Soil Nutrients, Nitrogen Mineralization, and Microbial Biomass in Organically Fertilized Potting Media under Greenhouse Conditions

The Winter Greenhouse Crop Organic Fertility Study seeks to determine best practices for organic winter greenhouse crop production and to understand soil dynamics in organic greenhouses systems. Most organic fertility amendments require some degree of microbial breakdown for nutrients to be available for plant growth, so this study focused on how nitrogen mineralization, microbial biomass, and media nutrient profile affected nutrient availability and subsequent plant growth to determine whether certain organic fertility options perform well under greenhouse conditions. Spinach (Spinacia oleracea var. tyee), day-neutral strawberry (Fragaria x ananassa cv. Albion), and a mesclun mix of arugula (Eruca sativa), mizuna (Brassica rapa var. nipposinica), and red giant mustard (Brassica juncea) were used as model plants in this study. Those crops were selected by local restaurant owners and grocers as having potential for local sales in the late fall, winter, and early spring. Fertility amendments used were fish emulsion (Drammatic 2-5-0.2 on Sunshine Natural and Organic Potting Mix), poultry litter (Sustane 8-4-4 on Sunshine Natural and Organic Potting Mix), a commercially available “all in one” potting mix (Purple Cow Potting Mix 0.5-0.2-0.4), and a mix commonly used by organic winter greenhouse growers (peat, vermiculite, leaf litter compost, greensand, rock phosphate, blood meal, and lime) in addition to a conventional comparison (CalMag 15-5-15 on SunGro LC8 potting mix). Fertilizer rates were calculated based on field recommendations for nitrogen in low organic matter soils for each crop. The “all in one” potting mix and the organic winter greenhouse blend exceeded nitrogen recommendations. Media was collected at planting and at harvest, and was analyzed for nitrogen mineralization, microbial biomass, and nutrient content. Fresh weight and dry weight of leaves and berries was taken at harvest. Although both the “all in one” potting mix and the organic winter greenhouse mix both exceeded crop nitrogen requirements, these mixes did not consistently outperform the others in harvest weight. In spinach, maximum yield was 175.62 g per 50 plants in the “all in one” potting mix and minimum yield was 63.29 g per 50 plants in the organic winter greenhouse blend. Nitrogen mineralization was measured using a 7 d incubation KCl extraction, microbial biomass was measured using chloroform fumigation, and the media nutrient profiles were measured using a mass corrected Spurway media test.  Preliminary data show media vary greatly in EC and micronutrients.  This experiment will provide much needed insight into nutrient dynamics in organic greenhouse cropping systems.