The soil fertility debate: Choosing between different recommendation systems

By Bill Bowden (Department of Agriculture and Food Western Australia)

This article appeared in the Australian Soil Club Newsletter in June 2004.


There has been a proliferation of fertiliser recommendation systems being offered to growers in Australia. The purpose of this summary is to address the guidelines for discriminating between the many systems on offer in Australia today.


There has been a proliferation of fertiliser recommendation systems being offered to growers in Australia. This has resulted in a heavy demand for “an objective or unbiased view” on the pros and cons of each and even an opinion on which is best. The nature of my training and background experience inevitably affects my outlook and means that I am biased towards the traditional approach. My credentials for this task are that I have had over 35 years of experience in trying to develop fertiliser recommendation systems which are adapted to WA conditions. These systems include soil and tissue testing, calculation from simple budgets to more complex modelling, trial strips and burnt windrow comparisons. At the WA Department of Agriculture, we have a range of fertiliser decision-making tools which we hope are used on a “horses for courses” basis.

The purpose of this summary is to address the guidelines for discriminating between the many systems on offer in Australia today; ie some of the rules for addressing the soil fertility debate.

The fertility debate

The main controversy has arisen from a belief by some growers that their soil fertility has deteriorated markedly as a result of using traditional fertility management practices. While there has been a lot of soil degradation in the past decades, it is simplistic to blame fertiliser management alone. Intensity of cropping, cultivation practices and greater productivity (and profitability) all have a role to play. And, in fact, many of the fertiliser management practices have had demonstrable, positive effects on soil fertility (eg soil P status, soil organic matter levels on light soils).

The debate has been fuelled by the introduction and offering of alternative approaches to soil fertility management. The main question is whether you should fertilise to improve the soil or to maximise cash returns from the crops. You can do both at the same time but there is a time dimension to cash flow which often means that these alternative philosophies come into conflict.

Overriding and confusing the debate is the credibility of different operators and the soil testing and interpretation methods they use.

Four features of credible fertiliser recommendation systems

Feature 1

Evidence that a recommendation system has been adapted for local farming conditions. Locally determined critical levels depend not only on how different crops were grown but also on things as mundane as the soil sampling and chemical analysis techniques which were used for those calibrations.

Why is local calibration so necessary and what is it?

The principles of soil fertility and plant nutrition are equally applicable anywhere in the world, but in any given environment, agriculture is unique and any recommendation system of any value has to pay more than lip service to that uniqueness. The yield responses depend as much on the growing conditions and crop demand as they do on the level of extractable nutrient. In WA, our unique mix of soils, seasons and crops mean that it is quite unlikely that critical levels determined elsewhere in the world might be relevant for us. I can give you examples of how such transference of results from one side of the world to the other have gone horribly wrong - even from one end of our agricultural areas to the other poses major problems for us.

Any one can analyse a soil sample for any element known to mankind. The problem is in making those analyses meaningful in terms of the likely responses of our commercial crops to remedial dressings of fertilisers or ameliorants. To determine whether our crops respond to nutrient inputs, we have to carry out trials with those crops in our environment. For each trial you get a soil test figure and a level of response. A set of data is developed from the results which can be used to determine a critical level in the soil below which you expect significant responses. Unfortunately there are few short cuts for this work.

Going along with this need for soil analyses to be related to local crop response, is a need for the sampling and chemistry to be the same as that used for determining the critical levels. If it is not the same, then the original trial work does not have to be repeated, but adjustments do have to be made by matching the new with the original soil test. We have soils in our soil bank (soils collected from ‘calibration’ trials and stored in South Perth – since about 1972-3), which can and have been used for this purpose.

In WA we have local soil test calibrations for the major nutrients: phosphorus (P), potassium (K), sulphur (S - CSBP). We have nitrogen (N) recommendation systems based on literally thousands of local trials. There are local calibrations/critical levels for the trace elements copper (Cu) and zinc (Zn). We have local data (soil pH and aluminium (Al) critical levels) for determining whether you need lime or not. And we have some local data on whether our soils with structure problems, are likely to be responsive to gypsum.

Feature 2

A credible recommendation system should be tailored to deliver advice for the highs and lows of crop demand, which vary according to management and season. Higher yielding crops need more nutrients (from either soil or fertiliser) than lower yielding crops. Several studies have shown that yield demand is equal to, if not more important than the soil supply of nutrients in determining the fertiliser requirements of crops.

Feature 3

A good recommendation system should provide a sensitivity analysis; i.e. it should show you the yield, quality and dollar consequences of spending more or less dollars on fertiliser, for different season or management scenarios. We found that this capability was invaluable in helping farmers in times of financial stress such as following several years of drought. What are the dollar consequences of cutting fertiliser rates? What if the coming season is a scorcher? The answers to the dollar questions are VERY individualistic; they vary from farmer to farmer, with stage in life and with short-term cash flow shortages or surpluses. A sensitivity analysis helps address the “how much fertiliser?” questions from an individual’s point of view.

Feature 4

A credible adviser or advisory system will try to give you not only short-term (“apply this amount of this fertiliser this year”) but also the more long-term implications for, and fertility requirements of, your soils. A recommendation delivered as a computer printout may look like it meets your individual needs, but can not replace a good adviser – although dollars may well dictate that you go for the former.

