A Brief Tale of Phosphorus

A brief post about Phosphorus continues our series of Plant Nutrient posts. Yesterday, we spoke about Nitrogen as the primary macronutrient needed by plants. Today we pick up the theme with the second most important nutrient, Phosphorus.

Phosphorus is another element on the periodic table of chemicals. It is second only to Nitrogen as a very important nutrient needed by plants. In fact, all organisms need it as it is the base structure of cell membranes, and the energy carrying molecule in living things. It is usually found in commercial fertilizer in the form of P2O5. naturally occurring rocks such as apatite contain large amounts of phosphorus and are the source, in soil of the element in nature.

The thing to remember about Phosphorus is that it is never water-soluble. As opposed to Nitrogen which is always leaching away with water, once Phosphorus is in the soil, it stays where it is put and binds tightly to soil particles. It is therefore commonly abundant to most soils and additional fertilizers containing it are seldom needed. A soil test of your garden soil will tell you whether the native soil is already well supplied with Phosphorus and if it is, you will not need to worry about the concentration in your fertilizer.

If you add Phosphorus that is not needed, it can bind to silt and run off into natural waterways causing troublesome extraneous levels of fertility in lakes and streams. Foaming along the edges of waterways is usually from Phosphorus runoff. This can cause an overgrowth of algae and seaweed that cause an imbalance in the body of water leading to depletion of Oxygen for fish, etc. Extra phosphorus is not only a source of pollution but is an unnecessary expense. In limestone based soil, which is most of the cultivable United States, you usually do not need to use anymore than 5% Phosphorus in your fertilizer. If your soil tests low in Phosphorus, use of up to 12% may be warranted but seldom more than that. Remember, pouring fertilizer on the top of the soil leaves a heavy layer of insoluble Phosphorus at the very top and not much penetration down into the rest of the root zone. A good way to apply it is when you are turning over the soil with a rototiller or plow and can incorporate Phosphorus deep into the field.

Phosphorus is the second number in the three number sequence on the fertilizer bag. It is an expensive component and makes up the majority of the cost of production of a fertilizer. The most expensive sources of Phosphorus are water-soluble forms specially created to be watered in. An example of this is Rapid Gro or Miracle Gro where the product is applied to individual plants by a watering can. These are only economical when used in very small batches applied to specialty crops such as vegetable gardens of flowers. They have been advertised over the years as extra good for flowers and fruit. Although plants do use a little more to make these structures, not a whole lot more are used versus a plant you are growing for foliage such as grass. The main response you see from applying water-soluble fertilizers to flowers and such is from the Nitrogen, not the Phosphorus.

While we are on growth responses from fertilizers, I’ll throw in another old wives’ tale. There is one stating that if your flowers do not flower and grow only green leaves, your fertilizer is out of balance and you have given too much Nitrogen and not enough Phosphorus. Usually the reason that your flowers do not flower is lack of sunlight. Both Nitrogen and Phosphorus are need for flowering and seldom is Phosphorus deficiency to blame for lack of flowers. At the most, the flowers may be stunted and smallish, but if it is going to have flowers, it will. More tomorrow on the third macronutrient, Potassium.


fertilizer? Let’s get some basics here

An alert reader heard about a friend who was trying to feed her plants necessary nutrients using the sports drink Propel. Propel had a small amount of potassium in it and I suppose, the person had heard that potassium was good for plants. I have to admire the creativeness while discouraging the idea for being impractical.

This has led to some ideas about blog posts regarding fertilizers. I have touched on some aspects of plant food in other posts, but this Propel question must lead to a better understanding on this blog. This may take a few days so hold on and let’s get started with Nitrogen before we get all the way to Propel and her Potassium.

Let’s do a little review of plant food. The first and most important plant nutrient is water. I think we can all agree that we can control the growth of plants and even their life by controlling water. This concept is so basic it is often overlooked as a crucial tool for growth and development. I say this not as an aside, but as a concept in plant nutrition before we get to the stuff you can buy in a bag. Remember, the water is the most important. Now here we go on the stuff in the bag.

Mostly what we talk about when we say “fertilizers” are the big three macronutrients, Nitrogen, Phosphorus and Potassium. These three, in that order, are listed in a standardised format on all plant food labels. There is a good reason they are in that order; they are listed in order of their importance to the plant.

