Phosphorus (P) is the most expensive nutrient (typically about $3.0/kg P) It is also a pollutant when it gets into waterways.
The key is to farm at the economic optimal soil Olsen P – farming below the optimal means less profit and farming above the optimal means greater losses of P to the environment.
Clover has a higher requirement for all nutrients including P, than grasses. Thus, increasing the soil P levels on P deficient soils will improve clover growth and hence the input of ‘free’ clover N.
The soil N status will increase as the clover N is returned to the soil and hence, given time, total pasture growth will increase.
Economic optimal Olsen P
For any given soil group (i.e., pumice, volcanic, peat, sedimentary, podzols, sand or recent) there is a relationship between soil Olsen P level and pasture production. It is a “diminishing returns” relationship. This means that when fertiliser is applied to a soil with low P there is a large increase in pasture production – the financial benefits from the increase in pasture production (a) are much greater than the cost of applying fertiliser (b) – it is economic. At high soil P levels the financial gain from applying fertiliser is less than the cost of applying fertiliser (a < b) – it is not economic. The economic optimal Olsen P occurs when (a) is equal to (b).
Not only does the economic optimal Olsen P depend on the soil group it also depends on how efficient the farm is – how many dollars are made per kg DM grown. Given that pasture DM is the cheapest source of feed costing about 2-3 cents/kg DM it can be assumed that most farmers will want to optimize pasture production and so generally the higher the production per hectare the higher the economic Olsen P. Economic optimal ranges Because soil tests are variable (see Farmfact 7-3 Soil testing) it is appropriate to aim at ranges for the economic optimal P levels as follows:
|Economic optimal Olsen P range|
|Soil group||Low producing
(< 1000 kg MS /ha)
(> 1000 kg MS/ha)
Increasing soil P
If the Olsen P is below the economic optimal range then capital P should be applied. The amount of P needed over and above maintenance, to raise Olsen P by 1 unit is shown below.
|Soil group||kg P/ha needed to raise Olsen P by 1 unit|
Maintaining soil P
Once the optimal Olsen P range is achieved maintenance P inputs are required to make good the losses of P from the farm. (The losses include milk and other products removed from the farm, P runoff, and movement of P to non-productive areas etc).
OVERSEER™ should be used to calculate maintenance P inputs in any specific situation. As a ‘rule of thumb’ the Pumice and Volcanic soils require about 6 kg P/ha annually for every 100 kg MS produced. For the Sedimentary soils about 5 kg P/ha is required. For example, a farm producing 1000 kg MS/ha on a Volcanic soil will require about 60 kg P/ha annually.
On a Sedimentary about 50 kg P/ha/yr is required. Maintenance P inputs are by definition the amount of fertiliser P required to maintain the Olsen P level. If there is a good soil-testing program on the farm (see FarmFact 7-3 Soil testing) P inputs can be adjusted over time to keep the Olsen P in the economic optimal range.
Decreasing soil P
There is no point in farming above the economic optimal Olsen P ranges – it is not profitable and it causes unnecessary P run-off to waterways. Many dairy farms are currently being operated above the optimal levels. In these situations fertiliser P should be withheld and the Olsen P levels mined down to the economic range. This will increase farm profitability in the short-term because less fertiliser is required and will not result in any observable loss in production. If no fertiliser P is applied the Olsen P levels will drop by 1-2 units per year. A good soil testing monitoring program is required so that a maintenance P fertiliser program can be commenced once the economic optimal Olsen P range is reached.
Common P fertiliser
|Superphosphate||9%||Soluble (plant available) P, also contains 11% S in the available sulphate form. Contains approx. 20% Ca as gypsum.|
|Di-Ammonium Phosphate, DAP||20%||Soluble (plant available) P, also has 18% N.|
|Reactive Phosphate Rock, RPR||13%||Slow release P, also contains 35% calcium. RPR has a small liming value. Contains no S and if S is added as elemental S this negates the liming effect. Rate of dissolution 0-30% and depends on type of RPR, particle size, soil pH and soil moisture. Most common P fertiliser used on organic farms.|
|Triple Super, TSP||21%||A soluble (plant available) P, also contains14% calcium. Does not contain S.|
|Serpentine Super||7%||Made by adding serpentine rock to super during acidulation. Contains about 5% Mg (most of which is plant available), 9% S and 17% Ca.|
|Reverted Super, Di-Calcic Super||4-7%||Made by adding lime to superphosphate making the P slightly less soluble. Can be sown down the spout with seeds. No better or worse than soluble P in terms of DM produced per kg P. More suitable on soils with very low anion storage capacity (phosphate retention) under high rainfall to reduce P leaching (i.e. peats, sands and podzols).|