PART 1 - Phosphorus, how much is enough?

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Phosphorus, how much is enough?


Farmers are told they need to keep adding P to their soils to replace what is being taken out from farming practices. (The concept of P maintenance). Yet on looking a little deeper the first real issue is to understand how much is being taken from your farms each year.

Simple maths shows New Zealand farmers are commonly applying up to ten times more phosphorus than the amount that is leaving the farm each year.

Figures released from Dr Eric Hillerton of Dairy NZ point to the fact that for every 100kg of milk solids that leave the farm, only .6 kg of P is in the milk. It doesn’t take much to work out that for a typical dairy farm with no inputs, producing 1000kg/ha milk solids per Ha that only 6kg of P per Ha is actually leaving the farm each year.

To take into account supplements bought onto the farm is another issue that has to be looked at, as Maize silage brings approx 3 to 4kg of P per ton of dry matter and Palm kernel around 7kg. On dry stock operations it has been calculated that approximately 3kg P/ha actually leaves the farm in meat or wool.

These figures certainly should raise some eyebrows as applications of sixty units of P per Ha per year are standard fare amongst high performing dairy farmers and 25 units on drystock properties. The nutrient budgeting software “Overseer 2008” states, in the “fertiliser advice” pages, as follows "Rule of thumb to estimate nutrient maintenance requirements for dairy farms on ash and pumice soils: “Apply 0.6 to 0.8 kg/ha 20% potassic superphosphate or equivalent for every 1 kg/ha milksolids produced”. Using this formula a farmer would apply approximately 63 kgs of P per Ha to replace 6 kgs of P removed.

So what does happen to all tge Phosphorus supplied in fertiliser applications?

On investigation there seems an absolute myriad of reasons where the P goes to, how it is stored and reasons that it doesn’t become available, or show up in your soil tests.

Are your soil tests showing a clear representation?

The starting point on the investigation seems to be the soil tests of which there are many. All of them state the amount of P in parts per million (ppm). These soil tests are built to mimic the P which should be available to the plant in the normal growing season.

The usual P test done here in NZ is the Olsen. Baking soda (sodium bicarbonate) is the actual extractant. The resulting Olsen P test states how many ppm of P is theoretically plant available using this method. Even though this test was actually designed for soils over pH 7 and really doesn’t seem to be the best test for our acidic soil conditions, we will continue and return to investigate this at another time.

Even with large applications of P the corresponding responses often do not seem to show in subsequent soil tests, so where did the P go and why?

P does not travel very far at all due to its ability to bond with major constituents of the soil such as Aluminium and Iron. This is where the majority of product that is applied in soluble acidic form ends up. These forms will not show up in your soil tests.
If P is applied as rock phosphate the Olsen P test is unable to access that particular form of P either due to the extraction method being inappropriate.

The only true way to find out how much P you have in your soil is to do a total P extraction. This is where the soil is totally broken down using strong acids and all the P can be measured. This will give you a much better idea of how much P you have in reserve and matched against the soil tests you can work out if applications of P are going to help or whether you need to look deeper on how to get what you already have into an available form.

Check out part 2 - making P available with other beneficial outcomes. Click here
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