ARCHIVE: Fertiliser Manual (RB209)

Target Soil Indices

The readily crop-available pool of these nutrients is measured by the analytical methods given above. The critical value for all three nutrients is related to a target Index. Fertiliser and manure applications should aim to raise the Index to the appropriate target for the rotation and then to maintain this target by maintenance applications.

The target soil Indices for soil P and K are given below.

  Soil P Soil K
Arable & forage crops, grassland Index 2 (16-25 mg/litre) Index 2- (121-180 mg/litre)
Vegetables Index 3 (26-45 mg/litre) Index 2+ (181-240 mg/litre)

Where crops are grown on soils below the target Index applying large amounts of phosphate and potash rarely produces yields equal to those where the crop is grown on soil at the target Index. This is particularly likely where soil P or K Index is 0 as shown in the following examples taken from data in the Rothamsted Research archive.

 

Yields of spring barley amd sugar beet on soils at different P Indices with and without freshly applied phosphate

    P Index
    0 1 2 3
    mg P/L
    5 12 20 30
Grain yield t/ha No added P 3.15 4.60 5.10 5.40
  125 kg P2O5/ha added 4.20 5.00 5.25 5.40
Sugar yield t/ha No added P 2.80 5.45 6.25 6.65
  125 kg P2O5/ha added 5.40 6.10 6.50 6.65

Note the lack of response in grain and sugar yields to added P at soil Index 3.

 

Yields of winter wheat and sugar beet on soils at different K Indices with and without freshly applied potash

    K Index
    0 1 2- 2+ 3
    mg K/L
Wheat   59 90 150 210 320
Grain yield t/ha No added K 8.00 10.80 11.35 11.40 11.40
  100 kg K2O added 9.50 11.00 11.35 11.40 11.40
Sugar beet   90 105 150 210  
Sugar yield t/ha No added K 5.00 6.00 7.00 7.60  
  310 kg K2O added 6.65 7.00 7.65 8.05  

Note the lack of response in grain yield to added K at soil K Indices 2-, 2+ and 3.

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