Let’s look at some characteristics in the soil of these three elements. I say elements because they are indeed elements on the periodic table of chemistry. Nitrogen (N) is used in the largest amount by the plant, then Phosphorus (Ph), then Potassium (K).

Nitrogen is the cheapest nutrient in most fertilizers, and will do your plants the most good. Nitrogen is water-soluble in soil and must be constantly resupplied to give your plants the food they need. Since it is water-soluble, rain will wash it away down into the soil and your plants will be sad until you give them more. Natural soil has various amounts of it and it comes from decomposing humus on the forest floor. As animals and plants rot away, the N is released and set free for plants to eat up. Also, rainwater contains small amounts that constantly feed earth’s plant cover a little bit. In a few plants, (mostly in the pea family) Nitrogen fixing bacteria take atmospheric N and convert it into usable N in the roots.

Commercially available animal manures or chemical fertilizers will give your plants the most improvement due to their Nitrogen content. Differences in the rate of release of N in different formulations can impact the growth of your plants. Let’s take the spectrum of N sources and look at their release rates. The most classic chemical source of N is Urea. Urea is named for urine (duh) and in the dry pellet form available in bags. It is relatively high in N (about 33% plus or minus a bit) and is immediately soluble in water and thus available to the plant. If you use too much, you will form a water solution with specific gravity higher than the juices inside plant cells. This can pull water out of the plant cells and burn the plant. You want to use this form of N sparingly and dilute with a lot of water. All the N from urea will go into the water and feed your plant and begin washing away. This gives a big quick burst of N and then nothing as the N washes down deep in the soil. If I were using this form of N for my plants, I would use it sparingly but frequently.

Nitrogen also comes in chemical fertilizers in forms called “WIN” which stands for water insoluble Nitrogen. WIN is a very good thing in fertilizers because it time-releases the N and constantly feeds the plants a little at a time. This is very good because it does not run off and cause pollution and your plants can happily feed on just a little N all the time which they really like. It is created by coating the little round balls of fertilizer with a slow dissolving coating. Sulphur coated urea is a common version. The fertilizer bag will say what percentage of WIN is in the mix. The more the better. The fertilizer manufacturers want as little as possible because it is an expensive type of N. Usually a fertilizer mix will have a little uncoated urea for a quick green up and then some WIN to keep feeding all season long. This is my favorite type. No need to keep applying it since the little prills are timed-release. This gives Infamous more time for blogging.

Another version of timed release is the use of natural animal manures. They contain Nitrogen bound up in the organic material of the manure. The manure has to be worked on by microbes in the soil before the N is released so by necessity, it comes out slowly, kind of “timed release” in a natural way. The only drawback is that manures are usually only about 2-3% N whereas you can get up to 33% in chemical urea. You need a whole lot of animal manure to equal a chemical product. It is also pretty expensive comparatively. On the plus side, you will probably never burn anything, so that is something. Any of you gardeners out here who are overly enthusiastic and tend towards a heavy hand, should probably use manure.

Well, that is all today about nitrogen. Tomorrow will be Phosphorus and then Potassium and finally Propel water. I guess wacky ideas about gardening can lead to good ideas for blogs.

New Category: Old Wives’ Tales

An alert reader had two observations from the world around him. He was in a new area and noticed that the horticulturists painted the trunks of trees white and they filled the cavities of tree trunks with cement. Both of these, I think, are in the category of old wives’ tales. Both are based on partial truths and a lot of tradition. So with apologies to the old wives, let’s examine these practices.

