Potassium and advice on its use – Ravensdown’s riposte to AgScience article

Ravensdown PR man Gareth Richards has expressed disappointment at Doug Edmeades’ “wholesale dismissal of the advice given by certified nutrient management advisors”  which was published in the latest issue of AgScience. 

Richards provided an article by Jeff Morton, a long-standing New Zealand Grassland Association member and scientist, for publication in the next issue.

That will be a few months away.

So we are publishing the Morton article here and on the NZIAHS official website.

He writes:

After phosphorus (P) and sulphur (S) which are not naturally supplied in the mineral form by New Zealand soils in their original state, potassium (K) is the next major nutrient required by pasture. The fertiliser K requirements depend mainly on soil type and farming system. Younger soils where there are the clay minerals present from which K can be released supply more than older soils and those without the appropriate clay minerals. Different soils have different abilities to retain K (cation exchange capacity) and this increases with clay and organic matter content. The K supply and retention of different soil groups is shown in the table below:


Soil groups K supply K retention
Pumices, podzols and sands Low Low
Peats Low High
Ash Low High
Brown sedimentary soils on terraces and hills Low Medium
Pallic soils on rolling downs and terraces and Recent soils Medium to high Medium



There is less efficient cycling and greater K losses by leaching from dairy cattle compared with beef cattle compared with sheep.

In practice, economically optimal ranges of soil K levels are recommended taking into account soil groups differences and farming systems and levels are maintained by different rates of fertiliser K as shown in the next table:


Soil groups Optimal soil QT K range in early spring Maintenance rate (kg K/ha/yr)
Dairy Sheep/beef Dairy Sheep/beef
Pumices (> 1500 mm annual rainfall), podzols and sands 3 – 4 3 – 4 120 – 150 30 – 60
Pumices (< 1500 mm annual rainfall) 7 – 10 4 – 6 70 – 100 20 -40
Peats 5 – 7 4 – 6 50 – 100 20 – 40
Ash 7 – 10 4 – 6 50 – 100 20 – 40
Brown sedimentary soils on terraces and hills 6 – 8 4 – 6 50 – 100 20 – 40
Pallic and Recent sedimentary soils 6 – 8 4 – 6 0 – 40 0 – 20


To minimise leaching of K, no more than 60 kg K/ha should be applied in one application. Maintaining soil K levels above the optimal ranges above will reduce farm profitability. If soil K levels are already above these ranges, then it makes economic sense not to apply fertiliser K. It is important to understand that achieving maximum pasture production by maintaining high soil K levels is totally uneconomic. To optimise K use, enough should be applied to ensure pasture is grown to meet your feed demand while retaining sufficient clover in the sward. Some advisers like to quote the probability of gaining a pasture production response to fertiliser K at levels well above the ranges above. While a response may be forthcoming, this is likely to be low (less than 5-10% at best) because it occurs on the flat part of the curve where increasing the rate of K fertiliser will only give a low increase in pasture production.

Over time, on average the cost of a kilogram of fertiliser K and N have been similar. In most developed situations you will grow an extra 10 kilograms of pasture dry matter per kilogram of N but only achieve this response from K on the high K loss pumices, podzols and peats which make up less than 10% of New Zealand soils.

My observations during 10 years of providing technical back up to fertiliser field staff in Ballance Agri-Nutrients and a little time auditing Ravensdown recommendations is that generally K recommendations are sound from an economic viewpoint. They can use the econometric model, AgResearch PKS Lime model, to assess what the economically optimal soil K range is. Of course, there are a few instances where insufficient K is recommended due to an inadequate fertiliser budget and some due to poor advice but no system is completely perfect. These observations are supported by the large soil test K databases held by the commercial testing laboratories (Hills and ARL) where a large proportion of the levels are in or above the economically optimal range. In addition, there is no evidence of a decline in soil QT K over the last ten years from the ARL data.

So in conclusion, although some advisors like to proclaim universal K deficiencies in pasture for whatever reason, there is no complete evidence that this is the case. This situation is a result of generally sound advice from the field staff of the two main fertiliser companies.

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