Main Effects and Interactions of N, P, and K Applied to Greenhouse Roses

Abstract

The floribunda rose cultivar Roswytha, grafted on to ‘Brögs Stachellose’ was grown in a factorial experiment with 3 rates of N, P, and K fertilizers during a six year period. The effects of the treatments on soil and leaf element contents as well as on yield and quality of the cut flowers were studied.

In the unfertilized plot the amount of available soil P increased during the first five years. Increasing the supply of N, P, and K resulted in significantly higher amounts of these elements in soil and leaves. Highly significant correlations between leaf and soil contents of N, P, and K were established using the modified Spurway-Lawton nutrient analysis method. However, the leaf-soil element relationship was much stronger for P and K than for N. Significant leaf-soil correlations were also found for Ca, Cl, and Mn.

Significant positive correlations were found between K, Ca, Mg, and Cl in the leaves and P in the soil. Foliar Ca, Mg, and B was significantly negatively correlated with soil K and foliar Cu was significantly negatively correlated with soil NO₃-N, P, Ca, and Cl.

All the leaf elements studied were positively correlated to the ash content of the leaves and the correlations were significant in all cases except for Cu and Na. Significant negative correlations were established between foliar K and Ca and foliar Mg and B.

The leaf content of N, P, and K varied significantly during the season, with the lowest values in the middle of the season and the highest at the beginning and the end. Similar trends were also found for Ca, Mg, Cl and ash.

Leaf area was significantly positively correlated with foliar K. Colour intensity of the flowers decreased with increasing leaf contents of N and K. This effect was most pronounced for K. The percentage of cut roses with bent neck increased considerably at low leaf K contents. It is suggested that K deficiency is one factor involved in the bent neck phenomenon of roses.

Plant fresh weight increased significantly up to the highest N rate and the single rate of K. At the highest rate of P and K, plant weight decreased. The total yield of cut roses was significantly influenced only by the nitrogen treatment. The number of flowers in the first grade, however, was affected by N, P, and K. High N and K significantly improved flower grade while high P significantly reduced flower grade. The highest number of first grade flowers was obtained in the N₂P₀K₁ treatment. The number of non-marketable flowers was significantly increased by N and P supply but decreased by K supply.

The gross return was calculated for each treatment. The highest return was obtained in the N₂P₀K₁ treatment. The element content of leaves in this treatment was as an average for the five flushes, 3.04, 0.29 and 2.14% for N, P, and K respectively. Considering the seasonal variation the following standard ranges are suggested for the rose cultivar Roswytha on ‘Brögs Stachellose’. N, flush 1, 3.4–3.6, flush 2 and 5, 3.0–3.3, flush 3 and 4, 2.6–2.8%. P, flush 1, 4 and 5, 0.28–0.36, flush 2 and 3, 0.23–0.28%. K, flush 1, 2.5–2.6, flush 2, 3, 4, and 5, 1.9–2.2% of dry matter.