Reducing root mat through stimulation of the carbon cycle

Year of study: 2014 to 2018

Agriculture Victoria, Bass Coast Landcare, Daryl and Margaret Hook

Introduction:

Root mat is a problem affecting many farms in Gippsland. These pastures become dominated by low production species such as bent and fog grass which form a dense mat of roots above the soil. The mat reduces water infiltration, and inhibits nutrient cycling,  decomposition and pasture productivity.  Causes are varied but include acidic soils, water logging and grazing management.

Objectives:

The objective of this on-farm demonstration is to evaluate compost and lime’s ability to invigorate decomposition and nutrient cycling, reducing acid root mat and improving pasture production.

Daryl Hook with root mat sample
Root mat close up

Project details

When Daryl and Margaret purchased this property 25 years ago, the property was divided into three paddocks, set stocked with little native vegetation. The majority of pastures had thick and dense acid root mat.

Daryl and Margaret decided against conventional pasture renovation (herbicide, lime, cultivation, nitrogen and re-sow) based on a desire for low cost, low input farming and a “biological” approach. The property now has 22 paddocks (average 5 ha), all divided by a double fenced line of native vegetation, predominantly swamp paperbark. 

Rotational grazing is undertaken with an occasional application of chicken or pig litter. According to Daryl the acid root mat has improved on many of their paddocks but the root mat is “chronic” and has not disappeared. The demonstration paddocks are amongst the worst on the farm.

Soil and pasture tests April 2014

Hook soil test results
Hook pasture test results

2018 Summary

  •  8 of the 12 treatments with lime showed a decrease in root mat depth.
  • pH improvement is likely due to lime + compost applications (up on 10 of 12 lime treatments with one unchanged), and compost alone (up on 10 of 12 with 1 remaining unchanged). The addition of nitrogen did not appear to make a significant difference.
  • dry matter measurements showed a clear response to applied nitrogen compared to treatments without nitrogen with the exception of the controls which performed better than all treatments.
  • compost and lime may have had a minor positive effect on rye grass, clover composition and dry matter yield.

Compost

Compost (a mixture of council green waste and chicken litter, screened to 40mm), made on site by Aussie Compost Company, Inverloch.

Pictured to the right is a combined table of 3 years compost analysis. Note: The drop in N value depending on stage of maturity and the C:N ratio which figures in the recommendations to address the organic root mat decomposition.

Assessment and recommendations below are from Declan McDonald – Senior soil scientist (SESL Ltd.)

“The overall nutrient profile of this compost is reasonably good and has acceptable calcium levels. It also has a useful contribution of trace elements. The high carbon content and low to moderate N in the compost, leads to a C:N ratio that is too high to achieve root mat decomposition. Nitrogen drawdown in pastures could occur, but more importantly this compost is not likely to have a major impact on the acid root mat which itself has a C: N ratio of 14:1. Ideally finished compost should be in the order of 12 or 15:1 whereas this compost has ranged from 24 to 28 to 1″.

“This highlights the need for supplementary nitrogen to offset possible N drawdown and activate the carbon in the root mat. In fact it may be necessary for an initial surplus of nitrogen at a rate of 150kg/ha to stimulate decomposition. This is an unacceptably high application rate for a single application, particularly because of the sandy loam texture of the soil and the risk of N losses through volatilisation or leaching. Ideally the fertiliser will be applied in three equal applications over the growing season. Note that this high rate is aimed at breaking down the acid root mat in conjunction with liming. It is not recommended that this level of nitrogen be applied to pasture for production reasons alone.”

Location details

Location and site details for Hook property

Experimental design and treatments

Capture
Jack D soil samples

Sulphur

New research shows a strong link between Sulphur, the breakdown of organic matter and the release of Nitrogen by soil organisms, particularly fungal organisms. It is worth keeping this in mind when dealing with root mat and nutrient cycling in general.

Species composition and Dry matter yield

Hook species % graph
Hook dry matter graph

Methods

All testing undertaken by Nick Dudley (Ag Vic), Dave Bateman/Joel Geoghegan (Bass Coast Landcare Network) with assistance from Daryl Hook.

Root mat Depth, Soil and Root mat Chemical Analysis

April 2014, and 2017. Root mat depth measured and t-test statistical analyses to determine change in thickness / persistence of root mat. Chemical analyses of root mat and mineral soil undertaken to ascertain changes in nutrient content of the two layers.

Methodolgy

A 125 metre transect established along each plot: Samples taken with Christie Hydraulic Soil corer to 30cm depth at 10 locations along each transect.
Each core measured for root mat depth and average calculated per plot.
Root mat separated from the mineral soil and bulked to form 1 root mat sample per paddock for chemical analysis. (2 samples)
10cm of mineral topsoil immediately below root mat separated and bulked to form 1 sample per plot for chemical analysis (14 treatment samples 2 control samples)
Sent to EAL laboratory (NATA accredited test)

Soil Bulk density EAL laboratory (NATA accredited)

Bulk density testing prior to lime and compost application to provide benchmark of the physical status of the soil. Assuming we get a response from treatments, testing in year 4 will show if a reduction in root mat increases or decreases the soil compaction risk.

