Lettuce Growth and Phosphorus Leaching in Media Amended with Phosphorus-treated Magnetite

 

A. Graham, J.H. Gillman and C.J. Rosen,

Department of Horticultural Science, University of Minnesota, St. Paul, MN, USA 55108

 

Citation

 

Graham, A., J. H. Gillman, and C. R. Rosen.  Lettuce growth and phosphorus leaching in media amended with phosphorus treated magnetite.  Acta Horticulturae (In Press).

 

 

Keywords: slow-release, fertilizer, eutrophication, runoff

 

Abstract

 

Magnetite was examined for use as a medium on which to bind phosphorus (P) for the purpose of reducing P leachate from container grown crops.  Magnetite was acid washed, pH adjusted to 5.0, and then shaken in a 10,000 mgP·L-1 solution.  The P-magnetite was chemically characterized to determine P content (176 mgP·kg-1 magnetite).  A greenhouse experiment was performed to determine the accessibility of the P from the P-magnetite to the plant and the amount of P leachate.  'Valmaine' Romaine lettuce Lactuca sativa was sown into 5.5 inch standard containers and treated with one of the following: 250 g P-magnetite (44 mg P), 150g P-magnetite (26mg P), 50g P-magnetite (8.8mg P), 150g untreated magnetite, 3g 15-9-12 Osmocote (119 mg P), 21-5-20 constant liquid feed (5.25 mg P/fert. event), and no treatment.  For four weeks, weekly leachate samples were taken and analyzed for P content.  At the completion of four weeks, the shoots were dried and weighed.  The constant liquid feed treatment had the highest P leachate and shoot dry weights.  Osmocote and 150 g P-magnetite had similar P leachate, and the shoot dry weights for the 150 g P-magnetite treatment were significantly higher than for Osmocote.  This study demonstrated that P from amended magnetite is accessible for uptake by plants and leachate was reduced using P-magnetite compared to a constant liquid feed.

 

Tables

 

Table 1. Treatments and corresponding Phosphorus content and irrigation

 

 

Treatment

P added (mg)/ treatment

 

Fertigation

250g P-mag

44

100 mgN·L-1 25-0-25

150g P-mag

26

100 mgN·L-1 25-0-25

50g P-mag

8.8

100 mgN·L-1 25-0-25

Control

0

100 mgN·L-1 25-0-25

150g untreated magnetite

0

100 mgN·L-1 25-0-25

15-9-12 Osmocote

119

Water

Constant Liquid Feed (CLF)

5.25/fert. event

100 mgN·L-1 21-5-20

 

 

 

 

 

 

 

Figures

 

Fig. 1. Phosphorus leachate concentrations. Mean separation among treatments within each sampling date by Duncan type differences at P≤ 0.05%. 

 

 

Fig. 2. Shoot dry weight results