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Phosphorus – too much of a good thing

Phosphorus fallacies – too much of a good thing

By Carolyn Elgar

It is natural for rosarians to do everything they can to keep their plants producing healthy blooms as often as possible. If we thought fairy dust would do this, we would apply it with enthusiasm. We are fairly certain fairy dust will not do this, but we have been told that fertilizer will give us the flowers we love. With our rosarian optimism we conclude if one or two fertilizer applications are good, then surely three or four would be better and give us more flowers. Likewise, if phosphorus (the P in NPK) gives the plant energy to produce blooms, then fertilizer with a lot of phosphorus is what we need for our rose plants. Right?

Wrong. Our passion to give our roses everything we think they need has led garden product marketing departments to create more things we think our roses need. Or maybe it’s American enthusiasm for the biggest, latest and greatest products that’s to blame. The fact is, high phosphorus levels in fertilizer do not promote more bloom, and it’s very possible these fertilizers may be doing more harm than good.

Fertilizer nutrients

A brief review of the macronutrients included in complete fertilizers: nitrogen (N) is involved in photosynthesis as part of the chlorophyll molecule and promotes vegetative growth; phosphorus (P) supports the transfer of energy throughout the plant for root development and flowering; and potassium (K) is an important part of plant metabolism, strengthening its overall health.

The need for fertilizer was produced by increased industrialization in farming in the early 20t century that resulted in increased food production.

Field soils were high in mineral content which tied up soluble phosphorus, leading to the theory that increasing the application of this necessary nutrient would ensure that at least some of it would be available to plants. In addition, if soils were cold, availability of phosphorus was limited, reinforcing the idea of “more is better.” 

Nutrient availability

Our neighborhood soils, while impacted (literally) by development, usually retain more nutrients than heavily used agricultural fields. Whereas nitrogen moves freely and can be leeched from the soil, phosphorus is not mobile. High levels of phosphorus stay in the ground and take on different chemical forms. Adding to the complexity of the issue, the amount of nutrients in the soil is not necessarily what the plant receives – availability is the key factor in nutrient uptake. 

Availability of nutrients to the plant is affected by a number of things: drainage, soil structure, and most importantly, soil pH, a number that indicates the acidity or alkalinity of the soil. Nutrients’ chemical forms are affected by a soil’s pH measurement based on a 1-10 scale with 7 considered neutral. The lower the pH, the higher the soil acidity; alkalinity is indicated by numbers on the high end of the scale. Different nutrients are impacted in different ways by pH levels. Phosphorus fixates, or becomes immobile, in the soil quickly after application. This fixation is minimized at a pH level of 6.0-7.0. So all the extra phosphorus in fertilizer will become unavailable at pH levels above or below that range. What looks like phosphorus deficiency may actually be due to the fixation of this necessary nutrient in the ground, due to a too low or too high pH.

Another question to consider is the amount of phosphorus a rose plant can use. Analysis of plant tissues indicate that the amount of phosphorus in those tissues is around 1/6 of the amount of nitrogen and 1/4 that of potassium. In other words, of the three, phosphorus exists in the smallest amounts, leading one to wonder how much extra phosphorus the plant needs. Research linking increased phosphorus levels to increased blooming is scarce, while there is sound research that demonstrates that only small amounts of phosphorus are needed for bloom. Commercial   growers of roses for cut-flower production typically use fertilizers with a 3-1-2 NPK ratio. Roses are not the only crop where this has been explored.

There is no plant that uses more phosphorus than nitrogen and potassium.

Dangers of too much phosphorus

All of this information becomes critical for the gardener when one learns about the negative effects of too much phosphorus in the soil. Phosphorus carries a chemical charge that competes with other micronutrients for plant uptake. Excess amounts result in leaf chlorosis because iron, manganese, and zinc are not available to the plant. Leaf tissue turns yellow while veins stay green. Adding more of these deficient nutrients is useless because they already exist in good amounts in the soil. Foliar sprays of micronutrients such as iron can help because they are directly applied to the weakening plant tissue rather than absorbed by the roots. 

In addition, phosphorus inhibits the growth of mycorrhizae in the soil around the rose plant. Rose roots form a symbiotic relationship with these beneficial fungi, providing carbohydrates in exchange for phosphorus which the fungi can find beyond the plant’s roots. When the rose has more than enough phosphorus it doesn’t need the mycorrhizae and withdraws its support. Mycorrhizal growth plays a key part in healthy soil.

Limiting phosphorus application

What you have read so far should convince you of the importance of soil testing before you diagnose nutrient deficiencies. The plant may look like it is suffering a deficiency because of limited availability instead of low nutrient levels. Where this becomes ironic is when the gardener decides to dose the plant with something it already has too much of, such as phosphorus.

According to the University of South Florida IFAS Extension, phosphorus will accumulate to toxic levels when applied as often as nitrogen and potassium. Many other extension agencies concur and suggest alternating fertilizer applications between one with no phosphorus and one with low levels of NPK in either equal amounts or at a 3-1-2 ratio. Phosphorus buildup is caused by heavy use of inorganic fertilizers or the use of composts or manures that are heavy in this element. If soil test indicates that your soil has too much phosphorus, you may have to monitor anything you use for its phosphorus levels.

So, it’s time to throw out the bloom boosting fertilizers with their high phosphorus levels. Any synthetic fertilizer with a high middle number (P) is going to overload your soil with phosphorus. As summer progresses and the leaves of your rose bushes turn yellow, you may be tempted to add something to the soil, such as iron or magnesium (Su-Po-Mag). By doing this, without testing your soil, you may end up worsening the problem.

It’s hard to hold back fertilizing your roses, especially when we know that roses are heavy feeders. Nitrogen does need to be replaced to alarger degree than the other two major nutrient because it is very mobile, but too much nitrogen will result in lots of leaves and fewer blooms. Some rosarians claim that dropping nitrogen levels will increase bloom because less plant energy will be spent on foliage. However, this does not mean the other two nutrients should be increased. Using natural sources of nitrogen, such as alfalfa meal, keeps levels of N low and will not hurt the environment as much as chemical ones will; in addition, natural sources usually have other trace elements that support rose health.

The bottom line? What you put in the ground is not necessarily what the rose will take up – availability of nutrients are affected by pH and other soil factors. If you think you have problems with nutrient deficiencies, get your soil tested. Finally, don’t use anything that has a phosphorus number higher than nitrogen or potassium. Your blooms don’t benefit and your soil will be overloaded.

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