Interpreting soil test results 2023
Following the wet season of 2022, it is more important than ever to undertake soil testing.
Why?
- the high N uptake year
- what are the waterlogging effects from 2022.
With average to above average grain yields over the past three seasons soil nutrient removal is predicted to be high. The table below provides an indication of the levels of nitrogen (N) and phosphorus (P) likely removed from the soil in different yield and protein scenarios. Protein levels in harvested cereal grain provides a good indication of the N status of the soil. Wheat with greater than 11% protein and barley with greater than 10% protein last season would most likely have had sufficient N applied to ensure yield potential was not limited by N. In these lower protein paddocks, consider a legume crop to improve soil N status.
Table 1: Nitrogen (kg N/ha) exported from paddocks from cereal crops
|
Cereal Yield |
Protein (%) | ||||
| 9.50% | 10.50% | 11.50% | 13% | ||
| Yield (t/ha) | 2 t/ha | 33 kg N/ha | 37 kg N/ha | 40 kg N/ha | 45 kg N/ha |
| 3 t/ha | 50 kg N/ha | 55 kg N/ha | 60 kg N/ha | 68 kg N/ha | |
| 4 t/ha | 66 kg N/ha | 73 kg N/ha | 80 kg N/ha | 90 kg N/ha | |
| 5 t/ha | 83 kg N/ha | 91 kg N/ha | 100 kg N/ha | 113 kg N/ha | |
| 6 t/ha | 99 kg N/ha | 110 kg N/ha | 120 kg N/ha | 136 kg N/ha | |
P fertiliser rates: Coming off above average yields in 2022, double check that your standard replacement rates will be sufficient to cover the P removed in 2020.
Recent research, (H. van Rees 2021) has indicated that wheat yields increased by 0.3t/ha from 5.8t/ha to 6.1t/ha when P rates increased from 17.4 kg P/ha (80kg/ha MAP at replacement rate) to 35 kg P/ha (161kg/ha MAP), which increased the gross margin by $22/ha. Based off this finding and the high amount of P removed in 2022, do not reduce P rates in the 2023 season.
Table 2: N and P removal across a range of crop types and yields
| Crop | Yield (t/ha) | Protein (%) | Nitrogen removed (kg N/ha) | Phosphorous removed (kg P/ha) |
| Wheat | 6.0 | 10.5 | 108 | 24 |
| Wheat | 4.0 | 10.5 | 72 | 16 |
| Wheat | 2.0 | 10.5 | 36 | 8 |
| Barley | 6.0 | 10.0 | 102 | 18 |
| Barley | 4.0 | 10.0 | 68 | 12 |
| Barley | 2.0 | 10.0 | 34 | 6 |
| Canola | 2.0 | 76 | 18 | |
| Lentils | 2.0 | 86 | 12 | |
| Chickpeas | 2.0 | 78 | 12 | |
| Field peas | 2.0 | 90 | 12 |
Breakeven point of soil sampling:
The cost of sampling a paddock = $150 sampling (members price) + $129 lab costs (N + P + moisture for 3 depths) + $10/sample (postage) $30 = $309 per paddock.
To break even on the cost of soil sampling, you would only need to grow an extra 1kg/ha of wheat per hectare.
When to soil sample: Ideally, sample as close to sowing as possible. Allow at least a month to get the samples to the lab, receive results, and interpret your results into a fertiliser recommendation. To have done this by ANZAC Day, get your soil sampling done by the 25 of March.
Converting soil test results to kilograms/hectare
To convert N, S and K results to kg/ha, use the following formula for each sampling increment:
Nutrient result x Bulk density x (layer depth in cm divided by 10 ) = soil nutrient kg/ha
___mg/kg x ___g/cm3 x (___cm/10) = ___kg/ha
Then add the results from each increment together:
Layer 1 + Layer 2 _ Layer 3 + layer 4 = Total nutrient in soil
___kg/ha + ___kg/ha + ___kg/ha + ___kg/ha = ___kg/ha
Use the table below to pick an indicative bulk density multiplier for your soil type:
| Soil texture | Bulk density (g/cm3) |
| Coarse sand | 1.3-1.8 |
| Fine sand | 1.3 |
| Light sandy clay loam | 1.3-1.6 |
| Loam | 1.1-1.4 |
| Sandy clay loam | 1.3-1.6 |
| Clay loam | 1.3-1.6 |
| Clay | 1.3-1.5 |
| Self mulching clay | 1.2-1.3 |
Importance of knowing soil N before sowing: Understanding soil N levels coming into the 2023 cropping season will allow growers and advisors to make targeted decisions on N application rates, N application timings and crop rotations. Research from a five-year experiment in Curyo found that N bank strategies of 125kg N/ha or Yield Prophet® 50% are the most profitable over time (Hunt et al. 2022). This work was supported by the Mallee Catchment Management Authority through funding from the Australian Government’s National Landcare Program. Coming into the 2023 season, knowing the starting soil N allows growers to make targeted decisions that lead to better returns, such as deciding to grow a pulse in low N paddocks rather than applying large amounts of urea, even though urea prices are coming down) to meet yield potential.
When soil N levels are low (<~40 kg N/ha) research has shown there are opportunities to increase wheat yield by applying some N prior to Z30. Waiting until post Z30 allows for a better understanding of the season’s yield potential and creates logistical flexibilities. Prioritise applying some N at sowing on low N paddocks to increase tillering to maximise yield potential. This could be undertaken through the seeder at sowing or broadcast in front of the seeder to allow for higher rates while minimising risk of fertiliser toxicity and also allows you to spread out some tasks on farm.
Book your soil sampling here or call Thomas Jones BCG on 0458 409 205.
