Optimizing Substrates for Organic Tomato Transplant Production

Abstract

Organically grown transplants required by the National Organic Program are rarely commercially available and are usually produced on-farm from locally available, inexpensive substrates. Six experiments were conducted to study the effect of potting mixes and their components on seed germination and seedling growth of tomato (Lycopersicon esculentum Mill cv. 'Celebrity') and to compare existing commercial organic and conventional substrates with a sample grower mix (GM20) (peat, perlite, vermicompost, feather meal, kelp meal). Variations on GM20 were added in some experiments including both higher and lower rates of vermicompost and feather meal in order to optimize seed germination and nitrogen nutrition. Although soluble organic fertilizers are often applied to conventional transplants after several weeks, no additional fertilizers were added in this study because allowable soluble organic fertilizers are generally cost prohibitive and are usually not locally available.

Analysis of pH, salts, physical properties, nutrient content, and distribution of nitrogen between ammonium and nitrate forms was conducted on each potting mix and on the components of the grower mix. Tissue nutrient content was analyzed weekly from each treatment to determine plant nutrient status.

Seed germination was consistently high in the original commercial organic and conventional media (73-89%). Germination in two additional commercial organic mixes was significantly lower (76-82%) in all but one case. Germination in the original grower mix was lower than the original commercial substrates, except in Experiment 4, where germination was higher overall compared to earlier experiments. It was not clear which components of the grower mix were responsible for reducing germination. Reducing vermicompost from 20% to 10% did not consistently improve germination. Increasing feather meal as a component by 50% reduced germination in one of two experiments while doubling its rate reduced germination in both experiments. Eliminating feather meal resulted in comparable germination (87-92%) to the original commercial mixes and higher germination than mixes containing feather meal in one of the two experiments. There was also no consistent relationship between media pH, salt levels, or physical properties and germination rates. Components of the organic mixes and the organic mixes themselves varied considerably between experiments in EC and pH except for the original commercial organic mix which was consistently low in EC and high in germination.

In 3 of 4 experiments, transplants grown in GM20 had a significantly higher dry weight than those grown in the original commercial organic and conventional potting mixes. Nitrogen deficiencies were present by the final week of transplant production in all commercial and custom mixes during at least one experiment. Addition of higher rates of feather meal did not consistently eliminate nitrogen deficiencies. Over all experiments, only one phosphorus deficiency was found in a custom mix including vermicompost. The most consistent source of potassium was the conventional mix, with deficiencies showing up by week 4 in most of the organic treatments. Even in the conventional mix, transplants were at the critical level for deficiency in one experiment.

Results of this study indicate that it is possible to add sufficient phosphorus into potting media for the period required for transplant production through the utilization of vermicompost, however, all media tested would require additions of soluble fertilizer in order to provide sufficient nitrogen and potassium. All media and components tested showed some variation with time and batch, but variation in the vermicompost was particularly striking. Thus, when utilizing grower mixed substrates it is important to have nutrient analyses performed on the substrate prior to use. Germination was sometimes reduced in media containing higher levels of nutrients, so care should be taken to optimize conditions during germination.