Abstract
The effects of Mg and P application on Mg concentration and dry matter production of orchardgrass (Dactylis glomerata cv. Kitamidori) were studied on site of volcanic ash soil low in available Mg. A randamized split plot design with three replications was used. The main plot received 20, 40 and 80kg P_2O_5 per 10a as double superphosphate, and subplot received 0, 5, 10, 20, and 40kg MgO per 10a as magnesium sulphate. The results were summarized as follows: 1) Each additional increment of Mg fertilizer significantly (at the 1% and 5% levels) increased herbage Mg concentration, and the maximum Mg concentration was obtained at the highest level of Mg applied. But no effects of Mg application were observed on the herbage grown in low temperature season ; yet the percent magnesium in herbage remained at low level. It was considered that the suitable amount of Mg fertilizer was 10kg MgO/10a each year to ensure the 0.22% level of herbage Mg concentration recommended for ruminants by ADAMS. And further it seemed to be necessary to maintain the exchangeable Mg level of 8mg per 100g of soil. 2) The Mg concentration of herbage decreased with increasing P application which should be closely related to applied P_2O_5/MgO ratio. The suitable range of the ratio was estimated from 2/1 to 1/1. 3) The application of Mg fertilizer evidently increased the dry matter yields compared with no magnesium treatment, however, there was no significant difference between the Mg applied treatments above 5kg MgO/10a. It seemed that herbage growth response to Mg application could not be expected when there was more than 5mg of exchangeable Mg per 100g of soil.
