Biological vs economic optimum soil fertility levels

Fert Options
with Robin Boom
Agronomic Advisory Services

There is a lot of historical research which has been undertaken in New Zealand from which the biological optimum levels for most of the major elements and some trace elements for pastoral farming have been determined. The Biological Optimum levels are attained when approximately 97 per cent of the potential production for that particular element has been achieved.

If we take phosphorus for instance, the Biological Optimum levels using the Olsen P test is achieved at 20ppm for sedimentary soils, 22ppm for volcanic ash soils, and 35 ppm for pumice and peat soils.

However when it comes to the economic optimum levels, these can differ for different farming enterprises.

For high producing dairy farms, the economic optimum Olsen P level may be 25 for sedimentary soils, 30 for volcanic ash soils and 45 for peat and pumice soils, assuming all other elements are in the optimum range.

Conversely for an extensive sheep and beef farm, the economic optimum Olsen P level may be 15 for sedimentary soils, 18 for volcanic soils and 25 for peat and pumice soils.


The same can be said for potassium, where the optimum level for dairy farms is between 140-200 ppm (MAF Quicktest units of 7-10), but for sheep and beef farms 120-160 ppm (MAF Quicktest units of 6-8) are generally regarded as adequate.

However again there are variances with different soil types with peat and pumice often having lower potassium levels, and some sedimentary clay soils having lots of reserve potassium which standard Quicktest analyses do not measure.

There are also animal health implications with potassium which need to be considered for cows in particular, so keeping potassium levels at the right ratio in relation to calcium, magnesium and sodium levels are important.

If milk fever, grass staggers or bloat problems occur, this is often as a result of a potassium imbalance in the soil with these other major cations.


Soil pH is another variable when it comes to optimum levels. PH stands for potential of hydrogen, so it is a measure of soil acidity, and not an indicator of the need for calcium as some people wrongly believe. Lime (calcium carbonate) will lift the pH, but so too will magnesium carbonate and other carbonates, as it is principally the carbonate in lime which raises the pH and not the calcium. Calcium has a very small impact on soil pH. When it comes to soil acidity, different plants have different requirements. White clover and lucerne perform best at a 6.3 pH whereas ryegrass only needs a 5.8 pH, so as a general rule of thumb, a 6.0 pH is considered optimum for dairy pastures. Too high pH can lift molybdenum levels too high and suppress the uptake of some essential trace elements. For sheep and beef where lime can be trucked on a 5.8-6.0 pH is considered economically viable, whereas when needed to be applied by plane, the sheer cost of flying on bulk quantities of lime can make it uneconomic to lift the soil pH above 5.6.


Again different soil types have different pH requirements, with peat and pumice soils not responding to lime as much as sedimentary and volcanic soils once pH levels get above 5.5 based on historical research.

However when it comes to lime I think there are other factors to consider such as aluminium toxicity and also the molybdenum status of the pastures.

I have had hill country sheep and beef clients observe pasture responses to lime by getting their pH levels above 6.0.

A trial I ran on a hill country property in the western Waikato gave an almost linear response to lime from lifting the pH from 5.0 to 6.1, but this particular site had aluminium toxicity and successive annual drought vulnerability, with lime giving a huge boost in the vigour and quantity of ryegrass grown as well as white clover, and suppression of the flat weed content of the pasture.

Marine clay soils on the Hauraki Plains can respond to lime even though the pH level appears to be good, and this is because these soils have a very high magnesium component making them sticky and plasticine like and it is the flocculation affect of the calcium in the lime which improves the porosity of the soil, improving the nitrogen cycle and nutrient cycling generally as well as root growth.

On such soils just using pH to gauge the need for lime will fail to detect the calcium:magnesium imbalance.

For most other elements, the biological optimum and economic optimum levels are similar, although as in all soil fertility issues, the fertiliser budget should focus on the most deficient elements and the most cost effective way of addressing these.

Robin Boom

Member of the Institute of Professional Soil Scientists


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