Phosphate Fertilisers: Organic or Inorganic?
Phosphorus is essential to all life. In fact, you may think of phosphorus as the “energy element” because phosphate bonds hold and release the energy that drives all life on the planet. Phosphorus is one of the big three in plant nutrition (NPK), and lack of available phosphorus is often the limiting factor for plant growth. If a plant doesn’t receive adequate phosphorus, its energy needs can’t be fully met. Its new growth will be stunted, both at the roots and at the shoots. And as the phosphorus deficiency worsens, the plant will eventually shut down completely and die.
Phosphorus is not very mobile in the soil. It is easily adsorbed or locked up with other minerals, making it unavailable to the plant. Very little plant-available phosphorus is actually dissolved in the soil solution. In fact, adsorbed phosphates on soil particles are often hundreds to thousands of times greater than phosphates in the soil solution! As the plant takes up the phosphates from the soil solution, the adsorbed phosphates slowly take their place, but sometimes not fast enough to meet the growing needs of the plants. So plants must have a steady supply of phosphorus, seed to harvest. Phosphates can be particularly limiting during early growth, when plants need extra phosphorus for root development, or during fruiting and flowering, when the plant needs to store up all of the energy it can!
Supplemental phosphorus can be provided to the plant in either organic or inorganic fertilizers. Some of the best sources of organic phosphate fertilizers include bone meal, high-phosphorus bat guano, colloidal rock phosphate (soft rock phosphate), and crab shell meal. These organic fertilizers are a good source of phosphorus, but they must be broken down by microorganisms in the soil before they become available to the plant. Mycorrhizal fungi are particularly good at feeding phosphorus to the plant. The fungi colonize the root cells and send out a web of filaments into the surrounding soil. The filaments (called hyphae) absorb phosphorus from the surrounding soil and transport it to the plant roots. In return, the plant feeds the fungi with sugars it produces through photosynthesis. It’s usually a good trade. In fact, if there is a phosphorus deficiency in the soil, plants will actually send out signal molecules to attract mycorrhizal fungi, even though the plant may need to spend as much as 20-30% of the sugars they make to feed the fungi!
Inorganic phosphate fertiliser, on the other hand, is immediately available to the plant. In fact, if a plant has plenty of water-soluble phosphorus in the root zone, the plant will actually exude enzymes to repel mycorrhizal fungi! After all, why should the plant feed the fungi 30% of the sugars it makes and get almost nothing in return? It’s a bad trade. So when there are adequate phosphates available to the plant, the plant will treat the mycorrhizal fungi as a pathogen instead of a partner, hoarding most of its carbohydrates for its own needs. That’s why in hydroponics there are fewer root exudates than in soil. In hydroponics, all of the minerals, including phosphorus, are provided to the plant in their ionic, water-soluble form. The plant immediately takes up the phosphates that it needs so it can grow and mature much faster. Mono-ammonium phosphate is one of the most water-soluble forms of phosphate fertilizer available, and it can be applied at the root zone or as a foliar spray.
Harley N. Smith
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