The Demonstration Farm certainly fits this category – confirmed by regular and frequent soil and pasture analysis which has shown consistently high levels of all nutrients tested. Given the big drop in payout, it is not surprising we are taking a critical look at fertiliser inputs and asking – can they be reduced, or eliminated in the short term?
Obviously this is being asked by dairy farmers nationwide – for most farms fertiliser is likely to be the major item of expenditure. Furthermore, particularly when soil fertility is good, it can be difficult, if not impossible, to see tangible gains from fertilisers. Back in my MAF days colleagues in the Research Division reckoned that with fertiliser plot trials it would take at least a 15% response to be able to be picked up visually by a trained operator.
If asked whether a farmer should cut fertiliser to help balance the budget, for some it should be the first thing to prune, for others the last. There are so many variable factors relating to the individual farm that the best fertiliser policy is likely to be highly individualistic.
Soils are complex, even more so are the chemical and biological reactions and interactions occurring within them. Consequently, while soil scientists and agronomists have accumulated a fund of knowledge there probably always will be areas of uncertainty and incomplete data. However, enough has been accumulated to enable informed and reliable decisions on fertiliser policy.
Unfortunately, however, there is also a lot of pseudo-science and even non-science aimed at selling material of dubious and unproven efficacy. Perhaps I can best summarise it by quoting the response of a well-informed colleague some years ago to the question of what he thought of a certain so-called fertiliser – an expensive way to buy 44-gallon drums.
Thus my aim in this article is to urge anyone contemplating changing their fertiliser policy to seek qualified advice before doing so. The emphasis must be on the word qualified. Look for relevant academic qualifications. Bachelor of Agricultural Science is a good one but there are others. They should be employed by a reputable and preferably well-established organisation. Experience is also desirable, and beware anyone who can’t lift their head above the computer screen and get their feet on the grass, and the eye on the pasture. Despite the economic squeeze don’t hesitate to spend money, if required, to obtain this advice. If from a qualified source any small outlay can be hugely returned.
Now to give you a bit of the established science on soil fertility and fertilisers, to aid understanding what is required for your farm.
Early scientists in this field developed the principle of limiting factors. This states that the level of crop production can be no greater than that allowed by the most limiting of the essential plant growth factors. These are things like water, temperature, light, and nutrients. To take an obvious example, if insufficient water is limiting growth – drought – increasing any or all of the other essential factors will not increase growth. Pouring fertiliser onto drought-stricken pasture would be an exercise in futility.
Where nutrients are the limiting factor, the principle continues to apply. Of the 16 elements known to be essential for plant growth, the nitrogen, phosphorus and potassium obtained from the soil are the most familiar, and in terms of fertiliser use, the most critical.
Taking nitrogen first there is a very common conception that nitrogen is a sort of universal stimulant to plant growth. This is not the case. Nitrogen responses will only be obtained when nitrogen is the limiting factor to plant growth. This is quite likely to be the situation in the spring, after winter rains have given leaching losses and the biological processes fixing atmospheric nitrogen or freeing organic matter nitrogen have been limited or non-existent. So in a fertile soil, in the spring, nitrogen responses are likely – indeed probable. The emphasis here is on fertile. If, for example, growth is limited by low phosphate levels, no amount of nitrogenous fertiliser will increase it. Similarly frozen winter soils, or dried out summer soils will give no response to nitrogen, or any other fertiliser – because temperature and water are respectively the limiting factors.
This brings us to phosphorus. Without doubt phosphorus is the main deficient nutrient in NZ soils. It would be interesting to know how much income super phosphate has earned for the NZ economy over the years. It has to run to billions.
On being applied to the soil, phosphate molecules move from the soil solution to become weakly bonded to the soil particles, from where they are removed by the plant roots as the plant requires them. With the passage of time, this bond strengthens, so that the plant finds it increasingly difficult to remove the P it requires – and ultimately impossible. This process, whereby phosphate molecules get locked up in the soil is known as phosphorus fixation or phosphorus retention. This dictates a need to be continually applying phosphorus to the soil to prevent it becoming the limiting factor for plant growth. The severity of this process will vary with soil type – but with many other factors as well. Phosphorus retention is particularly high on ash-derived soils, and these will require a different fertiliser regime.
The obvious question is, with continual application of phosphorus, surely over time this fixing process must become saturated, so the only phosphorus that need be applied will be just enough to replace that which the plant has removed? Don’t hold your breath. It has been calculated that for some ash soils it would take at least 125 tonnes of super phosphate per hectare to saturate this mechanism – and that’s only to the depth of a furrow.
However, with continual application of phosphorus, the fixing process does weaken, and more of the applied phosphorus will become plant available, over a longer period, and so a phosphorus reserve will be developed.
The level of this reserve can be easily established by a soil test, which will give values for both plant available phosphorus, and for phosphorus retention. Where this phosphorus reserve is at a satisfactory level – there is no one size fits all for this – fertiliser policy need only have the aim of maintaining this. When the reserve is very high it can, in effect, be “mined” by skipping a routine application, allowing a decline in available phosphorus levels – but staying safely above the limiting factor zone.
Many farmers get concerned if, for any reason, they cannot get their fertiliser on when planned – in the mistaken belief that applying it will give a boost to pasture production. This will only be the case where the elements being applied are the limiting factors to growth – and this is unlikely on a fertile soil that has been regularly topdressed. In this situation the objective is not to boost production – but to maintain it. Obviously, if phosphorus or potassium levels are the limiting factor withholding fertiliser will reduce pasture production.
Potassium is a different beast. It is not readily held in the soil and is easily leached so it is difficult to build a substantial soil reserve, so that as removal by stock is also high, annual, or twice yearly applications are generally the rule.
In summary:
Be sure that nitrogen is the limiting factor before applying it.
Where fertility is good and soil tests indicate a good phosphorus reserve, at least one phosphorus application can be skipped.
Don’t skip potassium. If phosphorus is having a holiday, straight muriate of potash can be used. Alternatively, to avoid cutting out phosphorus completely, 50% pot super.
Most important – get qualified advice.
For some farmers cutting back on fertiliser should be the first thing they should do – for others the last.
To provide clarity I have simplified some very complex processes. To any purist who thinks I have over-simplified I can only apologise.
