Too much phosphorus can be harmful to plants
Bill Sevier: Ask a Master Gardener
Saturday, February 27, 2016 1
Q: Do my roses need a lot of phosphorus to
produce more blossoms? S.R., Tulsa
A:
In a word, no. The
effect of phosphorus on a plant’s metabolism and the amount needed by all
plants is greatly misunderstood and misstated.
At least 16 mineral nutrients are needed for
healthy plant growth, and the three needed in the largest amounts are nitrogen,
phosphorus and potassium. These are the three numbers on fertilizer containers
and are often abbreviated NPK.
Nitrogen is important for leafy growth and
energy production, phosphorus is key for the storage and transfer of energy (in
plants, as well as animals), and potassium is essential for many aspects of
metabolism.
Nitrogen is water soluble and that which is not
used by plants may be leached from the soil. Phosphorus is tightly bound by
soil particles and remains in place unless used by the plant or is washed into
gutters and streams. Potassium is bound to soil less tightly than phosphorus
and potassium excesses are not usually harmful.
There is a belief by many that large amounts of
phosphorus are needed for root growth and bloom production. It is often
recommended that when planting roses or other ornamentals that a cup of bone
meal or other preparations high in phosphorus be added to the planting hole.
Well-qualified horticulturists have reviewed the
status of phosphorus as it relates to plant function and have stated there is
no scientific evidence that excessive phosphorus is needed by roses for any
reason. There is no evidence that excesses have any beneficial effect on
blooming or healthy roots of plants in general — in fact, too much appears to be
harmful.
No doubt all plants and animals
need phosphorus for normal function. In commercial agriculture, phosphorus fertilizer is needed
yearly. It is tilled into the soil but only in amounts needed for that growing
season. If it were simply added to the top of soil, it would remain in place
for years.
Soil tests from Tulsa-area homeowners’ lawns and
gardens, performed by OSU, show adequate or more often excessive amounts of
phosphorus in more than 75 percent of the tests. These excesses of phosphorus
have several undesirable effects. It has been shown to interfere with a plant’s
absorption of iron, manganese and zinc, resulting in yellowing of leaves and
poor health of the plant.
Excesses may also interfere with the growth of
beneficial fungi, called mycorrhizae. These fungi are normally present on most
plant roots and assist the plant in absorbing water and nutrients. Without
these fungi, plants must work harder than they would otherwise.
So before using bone meal or a high phosphorus
fertilizer, do a soil test. If there is no documented need, do not use
phosphorus fertilizer because it may be detrimental to your plants, and if
washed into the waterways, it will result in pollution.
For more information or to ask a question about
gardening, contact the Master Gardeners at 918-746-3701 from 9 a.m.-4 p.m.
Garden tips
§ The normal window for pre-emergent herbicide use
is mid-February to mid-March. With the unusually warm weather we have had, it
may be prudent to put out the pre-emergent earlier, rather than later, to
prevent early weed establishment related to the weather.
§ By mid-February, many cool-season vegetables like
cabbage, carrots, lettuce, peas and potatoes can be planted.
§ Spray peaches and nectarines with a fungicide to
prevent peach leaf curl before bud swell.
§ Collect and store graft-wood for grafting pecans
later this spring.
