Phosphorus

The phosphorus index is based on soil results and differs slightly for grass and tillage soils as outlined in Table 4.

(See here for extracts from the Teagasc Green Book of Major and Minor Nutrient Advice for productive agricultural crops ) 

Table 4: Phosphorus index system

Phosphorus index system (Table 11- SI No.  393 of 2022)

The phosphorus offtake by crops where the straw is baled or where straw is chopped are listed in Table 5.  The phosphorus return from chopping straw is significantly less than Potassium and a 9 ton/ha crop of winter oats, when straw is baled removes 34 kg P/ha (9 x 3.8kg) while a crop where the straw is chopped removes 31 kg P/ha (9 x 3.4 kg).

Table 5: P off takes in cereal crops (kg/ha) per tonne of grain yield

Once you have determined your index, use Table 6 to calculate your P allowance.  Where higher yields above a reference yield 6.5kg/ha (MC 20%) can be proven, an additional allowance of 3.8kg P per additional ton is available.  For example, a 10ton/ha crop of winter wheat on an index 3 site, has a P allowance of 38 kg/ha (25 kg from Table 6 + 13 kg additional allowance (from 3.5 additional tons x 3.8kg/ton)).  It is important to review the conditional notes under Table 6, as P availability can vary significantly on different soil types.

Table 6: Maximum Phosphorus rates on tillage crops

¹ The fertilisation rates for soils which have more than 20% organic matter shall not exceed the amounts permitted for Index 3 soils.

² Where proof of higher yields is available, an additional 3.8kg P/ha may be applied on soils at phosphorus indices 1, 2, or 3 for each additional tonne above a yield of 6.5 tonnes/ha (for Cereals only). The higher yields shall be based on the best yield achieved in any of the three previous harvests, at 20% moisture content.

³ Where pH is greater than or equal to 7, 20kg P/ha may be applied on soils at phosphorus index 4

⁴ Must be incorporated prior to or during sowing.

⁵ For winter cereals on soils of P index 1 and 2, 20 kg of the maximum P fertilisation rate may be applied up to 31st October, which must be incorporated prior to or during sowing

⁶ Potatoes - Where soil P levels above 15ppm no P fertiliser required.

Timing of P application

Winter cereals have a higher P demand during the growing season compared to spring cereals due to their higher yield potential.  For this reason,  a winter cereal crop on very low to low P index soils, can and should receive a P build-up application for index 1 and 2 soils of 20 to 10 kg P/ha respectively at sowing time up until the 31st October.  However, it must be incorporated or combined drilled to ensure it is in the correct zone for root access and reduce loss risk to water.  In early spring the remaining crop P requirements can be broadcast.

For winter crops sown on P index 3 soils crop P requirements should be broadcast in February/March typically with the crops 1st N and K requirements. This will replenish soil P reserves and supply in season crop demands.  At soil P index 1 sites or sites prone to P lockup it is advisable that P be incorporated at or before sowing. Restricted periods for fertiliser application as set out in SI 113 of 2022  must be respected.