| Summary: | 博士 === 國立臺灣大學 === 農業化學研究所 === 99 === Organic fertilizers are necessary to sustain soils ability to crop production and to help reduce environmental pollution especially due to nitrogen (N) and phosphorus (P) leaching. But heavy application of these in excess of crop needs can buildup N and P levels in soil affecting soil and water quality. Composted manure is known as an environmentally safer organic fertilizer but leaching and crop recovery from composted manure applied soils is less well understood and this knowledge could help managing compost application in high rainfall areas. A green-house experiment conducted at the National Taiwan University, Taipei from April 2006 to June 2008 evaluated the effects of compost and urea combinations on dry matter yields (DMY), apparent crop recovery of N and P, residual effects on soil and changes in soil quality. Treatments consisted of urea (F250), compost (M250), and combination of these two (F125M125) –all supplied 250 kg N ha-1 while two others (M500 and F250M250) doubled the N supply and an untreated Control (Ctrl) for comparison. M125, M250 and M500 corresponded to 8, 16 and 32 Mg ha-1 of composted manure application, respectively.
Cabbage (Brassica oleracea L.) and corn (Zea mays L.) crops were grown in standard Wagner pots in annual rotation using 12.9 kg (dry weight basis) of acid sandy clay loam soil. Soils were leached fortnightly applying 80 and 60 mm of water to cabbage and corn corresponding to 2000 and 1500 mm average annual rainfall, respectively. Leaching of mineral N (MNL) and inorganic water soluble P (WSP) were monitored in leachate water samples during growing seasons for a total of 28 times in 2-y. Although composted manure application was not repeated, blanket of NPK fertilizers were applied to all treatments except the Ctrl from crop2 to crop4 seasons from mineral sources in order to help maintain normal growth of the test crops.
With some variations among treatments, increasing rate of N application from 0 to 500 kg ha-1 mostly increased DMY of cabbage and corn. A tendency of high compost (M500) favoring cabbage DMY, and corn DMY indifferent of urea/compost combination was apparent in both years probably due to the effects of crop-rotation and physiological differences between crops. Demerit of high MNL outweighed the merit of high crop N recovery in urea treatment (F250). Apparent crop N recoveries were higher in urea and compost combined treatments in 4 of 4 crops. But there were outstanding results from higher compost additions such as the large reductions in MNL due to reduced volume of leachate outflows, greater utilization of water for higher crop growth, as well as higher N and P retention and addition of organic carbon in soil. An annual application or higher compost rate appears useful to increase soil pH and nutrient availability. Although inorganic WSP leaching was not high, the results suggested the need to evaluate other forms of P leaching such as total P and algal P from this compost applied soil.
The assumptions of 50% N mineralization and 30% P release rates in this compost could not be substantiated. The results implied that nutrient management goal in agriculture would be more realistic if N and P credits in this compost are considered to be approximately 25% and 15%, respectively for the 1st-y of application. Over assumption partly explained why crop DMY were relatively low in this study compared with other pot studies that did not involve leaching operations. Use of soil management assessment tool for deriving soil quality index (SQI) did not show statistical differences among compost and urea treatments. However, the tool was found useful because SQI correlated with DMY in 4 of 4 crops. Taken soil, leachate, and crop parameters together, evidences show that an application of 32 Mg ha-1 of compost outperformed all other treatments although combined application of compost (16 Mg ha-1) with urea (250 kg ha-1) was the best candidate for apparent crop recovery of N and P and MNL. The results of this pot study were found consistent and encouraging; however a field-scale study would be helpful to validate these conclusions.
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