Soil organic carbon(SOC) can be increased by application of N fertilizer. And SOC can also be increased by application of manure in the soil. If you compare between manure and N fertilizer, in this case manure application is more appropriate to increase SOC .......

The major source of N in soil is from soil organic matter which upon mineralization release mineral N as ammonical- and nitrate-N which can be considered as contribution from soil . The rate of N mineralization and the quantity of such mineralized soil N available for crop use in a soil depends on agroclimatic condition. At any given location, with variedly maintained organic matter levels and added fertilizer N , nitrogen contributed by native soil organic matter or C can be estimated and regression analysis using soil organic C as an independent variable can be done to predict addition of fertilizer N to achieve desired yield level.

I agree with Prakash Chandra Srivastava . I only add that it is sure that if the soil has less organic matter, it has lower N content. However, N availability for a crop in certain date depends on mineralization, then it depends on the temperature, soil water content and activity of microorganisms, among other factors.

The natural availability of N will depend on the C content of the soil. In addition, many other factors already mentioned here are important, as they influence mineralization.

Organic carbon can contain different fractions (recalcitrance). In your question, do you consider total organic carbon?

Some fractions have a higher correlation with the mineralization rate and it would be more interesting to use them.

I endorse the opinions of Prakash Chandra Srivastava and Corina Graciano. Most of the soil nitrogen occurs under organic form.Generally, organic form of N constitutes more than 98% N in soils.Hence, in many countries , organic matter content of soil is used as an index of its nitrogen status , grossly as high, medium and low .This grouping is used as an easy index for nitrogenous fertilizer application.This system is used in India also.

In summary, increasing soil organic matter will increase soil N availability pending its quality and ability to mineralize. The ability of soils to mineralize N from SOM is clearly related to environmental considerations including soil temperature and moisture. as well as in some cases texture.

The other aspect that is the quantity of mineral N (NH4+ and NO3-) which are the form taken up by plants in the soil. since about 98% of the N in soil is in the organic form, these inorganic forms will make up about 2% of the total N. The inorganic N is the form that most directly affect plant growth and yield. In warmer, drier climates,inorganic N can accumulate from organic N mineralization or as a residual from previous fertilizer applications. Under some conditions, inorganic N can

be quite high as long as it isn't leached out of the osil during periods of heavy rainfall or irrigation.

So, determining only Soil Organic C content without knowing its N content or determining total N without understanding the mineralization rates of soil organic N or residual soil inorganic N can often be misleading..

In order to understand the N availability of the soil, one needs to have information on the SOM content, total soil N content, Inorganic N content in the rooting zone of the crop of interest, and some idea of the N mineralization rate from the SOM in order to able to make decisions of fertilizer N availability to a crop.

Soil N processes in soils are very dynamic and even though soil N has been studied for a long time, there is still a lot we don't know about it. this keeps us in a job!

Kunal .

In my many years of experience (my previous job) conducting soil and plant tissue analysis for farmers and researchers, I have regularly observed soil samples with low organic carbon content but which contain a sufficient amount of nitrogen. This was particularly true for soil samples from intensively cultivated farms that regularly receive high doses of chemical fertilizers. Thus, using organic carbon as the sole basis of N fertilization is not correct.

Organic matter plays a vital role in N in the soil but also soil its self is difficult to predict we can only suggest

Considering the variabilities in different agroclimatic regions and management practices there is no universal single foolproof soil index. Dr. Luz is correct that under intensively cultivated areas receiving high N doses, redidual Inorganic N staying in soil prior to sowing is important and needs to be taken in consideration and same is also true for arid and semiarid region where mineralized N stays in the soil due to very low rainfall.

Increasing soil organic carbon content increasing not only N but also increasing others macro, secondary and micronutrients as per our research conducting in fields

Obviously there is a relationship between humus content (soil organic-C) and Nitrogen. The C/N-ratio indicates how much org. carbon in relation to Nitrogen is contained in the organic matter. A low C/N-ratio means that the organic matter is relatively high in Nitrogen which will be easily liberated and made plant available by degradation. Less Nitrogen fertilisation will be needed. Conversely a high C/N-ratio shows that the organic matter contains much organic carbon relative to Nitrogen and accordingly the N liberation after degradation will be low indicating a higher need for organic and mineral Nitrogen fertilisation.

The relationship in a nutshell depends on the ratio of carbon to nitrogen, as some professors have previously indicated

The majority of soil nitrogen is related to soil organic matter. But its availability is very limited. Only 1-2% of total nitrogen may mineralize in the soils depending on agricultural practices, soil, and climate conditions. There N fertilizer suggestions should take into account this mineralized nitrogen.

Thanks to all for the eminent responses.

I have some different question. As we all know that application of fertilizer in field is generally based on soil test recommendations. Like, we apply N fertilizer in different doses during experimentation based on the residual available N content in soil. Some studies reported that fertilizer N can be applied based on soil organic carbon content (analysed by Walkley and Black method) and they prepare a threshold carbon content level at which the crop's response to fertilizer N was very good. But below and above that threshold level the crop yield reduces. However, in our work we have similar range of threshold but we did not find any relation between organic carbon content of soil and crop response to fertilizer N.

Kindly guide.

No!

Fertilizer N can be applied based on soil organic carbon content and they prepare a threshold carbon content level at which the crop's response to fertilizer N was very good. But below and above that threshold level the crop yield reduces. As organic matter is decomposed, nutrients are converted into forms that plants can use directly. CEC is produced during the decomposition process, increasing the soil's ability to retain calcium, potassium, magnesium and ammonium. Nitrogen fertilizer means a chemical compound in which the percentage of nitrogen is greater that the percentage of any other nutrient in the compound, or when applied, results in an average application rate of more than twenty (20) pounds of nitrogen per acre over the field to which it is being applied.