Abstract
The six cool temperate grasses had been grown under mimic grazing system for 6 years on field plots treated with two levels (N-P_2O_5-K_2O: 1-1-1 and 4-4-4kg/a/year) of fertilizer supply respectively. Chemical analyses of macroelement were made for the samples obtained from each cut of the above trial and analytical data were examined in relation to occurrence of summer depression using statistical method. The main findings were as follows: 1) Yearly or seasonal fluctuation of macroelement content in herbage showed that N, P, Ca and Mg were related to occurence of summer depression. Therefore, taking into account the effect of these elements on the development of root, it was possible to suggest a definition of root value as follows: [N/P×1/(Ca+Mg)% ratio] of top. 2) Close relation was recognized between occurrence of summer depression and root value of each species in spring. Increases in the value tended to accelerate its occurrence in case of all species. The extent to which each species was affected by the root value decreased in the order as follows : perennial ryegrass>orchardgrass>tall fescue>Kentucky bluegrass>red top Above tendency well reflected the susceptibility to summer depression of each species. 3) Investigating each effect of N/P and 1/(Ca+Mg) which were obtained by separating the root value into two parts, it was found that concerning summer depression, orchardgrass and perennial ryegrass were remarkably susceptible to N excess or P defficiency and soil acidity most severely affected perennial ryegrass which was followed by tall fescue in severity. 4) The occurrence of summer depression was closely related to both root value and meteorological factors such as precipitation during rainy season and temperature in July. The equations which indicated the relation between above factors showed that severity of summer depression varied markedly due to difference of soil fertility even under similar meteorological condition and orchardgrass and perennial ryegrass were much more susceptible to both factors than the other species were. It was concluded that additional application of P, Ca, and Mg should be done based on meteorological factors for longer and more efficient utilization of grassland where cool temperate grasses were dominant under such specific condition as Japan located in the monsoon zone in which P deficient and acidic, volcanic 'ash soils were widely distributed.
