Usually the coming summer months after the harvest are the quieter months of the year and we do look forward to them. However, we shouldn’t be too relaxed, because important decisions are made during these months that affect next season’s crop productivity and profitability. Stubble management, weed control, decisions about crop rotations and varieties are some of them. Fertiliser decisions are in my mind decisions that should also be tackled during the summer months. Soil and plant nutrition will greatly impact crop yields and this can be managed if your soil nutrient status is known at the start of next season. Thus soil testing provides you a useful benchmark as a starting point for your early season fertiliser decisions. In fact, it is the best management practice due to proven relationships between soil nutrient status and crop yield responses to fertilisers, if sampling is done correctly.
A testing time
CSBP commercialised soil testing in the early 1970’s as a “trouble shooting” tool for poor performing paddocks. Over the years this tool has turned into an objective tool to monitor soil fertility and to adjust fertiliser and soil amendment inputs to optimise returns. Nowadays this aspect of soil testing is becoming even more important as rotations are changing, soil cultivation practices are changing etc. In other words: every year the soil nutrient starting point could be changing too. So, what would be a good soil sampling strategy? Taking soil samples is easy, but we need to keep the end goal of relating the soil sample to fertiliser responses for crop yields in mind and develop a strategy towards that.
A good soil sampling strategy should consider:
Firstly, CSBP demonstrated 20 years ago in whole farm soil testing projects that better soil test-crop fertiliser response relationships are achieved if samples represent the paddock area. This means taking every two to four years a minimum of 15 cores per sample (ideally 20-25 cores) at each site and having a minimum of six sites per paddock area, not sampling urine or dung patches, headlands, paddock edges or parts of the paddock that are not representative of the site. Consider chaff lines, “windrow” effects, areas of low proteins etc. and utilise gamma radiometric maps, biomass imagery or yield/protein maps from your header to define a representative site. If not too late, maybe take grain samples to confirm for instance any suspicions for trace element deficiencies, because those deficiencies are more difficult to be picked up reliably with a soil test. If germination was patchy then maybe also request a water repellence test (MED) when submitting samples to the CSBP Lab.
Secondly, relatively recent research efforts have focussed on increasing the sampling depth for nitrogen, phosphorus, potassium and sulphur to a depth of 30cm instead of 10cm. This too strengthens the soil test – crop response relationship.
Thirdly, the time of sampling and the laboratory analytical methods may influence your soil test results and both should, if logistically possible, therefore be kept consistent, i.e. always apply the same sampling technique in the same month and send the samples to the same laboratory. Regular sampling and monitoring over the years takes out some of the variation and is likely to provide better (long-term) information about soil fertility trends (using indicators like Organic Carbon for soil health, pH and Electrical Conductivity for farming sustainability and phosphorus and potassium for long-term fertiliser management) than a single snap-shot of soil sampling. Regular soil sampling can also detect when nutrient depletion is likely to occur if the fertiliser management is not adjusted. This is particularly true for phosphorus and potassium when compared with more mobile nutrients in the soil (nitrogen and sulphur).
Fourthly, CSBP soil analysis and interpretation (NUlogic) takes into account a balanced nutrition for optimal plant production for WA soil and climate conditions. It also offers a nutrient recommendation based on the maximum gross margin. Therefore using this service will give growers the best outcome from the soil sampling exercise to increase productivity and profitability. The interpretation is a result of continuous CSBP field agronomy trial data. All CSBP advisers are trained in the software package and can value-add soil test results by using this leading WA soil and plant interpretation service.
In summary, soil sampling is the best scientifically proven method to base fertiliser decisions on, in order to increase productivity and profitability from fertiliser inputs. Correct soil sampling considers representative sampling, sampling to depth, regular sampling, an ASPAC accredited laboratory and an advanced interpretation service developed on local conditions.