Saturday, February 27, 2016 By: Ask A Master Gardener

Excess Phosphorus may be Harmful to Plants

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.


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