One of the important administrative jobs on any farm is to have an accurate record of all fertiliser purchases and where the fertiliser was used. Nutrient use must be accounted for and this information is essential for cross-compliance.

These records must show where individual fertilisers were used and the total rates must not exceed the individual field or crop allowances. Assembling these records is not a simple task, but neither is it an impossible one. Growers should be well capable of assembling their own nutrient records but many choose to leave it to others.

The process requires some basic bits of information such as recent soil test results, the previous crop to determine soil indices, an adequate number of soil tests, the land areas that apply to the individual test results, a target market for the crop and a knowledge of the maximum allowable application rates for each crop. The latter can be found in the Teagasc publication “Major and micro nutrient advice for productive agricultural crops”. This can be viewed as a physical publication or downloaded from the Teagasc website.

An example

To help understand the intricacies of what is involved, it is necessary to work through an example. In this case we are using a farm unit of five fields, which could be part of a farm or a full farm. A simple explanation of what is involved initially is shown in Table 1.

This example is worked on a simple five-field section of a farm. The unit is just over 58ha in total. It is a combination of large and small fields. It gives the crops grown last year and this year.

A soil test should apply to 4ha maximum but this can be up to 5ha in a uniform field. This means that it is necessary to have more than one soil test result per field, except for Westland (see Table 1).

Recommended nutrient use involves the replacement of the nutrients removed plus additional nutrient where it is necessary to build soil fertility.

Nutrient use recommendations cater for increased offtake where high yields are being achieved. But it is essential to be able to verify a higher farm yield from an individual crop in any of the three most recent years.

Recommendations for N, P and K are based on different yield levels so individual crop and parcel recommendations may need to be adjusted relative to the target farm yields shown in Table 1.

Crops being grown after cereals, maize or vegetables receiving less than 200kg N/ha fall into Index 1 and can receive the highest nitrogen rates allowed.

Crops planted after oilseed rape, potatoes or protein crops, or vegetables receiving more than 200kg N/ha, fall into Index 2, so can receive the second highest rate of nitrogen (see Table 2).

Where temporary grass is used in the rotation, the folowing crop falls into Index 2 if the grass ley was only in place for one to four years.

Crops following long-term leys or permanent pasture gradually move from Index 4 to Index 1. Index 4 slots are the first or second crops out of grass, which were grazed only, and those in Index 1 are the fifth crop after permanent pasture or long-term leys.

Table 2 shows the amount of nitrogen allowed on all cereal crops according to index. These rates are based on reference yields (shown in brackets). Based on 20% moisture content, the reference yield for winter wheat is 9t/ha, while spring barley is 6.5t/ha.

Where a farmer can show proof of yields higher than the reference yield in any of the past three years, he/she can apply 20kg N/ha for every tonne over the reference yield. The proof of yield needs to be shown in sales from the farm.

An additional 30kg N/ha can be applied to milling wheat crops, while 20kg N/ha extra can be applied to malting barley crops to address proven low-protein contents.

Guidelines outline that a 6.5t/ha cereal crop in an Index 1 soil for P requires 45kg P/ha. There is an allowance of 35kg P/ha in Index 2 and 25kg P/ha in Index 3. Proven yields above 6.5t/ha can apply an additional 3.8kg P/ha per tonne above 6.5t/ha. So a 7t/ha crop of spring barley grown on an Index 1 soil can receive 46.9kg P/ha.

No P can be applied at Index 4, apart from where soil pH is 7 or over; in this case 20kg P/ha can be applied.

Potassium can be applied according to the K index in Table 3. Rates vary by crop and these should be increased or decreased if previous crop yields are higher or lower than the reference yields shown for each crop. The per-tonne K adjustment also differs between crops (see Table 3).

Working out farm nutrient allowances goes back to each individual soil test zone in each field. Each individual allowance is influenced by the area attributed to the zone, the crop grown, the soil test result and the proven yield target. These are all shown for the different fields in Table 4.

Each zone follows a similar process with the field designated by the code letter in Table 1 and the individual zones shown by the number that follows it. The three “M”s are the zones in the Mooreland field and it is all sown to winter wheat.

The M1 zone is 4ha. It is N Index 2 because it follows spring beans. This allows up to 180kg N/ha to be used in total. But because this allowance is based on a 9t/ha reference yield and we are targeting 10t/ha, we can use an additional 20kg N/ha on this zone – up to 200kg N/ha. This zone is Index 4 for P and K (located close to a farmyard) and so it does not need applied P or K.

M2 is also 4ha of winter wheat and its N story is identical to M1. It is Index 3 for P and K. Winter wheat has a 25kg P/ha allowance but this is for a 6.5t/ha reference yield and we are targeting 10t/ha. There is an additional 3.5t/ha of output and we can apply an extra 3.8kg P/ha for each tonne above 6.5t/ha so we can add an extra 13.3kg P/ha for this zone.

On the K allowance, Index 3 leaves an allowance of 110kg K/ha but this is for an 11t/ha crop and we are only growing 10t/ha so it needs to be reduced by 9.8kg to compensate for the missing tonne. Hence there is a net allowance of 100.2kg K/ha.

The M3 zone also has the same N story but it is Index 2 for P and K. The Index 2 P allowance is 35kg P/ha, plus the additional 13.3kg for the higher yield (48.3kg). The K (Index 2) allowance for winter wheat is 125kg K/ha but our allowance must be reduced by 9.8kg because of the lower 10t/ha target yield.

Calculating the allowances

Now that we have worked out the individual allowances for each nutrient in each zone, we can calculate the maximum allowance for each zone and the five fields. The zone allowance is shown for each nutrient in Table 5, eg 800kg of N for M1 based on four hectares at 200kg/ha.

All the zones are then added together to get the ‘farm’ allowances for N (10,506.2kg), P (1,900.1kg) and K (4,955.2kg). This amount of nitrogen would translate into 35.9t CAN (27%N or 270kg/t) or 22.84t of urea. The 1,900.1kg of phosphate translates into 11.88t of Super Phosphate (16% P or 160kg/t). The 4,955.2kg of K would be 9.91t of Muriate of Potash (50%K of 500kg/t).

We looked at using compounds to supply an amount of the fertiliser requirement (Table 5). If we used 10:10:20 it would take 105t to supply all the N but this would massively oversupply both P and K. However, 19t of 10:10:20 would supply virtually all the P but leave a deficit of 8,606kg of N (31.87t of CAN) and 1,155.2kg K (2.31t of Muriate as a top-up).

A similar exercise was conducted using 15:3:20 and 12:5:25 as the compounds. Both supplied all the K with the need to top up using additional CAN and Super Phosphate. A grower might then price the different combinations (Table 5) and decide on where the value is in the market.

These allowances are based on maximum allowed rates and a compound could still oversupply on some zones. These rates might also be reduced based on the risk of not achieving target yield levels in individual fields, given the high fertiliser prices.

  • Current/planned crop.
  • Previous cropping history.
  • Soil sample results.
  • Slurry or organic manure use.
  • Proof of individual crop yields for past three years.
  • Copy of the allowance tables.
  • Time and patience..