THE DIAGNOSIS AND RECOMMENDATION INTEGRATED SYSTEM (DRIS) FOR DIAGNOSING THE NUTRIENT STATUS OF GRASSLAND SWARDS - I - MODEL ESTABLISHMENT

Herbage analysis offers a definitive means of determining the N, P, K and S status of perennial ryegrass swards. Unfortunately, the results of such analyses can be difficult to interpret, simply because the min imum or 'critical' concentration of a nutrient in plant tissue for opt imum growth, varies both with crop age and with changes in the concent rations of other nutrients. The Diagnosis and Recommendation Integrate d System (DRIS) could help to improve the reliability of such interpre tations. Diagnoses made using DRIS are based on relative rather than o n absolute concentrations of nutrients in plant tissue, and as such sh ould be comparatively independent of crop age. The aim of this study w as to establish and test DRIS methodology for high-yielding perennial ryegrass swards. Because of prohibitive costs, setting up a whole new series of field experiments to evaluate DRIS model parameters for pere nnial ryegrass was out of the question. Instead, the diagnostic norms and associated coefficients of variation for the model were evaluated using data from a single (large) multi-factorial glasshouse experiment . Of the nutrient ratios selected to form the diagnostic norms, K/N an d S/N had the dearest physiological rationale, whereas those involving Ca and Mg in combination with N, P, K and S appeared to have little p hysiological basis. It was reasoned, though, that because Ca and Mg up take by plants are largely passive processes (ultimately governed by p lant growth), the DRIS indices for these nutrients, together reflected the degree to which growth may be limited by non-nutritional (environ mental) factors relative to nutritional ones. Both indices were combin ed to form a single reference (R-i) index. Without such an internal re ference, plant growth could be limited by multiple nutrient deficienci es, and yet N, P, K and S indices might all be close to, or equal to z ero (i.e. the optimum), simply because the absolute concentrations of each nutrient (while low) had been in the correct state of balance. Mo reover, by effectively using Ca and Mg as internal reference parameter s in DRIS, 'nutrient concentrations' which previously formed the basis of the critical value approach, were essentially incorporated into th e DRIS model, thus combining the strengths of the two diagnostic appro aches; the only difference being that Ca and Mg, and not dry matter, w ere the internal references against which the levels of the major nutr ients were compared.