Potting media, transplants and yields in the production of glasshouse tomatoes

Samples of potting composts for raising tomatoes on holdings in the South Holland glasshouse district were collected in 1960 and 1961.<p/>In propagation experiments with soil blocks, fresh weight of young tomato plants were found to vary widely. Some 3/4 out of the investigated composts were not suitable for raising plants.<p/>Soil analyses proved that under the given conditions many composts had an insufficient amount of plant available nitrogen, measured by means of N-value which is defined as mg water soluble N per 100 ml soil. Further investigations on composts for raising tomato plants in soil blocks showed that the relation between the optimal nitrogen dressing ( <em>N _d</em> in mg N per l, or g N per m3) and N-value is given by:<br/><em>N _d</em> = 10 [35 - N-value].<p/>Plant growth in the growers' potting composts was also influenced by soil physical condition measured as the air filled pore content at pF = 1,0 or as the organic fraction&gt;1000μin % of the total organic matter. The respective content should be at least 6 % or 30 %.<p/>Experiments with series of transplants from different compost qualities showed that heavier plants made a better crop and produced a higher early and total yield. Overall stem height and fresh weight of the transplants were most accurate for predicting yields by means of regression equations, while the number of leaves at least 3 cm long and the stage of development of the first inflorescence were less accurate.<p/>It is supposed that in the Netherlands the variation in yields of glasshouse tomatoes is partly due to poor potting composts for propagation; the decrease in potential during 1960-1965 <em/> is estimated at about 20 million guilders a year