Dr.Jat,good comments.For P and K one should not go for oven-dried samples.The amounts of extractable/available P and K will be affected by temperature.Even on air- dried samples seasonal changes in temperature affect the amounts of extractable nutrients in soil testing laboratoty.

Most of the organic C is associated with the organic matter fraction, whereas the inorganic C is usually associated with the carbonate minerals. However, for laboratory analyses, the soil organic carbon generally includes materials that accompany soil particles through a 2-mm sieve (Nelson and Sommers, 1982). For proper analysis of soil carbon we have to use the air-dried soil NOT the oven dried soil as this may cause loss of some weak fraction carbon while drying and hence, the estimates may be wrong. Certainly the N analysis from oven dried soil will have the differential results as compared to air-dried soil due to volatilization losses of N from soil but for P and K this could not affect much. Hence, for PK analysis oven dried soils may be used but not for N and specially its fractions like ammonical and nitrate N. So, for carbon and N analysis air-dried soils gives more accuracy compared to oven drying.

Dr.Jat,good comments.For P and K one should not go for oven-dried samples.The amounts of extractable/available P and K will be affected by temperature.Even on air- dried samples seasonal changes in temperature affect the amounts of extractable nutrients in soil testing laboratoty.

Dear G.A. Rajanna,

as Shankar already wrote you can use air-dried soil, but also oven dried soil. The advantage of using an oven is that you can control temperature. Commonly soil scientist dry soil at 40°C to dry soil while preventing loss due to VOCs (just have a look into some soil research publications or soil science guidelines). In case you are interested to assess NPK you can use dry soil for K and P and total N, but NOT for NO3 and NH4 fractions. Drying affected composition of NO3 and NH4...

Best

Nils

Interesting discussion . We have   observed a substantial difference  in ammonium acetate extractable-K in   air dried versus oven dried  smectite soils . Question is really pertinent 

Dear Rajanna,

Increased temperature causes oxidation loss of soil organic carbon. Field studies in the US Great Plains (Follett et al. 2012) have shown that soil C and N stocks are strongly negatively related to mean annual temperature and positively related to the ratio of mean annual precipitation to potential evapotranspiration, suggesting that they are equally vulnerable to increased temperature and decreasing water availability. Based on these empirical relationships, a 1 °C increase in mean annual temperature could cause a loss of 1900 kg soil organic C and 180 kg soil organic N per ha from the top 10 cm of soil over 30 years, but the decrease will be mediated by water availability.

These findings indicate that one should not use oven-dried soils for reliable assessment of NPK availability in soil.

Dear Dr. Rajanna

Its better to use air dried soils for analysis than the oven dried soils.  If in case air drying is not possible, in this situation we have to go for oven dry soils. But here drying temperature should not exceed 60 degree celcius for 1-2 days. if it exceed this temperature carbon burning will takes place. so it leads to inaccurate % OC values as well as Available N content of soil. 

Warm wishes

I guess this is dependent on the range of temperature involved. At temperature range less than 40 and not more than 100 degree celcius, results  could show that SOC and the cumulative SOC increased with increasing  temperatures

Normally use air dried soil for determination of Total C%(chemical digesion method,Walkey & Black,1939), total N%(Kjeldal technique), total P etc. you have to air dry and sieve by 2mm and store. In addition can use Loss on ignition (burning of different temp. and determine weight loss) fro quantify different C fractions. refer Tropical Soil Biology and Fertility: A hand book of Methods by J.M.Anderson and J.S.I.Ingram, 1998, CABI Pub.

Thanks for nice suggestions and valuable information which is discussed above.

I could not above to get the specific information from the above discussions regarding the LABILE/NON LABILE /VERY LABILE Carbon stocks in the soil as influenced by oven dry temperature. Can any body has research papers regarding this kindly share me.

New year wishes to all

Labile pool may change due to temperature. that is the free litter fractions will burn with temperature range 150-200oc (Ratnayeke et al., 2009?)

Interesting discussion..

THERMAL TRANSFORMATION OF SOIL ORGANIC MATTER BY NATURAL FIRES AND LABORATORYCONTROLLED HEATINGS

Abstract: The results of a multi-approach analysis on the effects that wildfires exert on soil organic matter (SOM) properties are presented. The methods used included wet chemical oxidation and therrnal degradation by flash pyrolysis, as well as Fourier-transformed infrared (FT-IR) and solid-state l3C and 15N nuclear magnetic resonance (NMR) spectroscopies. Such destructive and non-destructive analytical techniques were used for the assessment of heat effects to a molecular level applied to representative Spanish forest ecosystems and for the study of the alteration of different organic materials in the course of progressive heating in laboratory simulation experiments. The material s studied included: whole soils and sapric peat, isolated soil humic fractions, lignocellulosic biomass and preparations of cellulose. Among our findings, it can be emphasized: (i) the importance of the abiotic transformation of aliphatic precursors into aromatic macromolecules under present-day environmental conditions, (ii) a therrnal neoforrnation of heterocyclic N-forms, (iii) changes in the solubility properties of the soluble and colloidal SOM fractions, (iv) a preferentialloss of oxygen-containing functional groups and O-alkyl aliphatic structures, and (v) changes in the SOM macromolecular structure, probably related to the accumulation of a resistant alkyl moiety. In general, our results from laboratory experiments agreed with those obtained from soils affected by wildfires in different Continental Mediterranean forest forrnations. The results are discussed in terrns of the natural stabilization mechanisms of the most refractory SOM forrns, which is of particular interest in the study of the global C and N biogeochemical cycles. Source ; by F. 1. GONZÁLEZ-VILA and GALMENDROS  ..PDFs enclosed fro further reading...