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Soil tests results are a required input for the OVERSEER® nutrient budgets model. Soil tests required are Olsen P and QT cations (QT K, Ca, Mg, and Na). The other tests listed below are optional unless indicated, and have default values assigned to them. These default values can be overridden if measured values are available.
Sampling depth
Sampling depth should be 0-7.5 cm for pasture and 0-15 cm for crop and horticultural soils.
Default values
Soil group, soil order or soil type (the soil classification) is used to set default soil test values for K reserve test, ASC (anion storage capacity or PR or phosphate retention), carbon (%), clay (%) and field bulk density. These default values are based on typical values for a given soil group, order or type. Generally soil type gives more precise estimation, although data is not available for every soil type. It should be noted that within a soil order or soil group, there are a wide range of values for each test, and hence the typical value may not be a good value to use for a given farm.
Measurement frequency
Olsen P, QT cations, and soil pH should be measured regularly as part of a soil fertility monitoring program. Within the OVERSEER® nutrient budgets model, a soil test database has been added so that soil test values can be stored and plotted out over time. Trends in soil test over time are an important monitoring tool, and interpretation information is also provided on screen.
Optional soil test values should be measured if they affect an output of particular concern. For the K reserve test, the typical data is from historic samples, but it is known that there has been a slow decline in reserve K status over time on some farms. This and the high variability within soil classification groupings suggest that its measurement may be desirable.
Individual tests
Within the OVERSEER® nutrient budgets model, the following soil tests are used as inputs:
| Olsen P |
used to define the amount of plant available P, and hence in the calculations of plant P concentrations (and thus animal transfers around the farm), immobilisation and soil P runoff. |
| QT cations |
used to define the amount of plant available nutrient, and hence in the calculations of plant nutrient concentrations (and thus animal transfers around the farm) and nutrient leaching. |
| Organic S |
required input for the pastoral model only. Used to estimate the degree of buffering of plant available sulphate by mineralistion and immobilisation of organic S. This test should be done initially to determine a typical value, then less frequently as it tends to change only slowly over time.
Note: Although the model provides a method to estimate organic S from sulphate S, this is not always reliable. |
| K reserve test |
TBK K reserve test is used in the calculation of slow release K. A default is based on soil classification. It is recommended that the test is done every 10 years. The double nitric acid test (kc) result supplied by some laboratories should not be used directly but it can be use to set the K reserve status in advanced soil settings based on the following table:
| Kc |
K reserve status |
| > 0.5 |
very high |
| 0.35-0.5 |
high |
| 0.2-0.35 |
medium |
| 0.1-0.2 |
low |
| < 0.1 |
very low |
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| ASC |
used in the calculation of P concentration in runoff waters. It is recommended that this be measured once per block, or occasionally on some peats as the stage of development affects ASC values. A default is based on soil classification. |
| pH, carbon % |
used to estimate the effect of carbon change on maintenance lime requirement in the pastoral model. The model assumes a rate of carbon change based on development status, and a typical pH value. Otherwise, soil carbon (total or Hot water soluble) is tested regularly (every 3 years) over a 10 year period to determine more accurately the rate of change. |
| Clay % |
along with carbon and ASC, clay is used to calculate a structural integrity index, in the calculation of a transport term as part of P runoff indexes within the pastoral model. A default is based on soil classification. |
| Mineralisable N |
optional input (as kg N/ha) for the crop model only and is used to estimate amount of N available prior to start of crop growth. If omitted, then an estimated value is calculated from input information. Sampling depth is typically 0-60 cm although this can depend on the crop and soil. |
| Bulk density |
field bulk density (kg/m3). Optional test in the pastoral model used to estimate the effect of carbon change on maintenance lime requirement (see pH, carbon). A default is based on soil classification. |
Soil texture is also an optional input used in the calculation of a transport term as part of P runoff indexes. A default term is based on soil classification.
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