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The Mystery of Phosphorus
by Jack Shoultz

Originally published in American Rose
March, 2008

by Jack Shoultz

Of the three major nutrients, Nitrogen (N), Phosphorus (P), and Potassium (K), phosphorus is the most mysterious. To understand phosphorus and what it does in the garden, look at how phosphorus is treated in the soil. Phosphorus is essential for strong root growth, for the production of flower and seeds and for greater resistance to diseases. It is also necessary to the production of plant sugars and is the mechanism by which sugars move within the plant. Symptoms of phosphorus deficiency are: slow growth, dwarfing, purplish leaves, stems and branches, poor flower productions. Excessive phosphorus can cause problems, especially in bodies of water where it is introduced, usually through runoff. Among these are eutrophication (an increase in chemical fertilizer which causes excessive growth, depleting oxygen and reducing populations)and algal blooms (an increase in algae growth). Although, the percent of P to N is needed in smaller quantities, it is vitally important to good root growth. It will affect the plant growth if there is a deficiency. Phosphorus can become locked up in the soil and is the most difficult to be released. Most plants cannot extract nutrients from unbalanced soils. If the pH is to high or to low, phosphorus becomes unavailable. This is called phosphorus fixation and is especially prevalent in acid soils.

Phosphorus does not travel far through the soil. If the soil is dense or compact the roots of the roses can not travel to the areas or pockets of phosphorus. Helping your soil to become less dense will allow air, nutrients and moisture in the garden. With improved soil structure the roots can access the pockets or areas of phosphorous by growing to them. By contrast, a porous soil does not have sufficient texture to give the roots anything to grow to and does not retain moisture or nutrients. Much of the nutrients in the soil are the result of decaying organic matter. Working organic matter into a new bed will provide much of the nutrients for the beginning season. Compost, besides containing nutrients, also has microbes and biomass which is so important in building up your soil. Including phosphorus at the time you prepare the bed or when you plant a plant will ensure that there is sufficient phosphorus.

Bone Meal has the most phosphorous of the organic fertilizers. Other products with smaller amounts, but still contain phosphorus are fish meal, seaweed and animal manures. Sedentary rock is where the majority of phosphorus is found and this is in the form of rock phosphate. Phosphates in the form of phosphate salt can be released in trace amounts to a film surrounding nutrient laden soil particles which can then be accessed by the root hairs as they make their way through the soil and reach the phosphorous in these new zones. This is another reason that the soil you plant in needs to be full of organic materials. Rock phosphate, if finely ground, is soluble in water and lasts for a few years in the soil. Bone meal takes longer to break down to a usable form.

To help break down phosphate rock to a usable form for plants there are limited bacteria and fungi that can accomplish this process. The larger the number of these organisms along with proper conditions in the soil, allows the best production of soluble phosphate. In several scientific studies, it was found that these bacteria (which seem to be the most effective in breaking down phosphate) increased quicker and with higher concentration when compost was added to the mix.

Phosphorus has been tested and shown effective in treating and inhibiting plant diseases, such as downy mildew. Foliar feeding can be used to enhance the buildup of nutritious value in plants so spraying directly on leaves can increase not only the phosphorus content, but may increase the ability of the plant to resist disease. Check the P percent on the label to see what amount of phosphorus is included. Many products which include ingredients from the sea include quantities of phosphorus. Phosphorus is part of the runoff and settles on the ocean floor. It is then taken up by plants and sea life in the ocean and in turn harvested and processed into fertilizers. There are those that say Superphosphate is the same as using organic phosphorus. If all you look at is results in growth, this may be true, but if you look at how it is processed and the reaction in your soil, you will realize this is not so. Rock phosphate is treated with sulfuric acid to produce a compound known as superphosphate Ca3(PO4)2 + 2H2SO4 ? Ca(H2PO4)2 + 2CaSO4. A result of using superphosphate is the creation of an imbalance in the microbial population and a buildup of salts. Many trace elements also become inactive. Within the soil are a group of fungi that works with the roots of the plants known as mychorrizae. With the use of superphosphate, mychorrizae are inhibited, therefore reducing the ability of the roots to expand beyond their general area. The mychorrizae would be essential in delivering micronutrients and moisture. Without it, the makeup of the soil does not allow for a continued growth that would allow for the full potential. In fact, it could give the opposite effect, a dense compacted soil with slow activity.

The ideal process for including organic phosphorus in your soil would be as you prepare the bed. Incorporating organic matter along with the phosphorus would invigorate the planting medium and give your plants the best choice. If you are adding plants to an existing bed, add some organic matter in the bottom of the hole along with a supply of whatever type of phosphorus you wish. There will need to be a continuing of renewal by applying organic fertilizers and allowing them to be worked into a quality soil that will continue to be improved by additions of organic matter promoting increased microbial activity in the soil. With the increase of mychorrizae pushing its extended roots through the soil it will reach the phosphorus and continue bringing it to the plants from where ever it is.

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