Soil analysis is the starting point in determining the balance of nutrients required to maximise both grazed or ensiled grass. Soil fertility changes slowly over time and it can be difficult to see an underlying deficiency of either P or K, especially in grassland.
Therefore, taking soil samples on a regular basis will provide up-to-date information on the fertility status of the soils on your farm.
For example, an up-to-date soil test report will provide advice on the type and rate of lime required to maintain the pH in the target range. It will also provide the basis to determine crop P and K requirements on a field-by-field basis.
A small annual investment (€0.50/ac/year) in a standard soil test for P, K, and pH will help ensure the long-term productivity of your soils.
Getting pH in right range can be crucial
Step 2 – Apply lime as required to increase soil pH up to the target pH for the crop
Soil pH should be maintained at the target soil pH, which is essential to ensure optimum nutrient availability. Fertiliser efficiency will be reduced where the soil pH is incorrect.
The optimum use of applied nitrogen (N), P and K is obtained when at a soil pH 6.2 to 6.5 in mineral soils and a soil pH 5.5 in peat soils (see Table 1).
Lime will increase the release of soil nutrients, increase earthworm activity and improve soil structure.
Test soils every three to five years to check soil pH and apply recommended lime rates as per soil test report.
It has been estimated that grassland soils maintained at pH 6.3-6.5 will release approximately 60-80kg/ha more N per year than soils with pH 5.0. This difference is worth approximately €60-85/ha.
Calcium limestone is the most common form of ground limestone available.
Magnesium limestone can also be used, and is recommended where tested magnesium levels are less than 50 mg/l.
Phosphorus and potassium levels change very slowly over time.
The aim is to maintain soils at index 3 (see Table 2) by replacing P and K removed in grazed grass or grass silage.
For example, 1,000 litres of milk will remove approximately 1kg/ha of P and 1.5kg/ha of K. A cut of silage yielding 5t DM/ha will remove 20kg/ha of P and 120kg/ha of K.
Soils at index 1 and 2 have a lower nutrient supply and will require additional annual nutrient applications to reach soil index 3.
Soils at index 4 have a high nutrient supply and offer the opportunity to save on fertilisers.
It is recommended to omit P applications for a number of years and re-test soils to monitor soil P changes.
For K it is recommended to omit K for one year and revert to index 3 advice (especially on silage ground) until soils are re-tested.
Standard off-take values of P and K in grassland and tillage crops can be calculated using the online fertiliser calculator available at www. fertilizer-assoc.ie/fertilizer- calculator
Choose organic manures over artificial
Step 4 – Use organic fertilisers as efficiently as possible
Organic manures are a valuable source of N, P and K and can effectively replace artificial fertilisers.
To maximise the nutrients in organic manures, it is essential that they are applied at the correct time of the year.
The nutrient content of cattle slurry at different dry matter contents are shown in Table 3.
Therefore, good management at the time of application such as well agitated slurry with even and accurate application will improve the fertiliser value.
Remember to adjust slurry application rates depending on slurry dry matter to deliver the correct levels of P and K to meet crop nutrient requirements.
Applying organic manures to the same field year after year without knowing the soil fertility status may not be the best use of these valuable nutrients.
Consult soil test results and identify fields on the farm with the lowest fertility (Index 1).
This may result in increased fuel costs for transporting slurry to fields that are further away, but may still be economical when offset by reduced fertiliser costs on these fields.
Get the balances right with your fertiliser usage
Step 5 – Make sure the fertilisers used are properly balanced
A balanced plant nutrient supply is very important during the growing season. It is especially important for productive ryegrass swards that soil reserves can supply sufficient nutrients to meet the peak demands at particular stages during the growing season.
Soil analysis plays a key role here as it gives a measure of the soil's ability to supply both major and minor nutrients. Fertiliser applications can then be tailored to supply the correct nutrient balance.
The type of fertiliser selected will depend on the soil nutrient status, the crop demands and the rate of organic manure application.
For example, the nutrient requirements for a grazed sward will be quite different to that of a grass silage sward.
A grazed sward will often require a fertiliser to deliver a P:K ratio of between a 1:2 to 1:4 and fertilisers such as 18-6-12 or 24-2.5-10 supply P:K within this range.
However, a silage sward will require a P:K ratio of 1:6 and fertilisers such as 15-3-20 or 10-5-25 supply P:K within this range.
Fertilisers with appropriate N-P-K ratios need to be used for these different grass crops to ensure the correct delivery of N, P and K to meet early and seasonal crop nutrient requirements to maximise grass growth.
Soil fertility needs to be managed on a long-term basis with the aim to maintain soils at P and K index 3 for optimum production.
It is very worthwhile comparing old soil test results to current soil test results. This will provide a sound basis for tailoring a fertiliser spreading plan for the soils on your farm.
It will also help identify fields that need extra nutrients, for which slurry and farmyard manure is a cost-effective way of building these soil fertility levels.
Soil fertility changes very slowly over time so a small annual investment in lime, P and K now will pay long-term dividends in the future.
How to optimise your soil productivity with lime
The weather conditions last autumn have facilitated the application of ground limestone to both grassland and tillage soils.
Liming Irish soils is very beneficial as our soils tend to be naturally acidic and require regular lime application to maintain their productive capacity.
Maintaining soils at the correct soil pH will increase the release of soil nitrogen (N) and improve the availability of soil phosphorus (P) and potassium (K) following application.
The following are answers to some of the most commonly asked questions in relation to lime and managing soils that have been recently limed.
Q Are there different types of limestone used for making ground limestone?
A Yes, there are two types of ground limestone – Calcium (Ca) and Magnesium (Mg) limestone.
Where soils are low in Mg apply magnesium limestone. There are also a range of granulated limes on the market which are based on either calcium or magnesium limestone depending on the product.
Q How much should I apply?
A A recent soil report will show the rate of lime required depending on the soil type, soil pH and crop type.
Q How long will it take lime to work?
A The fine lime will work relatively fast, and the coarse lime particles will react more slowly and help maintain soil pH for a number of years.
Q What is the maximum rate of lime in a single application?
A Apply a maximum of 7.5t/ha (3.0t/ac). Where more lime is recommended, apply the balance after two years.
Q Can slurry and lime be applied at the same time?
A Applied lime can increase the loss of N to the air after slurry application.
If slurry is first applied, leave a week before spreading lime. If lime has been applied, avoid slurry application for three months.
Q How long should one leave between liming and applying urea?
A The N in urea and cattle slurry are in the same form.
Treat urea the same as cattle slurry as described in the previous question.
Q How long should one leave between spreading 18-6-12 or CAN and lime?
A There is no need to leave a gap with CAN or N P K compounds.
Q What is the target pH for grassland where my land is in a high molybdenum (Mo) area?
A Maintain a soil pH 6.2 on these soils.
Q Will recently limed tillage soils be more prone to manganese (Mn) deficiency?
A Yes, recently limed tillage soils are more prone to Mn deficiency.
To reduce the potential for Mn deficiencies, ensure lime is well incorporated during soil cultivations and seedbeds are well consolidated after sowing.
Q When is the best time to apply lime to silage swards?
A Generally, lime can be applied at any time of the year, provided the grass sward is low to avoid excess lime sticking to herbage.
For silage it is better to lime after cutting, as high uptake of lime can increase the pH in the silage pit which affects silage preservation.