Dear Kinfe Tekulu Perhaps this is a chemical reaction of fertilizer and humus soil

Regards, Sergey

The application of earthworms may produce acidic compound by-product, which resulted in lowering of pH. Moreover, pH has a significant role in concentration of nitrate, where high pH causes nitrogen being lost as volatile ammonia, while neutral pH stabilizes the content of nitrates

The saliva of earth work is quite acidic in nature. It may change the pH

This article is in par with your statement.

https://academicjournals.org/article/article1379924624_Azarmi%20et%20al.pdf

The breakdown of organic material by microbes into ammonium (mineralisation) is responsible for the increase in pH which could be temporary.

There should not be such differences in pH. However, irrigation (especially the occurrence of long term wet soil), plant type (rhizosphere effect), fertiliser type and organic matter addition could affect pH in relatively smaller range. Such effect may be pronounced by coarse texture. Indeed the sampling and measuring chronology of different samples sampling time may arise some time dependent changes. Be sure that if you have the initial sample make a simultaneous measurement of initial and post-experimental soil. This will alleviate time- and calibration-dependent errors. You then probably have better reasoning for pH differences.

You should also consider the decrease in redox potential induced by organic matter addition that wet-soils typically in the irrigated fields have a tendency towards a neutral pH. This proccess is further enhanced by fresh organic matter addition such as plant residue, framyard manure and vermicompost. You can find some data in the enclosed paper. Despite it is in Turkish the Tables will help you.

Article Effect of redox potential induced changes on Fe and Mn avail...

NPS fertilizers containing ammonia (or urea) will have decrease the pH around the roots. When plants uptake nitrogen as ammonia, the plants is mainly uptaking positiv charges elements. The plant however can not be electrical charged so it will release H+ which than will decrease the pH.

If nitrogen is mainly provided as nitrate the opposite will happen, the roots will release HCO3- and pH will increase.

The effect is more noticeble in substrate where the rootzone is limited. Most growers are afraid of using ammonium since the pH can even drop below 4.

The vermicompost might be having a high percentage of carbon and nitrogen and further addition of chemical fertilizers might have enhanced the activities of autochthonous and zymogenous microbes which might have led to the acidification of the soil environment. This may be one among the reason for the slight dip in the pH of the soil and at the same time, this is not the only reason for the dip in the pH levels.

Studies have shown that the application of chemical fertilizers (NP) reduces soil pH.

Generally , vermicompost application has a buffering effect on soils, unless soil condition is anaerobic. It is more likely to be due to use of fertilizers. Please specify the names of fertilizers used. Ammonium fertilizers are lnown to result in soil acidity. Similarly, sulphur fertilizers are also acid producing. I shall provide you details if you want to know more.

Release of certain organic acids

The release of organic acids during composting could have lowered the soil reaction. The chemical fertilizer could have temporarily lowered the pH

As i know some of the chemical fertilizers like Ammonium sulfate and Ammonium nitrate can reduce the soil pH temporarily. Besides this there is free product during decomposition of organic matter which is carbon dioxide. Remember carbon dioxide also decrease the soil pH.

NPKS containing fertilizers are well known for their acidic effect on soil. Sometimes these fertilizers are familiar as Acid Forming Fertilizers. Kindly have a look at this webpage for more information:

https://www.cropnutrition.com/resource-library/fertilizers-and-soil-acidity