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By P.W. Burton

At the meeting on regenerative pastoral farming held recently in Gore, the topic of appropriate Olsen P levels was raised.   A farmer participant was a little disturbed that a featured, recently- developed and highly productive grazing block close to Lincoln, had an Olsen P figure of 7.

Phosphorus and its importance were not part of the day’s agenda, however if it’s an issue with one farmer it will be exercising the minds of others and therefore a topic worth revisiting.

Phosphorus is an essential element.  When plant available phosphorus is below a certain level total pasture yield is restricted.  With less leaf surface area the ability of plants to convert sunlight to usable energy is restricted, and soluble sugar levels in the leaf are lower than optimum.

Quantity and quality go together, when less is grown quality declines and the reverse is also the case.

Soils in their natural state contain phosphorus, and work carried out in this country indicate that there is usually around 500kgP/ha in the top 150mm (6 inches) in situations where no phosphate fertiliser has ever been applied.

At the other end, figures in excess of 1,000kg/ha are not uncommon on intensive dairy land with a long history of regular superphosphate inputs.  These figures have been obtained from Total P tests that measure almost all soil held phosphorus.

The Olsen P test was designed to measure plant available phosphorus and although there is debate about appropriateness, it’s been used for long enough that its usefulness is not in question, particularly where there are good long-term records of phosphate applied.

The total annual plant uptake of P by pasture, based on historic research, in most situations is no more than 60kgP/ha, with the majority returned to the soil via dung, litter and roots. A 2007 article by Tim Jenkins, then of the Biological Husbandry Unit at Lincoln stated that the annual P losses in a typical beef and sheep unit may be as low as 4 – 8 kg P/ha, with dairy 10 – 16kg P/ha.

Typically, 50kg P/ha is applied to intensive dairy each year with 20kgP/ha applied to dry stock properties, hence the reason for the steady accumulation of phosphorus in topsoil under grazed pasture.

With phosphorus becoming increasingly expensive, and phosphorus of suitable quality becoming scarcer, can high performing properties operate with less inputs?

The key to applying less and being able to produce the volume of pasture required is dependent on several factors.  The speed at which phosphorus is cycled is the key to ensuring enough phosphorus is available for optimum pasture growth at all times.

Clovers being less competitive than grasses for all nutrients, was the justification the for higher than maintenance P inputs in the past, however trials by research institutes here show that an Olsen P of 10 is all that is required for clover survival.

Other long-term work shows that high Olsen P levels do not necessarily increase total pasture production, however higher levels reduce the likelihood of poor growth.

The speed of nutrient cycling is the most important factor in ensuring optimum, or close to, pasture performance.  This is dependent on physical soil structures which is determined by the activity of earthworms, beneficial fungi and bacteria, as well as myriad of other soil dwellers.

Only a very small percentage, often as little as 1% of all soil held nutrient, including nitrogen, is available for plant uptake at any given time.  It’s the ability of soil life to provide a continuous supply of nutrient that determines the true success of any growing system.

Those farmers already incorporating this aspect into their operations are capable of, and achieving, higher total farm production with significantly lower costs than those fixated on soil chemistry and conventional water-soluble nutrient inputs alone.

For more information contact Peter on 0800 843 809.

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