Brassica crops are most productive when grown on land with an approximately neutral pH. The ideal is pH = 6.5 for mineral soils and pH = 5.8 for organic soils. This rule should be altered where soil-borne pathogens are present, especially Plasmodiophora brassicae, the causal agent of clubroot disease. Land where even very low levels of infection are present should be raised to pH
values in excess of 7.0. Brassica crops vary in their sensitivity to acidic pH and the point at which crop productivity begins to diminish, as shown in Table 5.7.
Lime requirements of acidic soils are expressed in t/ha of ground limestone or ground chalk. The amount of lime recommended for soils of similar pH may vary with soil texture and soil organic matter content. Usually, the recommendations aim to maintain the top 20 cm depth of mineral soil to a pH of 6.5 and an organic soil to pH 5.8. Where there are variations in soil acidity across the profile, then larger applications of lime may be needed. Applications of lime in excess of 12 t/ha should be made as several separate dressings. Lime should be applied well before sowing or transplanting. Several months are required for changes in soil acidity to take place. There is an increasing tendency for growers of brassicas using highly intensive systems to use calcium oxide (CaO) as a liming agent. This has the advantage of acting very quickly to alter pH and is applied at about one-third the rate of carbonate forms. Also, the liming effect on pH is lost by the end of the season and this permits growers to plant potatoes in the following year with lower risks of infection by Spongospora subterranea and Streptomyces scabies, the causes of powdery and common scab diseases, respectively. Normally, Brassica vegetables should not be grown immediately after liming a very acid soil (pH <5.0). Where a crop is failing because of slight acidity, however, then some improvement may be achieved by top dressing across a standing crop. This is most likely to be successful where calcium is applied in readily accessible forms such as calcium cyanamide or calcium nitrate. Overliming, bringing the pH beyond 7.5, should be avoided, especially on sands, light loam and organic soils since this can result in induced deficiencies of trace elements such as boron and manganese.
The results of soil analyses reflect the quality of the sampling methods used. Samples must be representative of the area and taken to standardized depth, usually 15 cm. Areas that differ significantly in soil type, previous cropping and applications of manure, fertilizer or lime should be considered as separate samples. Small areas that are known to differ significantly from the rest of the field should be excluded from the main samples and tested separately. A minimum of 25 individual subsamples (auger cores) will be adequate for a uniform area.
Table 5.7. Guide to pH values below which crop productivity is reduced.
Subsampling points must be selected systematically and evenly distributed across the area. This is usually achieved by following a 'W' pattern and taking subsamples along the legs of this pattern at regular intervals. Samples should not be collected from the vicinities of gateways, headlands or close to trees and hedges. Fields used for Brassica production should be subjected to soil analysis a minimum of once every 3 years and more frequently where the land is cropped several times in one season.
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