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Maintaining correct P and K levels on cereal ground


While fertiliser prices may be tempting many growers to reduce or omit phosphorus (P) and potassium (K) applications this year, reductions will affect soil fertility. Initially, there may not be visual signs to reducing P and K applications compared to a reduction in nitrogen (N) applications. However, soils that reach low fertility (Index 1) will produce lower grain yields and quality.

So the only way to safely reduce fertiliser inputs is to establish the fertility status of the soils on your farm. Bear in mind that over the past 25 years the grain yield potential of winter and spring cereals have increased dramatically, resulting in higher seasonal demands for P and K.

The national fertiliser use survey shows that, on average, only half the P and K requirements are actually applied for winter cereal crops. This is being reflected in the results of soil samples taken on farms. These show that over the past four years soil fertility levels in tillage soils have been declining (see table 1, right). These results show that the proportion of soils at P Index 1 and 2 has increased from 46 to 57pc. Soils tested at the optimum soil P Index 3 have declined from 26 to 21pc.

The soil index system divides soils into one of four soil Index levels based on the soil test result and the expected response to nutrients applied annually (see table 2, below right). The ideal is a soil Index 3, but fertility changes slowly over time. For example, it often takes years to build Index 1 and 2 soils up to an Index 3. The rate of soil build up will depend on factors such as soil type, nutrient application rate, and the level of nutrient removed.

Phosphorus allowances for cereal crops can now be adjusted to take account of higher grain yields. An additional 3.8kg P can be applied for every 1t above the reference yield of 6.5t/ha for cereal crops (see table 3, below right). For example, a 10.5t/ha winter cereal crop grown on an Index 3 soil can now receive an application of 40.2 kg P/ha (25 + 4 x 3.8 = 40.2 kg/ha). For soil P build up, add an additional 10kg P/ha per soil P Index below Index 3 (see table 3, below right). Apply additional P allowance where soil P levels are declining or target grain yields are not being achieved.

The incorporation of straw will recycle relatively small amounts of P. For example, a 10.5t/ha winter wheat grain crop will return approximately 4kg/ha of P to the soil. The majority of the P in the crop leaves the field in grain and needs to be replaced.

Cereal crop K advice for wheat, barley and oats is shown in table 4 (below right). Higher yielding crops will have a higher K demand during the growing season and higher K off takes in grain and straw. Adjust K advice based on crop yield potential. For example, for each 1t increase/decrease, adjust K rates by 10kg/ha for barley or wheat and 15kg/ha for oats. Compare previous and current soil test results to monitor changes in soil K levels and effectiveness of previous K applications. This will give a good indication of the soil's ability to release/supply crop K requirements.

Soil K build up will fall faster than soil P as the crop removes up to 3-3.5 times more K than P. On Index 4 soils, K applications can be omitted as there is no response to freshly applied K. It is recommended to omit K for one year and then revert to Index 3 advice until soils are tested again.

Straw incorporation will help reduce K fertiliser costs as straw will return significant amounts of K annually. For example, a 10t/ha winter wheat crop will return in the region of 50kg K/ha. Straw value and fertiliser prices should be considered when deciding to chop or bale straw.

K fixing soils have low soil test K levels, and applied K usually becomes unavailable over time. It is virtually impossible to build the K status of these soils. Therefore it is recommended to apply maintenance Index 3 levels each year.

Nutrient balance soil test results will show the soil pH and the lime requirement. Aim to maintain the soil pH at 6.5 for cereal crops. In addition, check other nutrient levels such as soil magnesium, manganese, copper and zinc that are equally essential to cereal crops, even though required in small amounts compared to P or K. For example, the most effective way to improve soil magnesium levels is by applying magnesium limestone when liming soils. Cereal crops are most responsive to sulphur (S) on light sandy soils. It is recommended to apply 15kg/ha of S each year.

Mark Plunkett is a soil and plant nutrition specialist at Johnstown Castle, Wexford

Indo Farming