Assessment of the Need for Nitrogen, Phosphorus, Potassium, and Sulfur Preplant Nutrients for Plug Seedling Growth

Abstract

Salinity guidelines for seed germination substrates call for low, difficult to attain levels. This study was conducted to determine the value of N, P, K, and S as preplant nutrients, with the anticipation that some could be eliminated to alleviate the high salinity problem in many substrates. Impatiens wallerana L. `Accent Rose' and Gomphrena globosa L. `Buddy' were tested in 288-cell plug trays containing a substrate of 3 sphagnum peat moss: 1 perlite (v/v) in two experiments. Seven preplant N, P, K, and S treatments, including none, all (each at 100 mg·L-1 of substrate), all minus one of each of the nutrients, and N at one additional rate of 50 mg·L-1, were tested. Postplant fertilization was applied to all treatments as 13 N-0.9P-10.8K at 50 mg·L-1 N beginning 1 week after sowing and was increased to 100 mg·L-1 N when the fourth true leaf appeared. The resultant seedlings were transplanted into 48-cell flats and grown into marketable stage for further evaluation. For maximum potential growth, N, P, K, and S were needed as preplant fertilizer. However, compact shoots, not maximum growth, are generally desired in commercial production. Thus, K and S can be omitted since their contribution to growth was only occasional and small. To further ensure a consistently desirable level of compactness, it was necessary to omit N or P or both in addition to K and S. Omission of N alone yielded the most desirable compact plant qualities, including suitable reduction in final seedling size. Omission of P alone yielded larger reductions in height and shoot dry weight of seedlings compared to N omission, and a delay of 2 to 4 days in flowering of bedding plants. Omission of the four nutrients, compared to -P, resulted in similar seedling growth reduction and bedding plant flower delay. Since N omission lowered the salt (electrical conductivity) level of substrate more than P omission and had no negative impact on subsequent bedding plant flowering compared to the other two treatments, N omission would be the more desirable of the three. However, N omission resulted in chlorotic seedlings, but these quickly turned green upon restoration of N. Omission of P or all four nutrients resulted in desirably deep green seedlings. Growth of gomphrena seedlings, a high-fertilizer requirement category of taxa, was suppressed when the preplant rate of N was 50 mg·L-1 compared to 100 mg·L-1, while growth of impatiens, a low-fertilizer requirement category of taxa, was essentially equivalent at these rates. Preplant additions of nutrients applied at 100 mg of nutrient element per liter of substrate lasted for the following numbers of days; NO3-N, 18-25 days; NH4-N, 12-20 days; K2O, 27 days; PO4-P, >35 days; and SO4-S, >35 days.