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Organic phosphorus and plant nutrition

The fraction of organic phosphorus that is available for plants is very small and not enough for the plant to grow properly. Compared to nitrogen (N), carbon (C) and sulphur (S) which is part derived from the atmosphere. All organic P comes from available inorganic P in the soil. So it seems that added organic material to the soil, with all the benefits coming from it, organic material has a way of binding P to the unavailable organic compounds. That sounds complicated, but organic material binds freely available inorganic p so plants cannot use it.

The organic P is very important to plant nutrition as it becomes available through mineralization. The rate at which unavailable P becomes free through mineralization depends on various factors such as temperature, wet dry periods, bacterial activity in the soil, soil aeration and structure etc. The table below shows how variable P is in the soil

SoilP mineralized @ 25°CP Mineralized @ 35°C
A ploughed3 ug/kg14ug/kg
A Natural37 ug/kg69 ug/kg
B ploughed2 ug/kg13 ug/kg
B Natural6 ug/kg25 ug/kg
Effect on soil type and climate on phosphor mineralization

It is interesting to note that 50% of the N in fertilizers are used within the season but only 20% of P fertilizer. The rest of the P is absorbed into the organic form, which is not readily available to the plant, or stays in soluble form which the plant can use at a later stage.

Phosphorus is more readily available in warmer climate natural soils as mineralisation plays a significant role in breaking down the organic compounds. Interestingly P is mineralised faster in added compost than in the organic material of the soil. The Russians did try to use a bacteria called Bacillus megatherium on seed to increase initial P availability. They claim 10% yield increase but these results could not be repeated anywhere in the world. Maybe they had good strains or severe P deficient soils.

Unfortunately P soil analysis cannot provide a clear picture of how much P is available for plants as these soil analysis methods also measure unavailable organic P. At least a soil analysis will give a general indication of the P content. Always add P according to how much the plant uses or extracts from the soil. You don’t want to deplete P from your soil.

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