Guideline Index

Chapter 8: Assessing Soil Nutrients

8.7 Fertiliser test strips

Fertiliser test strips are useful for determining what nutrients to apply but are less useful for determining the appropriate application rate.

Test strips may be used to check results of soil tests or as a cheap way to test soil fertility. They may be of limited use on high-fertility pastures where there are no obvious nutrient limitations to plant growth. To determine the application rate, it is often more useful to prepare a budget and evaluate the costs of nutrients (discussed in Chapters 14 and Chapter 15 ).

There are basically two ways to set up test strips:

  • small hand-spread strips (20 m x 2 m), or
  • longer more commercial tractor/machine-spread strips the length of a paddock.

Whichever system is used, stock need to be kept off the strips for a period of time (at least 4 to 8 weeks, depending on the season) to allow the effects of the fertilisers to be seen.

A 20-m x 2-m test strip is equal to 1/250th of a hectare. Therefore, to apply the equivalent of 250 kg/ha, you need to weigh out 1 kg of the product to be applied. To ensure an even spread over the hand-spread strips, split the required amount of fertiliser in half and go over the plots twice. It can be helpful to apply the selected rate/ha to half of the plots and double the selected rate to the remaining plots in order to establish where the response finishes.

Small hand-spread or 4-wheeler spread strips are easier and less costly to set up and, provided they are around 20 m in length, will cover sufficient good and bad areas of pasture to allow a comparison to be made. Several sets of test strips may be needed around the farm to help in determining the final fertiliser strategy.

8.7.1 Site selection

  • Test strips are best sited towards the centre of a paddock or at least 3 metres from a fence line. Run the strips at right angles to the fence line.
  • Choose an area in the paddock that is typical of the diagnosed problem or where the change of fertiliser practise is to be and, if possible, a pasture with some clover or other legume present.
  • Avoid fence lines, trees, gates, stock troughs, haystacks, old firebreaks, corners of paddocks, stock camps or poorly drained areas.
  • Test strips will be of more use if they are put on an area that has not been top-dressed that year. Alternatively, top-dress the strips after the paddock has been top dressed and evaluate the potential for additional response above what is to be gained from the paddock topdressing.
  • Run strips up and down a slope, rather than across it. Surface runoff immediately after topdressing can shift fertiliser from one strip to another.

8.7.2 Assessment of the strips

The control (no fertiliser) strip is the most important. Without this, it is impossible to compare treatments to determine whether the fertiliser has had any effect or not.

When comparing the strips consider:

  • Pasture height and density.
  • Size and colour of clover leaves.
  • Botanical composition.
  • Evenness of pasture.

The strips should be regularly checked throughout the year and observations recorded, such as regrowth after grazing. The final assessment of the site will be made before the grass seed-heads emerge. If the paddock is going to be grazed, then the test strips will need to be fenced off. To see the effects of the fertiliser, it is important to keep stock off because they will preferentially eat the good strips where there is a response, giving the observer the incorrect answer for responses on the site.

Sometimes it may be preferable to graze the strips off and allow them to grow again to evaluate the regrowth, when the best response is visible.

If nitrogen is applied to any strip, then evaluate it regularly from 2 weeks after topdressing.

The strips can be inspected in the following years to observe carryover effects on pasture production and changes to botanical composition. In longer term trials the accurate marking of strips will be essential. The benefits of some fertilisers may not appear until the clover or legume content of the pasture has increased, so sometimes responses are not evident until the second year. If the strips are to be observed in the second year, make sure that the test site is grazed down similarly to the rest of the paddock in between seasons. Remove the fences to allow better grazing and to reduce the likelihood of stock camping on the plots.

8.7.3 Interpreting the results

If there are clear differences in pasture growth between strips, you will be able to assess which nutrient or nutrients you require to improve pasture production.

A 20% or greater difference in growth rate can be visually detected, whereas a pasture meter can detect about a 10% difference.

If there is poor growth on all strips, it may be due to other factors, such as poor soil structure, soil acidity, plant diseases, pests, waterlogging, salinity or lack of productive pasture species (see Excercise 1 ). Usually these factors have all become evident before the test strips were even established.

In areas of reasonable soil fertility, fertiliser test strips may indicate that no fertiliser is needed or be used to evaluate a range of selected fertilisers. In fact, even though a test strip has shown nil response, paddocks may actually respond to fertiliser application. An experiment at DPI Ellinbank compared paddock application versus strip application of superphosphate. Although the test strips indicated that fertiliser was not required, the whole-paddock application did result in an increase in pasture production and animal gain! The reasons for response in the paddock as against this lack of visible response in the test strip paddocks are probably due to:

  • The greater availability of recycled P.
  • An interaction between recycled N and K with higher soil P levels.
  • Selective grazing of test strips, thereby retarding regrowth due to lower pasture height.