• Nick Lyttle

Fertiliser Fear


Beware Fertiliser Decisions Driven by Fear or Habit

With drought conditions only just breaking over much of the South Island and a low dairy pay out, there are several reasons to control costs more closely this season.

There are many decisions that must be made on a farm but fertiliser choice is a big item (third largest expense according to Dairy NZ). It often gets left to the supplier’s advisors or the “let’s keep doing what we’ve always done – it’s worked till now” philosophy is followed.

Not enough fertiliser will become apparent within 1-3 years, however too much is rarely measured, often never detected, and infrequently associated with common animal performance issues.

Not only is this spend preventing other areas of investment on the farm, but it can also cause an increase in other costs such as higher metabolic disease, lower reproductive performance, or in extreme cases death. The latter is not as rare as you might expect. Just this season a client lost 12 lambs with nervous disorders from grazing a finishing crop that had been over fertilised with sulphur, as part of a foliar application to drive DM yield/Ha.

Trust the Science, Acknowledge the Art

Soil and fertiliser advisors often quote science as their guiding principle, but this science (the soil analysis) is not exact. The soil analysis in the laboratory is repeatable, at reputable labs. Their testing is done to international standards and their equipment is regularly calibrated. However even with all this technology the Olsen P test itself, as an example, has an accuracy rate of + or – 20%.

The portion of the sample sent to the lab and actually tested is less than a couple of teaspoons (10 grams). A 2 hectare paddock contains 1.2 million kilograms of soil just in the top 7.5cm of surface soil when it has a volume weight of 0.8 (clay dominant). The sampling itself is therefore the greatest source of variability.

Consider the variation in Olsen P tests obtained from this grid sampling (diagram below) of a dairy farm in Victoria. Each number represents a grid from which 20 soil plugs were collected.

So the sampling art is understanding how the paddock is grazed, looking at trough and gate placement and being aware of slope and underlying soil type changes relative to total paddock or block area. Soil generally has a large capacity to buffer in the short term and so the challenge here is to sample from a consistent transect, and observe the trend over several years. Watch grazing behaviour and stock congregation within a paddock and look at plant growth – all things only observed with time on the farm.

Knowledge Gives Confidence

Just because a soil test shows a certain level it does not mean the pasture will reflect that. However if the picture is mirrored then you can be more confident of your fertiliser decision. Taking paired soil and pasture samples is a good way to gain confidence about your decision and is very helpful to avoid fear of failure when a decision not to apply fertiliser is made.

A potential example of such a situation arose when reviewing the recommendation given to a North Island farmer for a per hectare application of the following product:

675 kg 20% Potash Serpentine Super which delivered the following per Ha kg nutrient levels -

P K S Mg Ca

36 67 45 27 89

When you consider the growing focus on nutrient management alone there is reason to pause, however, when you consider the information below it is hard to understand.

  • There is no need for more phosphorus in this current year – not only is the Olsen P above requirements but there are large reserves, a moderate ASC and top end of acceptable pasture levels.

  • Potassium is also in excess with a MAF quick test at 13 and pasture levels above upper end ideal of 3.0.

  • Sulphur soil levels are acceptable although the sulphate sulphur test is very high (this test can vary by + or – 40%) so it is given little attention.

  • Pasture magnesium levels could benefit from being higher, despite soil levels being adequate, however a reduction in plant available potassium would benefit.

Before reading on – what would you do if this was your farm?

Our Recommendation

Addition of lime and salt is the simplest, easiest and most cost effective current intervention here, which addresses both soil and animal health concerns, not to mention freeing up approximately $10,000 from the budget in a year where it is welcome.

#Fertilisercosts #Fertiliserrecommendations #Samplingvariation #Soiltestaccuracy #OlsenPvariation

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