“Feed the crop” versus “Feed the soil” debate

Many farmers crave for more than simple fertiliser advice. They want more holistic fertility advice. There is a school which plays up some of the soil destructive effects while playing down the positive, productive effects of simple crop orientated fertiliser systems. They say that if you fertilise/ ameliorate to bring the soil up to target fertility levels, then crop health and production will take care of itself. Such soil fertility targets, particularly when they have been established elsewhere in the world, should always be questioned for local applicability and economic viability.

Having highly weathered soils of low, natural, nutrient status, means that most of the chemical fertility in WA soils, has been developed using producers’ scarce funds. If you are tight for funds then you have to get a dollar return relatively soon after you have invested the dollar in fertility. The most immediate dollar return comes from responses in the current crop and I guess that is why our major research investment has gone into understanding crop rather than soil, requirements. Responding to short- term (immediate crop demand) versus long-term (run down of soil nutrient status) issues is a continuing bone of contention between the competing recommendation systems. Macronutrient cation (potassium (K), calcium (Ca) and magnesium (Mg)) nutrition is a case in point.

For example, back in the ‘60s, we knew that K deficiency of crops and pastures was going to become more widespread, because K fertility is low on the light soils in WA and we were running down stocks in the products being removed. However, we did not adopt a system to replace the potassium we remove until we knew we were running into danger of having major losses of yield due to K deficiency. You do not put K dollars out onto our K fertile soils just because the crop removes K – it might take 100 years before you get a dollar back for that investment. Now that much of the agricultural areas have run down, we must use K fertilisers. And because our diagnostic systems are not foolproof, we should be considering insurance dressings of K before the critical soil fertility levels are reached. In WA, Ca and Mg nutrition will follow the same history as K. Already we are seeing occasional Ca deficiency in sensitive crops such as canola and if we continue the trend towards using Ca free fertilisers, paying responses will become more frequent.

This short-term thinking obviously goes wrong when early treatment could stop a long term, expensive-to-cure problem from arising. Such is the case with liming to prevent subsoil acidification. Other soil ameliorations sometimes take time to have a paying effect. Overcoming surface sealing, rainfall infiltration and soil structure problems using gypsum may not give a return until a season comes when having an “all week soil” rather than a “Sunday soil” lets you seed a few hundred hectares which otherwise would have been left out of the cropping program.

Calcium/magnesium ratios

The use of critical soil calcium to magnesium ratios is sometimes used as a determinant of soil health and fertility. Ratios outside certain limits are used to justify the use of large quantities of lime, dolomite and gypsum.

For crop nutrition where you think in terms of 10s or maybe 100s of kg/ha, I reckon you fertilise the crop according to the sufficiency theory (which says use fertiliser when your soil nutrient levels are below some “critical” point). There is no solid local, or, despite decades of research, overseas evidence that the ratios are any better than the sufficiency levels for determining nutrient requirements.

When it comes to ameliorating soils (lime, dolomite and gypsum), you have to use tonnes/ha to have an impact on soil properties (unless you only want to change the nature of small proportions of the soil).

Traditionally we have used pH and aluminium tests as well as crop bioassays, to determine whether we need lime. Ratios do not help us much in this decision although you might change your lime to dolomite if Mg levels are low and the dolomite is sufficiently cost effective.

For changing soil physical properties such as surface sealing and soil structure, we traditionally recommend gypsum if soil clods disperse in rainwater. For soils of low pH (and high buffering?) you could use lime rather than gypsum to provide the calcium to stop this dispersion. I have yet to see the evidence for the value of using lime on soils of pH 7 and above where lime is up to 100 times less soluble than gypsum. There is anecdotal evidence that lime has a longer lasting effect than gypsum under such conditions but I do not know why and would welcome any data and suggestions on how it works.

Farmers who I respect have told me that they believe the Ca/Mg ratio story because in their experience “it works” and it explains things like “sticky when wet and sets like a brick when dry” and that their “best and most productive soils have the right ratios”. I admit to having a problem with anecdotal evidence and testimonials. The observations are usually correct, but often the comparisons are invalid because of the confounding of several variables (eg “best and most productive soils” have a lot of good properties, other than the right ratios, going for them). As a result, the interpretations can be quite misleading.

Again, nothing beats having some local data which justifies the claims. In WA, I know of only two efforts to test the ratio claims in a relatively valid way. One was at Kojonup where recommendations based on the 2 main methods were tested. The ratio method costing approximately $700/ ha over several years, gave the same or less returns than the local recommendation costing about $60/ ha. The other was at the WANTFA site at Meckering where the Ca/Mg ratio was changed down to less than 1 with 17 tonnes/ha of Epsom salts (magnesium sulphate) and up to 16 with 14t/ha of gypsum (calcium sulphate). Once the excess salt was washed out of the soil, there was very little difference in the performance of barley or wheat across the ratios. There is a trial testing the use of lime on poorly structured clays near Gnowangerup and it is yet to show any effects in either the soil chemistry or crop performance – still, early days!

Credible fertiliser recommendation systems use locally calibrated diagnostic methods, adjust fertiliser rates for different yield potentials and allow you the grower to see the yield, quality and dollar consequences of your fertilising decisions. Whether you invest your dollars in short or long-term fertility is a personal decision which can be made with the help of good advice from credible nutritional consultants. Ask your adviser on what he bases his recommendations and worry if they do not have a large component of local calibration/validation behind them. Testimonials are rarely credible authority for recommendation systems.


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