Slurry and chemical fertilisers are the two main components used on Irish farms to improve or maintain soil fertility and, in doing so, encourage grass growth.
Soil fertility is a subject which can appear complex from the outset, but when broken down and approached in a ‘bit-by-bit’ format, can be easier to understand and become a more manageable task to tackle on your farm.
It is important to remember that soil fertility is not something that can be corrected in a day, a week or even a year, and the reality is that on many farms, it could take up to five years to fully correct soil fertility.
The best place to start is with a soil test, and the first thing to look at on the result sheet is soil pH.
Soil pH is the foundation for building your farm’s soil fertility and once corrected, maintenance applications of lime every five to six years is all that would be required.
Assuming pH is at the optimum level of 6.3 for grassland on your farm, the next step is to examine your soils’ phosphorus (P) and potassium (K) index.
Your soil test results will fall under one of four P and K indexes and what your soil P and K index means is simple:
P and K Index Index 1 Very Low Index 2 Low Index 3 Target Index 4 Sufficient/High
As the table above indicates, index three for P and K is at the optimum level.
However, it is recommended that these fields are ‘topped up’ with P and K applications every year to keep them at a maintenance level.
Speaking at a Teagasc beef farm walk in Camolin, Co. Wexford last week, Teagasc drystock advisor Jack Murphy said: “Ground is full of P but only about 1% of it is available to the plant.”
However, Murphy noted that when pH is correct, “the soil is less acidic and more of the P in the ground starts to become available to the plant”.
Slurry and fertiliser
It is estimated by Teagasc that 3,000gal/ac of good quality cattle slurry (7% dry matter [DM]) will supply sufficient P and K levels to grow a crop of grass silage.
The table below gives an indication of the nutrient value per 1,000gal of different types of slurry:
Manure type N P K Cattle slurry (7% DM) 9 5 32 Dilute cattle slurry (3.5% DM) 8 3 15 Pig slurry (4% DM) 19 7 20
Where slurry is not being used and chemical fertiliser is being applied, 16 units of P and 100 units of K will maintain P and K indexes.
Silage ground should be prioritised for recieving slurry and dung produced on-farm.
Grazing ground
Land being used solely for the purpose of grazing for a year will only lose approximately six units of P and 10-12 units of K annually, at index 3 .
On an average stocked farm, 1.5 bags/ac of 18:6:12 in one of the rotations each year is all that’s needed for maintenance.
What should I give fields with index 1 or 2?
Fields showing up to be deficient in P and K will need more fertiliser to boost their nutrient levels.
Target these fields with dung and slurry or additional compound fertiliser to build up P and K reserves in the soil.
Concluding, Murphy, Teagasc’s drystock advisor, noted that farmers should make a plan on how they intend to address soil fertility and from that plan, work around it to allow for unfavorable weather conditions.