The effects of fumigating field soils before replanting apple are frequently predicted from bioassays where apple seedlings are grown in pots, some containing untreated soil and the others containing chloropicrin-fumigated soil. The results from 418 such bioassays made during eight years are discussed with reference to the effects of certain physical and chemical soil features.Soil phosphorus (P) content and pH had large effects on growth in fumigated (but not untreated) soils; they therefore had large effects on the growth response to soil fumigation. There were no indications of important effects due to soil content of potassium or magnesium or to soil texture.In fumigated soils, the height of seedlings was generally greater, the greater the soil P content, but in untreated soils it was mostly independent of P (range 1 -156 mg P litre-' soil). The differing effects of native P on growth in untreated or fumigated soils were attributed respectively to the presence and absence of vesicular-arbuscular mycorrhizas : their absence in fumigated soils was considered (for apple) to be an artefact in the seedling bioassay. The results for untreated soils indicated that the apple mycorrhizal system was very efficient in retrieving P when there was little available.Growth responses to soil fumigation were usually greater in acidic than in alkaline soils for any given level of P. When soils were P-amended before testing, the effects of native P and pH virtually disappeared and the proportion of orchard soils with economically significant growth responses increased from 39% to 67%.

The effects of chloropicrin fumigation of field soils before replanting apple are frequently predicted from bio-assays using pot-grown apple seedlings. The results of 506 such bio-assays carried out during 12 years are discussed with reference to modifications which were made to the test method. The interpretation of the most recent and valid bio-assay results is considered in relation to the different purposes for which such tests may be conducted.When all test soils were amended with phosphorus (to compensate for the eradication of vesicular-arbuscular mycorrhizal fungi by fumigation) the proportion of old orchard soils yielding economically significant growth responses increased from 39% to 68%. When, additionally, plant growth was assessed by shoot fresh weight (rather than shoot length) the proportion further increased to 82%.The traditional method of expressing growth responses to soil sterilisation, as proportionate growth increases, was found to be subject to skewing effects of non-biological fertility factors; this feature was avoided when results were given as actual growth increases. The latter value therefore is considered preferable when assessing the relative disease status of soils for etiological purposes (as opposed to predicting the cost-benefits of field soil treatment).The data indicated that disease severity was on average diminished in the most acidic soils (pH 4.8-6.1), but did not support the generalisation that disease severity increases progressively with soil pH.