Lifting phosphorus levels on heavy soils

There are two ways to look at phosphorus (P) and potassium (K) requirements. Firstly, soils at index 0, 1, 2 and 3 need maintenance levels just to keep them at that level. Secondly, if index levels are low you need to apply more than just maintenance levels.

Soils at index 3 (index 4 ROI) do not require any P or K maintenance application as there are sufficient soil reserves to cover maintenance requirements. However, these soils should be sampled annually to make sure that levels do not drop below index 2 (index 3 ROI).

Secondly, in order to build up soil P and K reserves, additional fertiliser is required. How much to spread is determined by the soil index based on soil samples, soil type and grass growth requirements. Each project farmer has had soil analysis for all paddocks to determine soil fertility and from this, recommendations for fertiliser can be made. Of course, when we talk about fertiliser, we are referring to both chemical fertiliser and slurry.

From a cost perspective, building up soil P is expensive; however, the cost is justified with improved grass production throughout the year. On a number of project farms, improving soil P status is costing £100/ha, with 75 units of P required to increase an index zero paddock to index 2 (NI “P” test), this cost excludes the maintenance P required for the current grass crop and is based on NI P recommendations.

Slurry is a valuable source of P and K on your farm and can contribute to P maintenance as is the case on Kevin McGrade’s farm. Typically, every 1,000 gallons of cattle slurry contains five units of nitrogen, five units of P and 30 units of K. The nitrogen available in slurry decreases after March unless it is spread with a trailing shoe-type applicator.

Remember, nitrogen and phosphorus are the only nutrients governed by the nitrates directive. There are no restrictions on lime or potash use.

Be mindful the soil P test in NI is different from that used in ROI and therefore results in different recommendations.

Kevin McGrade

Our primary focus is on soil phosphorus (P) maintenance. Soil analysis taken at the start of the programme highlighted the grazing block around the farm was 70% optimal for soil P. It is now 90% optimal (see Figures 1 and 2). This year we got slurry on in early spring using an umbilical slurry system which will help with the maintenance of soil fertility based on a nutrient content of 5-5-30 units of NPK per 1,000 gallons. We applied 2,500 gallon/acre of 6% dry matter cow slurry, so we assume approximately 12 units of P from slurry alone. Only 10% of the farm is now below optimal for soil P, with a few paddocks at index 1 from the recent analysis taken January 2017. These paddocks need 35 units/acre to build soil P and a further 20 units/acre for maintenance based on my current stocking rate.

My fertiliser plan for the year has an application of 18-46-0 on index 0 and 1 paddocks this spring at a rate of 25 units/acre. This was applied at the start of April. I also plan to use a compound fertiliser rather than continually spreading CAN on each grazing rotation. I will need to get two-thirds of compound applications on during the grazing season, specifically 18-6-12 on silage ground and 25-5-5 on grazing land. This approach will provide nitrogen and potash nutrients in one application and an incremental build-up of phosphate throughout the year, while at the same time providing soil maintenance for paddocks already at the desired P index of 2.

David Brady

Soil analysis from 2015 highlighted significant deficiency in soil P, for all nutrients including pH, P and K. We had only 8% of the area at optimal soil fertility for growing grass. Soil P was a major part of the problem, with only 11% of the block in the optimal index 3 or 4. While we have been improving the grazing block with major drainage and sward improvement work over the past three years, soil fertility has been our primary focus last year and so far in 2017. Recent samples highlight an improvement in soil P, with 21% in or above the optimal index 3. The 21% optimal figure also included a block of rented land which had been very low in soil fertility. However, this land is now in a long-term lease and has received more attention to lift soil fertility.

I made the decision to invest in drainage and fencing alongside soil fertility investment on some paddocks. Last year, we spread lime to get on top of soil pH. We also got on with chemical P fertiliser, specifically a compound 18-6-12 and 24-4-0 which had been used twice in the grazing rotation in place of CAN (27% nitrogen).

This year, we plan to target all index 1 and 2 paddocks with DAP (diammonium Phosphate 18-20-0) We have got slurry on to the grazing block this spring but the index 1 paddocks still require 30 units/acre to lift to index 2. We plan to get this DAP on before the end of April, which will account for 50% of the P nutrient required for index 1 paddocks. We will go again with a P compound around June to provide another small boost of nutrients for these low P index paddocks effectively spreading the P requirement over two or three applications throughout the year. It’s costing €140/ha to build P over two years.