Mark Plunkett: Tips for balancing soil P and K in a high-yielding year

Inputs: Both winter and spring cereals demand a good supply of P and K during the growing season

Mark Plunkett

February 07 2023 02:30 AM

Over the last decade, soil fertility on tillage farms has improved at a rate of around 1pc per year, thaks to regular soil testing and attention to detail in formulating suitable fertiliser types to supply balanced crop nutrition.

In 2022, the application of both phosphorus (P) and potassium (K) fertilisers as reported by the Department reduced by about 25pc, as farmers looked to keep costs down amid mounting fertiliser prices.

A closer look at typical fertiliser types show that the majority of blends have reduced significantly compared to 2021, as shown in table 1.

Reducing P and K applications over time will run down soil fertility built up over the last decade, which may not be evident until fields are re-sampled.

Both winter and spring cereals demand a good supply of P and K during the growing season.

The good growing conditions in 2022 produced higher grain yields than normal for the majority of crops. Spring barley yielded exceptionally well with reports of up to 9.5t/ha, removing 36kg P/ha (29 units/ac) and 108kg K/ha (86 units/ac) between grain and straw.

Such crops may have been fertilised for an average grain yield of 7.5t/ha, thus removing 21pc more P and K at harvest time.

Additional P and K will be required to replenish soil fertility reserves — but the application can be deferred until the cost of fertiliser decreases.

Organic sources of P and K such as slurry, poultry manures and dairy sludge should be considered.

Take the example above with a higher grain yield of 2.0t/ha, which removed around 7.5kg P (6 units/ac) and 23kg K/ha (18 units K/ac). An application of 10m³/ha (1,000gals/ac) of pig slurry will replenish P and K removed in higher grain yields.

This will help maintain soil fertility levels, which is essential for the efficient of applied nitrogen, sustaining high grain yields in the years ahead.

With high fertiliser prices, taking extra soil samples will provide the basis to tailor crop P and K applications based on soil fertility levels.

On high-fertility soils (Index 4) omit P or K applications to mine soil reserves. For example, for P Index 4 soils (> 10mg/l) omit P for 2-3 years, while for K Index 4 soils (> 150mg/l) omit K for 1 year, and revert to K Index 3 advice until the next soil sample.

Where P or K applications were omitted in 2022, now is a good time to re-sample those fields to see the effects on soil fertility levels. Table 2 shows recommended rates of P and K for a spring barley crop yielding 7.5t/ha.

Practices such as chopping straw will help retain more nutrients on farm. For example 10pc of P and 50pc K is retained in the straw.

Cover cropping after harvest will help reduce losses over the winter period and retain major soil nutrients.

Where organic manures are available locally, they offer a great opportunity to supply valuable crop nutrients and add soil organic matter to improve soil structure and health over time.

Finally don’t forget lime: around 40pc of our tillage soils have a lime requirement, which is central to regulating soil nutrient supply and the efficient use of applied nutrients.

Mark Plunkett is a Teagasc soil and plant nutrition specialist based at Johnstown Castle

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Mark Plunkett: Tips for balancing soil P and K in a high-yielding year

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