There are two major reasons why we should test soils. One is to balance soil fertility and the second is that it is a legal requirement. But for commercial farmers, the former should be by far the main reason. The guidelines for nutrient requirements are taken from Teagasc’s major and micro nutrient advice for productive agricultural crops.

Estimating your farm phosphorus and potash requirements based on soil tests is complex but possible. In the example in Table 1, I have taken five different crops growing on specified areas using the most recent soil test results.

I have also taken a liberty in this example because, technically, I should have a separate test result for every 5ha but I am only using one per crop. I am assuming straw is removed in all instances. The yield levels used are verifiable for each crop on this farm.

P allowance

Look at P and winter barley first. The winter barley crop in parcel A is given a basic allowance of 35kg P/ha on an Index 2 soil. Because my yield is 9.2t/ha rather than the base of 6.5t/ha, I have an additional P allowance of 10.26kg, or 45.26kg P/ha. This means I have an allowance of 1,194.86kg of elemental P for that parcel.

In parcel C growing winter oats, my basic allowance is 45kg P/ha on this Index 1 soil. I get an additional allowance of 9.12kg because my yield is above the base 6.5t/ha. Multiplying it all up gives this crop an allowance of 1,488.3kg P/ha.

Do the same exercise for all the other parcels. My allowance for spring barley is zero because my pH is under seven at Index 4. Adding all these up gives me a farm P allowance for 2021 of 4,316kg of elemental P which translates to 25.85t of Super Phosphate (16%). This quantity would also translate into 41.36t of 10:10:20 or 68.93t of 18:06:12 for the farm, assuming no organic manures were used.

K allowances

While the amount of K used is not legally binding, following the same principle is good farming practice. This time I will look at parcel B growing spring wheat.

The basic allowance here is 115kg K/ha and there is an additional allowance because the proven yield potential is higher than the base official rate of 8.5t/ha.

Again, multiply the total allowance of 117.45kg by the crop area of 9.4ha and you get a parcel allowance of 1,104.03kg.

Doing the same with parcel D growing spring barley, I have a basic allowance of 85kg/ha on this Index 3 soil. However, my yield of 7.2t/ha is lower than the official reference of 7.5t/ha so my basic allowance is reduced by 2.94kg/ha. Then it is 82.06kg x 82.7ha to give a 6,786.36kg allowance.

Doing the same for all the parcels gives a farm allowance of 18,044kg of elemental K, which translates into 36.09t of Muriate of Potash (50%K) or 90.22t of 10:10:20 or 150.37t of 18:6:12 to supply the total K requirement.

Potash assessment is more complicated because each crop has a different base yield level. The adjustment to requirement is based on an increase or decrease of 9.8kg/ha per tonne relative to each base yield.

N assessments

The farm nitrogen allowance is calculated in the same way.

The individual parcel allowance is based on the crop grown, the yield level you can prove and the previous land use. Quantities can be calculated for every parcel to give a farm allowance.

Maximum allowances

Being able to calculate these requirements is important in terms of good farm practice and meeting your legal obligation. These are the maximum allowances but there is no obligation to use them up to these levels. However, they should not be exceeded.

In all instances, any nutrients applied to the crop from other organic sources must be subtracted from the calculated bag allowances.

Make every square metre count

Your headland run is the biggest round of the field come harvest. It sets the scene for field yield. Yet that outside 3m run by the hedge is often the lowest yield in the field on a metre-length basis.

The main reason for this is likely to be fertility. For a few decades now, you have been spreading fertiliser and lime along this edge without another outside lap to even off application rate. So, it may be well worth your while to take a soil sample from around one field to check on what has been happening. If you find no problem in one field, you may not have this problem in any of them.

If fertility is low, you might consider putting on a good dose of P and K with a wagtail-type spreader, set close to ground running at low revs to give a tight spread pattern. Do not put fertiliser in the hedges. And if pH is low you might have to consider doing the same with lime when it is being spread next time or apply granular lime now.

Chopping straw

Fertiliser use advice caters for the removal and non-removal of straw, with lower application rates needed for the latter. This is correct in theory as the return of straw means a quantity of P and especially K is not removed. However, in my experience, it takes quite a time for the nutrients returned in straw to become available again for crop uptake. So, if you are at Index 1 or 2 for P and K, I would be inclined to fertiliser as if the straw were being removed for a few years and then test again. Putting P and K back in straw is a bit like having money in the bank on a time lock – it is not immediately available.

In brief

  • Understanding the basis of nutrient allowances is an important farm and soil management skill.
  • P allowances for crops are based on a standard base yield of 6.5t/ha for all cereals and allowances can be increased for higher yield levels.
  • K allowances have different base yield references for every crop and they can also be adjusted for proven yield levels.
  • Nitrogen allowances are based on the crop grown, the proven yield potential and the previous use of each field.