RELEASE OF MINERALISED NITROGEN FROM SOILS AFTER INCORPORATION OF DIFFERENT ORGANIC MANURES
Use of organic manures as a nutrient source for crops is common practice inagriculture. Added advantages of using organic manures against syntheticfertilisers are the positive inf1uence on physico-chemical and biologicalcharacteristics of soils. Release of mineral nitrogen from organic manures couldbe used as an index to check the quality of organic manures in relation to nutrientsupply to the crops. Therefore, the objective of the present laboratory leachingstudy was to monitor the nitrogen release from soils after addition of selectedorganic manures.
Two surface soils, Reddish Brown Earths and Non Calcic Brown (0-30cm)collected from Mahallluppallarna and Aralaganwila areas respectively, were usedin this study. Soils were mixed with three organic manures; Poultry manure(PM), Glyricidia leaves (GM) and Rice straw (RS) at the rate of 20 and 40tons/ha. There were four treatments; 1). Soil only - control, 2). Soil--Plvl. 3). Soil+GM and 4). Soil + RS. Each soil-organic manure mixture was placed inleaching columns and incubated in dark at room temperature. Mineralisednitrogen (NH+4 - N and NO) -N) was extracted with O.OIM CaCI2 solution at 7,14, 21, 35 and 49 days after incubation using the method described by Stanfordand Smith (1972). The NH\ -N and N03 - N contents were determined by usingstandard Kjeldhal method. The physico-chemical and nutritive characteristics ofsoils and organic manures were determined bv using standard laboratory methods.Results of the study revealed that the application of organic manures haspronounced effect on the release of nitrogen. In both soils, 40 tons/ha levelshowed higher release of nitrogen compared to 20 tons/ha level. Out of threeorganic manures, PM treated soils showed a higher release of nitrogen followedby GM treated soils. The RS treatment showed a very low release of nitrogenwhich is caused by the immobiolisation in two soils during the incubation. It wasvery clear that the release and immobilisation of nitrogen are controlled by theCIN ratios of organic manure. Up to 7th day the nitrogen release from PM andGM treated soils was high against the control as well as RS treated soils. After 7thto 2 Ith day, the mineralisation and release of nitrogen was ..decreased andsubsequently it was increased up to 35th day and then again decreased at the endof incubation with few exceptions. In general, the proportion of NH\ -N to NO) -N was high at the early stage of the incubation compared to the latter stage ofincubation. This may be attributed to the transformation of more NH+4 - N to N03-N by the nitrification process operating at the higher rate at the latter stage ofincubation. The release of NH -N and NO -N from organic manures wassignificantly higher than the control at certain sampling times. However,according to this study, the use of organic manures such as PM and GM with CINratios will enhance the mineral nitrogen content in soils during the first week ofincorporation.