PJ Phelan: Make sure you get your P application spot-on

Recent fertiliser price increases have focused our minds further.

High yields in 2021 have also resulted in greater crop removals of nutrients. Those nutrients must be replaced unless you are prepared to allow soil fertility to drop.

Regulations tie you to application limits based on soil analysis for which soil results cannot be more than four years old.

You may assume index 3 for phosphorus unless there is an older soil sample showing that the soil was P index 4.

The phosphorus requirement for/following a 10t/ha crop (4t/ac) on a P index 1 soil is 58kg/ha P (46 units/ac); on P index 2 soil is 48kg/ha P (38 units/ac P); on P index 3 soil is 38 kg/ha P (30 units/ac); and on P index 4 soil is 0 unless the soil pH is 7 or over.

Applications lower than those levels will result in reduced soil fertility.

On Index 1 and 2 soils there is a substantial benefit from combine-drilling the phosphorus for spring cereals.

Research has shown ‘no consistent’ yield benefit to autumn application of phosphorus for winter cereals, even on P Index 1 and 2 soils.

Phosphorus application after September 15 is not permitted except for P Index 1 and 2 soils where 20kP/ha may be incorporated prior to October 31.

The closed period for chemical P application (and indeed nitrogen) extends to January 12th/15 or 31 depending on the region.

Spring application of P should take place as soon as possible once the closed period has ended.

I consider that farmers sowing winter cereals into poor-fertility/ heavy soils should either combine-drill or incorporate the 20kg P at/before sowing; if they fail to sow on time, they should wait until spring to sow.

Phosphorus is not mobile and will not move into the rooting zone, following top-dressing.

While organic manures are very valuable in a fertiliser plan they should not be relied on as the sole source of P/K on Index 1 or 2 soils as they are slower to become available for plant growth.

While soil P Indices limit phosphorus applications, the most important factor is the trend in soil P levels.

Dropping levels indicate inadequate applications and increasing levels surplus.

Surplus applications are ideal if you are trying to build levels, but once you reach 10ml P/l soil, the risk of soil run-off carrying P to water is not acceptable.

In addition building soil levels to greater than 10mg/l will result in some of the P becoming unavailable for plant growth — similar to lodging money to a bank and being caught with a negative interest rate.

While potassium (K) applications are not restricted by regulation, soil analysis results are equally important for crop yield.

K is a critical nutrient for making crops resilient to drought. In a dry year crops on high-K soils will outperform those on low-K soils.

The soil K requirement varies with soil constituents and some soils will lock up K. Therefore local knowledge is important in determining soil K rates and timing of applications.

The K requirement for/following a 10t/ha crop (4t/ac), where straw has been removed on a P Index 1 soil is 130kg/ha K (104 units/ac); on P Index 2 soil is 115kg/ha K (92 units/ac P); on P Index 3 soil is 100 kg/ha K (80 units/ac) and on a P Index 4 soil is 0.

Where straw has not been removed those levels, following a 10t/ha crop (4t/ac), may be reduced by 50kg/ha (48 units/ac).

Farmers who availed of the straw incorporation measure will therefore have saved themselves the equivalent of 1 bag/ac of muriate of potash for 21/22.

Without soil analysis fertiliser — the most critical input for crop production — applications are guesswork.

Check through your soil analysis results now and sample all soils where results are three years old or greater.

If you have not being doing analysis for magnesium, manganese, copper or zinc, I suggest that you do at least one soil sample for them on each soil type you are farming.

PJ Phelan is a tillage advisor based in Tipperary; he is a member of the ACA and ITCA