The phosphate story

Conventional advice maintains that there is no benefit in adding P to soils that contain available P of index 3 (26-45 mg/kg P in the soil). Beyond this level, there is unlikely to be an increase in herbage yield (Lockhart Wiseman, 1993). However, the experiment cited above (Newton, 1993) showed a straight-line response up to levels of available P of 103 mg/kg (index 6). The equation derived from this work suggested that for every increase of 10 mg/kg available P in the soil, 

One of the problems is that the phosphate fertiliser allowed by the organic regulations is rock phosphate, and research in Austria has shown (Lindenthal, et al., 2000) that the application of rock phosphate had virtually no effect on subsequent crop yields. 

Whereas any N and K that is applied to fields is immediately available to the growing plant and then tends to be washed down out of reach of the plant roots, a large proportion of the P that is applied is rendered unavailable to the plant by being bound into aluminium or iron compounds if the pH of the soil is low, and by calcium compounds if the pH of the soil is high. Although it may seem rather an expensive waste of money, at the time, that as much as 40% of the P applied in farmyard manure should be unavailable to the crop, it does stay in the soil and will become available to the crop in time. 

The beneficial effect of available P is further illustrated by two adjacent fields on an organic hill sheep farm in Wales and by two fields on an organic Shropshire dairy farm (Fig. 10.3). Fields A and B on the Welsh hill farm were both clay loams, and field A (with 27.2 mg/kg available P) yielded 13.41 dry matter per ha per annum, whereas field 

Welsh hill farm Shropshire dairy farm 

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