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Dicalcic Phosphate: Is it a Water-Safe Fertiliser Option?

Dicalcic phosphate has been around a long time, it is otherwise known as lime reverted super or DCP. Most commonly (and traditionally) to manufacture Dicalcic phosphate limestone is mixed with Superphosphate to de-acidify and change its chemical structure from monocalcium phosphate to dicalcium phosphate.

This chemical process turns the phosphate from a water-soluble form to a citric-soluble form. Citric-soluble phosphate is plant available but controlled in its release to the soil environment. This change in chemical status, and the fact that DCP is pH neutral, are its two unique features in comparison to other phosphate products like DAP, Triple Super and Superphosphate.

Traditional DCP has been made by blending 50% Superphosphate with 50% limestone, this results in a phosphate content of 4.5%. This low phosphate content has resulted in a higher transport and transportation cost. Together these costs have been the fertiliser co-operative’s biggest bone of contention for its use and probably fair enough.

Over the years, Fertco have designed systems to overcome this issue while retaining the unique features of Dicalcic phosphate and pH neutrality. Fertco produce high analysis DCP products containing either 8% or 12% phosphate therefore dramatically reducing costs of transport and spreading.

The benefit of altering the chemistry in the fertiliser phosphate applied to farms is the efficiency of that phosphate. Efficiency is brought about by less phosphate loss to the environment and less phosphate fixated in the soil; benefiting both the environment and farm finances. In the following article we dig deeper into losses of farm phosphate and where Dicalcic phosphate offers benefit.

Phosphorus has always been the most expensive of the major nutrients to apply as a fertiliser. As such, knowing your requirements from sound testing regimes and responsible educated advice is the best starting point. Only applying the required amount of phosphate may sound simple enough advice but Fertco commonly find soils with excessive phosphate levels. In that situation, Fertco advises farmers to apply lower rates of well-made dicalcic like Fertco’s Dical 12.

Loss of phosphorus from farming systems either as particulate phosphorus in overland flow (runoff) or via leaching in “leaky” soils is one of the contributors and likely the main contributor to eutrophication of waterways.

The blue green algae (cyanobacteria) that are largely responsible for waterway degradation can “fix” their own atmospheric nitrogen but only if supplied with sufficient phosphorous. In lakes, phosphorus resides in the mud and sediment on the lake bed; excess loss of soluble phosphorus and the activity of wind and introduced fish species like catfish and carp exacerbate this by stirring up the bottom sediments.

Direct loss of phosphate from animal excretion is a major loss. This loss factor is largely being taken care of from the diligent actions of our farmers.

The campaign by the Agricultural industry in recent years to fence off drains and ditches to stock has substantially reduced this mode of phosphorous loss.

There are three other main ways in which phosphate can be lost from the farming system, we will briefly consider each along with  suggested solutions below. Phosphate loss represents an economic loss to the farmer, and is a substantial contributor to environmental pollution in the form of algal and weed growth in waterways and lakes.

Where possible we make reference to New Zealand-based trial work and we are happy to provide further information on request.

1. Phosphate Run Off
Phosphate run off is the main environmental bad guy. The “Waipawa phosphate run off trial” showed us that dissolved reactive phosphate was the most prevalent and damaging form of phosphate loss from farming systems and this was most likely to occur post-application of water-soluble phosphate fertiliser.

The Waipawa trial compared phosphate runoff loss between Superphosphate and RPR (slow release phosphate). The trial reported “In the post-topdressing period both dissolved reactive phosphate (212% higher) and total phosphate (94% higher) losses were greater from Superphosphate treated catchments than from the RPR treated catchments”.

Unlike many serious ecological problems, the solution to this is simple, and cost effective. Changing the form of phosphate used with no penalty to cost or production. Farmers have the option to choose a controlled release form of phosphate such as Dicalcic phosphate.

Fertco don’t recommend RPR in most situations simply because RPR relies on acid soils to release it phosphate and productive sheep and beef land should have soil pH of 6 (close to neutral) where possible RPR is a good option in some hill country situations and therefore Fertco have quality RPR in our product range.

2. Phosphate Fixation
Fixation of phosphate has a financial ramification for farmers. Fixation losses of phosphate in the soil relate to the accumulation of phosphate in the soil that is too tightly bound to soil clays and/or organic matter to be available for uptake by plant roots; or has been  stable (resistant to microbial breakdown) soil organic matter.

The presence of soluble iron and aluminium in ash soils also play a role in removal of plant available phosphorous by creating new compounds which plants cannot take up. The addition of acidic fertilisers exacerbates this natural form of loss.

The worst ‘fixation’ offenders in NZ by far are the volcanic ash soils of the North Island and Southland, which contain a very highly phosphate fixing amorphous aluminium silicate clay called allophane.

These soils require typically half as much again to double the amount of phosphorus to maintain a given levelof production as non-allophanic soils.

Simple but undeniable proof that 30-50% of the phosphorus applied to these soils is being effectively ‘lost’ in terms of availability to the plant.

A solution to this is to apply a pH neutral phosphate source to avoid not activating the iron and aluminium in soils, thus causing less fixation. Acidic fertilisers such as Superphosphate with a pH around 1-2.5 (immediately adjacent to the fertiliser granule) increase the solubility of iron and aluminium and increase the rate of phosphate “fixation”. Dicalcic fertilisers have a higher pH and do not exacerbate this situation.

3. Phosphate Leaching
The leaching of phosphate from fertiliser and recycled phosphate through most NZ soils is not proven prevalent, but phosphate can be washed straight through the soil into the groundwater. On extremely weathered podzols of the West Coast and parts of Northland, 30% of the applied phosphate can leach through the soil beyond the root zone.

As with the loss from run off, phosphate leaching and the gradual release phosphate from Dicalcic phosphate offers obvious benefit.

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