To guess, to probe or to model soil water - an agronomist's dilemma

Costs

Capacitance type probe + data logger: $5000+; annual maintenance + mobile network connection: $350/yr.

Yield Prophet®:  $170/paddock.

Note both methods require soil sampling + analysis (available N and soil water): approx. cost $350/paddock.

Soil water is only part of the story – what about N?

The amount of soil water available to a crop is clearly of critical importance when assessing the risk of dryland crops being able to complete their growth cycle and in particular to fill grain at the end of the season.  However, water is only part of the story, the other critical factor is how much N is available to the crop and required by the crop to fulfil its potential.  Soil moisture probes do not provide information on soil/crop N status, and therefore, some kind of N budget is still required when deciding on whether the crop needs additional N in relation to how much water the crop has available to it.  APSIM simulations, through Yield Prophet® provide information for a crop’s daily water and N use, how much water and N are still in the soil, level of water and N stress exhibited by the crop at any stage during the season, and through probability functions, how much water and N is required to achieve a particular yield outcome.

What else?

Yield Prophet® output goes beyond assessment of soil water and nitrogen. Users have the ability to explore and compare a range of interacting factors that influence crop production, such as evaluating the least frost and heat shock risk of different varieties. The ability to simulate crop production by changing one or more factors provides the user with a learning platform that improves their understanding of crop production in a way that probes cannot.

So why the interest in soil probes?

Soil probes provide no additional information to that generated in the Yield Prophet® model, and are nearly 30 times the cost. Model supporters are challenged by the interest in probe use by farmers and some advisers when models have been doing the same for longer and for a fraction of the cost. Why are probes so popular?

Using Rogers’ adoption theory, a comparison of the uptake of different technologies can be made. Rogers describes five key criteria of new technology adoption. These are:

1. Relative advantage,
2. simplicity/complexity,
3. trialability,
4. observability; and
5. compatibility.

A subjective assessment can be made using this framework to help understand the adoptability of a new technology.  In the following assessment, the five criteria for technology adoption suggested by Rogers, are scored (1=low and 5=high) for probes and for Yield Prophet®.

1. Relative Advantage

For a new idea to be adopted, it must offer some relative advantage over the current practice. Value in understanding soil moisture has become more apparent in the last decade. Prior to the availability of either system, rainfall was the surrogate measure for soil moisture. Growers and advisers understood the importance of soil moisture but had little knowledge of just how much water their soils could hold and how much moisture was in reserve for crop production for the remainder of the season.

Both probes and Yield Prophet® provide information on soil water status. Both provide the user with information about the position of water in the profile. Unless the probe is calibrated, only Yield Prophet® provides information on plant available water (in mm).  Once a grower or adviser understands how much water is available in the profile, they are in a much better position to make management decisions.

Probes provide only a fraction of the information required to make the necessary range of management decisions. Yield Prophet® combines the basic soil water information with regional climatic data, crop phenology and soil available N to produce a range of yield outcomes based on historical rainfall or expected rainfall for the remainder of the season. The real strength and relative advantage of Yield Prophet® is as a learning tool for users to explore complex soil water and plant growth relationships.

While aspects of both probes and Yield Prophet® reports are simple, a full understanding in its use, interpretation and set up takes time, training and effort. However, because Yield Prophet® is much better integrated with all factors affecting crop production it is given a higher score.  Score: Probes – 4, Yield Prophet® – 5

2. Simplicity/Complexity

For an idea to be adopted, it must be simple. Complex ideas are hard to grasp and are less likely to be taken up.  Soil probes offer relative simplicity. Users simply log on and receive information about the soil water level and rainfall. There is no other information provided that clutters thinking and users are free to use the data as they wish and can continue to make management decisions as they have in the past.

Yield Prophet® soil water reports are also simple. The ‘bucket’ representation has been widely used in discussions on soil water for some time.  The soil water reports are embedded in the crop reports that contain much information and interpretation into probable yield outcomes. It is possible, that the inclusion of the soil water into the full crop report adds complexity and that first time users are overwhelmed at the information presented.

Yield Prophet® is more complex to use than probes.  Score:  Probes – 5, Yield Prophet® – 3

3. Trialability

For a new idea to be adopted, it must be easily trialable.  Probe installation requires expertise, however once installed the process of using them is straight forward. Many probe output users have been able to trial soil probes on their own or nearby properties through regional, state and federally funded programs that support their installation and training in use. In many cases, other growers and advisers can access probe outputs without bearing the cost of purchasing the unit or paying for installation.

