Tillage Signpost Farms Soil Test Results

Mark Plunkett Soil and Plant Nutrition Specialist reports on soil fertility levels on Tillage Signpost Farms.

Tillage farms in Ireland have a one of the lowest carbon footprints at 2.5t C/ha as reported in the Teagasc Farm Sustainability Report in 2021. These farms include a livestock enterprise such as a finishing beef unit.  Over the last number of decades, livestock enterprises have disappeared on tillage farms due to margins, labour availability etc.  This reduces the carbon footprint to ~ 1.0t C/ha on intensive farms compared to 9t C/ha for intensive livestock enterprises. For example there are 10 farms participating in the tillage signpost programme with 7 farms with tillage only and 3 with livestock enterprises (1 pig & 2 beef finishing enterprises)

In the autumn of 2021 tillage Signpost farms were soil sampled which is part of the National Agricultural Soil Carbon Observatory (NASCO).  Over the next four years the programme aims to balance soils to 90% optimum fertility to improve N & P efficiency while reducing nutrient losses.  Farm fertiliser plans are reviewed and updated annually to tailor all applied nutrients to ensure a balanced nutrient supply during the growing season.

Soils with Optimum Soil Fertility

Tillage Signpost farms have 42% of soils (figure 1) tested with optimum soil fertility (pH> 6.5, P & K Index3) this compares to the national average on Irish farms at 18%.  Optimising soil fertility is the first step to maximise the efficient use of all nutrient sources such as soil, organic and chemical fertilisers.  Our modern cereal varieties of today have a large yield potential for example winter wheat up to 16t/ha.  Unlocking this yield potential starts with good health, structure & soil fertility management.

Figure 1:- The percentage of tillage soils with optimum soil fertility (>pH 6.5, P & K Index 3)

Soil pH & Lime

Maintaining optimum soil pH (+6.5) is the first step to increasing nutrient efficiency.  Soils tested show that 61% (figure 2) of soils have a pH +6.5 this is the same as the national average.  Results reveal that the majority (88%) of soils have a lime requirement of 0 to 2.5t/ha. Nine percent of soils require a maintenance lime requirement of 2.5 to 5.0t/ha, while only 3% of soils require build-up application rates of 5 to 7.5t/ha.

Figure 2:- The percentage of soils with different lime requirements

Soil P & K Results

Soil test results show that 67% and 71% (figure 3) of soils are optimum (Index 3) or greater compared to 43 and 68% nationally.  On tillage Signpost farms 36 and 43% of farms tested are P and K Index 4.  Soils at P index 4 are an asset and utilising soil P reserves reduces fertiliser costs while protecting the environment.   Phosphorus applications can be omitted on these soils for 2 to 3 years, for example, 15% of soil samples at P Index 4 have greater than 15mg/l.  For K Index 4 soils omit K applications for 1 year and revert to Index 3 advice until the next soil test.  On further analysis of the data it shows that 37% of soils at K Index 4 have soil K levels between 200 to 250 mg/l.    

Figure 3:- The percentage of tillage soils at P and K Indexes 1 to 4

Soil Organic Carbon (SOC) and Tillage Soils

Soil organic carbon is a measureable component of soil organic matter. This is one of the ways to mitigate against climate change by reducing carbon dioxide in the atmosphere. 

Soil carbon analysis has been conducted in 2021 in the top 10 cm of soil.  Further analysis will be carried out in year 2 to establish soil carbon stocks deeper in the soil profile (remaining 0.5 metre).  Changes in soil carbon depends on soil type and annual input of organic matter and it takes time to build SOC. We will examine different soil types and there ability to either sequester or release carbon, changes are most likely to occur in the plough / cultivation layer as the majority of SOC is returned in this zone.  This will help us understand carbon capture, changes and storage for tillage soils depending on soil type and organic matter additions.  Tillage soils have a good ability to sequester SOC depending on their soil type for example heavier soils large ability to retain SOC from annual inputs of organic matter (organic manures / chopped straw etc.).  Soil test results show that in the 10cm of soil 20% of soils have soil carbon levels of <2%, while the majority of soils (80%) are between 2 to 6% soil carbon (see figure 4).

Figure 4:- The percentage of tillage soils in different soil carbon ranges