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Currently, there are a number of external factors, outside of our control, putting downward pressure on grain prices. The instant reaction is to cut fertiliser inputs where possible to protect the bottom line.
As fertilisers represent a significant cost in producing one tonne of cereal grain, there is much discussion to reduce or omit phosphorus (P) and potassium (K) applications for the 2017 crop. Currently, fertiliser prices (P and K) are back on 2016 prices and represent approximately 27% of total production costs.
Profitable crop production depends on three pillars; cost per tonne, grain yield and price per tonne. Soil fertility is a key driver of grain yield. Therefore, now more than ever, the focus must be on tailoring effective nutrient programmes to drive crop yield potential while keeping a close eye on our costs per tonne to deliver a positive crop margin at harvest time.
Soil pH, P and K fertility
Recent results from Johnstown Castle show that 85% of tillage soils are deficient in one or more of the major soil nutrients (pH, P or K).
For example, 55% of tillage soils require lime, while 59% of soils have very low to low levels (Index 1 or 2) of P and K. Twenty-one percent of fields have optimum P and K (Index 3) levels for cereal production, while 20% of soils are Index 4 for P or K and offer an opportunity to reduce costs in the current year.
This highlights the importance of knowing the soil’s fertility status and working from a recent soil test report to make best use of applied nutrients.
Target levels
Soils will have different levels of available P and K which have been built up from the application of fertilisers and manures over time. The level of available soil P and K at which yield response plateaus is known as the critical soil P and K level (see Table 1).
Below the critical level, there is a decrease in yield and a financial loss (Index 1 and 2). Above the critical level (Index 4), there is no justification to further increase soil P and K through fertiliser/manure applications and doing so can be an unnecessary financial cost.
It is generally understood that the critical level will not change, but there may be differences in grain yield depending on seasonal factors (weather) between growing seasons. Soils supplied with adequate (Index 3) soil P and K will generally not show a yield response in cereals to additional P and K either in manure or fertilisers.
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 (see Table 2). When soils are below the critical level, additional P and K above that removed in crop off-takes needs to be applied to build soil reserves close to critical levels (see Table 3).
Advice
P and K advice for cereals needs to be adjusted to take account of higher yields. Over the last two years, grain yields have exceeded the national average for winter and spring cereals crops by up to 2t/ha.
The extra yield will have removed more P and K at harvest time and will need to be taken into account in order to maintain the soil’s yield potential.
Phosphorus is critical for rooting and tillering in both winter and spring cereal crops, especially on low to very low fertility soils. The majority of P is removed in the grain at harvest time, while a small proportion of P is removed in straw.
Cereal crops have a large seasonal demand for K and significant amounts are removed in high-yielding cereal crops. At harvest time, 50% of the crop K is in the grain, while the remaining 50% is in the straw. Where straw was not removed from fields in 2016, K rates should be adjusted to take account of K returned in chopped straw.
Maintenance rates
Table 2 shows the rates of P and K required to produce a cereal crop depending on the target grain yield. Phosphorus rates are the same for all cereal crops with a base grain yield requiring 25kg P/ha to produce a grain yield of 6.5t/ha. For every extra 1t above the base yield, an additional 3.8 kg/ha of P is required.
Potassium rates are different, depending on the cereal type. For example, oats and spring cereal crops remove more K than either winter barley or wheat.
The rates shown in Table 2 will replace the P and K removed at harvest time depending on the grain yield. Where soil P and K fertility is at Index 3, these rates will maintain soil fertility at the optimum index for grain yield.
When deciding on the rate to apply, take the highest grain yield in any one of the last three years and use this as your target grain yield to calculate your P and K crop requirements for 2017.
Build-up rates
Building soil P and K levels requires additional nutrient to increase soils at Index 1 or 2 to the optimum Index 3. The rate of soil build-up will depend on the soil type and assumes soils are at the optimum soil pH (6.5).
Index 3 soils have a higher grain yield potential, as these soils have a higher supply of P and K, thereby producing higher grain yields. Nutrient advice for Index 3 soils is aimed at replacing P and K removed at harvest time in grain and straw and don’t require build-up rates.
Applying build-up rates of P and K results in building the soil fertility and yield potential of your soils over time.
The yield potential of winter wheat, for example, is very much related to soil fertility index. Recent research shows that an Index 3 soil will yield up to 1.75t/ha more than an Index 1 soil.
For spring cereals, P placement on low-fertility soils will increase the crop P utilisation and ensure that P is available at an early stage to develop the crop’s yield potential.
For example, spring barley develops yield potential in the first three to six weeks after sowing and having an available supply of nitrogen (N), P and K close to the seed/rooting zone is critical to develop the crop’s yield potential in terms of tiller production.
Winter wheat is different in that it develops its yield potential over a longer period and has good ability to compensate for yield by producing more grains per ear.
Table 3 shows the additional P and K required for building soil fertility. Building soil fertility is a long-term investment and it will pay dividends in terms of extra grain yield. Maintaining a soil pH 6.5 is a critical factor to improving soil P and K levels and should be a priority consideration when aiming to build soil fertility.
Table 4 shows the recommended rates of P and K for a 7.5t/ha (3t/ac) spring barley crop on P and K Index 2 soils, while Table 5 shows the recommended rates of P and K for an 11t/ha (4.5t/ac) winter wheat crop on P and K Index 2 soils.
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