Jim Wilson Speaks at the Foundation for Arable Research Combinable Crops Day

Jim Wilson is a farmer and also runs Soil Essentials, which is a Precision Ag and agronomy consulting company in Scotland. He came to NZ this month to speak at FAR Combinable Crops day and has been hosted by FAR and LandWISE to speak at numerous field walks and events while here.

The FAR Combinable Crops day is an annual event, and is a great place for Arable farmers to catch up on new technology and research in agronomy. Over 420 people attended the event at Chertsey this year, which is a new record.

Jim’s take home message was simple:

“Precision Agriculture (PA) is common sense.  Fields and crops are variable, yet we tend to ignore this when managing them.  I first got interested in the early 90’s when harvesting a field of spring barley.   The yield could change from 3 tonnes/ha to 10 tonnes/ha within 20 metres.  As it had cost the same to grow the low yielding as the high yielding area, I was losing money on the low area.  How can we change this?”

Jim spoke about how fields are variable and crops are variable and yet we treat them with blanket prescriptions of fertiliser and agrichemicals. This means we are losing money and wasting inputs on low yielding areas.

Low yielding areas cost the same to grow as high yielding ones. The other side of this is that we are under applying on the high yielding areas.

Some variability is inherent (e.g, variable depths of subsoil and topspoil) and some is man-made (like heavily fertilised areas where the truck is loaded, where mistakes are made or the where last part load is repeatedly spread in a paddock). In areas where crops grow better year after year, they remove more nutrient.

Poor yielding areas can lead to surpluses of unused nutrients in the soil. These can lead to problems in some cases. In all cases the surpluses represent waste. In high yielding areas removals can lead to a deficiency and if can lead to the nutrient in question becoming a limiting factor. This can lead to high yielding areas becoming low yielding areas.

These effects can add to the variability, and result in lower average yields per unit of input. Precision Agriculture offers some new options and Jim explained how we can use our eyes and other tools to create zonal management to address this variability. When we address variability with Precision Agriculture, we make improvements into the future.

Yield mapping

This is an excellent tool to quantify and locate limiting factors in your crops. Yield maps are useful for strategy, especially when multiple years are layered together. Their limitation is that they give you information too late for the crop they represent. Yield maps are excellent when reviewing what works or doesn’t work on your land.

Soil sampling

This can be used as an aid to finding limiting factors.  A basic strategy is to sample known high and low yielding areas. If you find significant variation between these you may choose to sample on a grid and then use the resulting map to program variable rate fertilising. A start point may be to split each field according to old field boundaries then split each into low medium and high yielding areas for sampling.

Soil EM mapping

Soil EM sensing such as EM38, measures the apparent electrical conductivity of soil. This is influenced by the amount of salt, water or clay in the soil. To achieve the most robust picture of soil texture (and likely water holding capacity) an EM survey is best conducted with the soil at or near field capacity. The EM map can then be used as a basis for placing soil moisture probes and for scheduling variable rate irrigation.

So What can I do once I find poor yielding areas?

Variable rate lime can be applied according to sampled ’tiles’. Jim uses a 50 by 50m grid with a number of samples aggregated from each ‘tile’ in the grid.

P and K variable rate applications can be designed by calculating offtakes from yield maps. Where high soil levels occur, an option is to apply none in that year.

Seed can be variably sown if soil moisture, or pests such as slugs are a problem. This has been found to dramatically increase yields for little cost.

These responses to poor yielding areas can correct variability. In some situations, causes can not be found or are too expensive to correct. In these cases a choice has to be made, either to stop farming these areas, to grow a different crop there, or to reduce the growing costs and bring them back into profit.

What about crop sensors?

Because the tools described above are based largely on crop history, the arrival of crop sensors brings some exciting options. Each plant becomes an indicator of present soil, water and nutrient conditions. The various sensors use a combination of visible and near infra red light. The use of the NIR band means that problems can be seen 7 to 10 days before they are visible to the eye.

A common index called NDVI (normalised distribution vegetation index) is calculated from the difference between red and near infrared bands.  Sensors can be fitted to machinery and gather NDVI data with each pass over the crop and this allows comparison with other maps to record changes during a crop cycle. This is known as scouting.

Before using sensors for making decisions about N applications, it is important to eliminate other limiting factors. If N is not the limiting factor, overapplication can result. This is a waste of money and can lead to increased risk of leaching.

In the UK, variable rate Nitrogen applications have been found to return 25-35 UK pounds per hectare. Another benefit is that crops are much more even and less prone to lodging, which makes harvesting quicker.


Jim suggests you approach Precision Agriculture in a structured, planned way.

  • Check that you have enough variability to justify the time and money you will spend.
  • Identify and correct as many growth and yield limiting factors as possible, using your eyes, agronomy and other tools available to you.
  • Target your biggest costs first.
  • Reduce crop growing costs in remaining low yield areas.

Once you have the main limiting factors corrected, look at using real time sensors for crop scouting and for programming variable rate nitrogen applications.

For more on this topic and Jim’s services visit www.soilessentials.com, also have a look at the LandWISE website www.landwise.org.nz or for Australian work on the subject, see www.spaa.com.au.

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