A laboratory aerobic incubation experiment was conducted to assess the influence of plant residue, soil moisture content, and soil types on mineralization of organic N. The experimental design was a 2 x 3 x 5 factorial arrangement in a Randomized Complete Block design (RCBD) with three replications. The three factors were (1) two levels of plant residue (0 and 4 ton/rai of grinded cassava shoot), (2) three levels of soil moisture (PWP, ½ AWCA, and FC), and (3) Five upland soil series of the Western region of Thailand, consisting of three main soil textures (fine, medium, and coarse texture). Nitrogen mineralized from native and added organic matter was examined at specific time intervals for 1 year. The data was fitted to a logistic mathematical model describing the relation between mineral N versus moisture content (θ), level of plant residue (PR), and time (t). The model was verified by predicting the amount of mineral N released under a given condition and the result was compared to the observed value under the same condition. Application of the model for field condition where soil moisture fluctuates was performed by differentiating the original model to obtain the relation between rate of mineral N production versus N (t), which was the implicit function of θ, PR, and t. Stepwise calculation of the cumulative mineral N with time was developed to predict the amount of N mineralized through time. The results revealed that addition of 4 ton/rai plant residue drastically increased mineral N by 3.6 folds. The effect of soil moisture content and mineralization of organic N confirmed the significance of water on microbial activities. A satisfactory result was obtained from the mathematical model verification. The b and R 2 values were close to 1.0 and the t-test were non-significant. A rather high value of Root Mean Squared Error (RMSE) was obtained contributing to the cycles of microbial population fluctuation. Application of the model to the condition of fluctuating soil water content was performed and compared to the observation value at a given level of plant residue application and soil water content.
<p>A system for sensing depth of penetration of a dynamic penetrometer was developed. It contained an ultrasonic<br />distance sensor working together with a microcontroller capable of sensing and recording distance of penetration<br />and field data necessary for the construction of the penetration resistance (PR) profile. The system was equipped<br />to the conventional dynamic penetrometer of the Department of Soil Science, Kasetsart University at<br />Kampangsan. The depth sensing system was found to work properly. Depth validation by means of t-test yielded<br />t value of 0.1713 as compared to 0.9800 of t-table under two-tailed test. In addition the RMSE and the MAPE<br />values were 1.2625 and 3.3473 %, respectively. Validation test on depth of penetration showed satisfactorily<br />insignificant difference between depths read by the invented device and by standard measurement. A computer<br />program for manipulating field data for PR calculation was constructed. The program was written in Microsoft<br />Visual Basic and imbedded into Microsoft Excel spreadsheet capable of accessing the field data being recorded<br />in an SD card of the ultrasonic distance sensing system. The data was manipulated by means of a user interface<br />that enabled the user to construct the PR profile with ease.</p>
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