Nutrient Management Research Database

Abstract

Fall fumigation and loss of beneficial mycorrhizal associations in soil planted to onions the following spring can result in stunted onions due in part to reduced P availability, especially in high lime soils. Fall P placement (broadcast or banded below and between double onion rows planted 4 inches apart the following spring,) in fall prepared beds was evaluated at the rate of 0 or 58 lb P 2 O 5 A -1 in fumigated (Metam Sodium® at 112 lb ai A -1 ) and non-fumigated Nyssaton silt loam with over 10% calcium carbonate equivalent. Fumigation reduced onion whole plant dry matter production and bulb diameter at early bulbing and maturity in all years. Plant P content was markedly reduced at early bulbing with fumigation but recovered somewhat by maturity. Adding P, regardless of placement, compensated for the fumigation effects in at least one year but failed to restore growth and P uptake in all years. Subsurface banding P into fumigated fall beds did not always prove as effective as broadcast P incorporated during the fall bedding operation. In contrast, subsurface banding P into non-fumigated soil was consistently as effective as broadcast P

This 3 year study in an onion cropping system investigated the effects of P placement and fumigation products on yield and onion top P content.

P was applied at rates of 0 or 58 lbs P₂O₅ per acre as triple super phosphate. P was either banded or broadcast applied.

The fumigation product Vapam (c) was applied at a rate of 35 gallons per acre after bed formation.

Onions were planted as double rows centered on 22 inch beds.

Bed fumigation consistently reduced onion yields, while plant tissue P concentrations were not effected. The activity of mycorrhizal fungi (which help platns access P) was stunted by fumigation. 

Fall P addiitons increased yield only 1 out of 3 years.

Plant concentrations of molybdenum, zinc, manganese, copper, and iron were reduced with fumigation.

No differences between broadcast and band applied P was found, and banding sometimes even reduced onion dry weight. However, this could be due to poor band placement, as banding P has been strongly established as the preferred application method.