how to reduce or increase soil ph value

Increasing soil pH
The aim when attempting to adjust soil acidity is not so much to neutralise the pH as to replace lost cation nutrients, particularly calcium. This can be achieved by adding limestone to the soil, which is available in various forms:
Agricultural lime (ground limestone or chalk) is used for soil liming. These natural forms of calcium carbonate are probably the cheapest form of lime for gardening and agricultural use and can be applied at any time of the year. These forms are slow reacting, thus their effect on soil fertility and plant growth is steady and long lasting. Ground lime should be applied to clay and heavy soils at a rate of about 500 to 1,000 g/m² (1 to 2 lb/yd² or 4,500 to 9,000 lb/ac). The concept of "corrected lime potential" [2] to define the degree of base saturation in soils became the basis for procedures now used in soil testing laboratories to determine the "lime requirement" of soils.[3]
Quicklime and slaked lime: The former is produced by burning rock limestone in kilns. It is highly caustic and cannot be applied directly to the soil. Quicklime reacts with water to produce slaked, or hydrated, lime, thus quicklime is spread around agricultural land in heaps to absorb rain and atmospheric moisture and form slaked lime, which is then spread on the soil. Quicklime should be applied to heavy clays at a rate of about 400 to 500 g/m² (0.75 to 1 lb/yd² or 3,600 to 4,500 lb/ac), hydrated lime at 250 to 500 g/m² (0.5 to 1 lb/yd²). However, quicklime and hydrated lime are very fast acting and are not suitable for inclusion in an organic system. Their use is prohibited under the standards of both The Soil Association and the Henry Doubleday Research Association.
Calcium sulphate (gypsum) cannot be used to amend soil acidity. It is a common myth that gypsum affects soil acidity.[4] However, gypsum does reduce aluminium toxicity. Because gypsum is more soluble than alkaline earth carbonates, it is recommended for the treatment of acidic subsoils.[5]
[edit]Reducing soil pH
See also Soil acidification
The pH of an alkaline soil is lowered by adding sulfur, iron sulphates or aluminium sulphate, although these tend to be expensive, and the effects short term.
Urea, urea phosphate, ammonium nitrate, ammonium phosphates, ammonium sulphate and monopotassium phosphate also lower soil pH.
Fertilizers often also affect a slightly more acidic soil since this increases uptake of nutrients.
Decayed vegetable matter, compost, stable manure, urea, etc.