Soil constraints causing a decline in sub clover productivity: Project key findings

Beef production in the South West’s high rainfall zone appears to be significantly constrained by poor sub clover productivity.

That’s according to a survey of 24 beef properties conducted by South West Catchments Council (SWCC) in partnership with Western Beef Association Inc., which aimed to give farmers a better understanding of their soil constraints. The results suggest that multiple soil-related constraints are causing a significant loss in the productivity of sub clover, nitrogen fixation and protein production.

SWCC’s Regional Agriculture Landcare Facilitator Peter Clifton said the first signs of poor nitrogen fixation were seen on plant roots.

“We dug up about 400 sub clover plants from 39 sites in August 2019 to see how well they were forming nitrogen-fixing root nodules,” Peter said. “Each plant was carefully extracted from a sod by washing away the soil to make sure we didn’t detach any nodules.”

Nodule health was scored using a protocol provided on the Department of Primary Industries and Regional Development (DPIRD) website. The average score was 2.7 with only 20 percent of sites nodulating adequately. Nodule health was moderate at 38 percent of sites, while the remaining 42 percent were scarce, rare or ineffective.

The validity of the assessment was checked by sending 43 images with scores to consultant Neil Ballard, who gave the sample a lower score on average, suggesting the results were conservative. However, samples were collected in August and may have scored higher if collected in September after some spring growth. It is also recognised that nodule scoring of individual plants is a relatively crude assessment of the mass of nodules per unit of area.

With poor nodulation reported in other parts of Australia, the results were not unexpected. They suggest there could be a significant shortfall in nitrogen fixation that could be causing a loss in pasture production and limiting pasture protein content for cattle growth.

The result may help to explain why some more intensive producers are increasing their use of fertiliser nitrogen. However, another issue with sub clover seems to be poor establishment, with one producer involved in the project dropping clover from his seed mix after noticing a decline.

One potential cause of poor establishment was also discovered on sub clover roots. As Peter explains, it was an unexpected outcome, but perhaps one of the more significant findings.

“We noticed that the lateral roots were dark in colour and sometimes quite short. I sent photos to UWA (University of Western Australia) researcher Daniel Kidd who said it was sub clover root rot. We often think of pasture as a healthier system compared to cropping, so I wasn’t expecting to see significant levels of disease.”

Peter forwarded the 43 images of sub clover roots to a leading authority in root rot, Professor Martin Barbetti at UWA, who indicated that about two thirds had severe root disease that can reduce clover density, productivity and nodulation.

The disease is a complex of several fungal pathogens that can kill seedlings both pre and post emergence, shear tap roots, or more commonly cause a loss of lateral roots. Treatment with fungicides is not highly effective due to the presence of multiple diseases that tend to be highly variable across farms and years depending on climatic conditions. Also, fungicides can kill rhizobia responsible for nitrogen-fixing root nodules.

One of the best treatments for the disease complex appears to be the use of resistant varieties. However, the challenge is to find and develop varieties with good resistance across the suite of soilborne pathogens. Another option is cultivation to 10 centimetres, which can reduce disease for four to 10 years. Cultivation can help by reducing soil compaction, which the survey found to be severe at a depth of 10 centimetres on 28% of sites. Less compaction can increase the rate of root growth and resilience to disease. However, there can be detrimental effects from cultivation, particularly soil erosion and loss of soil fertility, which is also critical for disease resilience.

Another factor affecting nodulation is molybdenum, an element that helps nodules fix nitrogen. Molybdenum was found to be low in sub clover tissue tests (defined as below 0.2 milligrams per kilogram) at 38 percent of sites that had enough sub clover to test, by far the most deficient micro-nutrient.

“We assessed molybdenum with tissue tests, which are considered more reliable for testing micronutrients compared to soil tests. Moly doesn’t come in standard tests so you need to ask for it, or select a comprehensive test. We sampled sub clover, which is more sensitive to nutrient deficiencies compared to ryegrass, and we also found that sampling around the start of spring when growth rates are high gave a better indication of nutrient deficiencies.”

One factor contributing to low molybdenum is likely to be low soil pH, which averaged 4.75 (in calcium chloride) in the top 10 centimetres across survey sites and tended to decline with depth in sandy soils (4.4 at 10-20 cm). Molybdenum becomes less available to plants as soil acidifies. However, the result may also reflect a lack of molybdenum maintenance, which is typically required every 5 to 10 years.

Low soil pH can also be detrimental to the nitrogen fixing rhizobia that form nodules on sub clover roots. While sub clover is relatively tolerant of low soil pH, the rhizobia that fixes nitrogen in root nodules are more sensitive. For sub clover rhizobia, the optimal soil pH is between 5 and 8. The majority of soils sampled were below pH 5 in the top 10 centimetres, with 30 percent of sites below a pH of 4.5. Sandy soils declined with depth to an average of 4.4 and 4.6 between 10-20 and 20-30 centimetres respectively, with a minority of clayey soils also reaching these levels. Also, pH may be stratified in the top 10 centimetres, as demonstrated in one soil with a pH of 5 (0-10 cm sample), but 6.1 in a 0-2.5 centimetre sample. This suggests that the apparently optimal pH soil is actually suboptimal below 5 centimetres and perhaps reflects a lack of lime incorporation in the region.

Nodulation can also be reduced by certain chemical residues and can be improved by inoculating with the most recent strains of rhizobia every 5 or so years that coincidently perform better in acidic soils.

In conclusion, the project has highlighted the poor productivity of sub clover in terms of nodulation and nitrogen fixation. This is likely to be related to several soil constraints, including fungal pathogens, low molybdenum and low soil pH. Pathogens may also affect sub clover productivity in terms of establishment, survival and growth, possibly accentuated by compacted soil that limits root growth and resilience to disease.

Management of these issues could take the form of sowing sub clover varieties that are more disease resistant, maintaining soil pH at 5 or above, maintaining molybdenum levels, inoculating clover with the most effective strains potentially every five years, and possibly cultivating every 4-10 years to 10 centimetres, which would treat compaction and incorporate lime to avoid stratification of pH.  The decision to cultivate should consider the level of compaction, disease, need for lime incorporation and potential to combine with seeding operations, compared against potential erosion and loss of soil fertility.

Assessment of these factors requires comprehensive tissue testing of sub clover near the start of spring every 5 years to monitor molybdenum (and other) levels or molybdenum application every 5-10 years, soil tests to monitor soil pH to at least 20 centimetres, possibly in 5 centimetre increments to capture stratification, assessment of compaction levels (e.g. with a spade) when soils are wet but not flooded, and the careful extraction of sub clover roots to assess nodulation and root disease. Assessment of sub clover root health may be a valuable indicator of overall soil health.

This project was supported by South West Catchments Council and Western Beef Association Inc, with funding through the Australian Government’s National Landcare Program.