Development and evaluation of preliminary DRIS norms for dallisgrass

Abstract

Preliminary DRIS norms were developed and verified for accuracy in diagnosing ? and ? deficiencies and sufficiencies, and in predicting yield responses to applied ? and ? by dallisgrass (Paspalum dilatatumPoir.). Dry matter yield and dry matter accumulation rate, both alone and combined, were used to delineate high‐yield populations. Data were obtained from five‐year soil fertility field experiments on Olivier silt loam, an Aquic Fragiudalf. Data from the first year were used for norm development and subsequent data for norm verification. In the ? study nearly all samples diagnosed were ? deficient, while very few samples in the ? studies were ? deficient. Yield increases due to ? applications frequently occurred where the ? index was negative but not necessarily the most negative index, indicating that all negative ? indices should be indicators of a true ? deficiency in dallisgrass. The most accurate DRIS norms resulted in 82% overall accuracy in diagnosing ? deficiencies and sufficiencies during four years. These same norms identified 100% of the actual yield responses to ? that occurred during the four year period. At both the low and medium soil test ? levels, the number of incorrect diagnoses of ? deficiencies was minimized when only the most negative ? indices were considered true indicators of a P deficiency. At low and medium soil test ? levels, the most accurate DRIS norms resulted in 75% and 83% overall accuracy, respectively, in diagnosing ? deficiencies and sufficiences during four years. There were not enough yield responses to ? for an evaluation of ? response prediction accuracy. Use of the dry matter index as a basis of discriminating between deficiency and sufficiency did not improve overall diagnostic accuracy in evaluating the ? status of dallisgrass but slightly improved overall diagnostic accuracy in evaluating the ? status. Rate of dry matter accumulation as a criterion for selecting the high‐yield population resulted in more young plants in those populations and appeared to improve DRIS norm accuracy in diagnosing the ? and ? requirements of forage with variable age among harvest dates

Notes

Approved for publication by the Director of the Louisiana Agricultural Experiment Station as manuscript number 88–09–2431.