P(mg/l) K (mg/l) Mg (mg/l)
Olsen P extraction Ammonium nitrate extract
0 0-9 0-60 0-25
1 10-15 61-120 26-50
2 16-25 121-180 (2-)
181-240 (2+) 51-100
3 26-45 241-400 101-175
4 46-70 401-600 176-250
5 71-100 601-900 251-350
6 101-140 901-1500 351-600
7 141-200 1501-2400 601-1000
8 201-280 2401-3600 1001-1500
9 Over 280 over 3600 over 1500

The Soil Index is a value which is provided for agricultural crops to optimise / maximise yield and is used to determine what might be needed to be added from a fertiliser to a soil/crop to replace that which has been taken up and removed by a crop.

Turfculture does not require optimum / maximum yields but as a general guide the Index values will typically be either set (in PQS tables) as 1 (e.g. fine turf) or 2 (e.g. for perennial ryegrass swards), whilst for many agricultural crops the Index will be 2. (See https://ahdb.org.uk/knowledge-library/rb209-section-1-principles-of-nutrient-management-and-fertiliser-use)

Once a soil is at its desirable index value, all that is needed is to replace what is taken away by a crop. If the index is high for a crop, then there is little point applying any more of that nutrient as crop response will not occur as there is already an adequate supply.

For the majority of turfgrass pitches (especially rugby and football) the mown grass is not taken away (i.e. the clippings are not boxed off and are just let fly) and is recycled so there isn’t really much nutrient loss.

For fine turf surfaces, such as golf or bowling greens, but also cricket squares and tennis courts the nutrients are taken away because they are mostly boxed off, so over time there will be a gradual depletion of soil nutrient reserves if they are not replaced.

Numerous factors can influence how much nutrient input is needed to raise the status of soil content for a particular nutrient, with monitoring the actual sward content and pitch performance being an important requirement, as nutrient inputs may not be as much as theoretically indicated due to soil weathering, atmospheric deposition, nutrients within top-dressing and nutrients within irrigation water. Undertaking regular soil tests along with sward analysis will help build up a more accurate and relevant trend for what might be needed to produce an optimum turf surface rather than basing calculated inputs from theoretical estimates, which can vary significantly due to the variables involved.

How do you determine the likely fertiliser requirement for these nutrients to reach the target index values?
RB209 Section 1: Principles of nutrient management and fertiliser use, page 29, provides some helpful figures. If an average middle value is used, then a general guide can be as follows:
• To raise a soil up 1 mg/l for P then 25 kg of P2O5 per hectare is needed.
(“… to increase soil Olsen P by 10 mg/l may require approximately 200–300 kg/ha P2O5”, so a middle figure is 250kg. To get to a single value of 1 mg/l just divide by 10, hence the value of 25 kg of P2O5 per hectare).

• To raise a soil up 1 mg/l for K then 8 kg of K2O per hectare is needed.
(“… to increase soil potash by 50 mg K/l, 300–500 kg K2O /ha as a potash fertiliser … may be required”, so a middle figure is 400kg. To get to a single value of 1 mg/l just divide by 50, hence the value of 8 kg of K2O per hectare).

Note: The Soil index is in the nutrient element only, i.e. P, K, Mg, whilst the fertiliser recommendation is the usual oxide molecule; this does not have any bearing on the calculations given here [page 26 of RB209, Section 1].

So, to decide on what a desirable Index value is to be and the chosen mg/l within that value, some working out is needed to provide some accurate figures for fertiliser needs.

Example:
A has been soil tested and the following results returned:
• P = 10 (bottom of Index 1).
• K = 60 (top of Index 0).

This example will only concentrate on P and K, as Mg is rarely deficient in turf and occasional small additions (if required at all) within a fertiliser will usually be satisfactory for most turfgrass situations.

For this situation (which hasn’t been defined) the assumption is that the ‘ideal’ Index is to be 2 for both P and K, and the chosen value to aim for within the Index is 20 mg/l for P (mid-range for Index 2) and 121 mg/l for K (bottom of Index 2).

The soil status therefore needs to be increased by 10 mg/l for P and 61 mg/l for K. (Note: If the grass sward is performing well, without any nutrient deficiency signs then the theoretical values for increasing might be wrong and not as much nutrient input is needed as anticipated.)

If the mid-range figures are used, being:
• To raise a soil up 1 mg/l for P then 25 kg of P2O5 per hectare is needed.
• To raise a soil up 1 mg/l for K then 8 kg of K2O per hectare is needed.

The target value does not need to be achieved in one attempt, so a phased increase can be planned, within maybe a 3-to-5-year period. If a yearly soil analysis is also carried out, then this will help to review what was previously applied and see how much of an increase in available nutrients actually occurred; this can be used to relate the theory to what is happening in practice.

What is the total theoretical amount of fertiliser needed, in this example?
• For P this is an increase of 10 mg/l, so the total required is 250 kg of P2O5 per hectare.
• For K this is an increase of 61 mg/l, so the total required is 488 kg of K2O per hectare.

A ‘traditional’ fertiliser for general winter pitches of 20:10:10 might be considered for application in April/May, in anticipation of post-season renovation.
If an application rate of 25g/m2 was planned this would see the equivalent of 250kg of total fertiliser being applied per hectare. This would equate to 50 kg/N (which is on the high side for general sports turf); 25Kg P2O5 and 25Kg K2O being applied per hectare.
If two applications of this fertiliser were planned per year this would mean the soil being ‘topped up’ by 50Kg P2O5 and 50Kg K2O, per hectare per year, assuming clippings are returned.
Using this fertiliser and rate would see a phased increase:
• over 5 years for P. To get to 20mg/l P in the soil this requires 250Kg P2O5 in total, therefore with 50kg being applied per year this would take 5-years.
• over 9.5 years. To get to 121 mg/l K in the soil this requires 488kg K2O in total, therefore with 50kg being applied per year this would take 9.5-years.
In practice, yearly soil tests would help show the actual trend which is occurring and the adjustments that might be needed to a fertiliser programme.
A different turf of fertiliser might, however, be required, with less Nitrogen per application, along with P and K, but an additional supplement of a single application of superphosphate (~20% P2O5) and sulphate of potash (~50% K2O) might also be given to boost the soil content of these two nutrients.
One additional application of superphosphate at 25 g/m2 of the fertiliser (or 250kg per hectare) would provide a (theoretical) 2 mg/l increase in soil P content per application. If the P2O5 content is 20%, this provides a total nutrient (in contrast to total fertiliser) input of 50kg (250kg x 20%) of P2O5 to the soil.
Note: if 25kg of P2O5 is required to raise the soil by 1 mg/l of P, then 50kg/25kg raises the soil content by 2 mg/l.

By contrast one additional application of sulphate of potash at 25 g/m2 of the fertiliser (or 250kg per hectare) would provide a (theoretical) 15.6 mg/l increase in soil K content per application. If the K2O content is 50%, this provides a total nutrient (in contrast to total fertiliser) input of 125kg (250kg x 50%) of K2O to the soil.
Note: if 8kg of K2O is required to raise the soil by 1 mg/l of K, then 125kg/8kg raises the soil content by 15.6 mg/l.