Phosphorus (P) - Understanding Minerals in your Soil

Posted by Neil Miles on

Phosphorus (P) – The energiser

Key Roles

Phosphorus is essential for efficient photosynthesis. It is the "energy mineral" required throughout the process of glucose production. This begins with adenosine-tri-phosphate (ATP), often called "the battery of life". However, the critical P link continues with a suite of phosphate-based enzymes that drive the sugar factories in the leaf (chloroplasts).

Phosphorus is also essential to plant immunity, as many of the processes surrounding this natural protection system are phosphate-based.

This mineral drives all stages of the crop cycle, from early root growth to the vegetative phase, and it is in even more demand for fruit and seed filling.

Key Characteristics

Phosphorus-based fertilisers are among the most expensive and unstable of all mineral inputs. This relates to the very low solubility of phosphate complexes. When phosphate anions react with cations in the soil, such as calcium, iron, aluminium and manganese, an insoluble compound is formed and you have effectively lost your fertiliser investment. It is estimated that 73% of applied phosphate is destined to lock up in this manner. It has now become part of the huge bank of insoluble phosphorus found in most farmed soils. The good news is that you can learn the tricks to reclaim this frozen reserve and you can also stabilise your P, to avoid these lockups.

Ideal Levels

Your soil should ideally contain between 50 – 70 ppm of this key mineral. It is common to see P deficiencies in broadacre soils while, conversely, gross excesses are almost the norm in intensive horticulture and home gardens. When P has been oversupplied, you can expect to see plant shortages of zinc, iron, potassium and, even calcium uptake is impacted.

Humic acid is the best tool to stabilise your soluble phosphate inputs to prevent lockups. The humic acid and water soluble phosphate combine to create a phosphate humate that remains stable and plant available throughout the season. Soluble humate granules should be combined at rates of 5% with DAP/MAP (i.e., 5 kg of soluble humate granules with every 100 kg of DAP/MAP per hectare – up to 10 kg/ha banded or 20 kg/ha broadcast).

Key Considerations

Some of the strategies to reclaim your locked-up phosphate include utilisation of compost, legume-based cover crops, fulvic acid, the introduction of mycorrhizal fungi and the use of stubble digestion programs. Cellulose-digesting fungi that can digest crop residues release acid exudates that increase the availability of locked-up P, so you can effectively kill two birds with one stone. You can turn residues into invaluable humus, while recycling your frozen P reserves.

Magnesium can stimulate the uptake of phosphorus, while excess potassium can inhibit uptake.

Slow-release phosphate fertilisers, like Nutri-Tech Solution's Soft Rock, are much more effective in crops like pastures and orchard crops that do not require fast-food phosphate. They will release their phosphate, calcium, silica and trace minerals over several years and provide a much better investment. Always remember that the water soluble alternative will only deliver 27% of their P lode, before locking up. However, there is a role for DAP/MAP in short term row crops, where instant phosphate is required. Just ensure that you stabilise this rapid-release form with humates.

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