PART 2 - Phosphorus, how much is enough?

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Making P available with other beneficial outcomes


Farmers that are doing total P tests are continually amazed at how much Phosphorus is in the soil.

Figures when calculated up from the total ppm of P, and using an estimated figure of 2000tons of soil per/ha in the top 150mm regularly find they can have between 5 to 7 tons of actual P per hectare in that top 150mm. Remember this is a total extraction test which completely breaks down the soil to get all the P in the soil.

Making the Phosphorus that's already in your soil available to the plants.

Without getting into any arguments about the relevance of any of the soil tests used, a figure of around 40ppm of P extracted by any of the accepted laboratory soil tests (e.g. Bray 2, Resin P, Mehlich 3, Olsen) is totally acceptable for optimum plant performance in any conditions. Remember these tests are to mimic the available P to the plant. Yet many farms are seeing that they can have very high total P levels and very low available P.

So why is this? There seems to be many reasons ranging from soil pH, anion storage capacity of the soil (ability for elements such as aluminium and iron to bind phosphate) plant species and biological activity to name but a few.

pH seems to be the most applicable to start with as this will have the largest effect on many of the other factors listed above.

There are numerous studies world wide that show for the best availability of P and actually most other plant and animal nutrients, that the soil pH should be approximately 6.5.
Yes there are arguments to state you can grow more dry matter at different pH, but the studies do not go on to look at actual farm production eg milk solids per hectare, effects on animal health, long term fertilisation costs and general profitability.

Just one of the major animal health problems, at this time of year is facial eczema.
This one farming issue causes massive production loses and also long term animal health issues, let alone the stress to the animals and the farmer. Yet recent studies are now backing what many old timers have said; that an application of lime can reduce spore counts by approximately 80%.

The reason for this is that the smaller hyphal diameter fungal spores, which tend to be toxic generally like a lower pH range and the application of lime takes the surface area of the soil and plants to a higher pH when applied. The author by his own account can attest to this on trials when farming on peat 25 years ago when spore counts went from close to 300,000 down to below 20,000 within a week of an application of lime.

Of course the secondary effect is to raise the pH of the soil which will in turn increase the availability of P and many of the other nutrients required to run a happy and healthy farm.
The long term effect can be so much better when done properly as the calcium aids the biology to break down the trash which harbors the spores. A by-product of reducing trash to humus is the release to plant availability of all the mineral plant nutrients that were unavailable in the trash. Thus many farmers are finding an application of lime can be a hell of a lot more beneficial than they ever expected.

Lookout for upcoming articles on: “The benefits of liming. It’s much more than just pH amendment”. and “Silica”. “ An effective tool to release locked up P” also “Mycotac / Mycowet. What the promoters of this anti fungal product don’t tell you”!
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