Depth of soil sampling will be hugely dictated by the purpose of your study. How shall we decide the depth of soil sampling. Simply by the  vertical  distribution of  major proportion of roots contributing towards nutrient uptake by a given crop..

Ideally  depth of soil pH measurement should represent the depth of  crop rhizosphere , if soil fertility evaluation is to be undertaken or else for pedological evaluation, horizonwise measurement of soil pH would be most effective...

Soil pH is likely to increase with depth in salt affected soils or decrease in acid soil zones as a subsoil acidity..

Depth of soil sampling will be hugely dictated by the purpose of your study. How shall we decide the depth of soil sampling. Simply by the  vertical  distribution of  major proportion of roots contributing towards nutrient uptake by a given crop..

what should be the criteria of collecting soil sample of any particular region for the study of soil properrties such as soil ph, conductivity and microbial studies?

is it enough to collect top soil for analysis , what will be the effect of depth over these properties?

For small, dry-land grain crops and natural to seeded grassland and for the purpose establishing the soil pH, we have sampled in various cases the visible root zone (5-15 cm depth).  For tree crops and forest trees the root zone cannot be estimated easily.

I would be interested in the purpose of your soil pH estimates. Liming requirements? 

Aimam, I said earlier also, depth of soil pH would measurement and possibly the lime requirement estimation would depend upon vertical distribution of roots of annual as well  as perennial crops. There is a huge data base for root distribution pattern of variety of crops, need not to worked out again...But where subsoil acidity is an issue, depth of soil pH measurement has to accommodate subsoil depth also..

Compliments to you Dr. Anoop, for explaining the science behind soil sample depth for soil pH measurement should represent the depth of  crop rhizosphere

"natural" soils as forestal or grasslands soils show different "horizons" and it is necessary to investigate on theses different levels if you want to understand pedogenesis and physico-chemical characteristics... it is true for pH measurement as for carbon or nitrogen content for example. Mix the different levels have no sense...

It depends on your study, in general, the dutch auger tool was used to collect the soil samples from the certain depth. The soil fertility properties can be analyzed from the upper layer of soil, while below was much more to physical and mechanical properties. In acid sulphate soil the pH was usually decrease from the upper to below. The acidity and alkalinity was affected by the exchangeable H and Al ions in the soil. the higher Al will give a lower pH soil and caused toxicity to plants. 

Dear Aiman

Many factors will effect method and soil depth sampling;

* If it is a pilot study: crop type and it's root growth type are the main factors. If its shallow, may be soil depth of 30 cm will be enough while, if crop with deep roots system you need 0-30, 30-60  and 60-90 cm ...etc.

* If it is a project with different crops, you need to more depth. Composite samples may be used.

At the first depth you must to clean samples from plant litter and other things to avoid over readings. 

Best

dear colleague,

that depends on the depth of the roots of the culture that will be grown, e.g. almost 80% of all grass roots develop and grow in the upper layer of the soil (10 cm) and take there the necessary minerals. It is clear that in this case the soil sample must be taken in the upper soil layer. For deeper rooting crops, soil semples must be taken at the proper depth.

Sincerly yours,

Lucien Carlier

The depth is also determined by whether or not the deeper roots may encounter acidity that would prevent their exploiting subsoil mousture

That's why we advocate always to consider effective rooting depth as per your objective .

This is related to your goal. The suitable criteria can be effective rooting depth.

Regard

I suggest that irrespective of the crop, you should take soil samples at 20 cm interval and up to 1 meter of soil depth. This will give you a clear picture of pH distribution in the soil profile. However, you can still increase or decrease the soil depth if your objectives are not fulfilled.

The soil pH measurement depends on the rhizosphere zone. 

if some one can suggest protocol for measuring electrical conductivity of soil sample?

and how deep a soil sample should be selected for that purpose?

Of course it depend mainly on rhizosphere zone of crops grown in the field .

We need to differentiate between soil sampling for fertility evaluation and soil taxonomy classification..??

i am asking about the methods, because in literature i found different methods such as saturation paste method, 1;5 ratio of soil suspension , 1; 1nd 1:2 ration of soil suspension.

some involve cell constant while determining electrical conductivity  where as some are not, so i want to know why need to know cell constant? i want to know how to calculate it for soil sample?

thanks in advance

Dear Aiman,

I suggest you should consult a local soil chemist for all your queries. Determining pH and EC are the basics of soil science. Tons of literature is available on this topic. In my opinion you just need somebody to explain all this stuff to you.

You should also consider the hostility of the deeper layers as potential constraints to uptake of subsoil water

This is related to your aims. Usually 0-25 cm is used based on effective plant rooting depth or effective root zone.