This test determines the total nitrogen (TN) content of the soil including that present as both chemically stable humus (or passive) and partially decomposed plant and animal residues (or active) organic matter fractions. The test gives some indication of the N supplying power of a soil, but it’s primary use is to enable the expression of other, related parameters (Organic C, AMN) relative to this property. Therefore it is acceptable to use soil total nitrogen as soil quality indicator for overall assessment of fertility status.

Dear Jamal Alawamy,

It is an interesting issue. 

Total nitrogen analysis measures N in all organic and inorganic forms. Total nitrogen does not indicate plant-available N and is not the sum of NH4-N + NO3-N. Total N is not used for fertilizer recommendations.

Nitrogen occurs in soils as organic and inorganic forms and soil testing may be performed to measure levels of either. Nitrate nitrogen (NO3-N) is most commonly measured in standard soil tests because it is the primary form of nitrogen available to trees and, therefore, an indicator of nitrogen soil fertility. However, soil concentrations of NO3-N depend upon the biological activity and may fluctuate with changes in soil temperature, soil moisture, and other conditions. Nitrate is also easily leached with rainfall or irrigation so current soil tests may not reflect future levels of nitrogen soil fertility.

Ammonium nitrogen (NH4-N) is also a plant available form of nitrogen in orchard soils and it can be determined with soil testing upon request. In general, NH4-N is not determined and reported with a standard soil test. Ammonium nitrogen does not usually accumulate in soil because soil temperature and moisture conditions that are suitable for tree growth are also ideal for conversion of NH4-N to NO3-N. Ammonium nitrogen concentrations of 2-10 ppm are common. Levels above 10 ppm NH4-N may occur in cold, wet soils or in soils irrigated with a water supply that is high in ammonium nitrogen.

Total nitrogen which is a measure of all organic and inorganic forms of nitrogen in soil can be determined with soil testing. However, it is not included in standard soil testing.

You can follow the attached link. It is following at Soil Science Laboratory, IRRI.

https://www.norweco.com/html/lab/test_methods/4500no3efp.htm

This test determines the total nitrogen (TN) content of the soil including that present as both chemically stable humus (or passive) and partially decomposed plant and animal residues (or active) organic matter fractions. The test gives some indication of the N supplying power of a soil, but it’s primary use is to enable the expression of other, related parameters (Organic C, AMN) relative to this property. Therefore it is acceptable to use soil total nitrogen as soil quality indicator for overall assessment of fertility status.

Dear M. N. Islam

Thanks for a long discussion. But I like to add that for wetland rice soil NH4-N should be considered as the indicator N fertility. However, rice plant can absorb NO3-N also in some cases specially upland rice. Another term potentially mineralizable nitrogen (PMN) is used to describe for soil quality indicator regarding N. 

PMN is the fraction of organic nitrogen converted to plant available (or mineral) forms under specific conditions of temperature, moisture, aeration, and time. Determining levels of PMN can provide an estimate of available N in the soil. PMN originates mainly from microbial biomass and plant and animal tissues—the main source of the organic nitrogen pool. It represents the fraction of nitrogen easily decomposable by soil microorganisms and is considered an indirect measure of nitrogen availability during the crop growing season.

Yes total nitrogen can be used as soil quality indicator.

Dear Suresh K Malhotra and Arvind Singh

Thanks for your answer. Total N can be the N status of a soil, but side by side potentially mineralizable nitrogen (PMN) which is a fraction of organic N should be considered as a soil quality indicator. Because this will be available during the growing season. I mentioned about this in my previous answer. Thank you sir. 

Dear colleagues,

Since organic carbon (OC) and total nitrogen (TN) contents of soils are strongly and universally correlated, and determination of organic carbon is much easier, it is preferred to include OC (instead of TN) as an indicator of soil quality. Besides indicating nitrogen availability, OC indicates biological qualities as well as availability of several other nutrients in the soil.

Total nitrogen could only be used as an index of soil quality indicator as long as it gives some about the organic carbon content  .On the other hand , total N content does not give any information about the more sensitive parameters like soil microbial load , soil microbial biomass nutrients, soil enzymatic activities as part of labile fractions of soil organic carbon..? 

Excellent discussion Dr Anoop K Srivastava. I also mention the fraction of organic N which is mineralizable during growing season and microbial load will be increased. This fraction of N also indicated the OC content od soil. So, the potentially mineralizable N (PMN) can  be used as indicator of soil quality regarding N status in soil.

Soil quality indicators should be responsive to the changes in soils, easy to measure and should not be strongly related to any other properties which is easy to measure. Actually minimum data set selection procedures heps to discrimnate most significant indicators for any case. Organic matter and total nitrogen are strongly correlated parameters. Thus measuring both of them may not enable extra information. But in studies dealing with organic matter amendments with differing C:N ratio, it would help our understanding about the soil quality. 

I  agree to the views expressed by Dr. Kundu. OC specially the  active fraction of soil C can be used as an indicator of soil health. A pdf is attached for information.

I am also agreeing to the views shared here and it is better to include OC for soil quality indicator as it will be easy to determine and have a strong correlation with available N