Hi Sufia,

soil pH is variable and it has important effect on the floral diversity. Usually soils in the raining areas are acidic and pH varies below 7. In the tropical countries with high or moderate raining more plants grow. But excessive acidic soil (pH8 and alkaline in nature as such. Some psammophytic plants grow here but not so good. So, acidic soils are preferable to the plants.

you can find more information here:

http://www.dlia.org/soil-ph

http://www.epa.gov/caddis/ssr_ph_int.html

In general the acidic environments are characterised by a reduced productivity, a lower number of individuals and in higher of lower correlation also with a lower number of species. The diversity index, however, depends on homogeneity of distribution of species in a community. If a highly specialised species predominates, it will be low. But it depends on local conditions. All must be found in the field and compared with adequate situations. There is not a simple scheme.

Plant diversity suffers from much nitrogen,  as in meadows  where not  many different flowers are present, dandylion prevailing.

After a few harvests of the meadows plants and possibly adding calcium the growing plant mass is reduced, but many flowers get a chance.

I am working from many years in  Mediterranean region with characteristic vegetation (South-Eastern France and Corsica): integrated biology of some biocenosis (microclimate and mesoclimate registrations-nature of soil-erratical sudden showers (during either spring, autumn or winter, and rare sudden showers in summer)- phenology plants hosting insects-one or another biocenosis including one phytophagous species and its predators and parasitoids. When rock under a very poor clayey soil is jurassic calcareous, pH is between 7 and 7.5 due to sudden showers which leach (from many centuries) fine particles of calcareous. When rock under soil is old vulcanic (primary era), soil is very poor and plant diversity is near but not the same as on jurassic calcareous, with pH between 7 and 6. In Corsica, plant diversity is also near  than the two previous one, except that plant species are not the same as in South-Eastern France: many plant species associations are either endemic of the Island or the same species as in Tuscany (western Mediterranean coastal region of Italy, and pH of soil is between 6 and 6.5 on rock of old vulcanism (primary era). So, a Mediterranean plant association depends mainly of pH of the soil and this pH depends greatly of rock under soil.

The Altmühl valley in Bavaria is known for its plant diversity too. The valley is carved  into the jurassic calcareous. Where wood is removed for pastures at the slopes of the valley, as done since centuries, many types of flowers blossom.

"In temperate and boreal regions, plant diversity has a strong positive association with soil pH. Consequently, in order to protect plant diversity effectively, a relatively large proportion of protected areas should be on high pH soils"

You may like to see the following article:

http://ac.els-cdn.com/S0006320704001508/1-s2.0-S0006320704001508-main.pdf?_tid=07fa8f88-a320-11e4-bb79-00000aacb35e&acdnat=1422032049_09d52bb005d633f3b700d79d9b9d19d9

You will find a lot of information in this publication : 

Soil fertility and biodiversity in organic farming

P Mäder, A Fliessbach, D Dubois, L Gunst…2002 - science.sciencemag.org

In southern Mediterranean part of France are basic pH soils (about 6,5-7) [email protected]  of the Jurassic period, which were from long-time liked by hot rain (so water is without soluble calcium carbonate underground when reaching level of roots). Wild vegetation is typical (trees and shrubs) of wild vegetation on primary pH soils far from 3 to 6 km (except for Quercus suber growing only on acid pH soil).

Some flower-bearing plants can only tolerate certain levels of soil pH. If the plant grows optimally in acidic pH, then its growth would probably be stunted in basic pH. On a larger scale, a forest in an environment with acidic soil would naturally predominantly contain flowering plants that thrive best in acidic soil. There would probably be flowering plants that grow best in basic soil but they would not be in the majority. The acidic pH-loving plants could then emerge as the dominant species and this would be reflected in the forest's floral biodiversity.