Soil pH plays a major role for the development of various flora and fauna and in general, for productive soil, it remains at near neutral (ranges from 6.3-7.2). Because at this pH various bio-chemical activities in soil organisms can function well.If soil pH becomes alkaline, addition of ammonium sulfate may be used to bring down the pH level to near neutral. Similarly, in acidic soil calculated amount of lime could be added. There are other chemicals that may be used for making pH control in soil.

The only way to decrease desertification is to promote aforestation. This will improve soil pH chemically and naturally.

To maintain the pH of a soil, you have to know how that particular soil works, without the anthropic impact, and then look for ways to reproduce those conditions, for example the microorganisms that live in it.

if soil become acid condition to add calcium and magnesium to neutralize the soil pH

if soil pH increase above 7.8 we have to add nitrogen fertilizer to increase soil acid condition

To maintain the pH of a soil, you will improve soil pH chemically and naturally. There are other chemicals that may be used for making pH control in soil such as ammonium sulfate.

If it is on the acidic side use lime and other alkaline material to reduce the acidity. If it is on the alkaline side use ammonium based fertilizers. If soil is neutral maintain its neutrality by using fertilizers such as urea which is neither basic nor acidic. For organic fertilizers coffee grounds add acidity whereas egg shells add alkalinity.

(i) In acidic soil adding lime or organic matter will help increase the soil pH.

(ii) In alkaline soil, some of those things that Srini Vasan said can be done.