9 out of 10 Grassland Soils Outside Optimum Ranges

Mark Plunkett & Stan Lalor, Teagasc, Johnstown Castle, Wexford

“More from less” This is a phrase that is currently widely used in many Irish contexts from the economy to the planned farm expansion as laid out in FH 2020. This phrase now more than ever is true for the latter in the absence of access to additional land to support expansion. To sustain additional farm output the productivity of existing land will have to be increased to meet the feed demand on these expanding livestock farms.

The Teagasc soils database shows that only 1 in 10grassland soil samples analyzedbetween 2007 to 2013 have the optimum balance of the major soil nutrients to maximize grass production(see figure 1).

Figure. 1:- Percentage of grassland soil samples analyzed through Teagasc in 2013 in the optimum soil pH, P and K levels. 90% of samples were deficient in either pH, P or K.

To maximize grass growth on good mineral soils a soil pH 6.3 and soil P and K at index 3 is required. Currently 90% of soil samples are outside of these soil fertility targets.

Thesemajor soil nutrients are required in relatively large amounts during the growing season for N use efficiency and driving grass growth.

For example where soil pH is low it will limit the availability of both soil nutrients and applied nutrients as either fertilizers or manures. For example soils at P index1 (very low) are estimated to have 1.5t/ha of DM lower yield potential than index 3 soils.

Silage swards have a high demand for K and swards that are incorrectly fertilized will reduce grass yields. These are often fields that are furthest from the farmyard and generally don’t receive an annual application of cattle slurry due to transport costs.

This article identifies 5 steps to consider in the improvement of soil fertility on your farm that will help maximize production and the response from applied manures and fertilizers.

Step 1 - Have soil analysis results for the whole farm

Soil analysis is the starting point in determining the balance of nutrients required to maximize both grazed / 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 soil the pH in the target range. It will 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.

Step 2 - Apply lime as required to increase soil pH up to the target pH for thecrop

Soil pH should be maintained at the target soil pH which is essential to ensure optimum nutrient availability. Fertilizer use efficiency will be reduced where the soil pH is incorrect. The optimum use of applied N, P & 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 3 - 5 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 which is worth approx. €60-85/ha.Calcium limestone is the most common form of ground limestone available. Magnesium limestone (also called dolomitic limestone) can also be used, and is recommended where soil test magnesium levels are less than 50 mg/l.

Table 1. Optimum soil pH for grassland soils
Crop / Optimum Soil pH
Grass / 6.3
Grass (High molybdenum soils) / 6.2
Clover / 7.0
Peat soils / 5.5

Step 3 - Aim to have soil test P and K in the target index 3 in all fields

Soil index 3 is the optimum soil index to maximize grass quantity and quality on an annual basis. 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 / grass silage. For example 1,000 litres of milk will remove approximately 1kg P and 1.5kg K, a cut grass silage (5t DM/ha) will remove 20kg P/ha and 120kg K/ha.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 fertilizers. 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 1 year and revert to index 3 advice (esp. on silage ground) until soils are re-tested. Standard offtake values of P and K in grassland and tillage crops can be calculated using the online fertilizer calculator available at

Table 2.Soil P & K Index System
Soil P & K index / Soil P (mg/l) / Soil K (mg/l)
Grassland / Grassland & Other crops
1 / 0.0 – 3.0 / 0 – 50
2 / 3.1 – 5.0 / 51 - 100
3 / 5.1 – 8.0 / 101 – 150
4 / > 8.0 / >151

Step 4 - Use organic fertilizers as efficiently as possible

Organic manures are a valuable source of N, P and K and can effectively replace artificial fertilizers. To maximize 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 time of application suchas well agitated slurry with even and accurate application will improve the fertilizer 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, however may still be economical when offset byreduced fertilizer costs on these fields.

Table 3:- Typical N, P and K fertilizer value of cattle slurry at different dry matter %.

Dry Matter % / Fertilizer Value kg/m3*
N** / P / K
Cattle Slurry / 7%
3% / 0.7 (6)
0.55 (5)
0.3 (3) / 0.6 (5)
0.4 (4)
0.22 (2) / 4.3 (38)
3.1 (28)
1.9 (17)
* units per 1,000 gallons are shown in brackets
** N fertilizer value assumes spring application with splashplate

Step 5 – Make sure the fertilizers used are properly balanced

A balanced plantnutrient supply is very important during the growing season. It’s 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. Fertilizerapplications can then be tailored to supply the correct nutrient balance. The type of fertilizer 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 fertilizer to deliver a P:K ratio of between a 1:2to 1:4 and fertilizers such as 18-6-12 or 24-2.5-10 supply P:K within this range. Where a silage sward will require a P:K ratio of 1:6 andfertilizers such as 15-3-20 / 10-5-25 supply P:K within this range. Fertilizerswith 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 maximize grass growth.

Soil fertility needs to be managed on a long term basis with the aim to maintaining 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 fertilizer spreading plan for the soils on your farm. It will also help identify fields that need extra nutrients, for which slurry / FYM 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.