Population of what...?

The number of families per unit area would only go to prove that the population is increasing. Fertility IS impacted by population, but environmental factors and soil physical/chemical properties are ultimately responsible for the quality of soil fertility.

Concerning animals, the quantity of them is controlled by food. Soil population is depending from the food (energy); but that is named SOIL ORGANIC MATTER. If you do the relationship between population and SOM (or SOCarbon) there is a perfect relationship. Others soil parameters could control de diversity, as soil pH, but usually the SOC content explain the amount of animals living in/on the soil (subsystem &, hence, in the total system).

Depending on the fertility treatments, you may need to target specific faunal groups that respond to either organic or inorganic fertility inputs in a short space of time.

As per your question, I think there is simple method to extracts population from soil sample is Tullgren funnel (1918) / Tullgren-Berlese method and then select systematic groups of them.

I understand now. You refer to persons, not microbiology or micro-fauna. Persons have impact on soil according to:

- Number of persons.

- Life standard.

Then you need to refer to number of person and after that to correct according the standards of life.

For example, in marginal societies all the goods & foods are coming from soil production. The impact is direct. But in developed societies the impact is downing because at lot of products are imported and the marginal soils are abandoned; in this way the impacts are not so direct. That is a general rule in all societies, from U.S.A. to Spain.

Thanks Juan G. Lancho for your very good view point. If I explain my view point like that "with increasing population, changes in the environmental quality takes place and so the irrigation water especially the canal irrigation water i.e there may be more addition of organic materials or polluted water into the canal water. So what happen to soil fertility when irrigation with this type of water takes place. Also in case of high population, there may be direct but unnoticed addition of organic material as well as the pollutants to the soil or there may be the addition of these materials through local winds to the soil" So I wanted to study what happen to soil fertility as well as soil quality in case of increasing population. Your comments please Mr Juan G. Lancho whether I should consider the population of the whole town or the population per square km or number of families living in the town or village and the city areas. Please Note that I have selected 11 places as my sampling locations out of which 6 are urban and 5 are rural in my district. Also, I will analyze both the soil as well as the canal irrigation water for all the chemical (nutrients plus heavy metals), physical and microbiological parameters. Your improvement in the study is awaited please.

Probably you should consider the people that truly impact on these soils.

I see you separate properly urban and rural persons. Some cities live independently from the surroundings; others lives from the field in an intensive way. For example, in Spain, Valencia depends from the citric plantations, more people means more intensive citric cultivations; but Caceres is merely a commercial city, this city has not any impact on the field, excepting refuse waters and little more.

Then, first thing is to establish if the urban people only affect the hinterland concerning contamination (refuse waters, load metals, etc.) or, in addition, all these persons are working and they depend from the soil resources.

In the first case you have in account only the urban population concerning to the contamination effects and the rural taking in account the population of the villages; but in the second you need to consider ALL the population (e. g., referred to square meter).

In any case, please taking also in account that contamination is directly related to the life style or standard; contamination is directly related to the consummation of energy of these persons, i. e., better life, higher pollution; meanwhile soil degradation depends of the impacts on the soil, having an important issue the soil management (e. g., replacement of the initial SOC level).

Nice discussion ..

i would suggest to please look at two things. Identify the two contrasting sites, thinly and thickly populated sites, evaluate the nutrient density of crops at both the sites and over a period of time , you will get back the difference in nutrient density of those heavily consumed crops as an indication of changes in soil fertility , and secondly monitor the changes in land use and consumption pattern , how does it relate to soil fertility ..

and third option could be per capita  income of areas from low fertility to high fertility soils...