Making More from Sheep Australian Wool Innovation Limited Meat & Livestock Australia
MODULE 7: Grow More Pasture
Tool 7.2
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The two key questions regarding fertiliser use in pastures are:

  • Which fertilisers will give a significant boost to pasture growth?
  • At what rate should such fertiliser be applied?

For the macro-nutrients phosphorus (P), potassium (K), and sulphur (S), there is no substitute for laboratory soil tests when it comes to assessing both the likely responsiveness and the best rate of fertiliser to use. Soil tests are also invaluable for diagnosing many soil health issues (see tool 6.5 in Healthy Soils).

However, laboratory soil tests are not as useful (in some cases not at all useful) for nitrogen (N), the trace elements or for soil conditioners such as gypsum, and this is where fertiliser test strips can be extremely valuable. They have the added advantage that they are ‘free’ and they provide specific information for your own paddock.

Test strips are ideal for testing which fertilisers will give a significant boost to pasture growth, but not as well suited to the question of fertiliser rate. It takes too many strips, and accurate cutting of pasture is required to assess an optimum rate. For this reason, test strips are usually given a higher than average rate of fertiliser to make it easier to see any responses. If you get a response in your test strips, seek local advice as to an appropriate rate.

Test strips are not suitable if a rapid decision is needed. Ideally, the test strips will be applied and assessed this year as the basis for fertiliser decisions in subsequent years.

For annual pastures, test strips are best put out before the break of season, but for high rainfall zone, perennial pastures, test strips can be put out any time between February and September.

There are two common methods of putting down fertiliser test strips:

Small, hand-spread strips.

A good size is 20m long and 2m wide – for this size, the sums are quite easy; simply divide the rate per ha you wish to test by 250 and apply that evenly to the test strip (i.e. 1 kg of fertiliser is equivalent to 250 kg/ha). Because the areas are quite small, it is advisable to put down two sets of test strips within a paddock. After you have weighed out the fertiliser, six of these small test strips can be put down in about 45 minutes.

  • Mark the four corners of each strip and run twine around them to ‘outline’ the test strip
  • Run the test strips down the slope to minimise the chance of nutrients washing from one strip to the next
  • Apply the fertiliser as evenly as possible – going over the strip twice usually gives better uniformity than a single application
  • Remove the twine but leave the corner posts so you can easily locate the strips (mark the strips on a map so you know what has been applied to each strip)
  • Run the test strips parallel to each other and leave a good distance (say 5m) between strips to make assessing the growth response easier. Narrow test strips are not well suited to a crisscross design because the area of overlap (2m by 2m) is too small.

Larger, machine-spread strips.

These should be at least 100m long (can run the length of the paddock) and the width of the spreading machine. [Note, this technique is only suited to ‘drop’ type spreaders as ‘spinner’ type spreaders don’t have a definite ‘edge’].

  • As for hand-spread strips, it is essential to mark the corners of the strips and draw a map so you know what has been applied to each one
  • Reasonably careful calibration of your spreader is important, as is cleaning out the spreader between different strips, especially if you are testing responsiveness to trace elements
  • Leave at least 10m between strips to make assessment easier
  • Machine-spread test strips (if the test strip is at least 5m wide) are suited to either parallel strips or to a criss-cross design. Criss-cross designs are useful if you want to test ‘combinations’ of fertiliser (say P and N, or P and molybdenum).

Assessing the test strips

Stock need to be kept off the test strips to make it easier to see the effects.

For annual pastures, the biggest effect will probably be seen in spring. Many producers close up the area where the test strips have been applied for hay cutting – this gives the best visual result.

For perennial pastures, paddocks can be closed up for hay, or for rotational grazing systems, and assessed before the stock enter the paddock each time.

Things to look for when comparing test strips to the rest of the paddock include:

  • Height of the pasture
  • Colour and ‘healthiness’ of the pasture sward
  • Amount or density of the clover
  • Size and colour of clover leaves
  • Evenness of the pasture
  • Any impact on pasture composition, including if there are more or less weeds in the test strips.