We have spent the last twenty years focused on soil fertility particularly fertility under grazed pastures and there are still times when parts of the picture are cloudy, sometimes even decidedly murky.
For an individual farmer or general farm consultant to be able to decide on the quantity of nutrient to be applied this spring, particularly deciding on whether the phosphorus input, should it be required, be applied in the form of rock phosphate, locally made single superphosphate, or perhaps DAP requires a great deal of information and a sound understanding of soil and plant requirements.
This is where models may be useful. A good model allows decision making to be relatively rapid and accurate. For a model to be useful it must contain correct information, particularly the initial information on which all further assumptions are made.
Should any part of the information on which a model is based be incorrect, the final outcome will almost certainly be nonsense regardless of the accuracy of the logic used.
An example of a logical argument based on a faulty premise is that land produces a base level of pasture without fertiliser nitrogen being applied and that each kilogram of fertiliser nitrogen applied provides an annual growth increase of 10kg DM.
A little nitrogen used strategically, when available nitrogen is the limiting factor and there are no other nutrients limiting production, may provide an increase far greater than 10kg dry matter for every kg of N applied.
More nitrogen does not mean a further increase of the same magnitude for each kilogram of nitrogen applied. Pasture response to the application of any nutrient depends on a large number of factors and there is no model available today that can accurately predict an outcome in every situation, and probably never will be which is why capable competent people in the field are so important.
A recent independent report on the Berryman property near Edgecumbe where the soil fertility programme has been based on DoloZest and CalciZest since August 2004 and has applied less than 20kgN/ha in each of the last four seasons states that it grew 30% more feed for the 2011/12 season than the “average” farm for the district.
The report also shows that the Berryman property produced 47% more milk solids per hectare than the “average”, had a substantially higher Gross Margin measured in $/ha, and a lower impact on the environment.
When any model or research provides a response that is favourable to our position, as has the independent on-going nitrate leaching work at the Berryman’s which showed average Nitrate-N leaching levels of less than 5ppm for last season, we tend to view it favourably, when it doesn’t the tendency is to be critical.
What we believe is essential is that models should never drive behaviour. Farming is a practical exercise and changes should be driven from the field with science providing the reasons for the outcomes, remembering that substantial change nearly always comes from the fringes rather than mainstream.