Now is a good time to test tillage soils for phosphorus (P) and potassium (K) and establish soil levels before winter crops are planted. Soil-available P and K are essential during the establishment phase for root development and tiller production.
Supply of K is also critical to maintain plant moisture in the growing crop and to maximise the efficiency of N fertiliser in the crop.
A well-developed root system is critical to supply both nutrient and water requirements during the growing season. Research indicates that cereals grown on fertile soils with an index of at least 3 are more resistant to root diseases, such as Take All.
Soils will have different levels of available P and K due to natural fertility and build up from the application of fertilisers and manures over time.
The level of available soil P and K at which yield response levels out is known as the critical soil P and K level (see table 2, right). Below the critical level, such as soil index 1 or 2, there is a decrease in yield and a financial loss. Above a critical level of index 4 there is no justification to further increase soil P and K through fertiliser/ manure applications, and doing so can add an unnecessary financial cost to the grower.
It is generally understood that the critical level will not change but there may be differences in grain yield depending on seasonal factors between growing seasons. Soils supplied with adequate soil P and K will generally not show a yield response in cereals to additional P and K either in manure or fertilisers.
Currently it is accepted that when soils are maintained at the critical levels (index 3) for that soil type, then the amount of P and K applied should replace crop off-take. When soils are below the critical level additional P and K needs to be applied to build soil reserves close to critical levels.
Winter Oilseed Rape
Winter oilseeds will be the first crop to be sown this autumn. Where soil P and K levels are low, P and K should be applied as shown in table 3 (right) and incorporated into the seed bed at sowing. Remember that all P fertiliser application must be completed by September 15 under cross-compliance. Sufficient P and K is required during the establishment phase for rapid root development and good early ground cover. Rapid crop establishment will help reduce weed problems and full ground cover will help reduce pigeon damage over the winter period.
Organic manures are an effective source of N, P and K and can satisfy early crop requirements. For example, an application of 22m3/ha of good quality (7pc DM) cattle slurry can supply a proportion of the P requirements and all the crop's K requirements (around 13kg P and 95kg K). Oilseed rape has the ability to utilise N over the autumn period as the crop can take up and store significant levels of N in above-ground canopy.
Winter Cereals: P and K Recommendations
Modern winter cereal varieties (wheat, barley, oats) have a very high yield potential (10t/ha plus) and therefore high P and K requirements. For example a 10t/ha winter wheat crop removes about 38kg P and 100kg K/ha between grain and straw at harvest time. To maintain soil fertility, nutrients removed in grain and straw must be replaced. Otherwise soil fertility will decline and future crop yield potential will be comprised.
Phosphorus advice for cereal crops is shown in table 4 (below) for a range of grain yields. P advice can be adjusted based on proven previous crop yields. The reference yield for cereals is 6.5t/ha and an additional 3.8kg P/t can be applied for every tonne above the reference yield. For example a 9.5t/ha cereal crop grown on an index 3 soil is now permitted 36kg P/ha (25 + 11.4 = 36.4kg/ha).
The K advice, as shown in table 5 (below), is for winter cereal crops where straw is removed. K applications need to be adjusted based on the crop's yield potential. For wheat and barley crops increase or decrease the K rate by 10kg/ha per tonne depending on crop yield potential. Potassium advice should be adjusted to take account of soil K release. Soil K release will vary depending on soil's clay content -- for example heavy soils will release in the region of 20-40kg K/ha/yr, while light soils will release relatively small amounts of K annually.
Soil K will fall faster than soil P as the crop removes more K than P. On index 4 soils K applications can be omitted as there will be no response to freshly applied K. It remains vital to test soils regularly and monitor the reduction in soil K in the absence of K applications.
K-fixing soils give low soil K reading and render applied K unavailable over time. It is virtually impossible to raise the K status of these soils to index 3/4. Apply rates close to maintenance (index 3) levels each year.
Farm P and K fertiliser programmes should aim to build soil fertility levels to soil index 3 for maximum crop yields. Organic manures are the most cost-effective route, where available, to build soil fertility. Regular soil analysis is essential to monitor changes in soil fertility levels. Fertiliser programmes should be adjusted based on current soil fertility levels and crop requirements.
Trace element deficiencies that occur most frequently in winter crops are manganese (Mn), copper (Cu) and zinc (Zn). Crops require trace elements in small amounts and insufficient supply will reduce crop yield potential through poor emergence and reduced ear numbers. Soil trace element levels will depend on soil type and parent material. Soil and seasonal factors, such as soil temperature, moisture content, seedbed consolidation, soil pH and soil compaction will inhibit availability.
Soil analysis will provide information on Cu and Zn levels and a preventative programme can be put in place. The soil test is useful in predicting Mn deficiency. For example, high soil pH will reduce the availability of soil Mn and Zn and this needs to be considered when interpreting soil analytical results. For example, Mn has a key role to play in cereal crops in terms of rooting and tillering. In addition, Mn will reduce the risk of take-all infection in barley and wheat crops. Consult with your local adviser to identify control measures where different trace element deficiencies are present.
Over the coming weeks, oilseed rape and cereal crops will be harvested and the yields will be useful information to identify fields that may have underlying soil fertility problems. Contact your local adviser to organise taking soil samples to establish soil fertility levels and use this information to plan crop type and crop rotations for next year. Where soil fertility levels are low, apply lime, P and K before sowing to ensure well rooted and tiller crops entering the winter period. This is the first step to establishing good foundations for high-yielding crops next year.
Mark Plunkett is a soil and plant nutrition specialist with Teagasc, Johnstown Castle