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 10 grassland soil samples
analyzed between 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.
These major 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 index 1 (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 the
crop
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 www.fertilizer-assoc.ie/fertilizer-calculator
Table 2. Soil P & K Index System
Soil P (mg/l)
Soil K (mg/l)
Soil P & K index
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 such as 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 by
reduced fertilizer costs on these fields.
Table 3:- Typical N, P and K fertilizer value of cattle slurry at different dry matter %.
Cattle Slurry
Fertilizer Value kg/m3*
Dry Matter
%
N**
P
K
7%
5%
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 plant nutrient 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. Fertilizer
applications 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 1:2 to 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 and fertilizers such as 15-3-20 / 10-5-25 supply P:K within this range.
Fertilizers 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 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.