This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is a by-product in the manufacture of Bessemer steel, and is also known as "basic cinder" and "Thomas's phosphate". It is a fine dark-grey powder, 80 per cent of the particles of which should pass through a sieve having 10,000 holes to the square inch. It is only since 1885 that basic slag has been used as a manure, having previously been discarded as a waste product. The estimated production for the whole world in 1885 was 150,000 tons, and in 1906 as much as 2,383,000 tons. Of this quantity Germany produces 1,510,000 - more than one-half; the United Kingdom being second with 300,000 tons.
Basic slag is an alkaline manure, and usually contains from 30 to 40 per cent of phosphate of lime, which is equivalent to 9 to 18 per cent of phosphoric acid. The phosphate in basic slag is in combination with lime, and in good samples the greater part of the phosphate can be dissolved in a dilute solution of citric acid. As the value of basic slag depends largely upon its solubility in citric acid, purchasers should obtain a guarantee as to its citric solubility, as there are inferior brands of basic slag in existence. If 90 per cent or more is soluble, the sample is a good one.
Basic slag is a valuable manure for all soils except those of a chalky or limestone nature. It is particularly valuable where large quantities of stable manure have been applied, and where there is a tendency to acidity. For fruits, flowers, and vegetables of all kinds basic slag may be used at the rate of 2 cwt. to 4 cwt. per acre. There is a general impression that it should be used only during the winter months. In practice it will be found useful if applied to the soil about three months before the crops are likely to require it. It yields up its phosphates slowly, but in the meantime the lime is acting in conjunction with the humus in the soil and excites bacterial activity. When finally potting Chrysanthemums, Zonal Pelargoniums, Begonias, and a host of other plants, a sprinkling of basic slag over the compost heap will produce excellent results in the way of early bloom, etc.