Cost is a factor that limits trialabilty. At approximately $5000/probe plus ongoing support costs, growers may consider probes too expensive to trial. Where this is a barrier, growers can share access to probe data and hence reduce the cost to an individual.  It must be remembered however, that the probe only provides information specific to the soil type and rainfall at a particular site.

Setting up Yield Prophet® requires a level of skill that requires first time users to undergo a reasonable level of training and follow up support.  Users are required to select a soil type that represents their own soil type; this requires a matching of known measured soil characteristics that are available in the Yield Prophet® soils database for local soils. Comparisons of chemical and physical properties need to be made and many farmers and advisers may lack the skill, and therefore, confidence to conduct such comparisons.  Selection of an existing soil type is becoming easier, with regional soil selection functions providing a short list of soils to choose from.

In summary, using Yield Prophet® as a way of increasing soil water knowledge is more difficult to trial than probes.  Score: Probes – 4, Yield Prophet® – 3

4. Observability

For a new idea to be adopted it must provide a reward in a reasonable time frame.

‘Pay-back’ time for either system is generally regarded as very good.  Both systems improve farmers/advisers knowledge and understanding of what is happening to soil water during the cropping season.  Both probes and Yield Prophet® can provide paybacks almost immediately which can continue throughout the season and beyond.

The added benefit of Yield Prophet® is the combination of soil water and soil available nitrogen data.  In addition Yield Prophet® provides information on crop phenology and climatic data to simulate probable yield outcomes and allows a comparative analysis in a systematic way. It provides a science-based rigor to complex decision, and therefore, making Yield Prophet® far more useful beyond knowledge of soil water.  Users of Yield Prophet® gain confidence in understanding all the factors and interactions involved in producing high water use efficient crops.  Score: Probes – 2, Yield Prophet® – 4

5. Compatibility

For a new idea to be adopted it must be compatible with current thinking.  It should not be too many steps ahead of current practice and thinking.

From the perspective of improving knowledge on soil water alone, probes are more compatible with many users. They are a simple extension of a rain gauge and provide a translation between rainfall received and water in the soil. Probes provide some learning and develop a users thinking and understanding of the behaviour of water in a soil profile.

Until a decade ago, many growers and advisers simply thought about soil water in terms of: the profile is ‘dry’ or ‘about half full’ or ‘close to full’.  Little if any consideration was given to soil water in terms of mm of plant available water (PAW). Probe outputs provide a picture of water in a soil profile which is readily compatible with current thinking. Calibration of the probe is conducted by some groups to convert readings to mm of PAW. This is done without regard to consideration of chemical constraints that may limit root production and root growth at depth. Users can relate this information to their previous experiences and refine rules of thumb they have developed over time to make decisions rather than have a computer model do much of the interpretation that may challenge existing ideas and concepts.

Use of computer models is also less compatible with farmer knowledge and current experience. Farmers don’t spend a lot of time using computer models to ‘think through’ a problem. They prefer to use their own experience to rationalise a concept and to make decisions. Probes are something they can purchase, operate, feel, see and touch, similar to a rain gauge or a piece of machinery. They make more ‘sense’ to growers than computer models.  Score:  Probes – 4, Yield Prophet® – 2

In summary, the growth in the use of probes to understand soil water is easily explained using the framework above.  Whilst not much difference exists in relative advantage of the two systems, probes perform better in the other assessment attributes, except for the observability test.  Both probes and Yield Prophet® are driving the discussion on soil water. They are improving the understanding of the amount of the valuable resource that can (or cannot) be used to produce grain. Yield Prophet® takes the understanding of soil water and its relationship to potential yield to a new level. It provides much more information and interpretation of how crops use that resource in a way that probes cannot.  Yield Prophet® integrates all the information required to make better decisions on crop production (Table 2).

Holy Grail

For optimum interpretation of soil water, combining probe outputs with Yield Prophet® could provide confidence, increase understanding and improve accuracy of soil water relationships between rainfall, evaporation and crop water use.  If the soil moisture probe was calibrated, it could verify site soil water characteristics such as DUL, CLL (and hence PAWC), rooting depth and through flow.  In comparison, the model calculates daily water and N use, provides information on whether the crop is water or N stressed, and critically provides an assessment of the risk, using probability, of achieving a particular target yield, and how much N is required to achieve the target yield.

Whilst the debate may continue as to which is the best, both systems have played significant roles in improving growers and advisers understanding of soil water.

Contact details

Harm van Rees

Cropfacts Pty Ltd

harm@cropfacts.com.au