June 23, 2012

1 - Phosphorus may interact with iron in the soil, but it is good for your garden.

Yard and Garden June 23, 2012


I live on an old pecan farm (16 acres)and just found 2 old 25 lb bags of triple superphosphate. What should I do with it? Should I use it in a vegetable garden, an iris garden, or under fruit trees??? Or, should I dispose of it? I researched a bit and I learned that phosphorus ties up iron in soil so I am nervous about using it!

Amy P.

From NMSU University-Wide Extension


Phosphorus is necessary for good flowering and fruit production in plants. Triple superphosphate can supply the phosphorus that plants need. It is not considered an organic source of phosphorus, but does provide the same nutrient that organic sources of phosphorus provide. In this form, it is often more soluble and readily available to the plants. In excess it can indeed react with iron in the soil, making the iron less available to the plants. However, the calcium that is common in most New Mexico soils also reduces availability of iron and phosphorus (as well as other minerals) to plants. The Triple superphosphate will be useful in your flower, fruit, and vegetable gardens (vegetables that produce fruit such as tomatoes, squash, chiles, etc., not leafy vegetables such as lettuce). You have a lot, so do not apply the phosphorus in excess, use the supply you have over several years, or widely spread over a large area.

It would be best if you would have your soil tested to see if phosphorus is lacking in your soil and how much to add in each part of your garden. Your local NMSU Extension Service County Agent can help you with regard to soil testing and understanding the soil test results.

As I mentioned above, the naturally occurring calcium in our soils will react with the phosphorus making it unavailable to the plants. Organic matter (compost, composted manure, organic mulch) added to the garden soil, sulfur, or other acidifying materials will help delay the loss of phosphorus (iron and other mineral) availability to the plants. Some plants, including some native plants, are able to change the chemical environment of their root zones to cause the release of bound minerals and will not need the additional acidifiers, but most garden flowers, fruits, and vegetables will benefit from additional organic matter.

For more gardening information, visit the NMSU Extension publications Web site at http://aces.nmsu.edu/pubs/_h, or to read past articles of Yard and Garden go to http://aces.nmsu.edu/pubs/periodicals.html

Send your gardening questions to:

Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd.
SW, Los Lunas, NM 87031.

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist emeritus with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.