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Phosphorus is critical to soil health – what are the ‘P’ options available to farmers?

Phosphorus is critical to soil health – what are the ‘P’ options available to farmers?
Outgro Fert - Stacked Bags of Phosporus
One of a few products that does not go through the Tow and Fert is Super Phosphate. There are alternatives for Phosphate application. Below, 2 companies; PFP Fertiliser and Fertilizer New Zealand explain the importance of Phosphate and the different products they offer that do go through the Tow and Fert Machines.

One of a few products that does not go through the Tow and Fert is Super Phosphate. There are alternatives for Phosphate application. Below, 2 companies; PFP Fertiliser and Fertilizer New Zealand explain the importance of Phosphate and the different products they offer that do go through the Tow and Fert Machines.

Supplied by PFP Fertiliser:

Testing for Phosphorus will tell you what maintenance or capital products to use. It is common knowledge that Phosphorus is one of the key 16 Nutrients required for optimal pasture growth and its levels present in the soil are commonly the first thing looked at on any soil test. Phosphorus levels are often a key driver in determining soil fertility so its importance cannot be overlooked.  

Traditionally, it is the Olsen P test that is used to gauge the soil reserves of phosphorus and Super Phosphate has been the go-to product for both maintenance and capital applications. Unfortunately, Super Phosphate is one of the few fertiliser products that will not go through a Tow and Fert. Fortunately, there are alternatives to Super Phosphate. In recent times there has been more interest in these alternatives as both the potential losses to the environment and the impact of acidic fertiliser applications on soil biology and pH has become more openly discussed. Other test methods are also now more commonplace, each with their own pros and cons around seasonal variations and accuracy. Hill’s Laboratories supply some good technical notes on this. In our opinion, at Outgro, alongside the Olsen P test, the Resin P, Total P and ASC (anion storage capacity) test should also be included in determining Phosphorus requirements as well as your operational situation and overall soil type.  

Once the requirements have been determined then comes the decision as to what product to use for either maintenance of existing levels or a capital dressing to lift the current levels.

For us at Outgro the ultimate P fertiliser has:  

  1. Low water solubility (minimise environmental losses),  
  2. High citric solubility (highly plant available),  
  3. Non-acidic (neutral pH),  
  4. Low in contaminants (cadmium, arsenic, fluorine, lead),  
  5. Is sustainable both in terms of results and in an economic sense.  

The form of Phosphorus that ticks most, and in some cases all the boxes above is Di-Calcium Phosphate (DCP). DCP is a fine particle option and is suitable for Tow and Fert applications. DCP provides good compatibility with other products enabling a comprehensive ‘one pass’ mix approach that will cover both maintenance and/or capital requirements.  

The other Phosphorus option that is worth mentioning is MAP or Mono Ammonium Phosphate which is a highly soluble product applied in a liquid form capable of being absorbed through the plant leaf rather than via the soil/root system. While this is not suitable for addressing soil P reserves, it works well in combination with other ingredients to enhance foliar uptake.


Supplied by: Fertilizer NZ

Phosphate is an important input into any farming operation. There must be sufficient phosphate to feed the plant, BUT it also must be in balance with all the other elements that make up the total food that the plant needs.  

Here is the major issue. How much is too much and what is too little? The way we have farmed for the past fifty years is about tipping on as much as we can afford and expecting all to be well. In some cases, it has led to an imbalance in the soil which instead of increasing growth has slowed it down.  

Consider this; a ton of maize contains no more than 10 kgs of phosphate, and in dairy pasture the percentage of phosphate is a lot less than that. So maybe the discussion should be about the ‘how-to’ of making the phosphate that we apply become more available.  

Some companies and individuals using scientific data have shown that they can grow the same amount of pasture or crops using less phosphate. If this is the case, then we should be working with them to advance farming in New Zealand. Given that a plant can only take up a small percentage of what is applied, the question to ask the bulk suppliers is what happens to the rest of the phosphate.  

The average soil test generally only shows individual levels but does not complete the picture of how these levels correlate to all the other elements.  

Soils have a range of structures and capabilities. Different types of soils hold differing amounts of water, and an example of this is that sand holds very little water but peat would hold a lot more. The same applies to phosphate. Different soils hold more phosphate than others. Using just one type of test to measure any element probably is not the best practice.  

When it is time to plan for a phosphate application, due consideration should be given to which type of phosphate would be best for your own farming operation and how this would be spread. For example, some phosphates are acid based and water soluble, while others are based on calcium. They may be a solid product, or could be in liquid form, and in various situations either would work. In several countries, liquid is a preferred option as it goes on cleanly with very little drift. It is applied efficiently and most of it is taken up by the plant, which is the reason why seemingly so little can do so much.  

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