FREQUENTLY ASKED QUESTIONS (FAQs)

Below are some of our most frequently asked questions and answers

What is Dicalcium Phosphate (DCP)?

• It is a phosphate molecule that has twice the amount of calcium atoms per phosphorus atom as there are in monocalcium phosphate (superphosphate).
• DCP is made by reacting superphosphate with an alkaline (liming) agent. The liming agent is mixed with superphosphate and a chemical reaction occurs. Once this chemical reaction (reversion process) is complete you have DCP.

How is DCP different to Superphosphate and RPR?

*Once RPR is fully dissolved (3 - 5 yrs under normal farming conditions)

What is the importance of pH neutrality for pasture growth and development?

Maintaining soil pH at 5.8 - 6.2 is an essential part of pasture management. Majority of soils in New Zealand are naturally acidic and will produce low yields in the absence of lime or lime based fertilisers. If the soil pH is too high or too low, certain nutrients required for plant growth are chemically changed to forms that are unavailable to plants. This results in plant nutrient deficiencies and poor yields.

What is the optimum pH for soil microbial activity?

• pH 5.8 - 6.5

Why is soil microbial activity important?

• The higher the levels of soil microbes, the greater the rate of breakdown of organic matter which means the greater the release of nutrients for plant growth.
• Microbes improve soil structure by helping to bind soil particles together which improves aeration and soil water storage capacity.
• Clover requires microbes to assist in fixing nitrogen. Therefore the higher the microbial population in the soil, the greater the capacity clover has to fix nitrogen. (Rhizobia provide nitrogen fixing function - reducing the requirement for urea - clover can supply up to 200kg N/ha/yr)

Why is soil earthworm activity important?

Earthworm activity improves the soil structure through providing;

• Better aeration
• Better environment for root growth (easier root penetration)
• Greater resistance to compaction and quicker return to production after adverse field conditions (pugging, flooding, drought etc)
• Influence on increasing organic matter decomposition and the ensuing nutrient availability
• Earthworm counts respond well to calcium and pH increases in a soil where these levels are suboptimal

How does soil microbial activity respond to applications of phosphate based fertilisers and urea?

• Soil microbial activity declines relative to decreasing pH levels from 6.0.
• Through the process of growing plants, a soil will naturally become more acid. The use of phosphate and nitrogen products such as urea and DAP stimulate plant growth which will cause the soil pH to decrease (in the absence of regular calcium applications). This has the effect of reducing soil microbial activity.
• Dicalcium phosphate differs from superphosphate as it has liming value (due to its calcium content) and this counteracts the lowering of pH through plant growth.

What is the difference of applying DCP over Lime and Superphosphate applied individually or as a mix?

• As soon as lime and super are mixed the reversion will begin and this can lead to physical handling problems such as wetting, setting and forming of lumps.
• With lime and super you have a powdered and a granulated product so it is difficult to achieve a consistent mix and this often results in banding (uneven spread of mix).
• As DCP has a liming effect, so you are much better able to maintain soil pH at optimum levels - rather than applying lime once every few years and going through the associated cycle of depletion and replenishment and the related changes in productivity.
• You can mix any amount of lime with DCP without lumping, caking or wetting reactions.

How does DCP compare with superphosphate in $/unit of P?

• Superphosphate will usually appear to be the most cost effective, but this is not an accurate basis for comparing the two products because of the different effects that they have on different soils.
• Superphosphate is highly water soluble therefore in situations where there is a surplus of water soon after application (esp on hilly country and light soils), this P can be lost before it is assimilated into the soil.
• In a high P retention soil (particularly at low pH) P in can be fixed by the aluminium and iron in the soil which makes it unavailable to the plant, so some of the phosphate does not get used to grow plants. DCP is a form that is less susceptible to P fixation because of its higher calcium to phosphorus ratio.
• This analysis also ignores the fact that in DCP you are also getting calcium that has liming effect, where there is none in superphosphate.
• If you take the liming effect into the equation, with DCP you get the two elements applied in the one application, whereas with super and lime you need two separate applications to supply your maintenance requirements.

How can I increase/maintain soil pH?

• You can apply lime on its own or you can use DCP (and get P as well).
• Continuous application of DCP can gradually increase soil pH (especially on hill country where there is a higher requirement of fertiliser per su).

Is lime required for animal production/growth?

Yes, about 15 kg per of lime per stock unit is required per annum to maintain pH.

Is DCP safe to mix with seeds?

• Yes, because DCP is pH neutral it does not burn seeds.
• This makes aerial oversowing much more cost effective when applied with fertiliser.

Can DCP unlock the fixed Phosphate in the soil?

Yes. It does this by helping to correct soil pH (P availability is at its maximum at around 6 pH). The calcium in DCP has a priming effect on phosphate and unlocks phosphate that has been fixed in soil colloids by aluminium and iron.

How does the Phosphate release pattern differ between Superphosphate, DCP and RPR?

• Superphosphate is immediately available due to the phosphate being highly water soluble (making it more prone to leaching).
• DCP is a medium release product as the phosphate has a water solubility of less than 5%. The release rate of DCP is mediated by plant growth - as plants take up phosphate, more P is released from DCP.
• RPR is a slower release product and dissolves at a rate determined by pH (the higher the pH, the slower the release).

What is the difference between the calcium in Superphosphate and DCP?

• The calcium in superphosphate is in the form of calcium sulphate and calcium phosphate, both of which do not have any affect on soil acidity/alkalinity.
• In DCP, some of the calcium is in the form of calcium carbonate and this form has the effect of reducing soil acidity.

The phosphate in DCP has to absorb acid before it can be absorbed by plants, which also helps to reduce soil acidity.