Legume-derived nitrogen still an important option to bolster pastures

In all but specialised high-input, high-production grass-only pastures, a legume as part of the pasture is a key factor.

Correct legume species and variety for a given soil (including sub soil) and environment, care with establishment, ensuring rhizobia bacteria are successfully established with the legume, attention to aspects like soil fertility and appropriate grazing management are all important for legumes being a strong part of any pasture.

Legumes can build soil nitrogen by around 25 kilograms a hectare for every tonne/ha of legume drymatter production. Plus they provide high protein, high quality feed.

Soil nitrogen is important for the grass component, as well as for crops if part of a rotation.

Pasture legumes form a symbiotic (mutually beneficial) association with specific soil bacteria (rhizobia) to meet their nitrogen requirements.

Nodules develop on the plant roots and house millions of rhizobia that convert nitrogen from air into plant available nitrogen in a process known as nitrogen fixation.

The association between host plant and its rhizobia is specific and pasture legumes must be inoculated with the correct rhizobia strain (or Group) for effective nitrogen fixation.

Soil pH is a major feature when determining what pasture legumes to select.

Each species has a specific suitability range.

Note pH is detailed in soil tests as either water or calcium chloride (CaCl). In NSW, mainly the latter is used.

In acidic soils it tends to be a lower reading than water pH. pH is an acidity alkalinity test. Some plants like acid soils, some neutral (half way between acid and alkaline, around 6.5), some alkaline.

Low pH is associated with aluminium toxicity. Pasture legumes that perform well in acidic soils tend to be tolerant of aluminium toxicity.

For example serradella suits soils (including sub soils) with pH as low as 4.2 and with aluminium levels quite high.

Biserrula may not be quite as aluminium tolerant as serradella but generally suits soils pH 4.5 to 7.5 (more alkaline soil tolerant).

Lucerne has low tolerance to aluminium toxicity and generally suits soil pH from 5.5 to 7.5.

Often paddocks are variable in lucerne density because of sub soil acidity.

Sub clover is variable in pH sensitivity, but most of the common species (subterranean), has good tolerance and suits soils with pH 4.7 (with moderate aluminium toxicity) to around pH 7.

Brachycalycinum sub clover types (Clare and Antos examples) suit more alkaline soils, pH 6.5 - 8.0.

Medics, mostly, but not all species, suit more alkaline soils, including the widespread naturalise burr medic. Barrel medic, with varieties like Caliph and Sephi, overlap with sub clover on neutral pH soils, but generally far better suit alkaline ones.

Balansa clover, a particularly water logging species has a soil pH range of 4.5 - 7.0. Gland clover's pH range is 4.5 to 8.0. Rose clover suits soil pH 4.5 to 6.0.

Other variety factors are important for long term persistence, especially appreciated after the long recent drought.

Varieties with high levels of hard seed, provided a pre-drought good seed set occurred, can commonly regenerate after a long drought or a moderate cropping phase where in-crop weed control by herbicides destroyed pasture legumes growing within the crop.

Maturity suited to a given environment, for example early flowering and seed set such as late July early August, also plays a role in long term persistence. In our own case Dalkeith sub clover King, Santorini, and Elgara serradella, and Casbar biserrula, are all hard seed early flowering varieties proving long term persistent.

Many other factors are important for long term persistence including grazing management (especially critical for aerial seeded varieties) and soil fertility.

Next week: Top wheat breeder recognised for outstanding varieties.