First, the white painting on tree trunks. It is my belief that this got started way back in Europe with lime sulfur. Lime sulfur is a slurry of stuff that coats insect eggs and kills them pretty well. If you have a tree in the dormant condition, you spray this noxious smelling gunk on them to kill scale and insect eggs. It works partially so it has become wildly popular again for non-chemical pesticide spraying. I would use it myself for scale as it dries the shell of the insect out without hurting the tree. The lime makes the tree kind of white-ish. The lighter color also has the effect of reflecting winter sun. Sometimes in the winter, the sun comes out on a cold day and shines warmly on the trunks of frozen trees. It is possible that the trunk warms up, and thaws. Later that night when the warm sun sets, the trunks very rapidly cool down. Since the outer layers of the trunk freeze faster than the center, the trunk can contract on the outside and crack, leaving a nasty open split. Painting the trunk white with lime reflects the sun, avoids the warming and the subsequent cracking. The weather conditions that cause this are very particular, and in practical situations, very rare. But rare or not, this CAN happen and lime can help with this. The lime spray has been used by orchardists for hundreds of years for weak insect control and frost crack prevention. In my opinion, lime is not very effective for either purpose but I do admit that it can help a little bit. In any case, lime sulfur would not generally be harmful so at worst it is a waste of your time, and at best it may be beneficial. Is it worth my time or trouble? No. Is it steeped in folklore of miraculous insect prevention and frost protection? Yes. You decide on whether to paint your trunks or not.

Now that is the original recipe for the whitewash for tree trunks. Without knowing exactly why, people have it in their heads that trunks need to be painted white. So then they use latex paint. Lime sulfur may be a harmless horticultural practice but latex paint is not. Latex paint can be suffocating to the tree cells underneath it. It can cause the cells to necrose and die. This can cause very bad dead areas in the protective “skin” on the tree and resulting rotten areas. So, tree trunks painted white? Not necessary and usually done by people who have a lot of time on their hands with nothing better to do.

The next old wives’ tale is cement in the cavities. First of all, if your tree trunk is so rotten that it has a big cavity that a squirrel or bear might live in and you want to fill it with cement, you need to get a new tree. Filling trees is an old-time tree technique that is avoided nowadays because proper trimming and tree removal when trees are unsafe are better strategies. Can you imagine the lawsuits that would be filed in this country if you left a street tree standing that had a big hole in it and then a storm knocked it down on someone’s house or car? No, a hollow tree is a weak tree and a tree filled with cement may be sturdier but it is brittle. Wind can knock over those kinds of trees in a big storm. I would not do it. Old wives’ tale #2.

Please send me your other dubious horticultural practices; I like this new category!

P.S. Old wives’ tale #3 was just discussed a couple of days ago. Ants are NOT necessary for peony buds to open. They just like eating the sugar.

Verticillium Wilt and the Redbud

See the dark rings in the heartwood?

Today we cut down an old Redbud tree that leafed out poorly this spring. The tree has been suffering for years with a pathogen called Verticillium wilt. The crosscut of the tree shows the bark around the edge, then the white sapwood, then the darker heartwood. Heartwood consists of sapwood cells that have died which is natural. Within the sapwood there are rings that are occasionally much darker than the dark heartwood. These near black streaks in the vascular tissue of the tree are where the pathogen Verticillium tried to plug up the tree. The tree grew around them many times in its life. Old age and pathogens plus a nice canker rotting the trunk overcame its defenses.

Stump of a Redbud Tree

Here is a picture of how small the leaves were this spring. The flowers bloomed but the leaves were not getting enough water through the vascular system to push out the leaf buds and develop normal leaves.

Small leaves are a symptom of vascular disease

Verticillium wilt is a fungal disease carried in the soil. Many many species of plants are susceptible to it. For a complete list, check a pathology book or ask in the comments. There are about a hundred susceptible species. When the fungus in the soil comes into contact with feeder roots, the fungus enters. The fungus grows and proliferates eventually making reproductive spores. Spores are carried around the whole plant by the water going up the stem. As the fungus spreads systemically, its main function is to block the vascular tissue. It can also create trunk cankers. Here is a picture of trunk damage.

This trunk was very rotten from a canker

Just an aside about the spring flowers it had this year. The flowers were not very affected. The buds were already formed last summer. The buds just needed a little water to flow into them this spring to telescope out the cells and form flowers. Just because your tree flowers, it does not mean that it is healthy. In fact, many times, the biggest burst of flowers come just before a tree dies. The tree wants to make as many babies (seeds) as it can to continue its species in the world. SO flowering heavily may be a sign that your tree is checking out.

There is no remedy for Verticillium wilt. Try to buy healthy looking nursery stock from cultivated fields. Once a tree has died in the soil from Verticillium
wilt, the soil will be contaminated with the fungus. Be sure to replant a resistant species or plant far away from that spot. The spores will last in that soil longer than your lifetime. I will be replanting an evergreen of some type.