Low pH and high EC is possibly due to oxidation of sulfides resulting in formation of sulphuric acid and consequent dissolution of some secondary minerals to cause high conductance or accumulation of soluble salts.

Probably may be present  an organic acid

The suggestion from Prakash Chandra Srivastava seems most likely if the deposit is likely to contain sediments that at some point have has a marine influence (coastal/ tidal) and accumulation of organic matter.·

Just to clarify, I am suggesting that seawater was the source of S in form of sulphate (though other sources occur) and organic matter source of carbon to create high oxygen demand and therefore reducing conditions to aid sulphide formation. Acid formation and pH in order of 3.5 occurs when soils exposed to oxygen (either through changes in drainage or uplift of land (or just by taking a soil sample and letting it oxidise).

In addition to any contribution from different ionic species in the soil solution, H+ is the ion with the highest equivalent ionic conductance (around 350 compared with about 76 of Cl- and 50 of Na+) and consequently with the highest contribution to EC, so just a pH=3.5 may explain a significant proportion of measured EC. 

Good reply Dr. Parkash, yes, low pH and high EC is possibly due to oxidation of sulfides resulting in formation of sulphuric acid or accumulation of soluble salts.

Dear Vitezslav,

The pH and EC values indicate that they are acid sulfate soils. Acid sulfate soils are naturally occurring soils, sediments or organic substrates (e.g. peat) that are formed under waterlogged conditions. These soils contain iron sulfide minerals (predominantly as the mineral pyrite) or their oxidation products. In an undisturbed state below the water table, acid sulfate soils are benign. However, if the soils are drained, excavated or exposed to air by a lowering of the water table, the sulfides react with oxygen to form sulfuric acid.

When drained, pyrite (FeS2) containing soils (also called cat-clays) may become extremely acidic (pH < 4) due to the oxidation of pyrite into sulfuric acid (H2SO4). In its simplest form, this chemical reaction is as follows:

2 FeS2 + 9 O2 + 4 H2O → 8 H+ + 4 SO42− + 2 Fe(OH)3 (solid) 

Dr.Srivastava's reply as further elaborated by Dr.Kundu appears to be more appropiate.Of course other colleagues have given good leads.The soil appears to be acid sulphate soil.Keeping the soil under submergence continuosly will help to raise the pH to some extent (say upto 4.5-5.0).

Yes, I agree with explanation provided by Dr. Srivastava, Dr. Kundu, Dr, Butler for low pH and high EC of the soil.

Interesting discussion . But , soil pH of 3.5 with an  EC of 1.5 , are not indicative of  two   processes  taking  place  simultaneously  in opposite directions...?

In Kerala state, there occurs what are locally called 'Kari soils' which in soil science parlance are designated as acid sulphate soils. They are low pH and high EC soils and waters are hard due to higher salt content and do not produce lather when soap is used. Such soils deserve to be reclaimed not by liming but by improving water retentivity in soils and preventing leaching in situ but providing drainage digging trenches all along the field borders as rain fall exceeds 1500 to 2000 mm annually.