Samples taken with Christie Hydraulic Soil corer to 30cm depth at three locations along each transect. 10cm of soil measured immediately below the acid root mat. Samples dried at 105oC. Each individual core tested separately. Average bulk density calculated per treatment.

Pasture species % transect (Ag Vic field officer)

Autumn and Spring 2014,15,16,17 before grazing to determine species composition changes and match composition changes with root mat changes.
Pasture stick 40 times per plot – record the plant species dead feed or bare ground touching the stick.

Dry Matter yield

Autumn and Spring 2015, 16 and 17, before grazing
Plate meter 40 times per plot to measure pasture height, converted into Kg/DM/Ha

Compost

10 handfuls of compost taken from various parts of the pile, mixed in a bag and subsample sent to SESL lab for standard chemical analysis . Sticks included in sample and are ground up and included in test.

Soil bulk density

EAL laboratory (NATA accredited test)

Bulk density prior to lime and compost application will provide bench mark information on the current physical status of the soil. Assuming we get a response from treatments, testing in year 4 will show if a reduction in acid root mat increases or decrease the compaction risk.

Samples taken with a Christie hydraulic soil corer to 30cm depth at three locations along each transect. 10cm of soil measured immediately below the acid root mat. Samples dried at 105°C. Each individual core tested separately. Average bulk density calculated per treatment.

Next steps

This site will be monitored until 2018. Interim results and final report will be released on the Gippsland Soil Trials and Demonstrations page, through further field days and Landcare communications channels.

Organic mats and soil testing: What you need to know

Some farms in Gippsland have organic mats in some of their paddocks and if/when they soil test these paddocks they unknowingly do so incorrectly. Organic mats or root mats are sometimes referred to as ‘acid’ root mats but not all organic mats are ‘acid’ mats. Organic mats can originate from bent grass root mats (Figure 1) or other species such as fog grass on sandy soils or paspalum on heavy clay loams.

These organic root mats build up over time due to underlying soil conditions being unfavourable for the normal process of organic matter breakdown within the soil (roots and root hairs) and/or a build- up of organic matter (mainly dead leaves, dead plants and plant crowns) on the surface of the soil. Organic matter build up on the surface is usually due to paddocks becoming waterlogged regularly and for long periods or from salinity.

Acid root mats most commonly occur as a result of the soil being very acidic (low pH) and often coincide with low soil fertility, particularly potassium, as well. This low acidity (less than pH5.1 (water) reduces the range and activity of soil micro-organisms which would normally break down the dying/dead organic matter into plant available nutrients and other products of plant decomposition. This process provides essential nutrients for plant growth as well as sustaining productive healthy soils and all the living organisms (bacteria, fungi, mites, ants, millipedes, beetles, earthworm, slugs and snails) which live in the soil.

In the acid mat scenario, the nutrient levels and pH itself is higher in the root mat than the soil under it. In soils, nutrients adsorb (attach) to clay minerals and organic matter and strongly so in the organic matter. This means that much of the nutrients in the organic mat are not so available to plants and ‘tie’ up a lot of fertiliser nutrients when applied. This characteristic has implications for soil test results. In soils with an acid mat, the critical problem being a low pH (maybe also low nutrient levels) may not be identified or the extent of the problem not so pronounced in the soil test results.

If a 10 cm soil test core is taken from the surface in a paddock with an organic/acid mat (Figure 2), the soil test results will indicate that the pH and nutrient levels are much higher than the actual case in the soil. The soil test results will therefore not indicate the true soil nutrient and pH of the soil. An example is a bent grass mat in grey sandy loam east of Inverloch tested in the mat at pH(w) 4.9 but pH(w) 4.5 in the soil itself. The pH scale is logarithmic meaning that the lower figure is substantially more acidic than the higher figure.

So when soil testing paddocks with organic mats, it is essential to remove the organic mat and soil core the soil only to 10 cm depth. Carefully remove the organic mat without removing any of the soil. Sometimes, this is not easily achieved and can be time-consuming but better than renovating and sowing only to have the problem return very quickly, also time consuming and expensive. Qnly will coring into the soil allow the soil test analyses to help you to identify the real problem in an acid mat soil, usually low pH and low nutrient

Direct drilling seed into the mat itself without addressing the real problem will result in plants germinating but then stop growing after some weeks and turning yellow from nitrogen deficiency as very little nitrogen is released in the mat due to minimal soil micro-organism activity. Acid mats also tend to develop allellopathic (poisonous) effects within the mat and this greatly affects plant growth, often killing them in the older mats. Therefore accurate soil testing on soils with soil mats is crucial to identify issues and determine remediation.

Root mat

The Healthy Soils Sustainable Farms project is supported by the West Gippsland Catchment Management Authority through funding from the Australian Government’s National Landcare Programme.

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