Every year kiwi farmers waste thousands of dollars on phosphate they don’t need. In my experience, some soils in New Zealand, like Taupo ash for example, have high phosphate availability. A dairy farmer I work with in Tokoroa currently has three times the optimum level of phosphate in his soil (Bray 2 test) and hasn’t applied any more for about six years. Plant tissue tests confirm phosphorus is still high in the pasture.

In that six-year period the soil calcium and magnesium base saturation has been balanced out. What we found was very interesting. Even though no phosphate was applied the level kept building and building. Provided there’s no fertility transfer off farm; he won’t need phosphate again for many years.

How phosphate truly works in the soil system is not very well understood and this has allowed a great deception (or ignorance) to take place by salesmen and consultants of all kinds, some with honest intentions. This problem exists in both the conventional and natural movement.

Most plants have a symbiotic relationship with the soil microbes. When photosynthesis takes place sugar is produced (liquid carbon). The plant feeds some of this energy source to the soil via it’s root system. The soil microbes in return provide the plant with nutrients, phosphorus being one of them.

This natural system applies to most soil nutrients but there’s something unique about the element phosphorous. From a chemistry point of view, it’s a triple negative anion. It is commonly thought that anions leach with rainfall. This is true in the case of sulphur, boron and nitrate nitrogen but not phosphorus.

When any phosphate type fertiliser is applied to soil it stays right where it’s put, for many years. (There are several important exceptions to this that I’ll cover later.) If this phosphate was applied in a water soluble form then a chemical reaction takes place and the phosphorus ties up with the positively charged cations, the main one being calcium but if the soil is high in iron or aluminum it will tie up with these elements as well.

Taupo ash is very low in iron. Clay soils with high iron (Some areas of Northland) seem to have a low phosphate availability (Bray 2 test) even though they’ve had a good fertiliser history. When soluble P is applied to the soil the plant can take up this nutrient directly via its roots and so no longer needs the microbes, which then die due to not being fed their liquid carbon.

Once this soluble P ties up, either more is needed to keep this dependency going (like a drug) or the farmer must find another way. Mycorrhizal fungi will recolonise quickly once soluble P is discontinued so it’s only a short term issue and so going ‘cold turkey’ doesn’t apply here like it does with nitrogen (another story).

Organic Reactive Phosphate Rock (RPR) works well because this bond between phosphorus and calcium has already taken place in nature. It measures high on a citric solubility test and also shows up on a Bray 2 soil test as building. The mycorrhizal fungi can go to work on this nutrient source and it doesn’t feed the plant directly, causing any disruption. But over doing it may lower the mycorrhizal fungi colonisation percentage (Soil Food Web test).

Olsen P is not the correct soil test for New Zealand soils. Dr Olsen created this test for alkaline soils in America with a pH greater than 7.5. We’ve only ever had to use this test once for an extremely over limed case. NZ soil scientists seem to get quite agitated by this fact about Olsen P.

I suppose I would too if I found out I had been using the wrong soil test my whole professional career. Fertiliser manufacturers don’t like this fact either. Take a look at their main products for sale. They probably make good money selling some fancy repackaged phosphate when Aglime and sulphur would have done the trick for far less cost to the farmer.

Call me cynical but this is the inconvenient and often awkward truth. So what are the exceptions to the basic principles above? Well cut and carry is a fertility transfer. If this transfer is off farm, then phosphate may need to be applied. In an all pasture system there’s very little phosphate leaving but with cut and carry lucerne for example, a huge amount of P can be taken off by the crop each year.

What is needed is the correct soil test in combination with plant tissue tests. The optimum range bar graph isn’t accurate on most plant tissue tests by the way. This is also part of the deception (or ignorance). The ideal percentage range for pasture is 0.25 – 0.4 for grass and 0.35 – 0.5 for legumes.

I very rarely find phosphorus deficiency in permanent pasture systems because the mycorrhizal fungi is usually functioning and lots P has been applied over the years which is often still available. There are four main limitations and/or disruptions to this soil/plant symbiotic relationship: soil cultivation, chemical spray, soluble P and mono culture cropping. This is the industrial farming model. Has anyone noticed the industrial age is over?

So if phosphate is low in the soil with a Bray 2 test and a lot has been applied over the years you have a more difficult soil type. It may be high in iron and/or aluminum but this is not always the case. The calcium and magnesium will need to be balanced out but this type of soil may need phosphate fertiliser for a few years or a biological stimulant of some kind to release that locked up nutrient.

There’s another type of test, not commonly used or understood, but which can give you a

better picture called a Total Nutrient Digest. This strong acid test will probably show you where all your money has gone over the years. That expensive phosphate applied is still sitting there ‘having a holiday’, as one of my clients puts it. So how does it end up in the water way you ask?

Another way phosphate leaves the farm is attached to the soil. In dry conditions and if the soil is bare, it will blow away in the wind or wash away during heavy rain. Soil needs to be covered and bound by plant roots. Put that unproductive hill country back into trees. Soil erosion is one of the main problems here. When driving through any hill country farming area look at all the hill sides slipping into the local river.

It was never economic to clear that steep land and was only done so because of government subsidisation. On the flip side, lots of good flat land in the central North Island is currently in pines that would be perfect for pastoral farming. Bush sickness was due to extremely low cobalt; this problem can be corrected with fertiliser.

My next article will be about over liming, why it happens in NZ and how to fix it. So if your soil tests show a pH higher than 6.4 you have probably over limed and created a magnesium and/or potassium deficiency.