Figuring Phosphate Losses from Soil Testing | Eurofins
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Figuring Phosphate Losses from Soils

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Through soil testing, we’re able to see that losses of phosphate from New Zealand soils can be much higher than expected, and are related to soil type, climate, and the form of phosphate (P) applied.

The amount of phosphate found in drainage water is strongly related to that found in soil, particularly soil solution P. According to many published studies, the amount of P found in soil solution is strongly related to the Anion Storage Capacity (ASC) of the soil, also known as phosphate retention (PR).

The ASC test gives an indication of the soil's ability to hold onto anions, particularly P and sulphur (S).

The major soil property determining the ASC is the amount of iron and/or aluminium oxides in the soil. A low ASC soil will have low P binding sites, conversely high ASC soils will have many binding sites. NZ soils generally have a range of ASC between 0 – 96%. Soils with ASC < 40% are more likely to be vulnerable to P loss.

Rainfall and drainage

The quantity of nutrients lost via subsurface flow is related to the amount of rainfall and drainage, according to Gordon Rajendram, Eurofins Agri-Testing soil Scientist. He did these studies when he worked for AgResearch in the 1990s and they have been included in the Overseer nutrient model.

The rainfall for most regions of NZ ranges from about 500 to 3000 mm per year, but the South Island’s West Coast can have as much as 5000 mm per year, if not more.

Studies done on low ASC soils in Northland have found on average 35 – 65% of applied P can be lost via drainage.

Recently, an AgResearch study on Waikato peat soils showed up to 46kg P/ha/yr with an ASC of 19%. The loss of 46kg of P is equivalent to what farmers would normally apply to their pastures in any one year (40 -70kg/ P/ha/year). As a consequence, these types of losses have a large financial cost (~$150/ha) to the farmer and have implications for the environment.

How to deal with low ASC soils

A highly soluble P fertiliser, even though it may be cheaper per kilo basis, is not the answer for lower Anion Storage Capacity soils. These low ASC soils can be found in all regions of NZ.

You cannot change the soil type or the climate, but you can change the amount and form of P applied to areas which are vulnerable, according to Rajendram. The best way to reduce P & S loss is to apply less water-soluble P & S fertiliser, which slowly releases P & S or apply a small amount more often as the pasture or plant requires it.

Sulphur is even more easily leached than P; it is like applying a whole year’s supply of food in one day that will just sit in the solution in lower ASC soils. Plants can only use a certain amount daily; the rest is at the mercy of the climate/rainfall of the area.

Why ASC is important

Anion Storage Capacity was developed in NZ by WHM Saunders in 1965. ASC is determined using a buffered sodium acetate solution at pH 4.6, containing 1000mg/L phosphorus. It is important to buffer the extracting solution to a pH of 4.6 to achieve maximum phosphate retention and provide repeatable results in the laboratory. The difference in initial 1000 mg/L added and the phosphorus remaining after 16 hours of shaking is used to calculate the % ASC.

Saunders found that the amount of phosphate extracted from the soil by the sodium acetate was negligible compared to the amount of phosphate in the buffer solution so excluded this from the calculation.

ASC is a great tool to determine different soil groups and provide an index for fertiliser recommendations. Like much empirical agricultural testing, ASC is good at providing a low, medium and high value, but it is less able to differentiate the phosphorus retention characteristics of similar soil groups.

Need more information? Talk to the experts

Eurofins Agri-testing is an independent soil, plant, feed and fertiliser testing scientific laboratory with over 30 years of experience in New Zealand. We help New Zealand farmers and farm advisors with the most accurate testing results using advanced techniques. Globally, Eurofins is a leading provider of testing services with more than 650 laboratories across 45 countries and over 45,000 staff. To find out more, contact us today.

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