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We are told it is hard to get enough available phosphorus into our garden soil, yet did you know it is also easy to get too much phosphorus?
Phosphorus is the "P" in NPK fertilizers, and plants need a lot of it to build proteins and to transfer energy within the plant's cells. There is a delicate balance necessary between nitrogen and phosphorus for both to be effective, and too much of either poses serious problems in the garden and in the environment.
Phosphorus is not very soluble and tends to be immobile in the soil instead of working with the nitrogen. Too much nitrogen gives you lush-looking tomato plants with no fruit, and nitrogen run-off also happens where there is excess nitrogen. The soil pH affects how easily or not the phosphorus can be altered into a form plants can use. When the soil is more acidic, the phosphorus is more easily broken down by microbes in the soil to a form the roots can take up into the plant.
Plants store phosphorus in the form of phytates in the bran and seeds, where it acts as a protective mechanism until time for the seeds to sprout. Many of those seeds become foods (and not necessarily good for us), although some seeds were always saved for planting another crop before Monsanto bought control of the seed market.
The human digestive systems lack the enzyme phytase which separates the phosphorus from the phytate molecule. We need phosphorus, just as plants do, but ingested phytates in beans and grains keep us from absorbing phosphorus and many other minerals unless we break them down before ingestion. (See my post on soaking grains and beans.)
Commercially raised non-ruminate livestock like pigs, chickens, turkeys, etc. are usually fed grains like maize (corn) and legumes like soybeans. They also cannot digest the phytates. When these non-ruminates eat seeds and beans (containing phytates) the unabsorbed phytate passes through the GI tract and elevates the amount of phosphorus in the manure.
If the bagged compost you buy has manure from non-ruminants, you can bet it probably has an excess of phosphorus. (If it's home-grown, and the non-ruminant farm animals are not fed grains and soy, you probably have great compost!) Excess phosphorus can kill off the mychorizzal-forming fungi who perform the process necessary for the plant's ability to absorb micronutrients; it also leads to eutrophication such as reduction in water quality and reduction of fish populations in our creeks, streams, lakes and rivers.
On the other hand, ruminates like cattle, goats, sheep, and deer with multiple stomachs are able to digest the phytates during the fermentation process that goes on within their stomachs (which includes chewing their "cud"). Their manure would be the better bet for compost as far as phosphorus content, if it is otherwise healthy.
Alert: You should never assume your garden needs NPK, or how much, without a soil test.
If you choose to be organic for yourself, your family, and the planet, as I do, there are many choices better than commercial chemical fertilizers. Chemical fertilizers are salts, and salts sterilize the soil... sometimes little by little, but ultimately chemically fertilized soil is mostly dead soil.
If your garden needs phosphorus, you can buy bone meal, acidulated or ground bone, depending on your needs and local availability. Bone meal can contain anywhere from 20 to 30 percent phosphorus, determined by whether the bones were steamed or not.
You could also use rock phosphate if your soil needs phosphorus; however rock phosphate takes a long time to break down when soils are more alkaline than acidic. I use soft rock phosphate in the form of CalPhos if I need phosphate because the plants can immediately use it. It's also great to put on the compost pile if you need phosphorus.
Again, be safe and have a soil test done!