SULPHATE AND PHOSPHATE OP LIME.

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time, absorbing carbonic acid but slowly from the air, isthe cause of this (174).

445. Chalk, shell-sand, and indeed shells themselves,which consist of carbonate of lime and a small quantityof animal matter, are useful additions to all soils whichcontain but little lime. Quick or caustic lime is espe-cially useful in soils rich in humus. Both lime andlikewise its carbonate are valuable additions to soilscontaining any quantity of sulphate of iron (210,309).That salt, which appears to be hurtful to vegetation, isdecomposed by lime; gypsum is formed, whilst theiron is converted into oxide or carbonate.

446. Sulphate of lime, or gypsum, is likewise a veryuseful addition to all soils which do not naturally containit. It is partly useful as supplying lime, and partly asserving to fix ammonia from the air (414), and thusyielding plants more of that substance than they couldotherwise obtain.

447. Phosphate of lime is found native as a mineralin Spain and other countries; it would certainly forma valuable manure for many soils. As phosphate of limeis generally found in nearly all plants, all substances con-taining it are useful as manures. The native phosphate oflime has as yet hardly come into use at all as a manure.

448. There are no mineral manures which containphosphate of magnesia; it exists in many soils, and in agood many organic manures. Bran contains a consider-able quantity of this salt.

449. Ashes of various kinds constitute an important