Within reason the more winter annual legume plants per given area (for example per square metre) the greater their potential for greater early production and building of soil nitrogen.
Greater density of winter legumes (for example a minimum of 70 aiming for up to 350 seedlings square metre) will be a product of many factors including soil fertility, species and varieties suited to soil type (often soil-type variable) over a paddock, grazing management, rhizobia health, pests like blue and red legged earth mites and seasonal conditions.
Now is a good time to assess stands for legume density, their health and their type. Where only a few legume plants can be found per square metre and if their health is poor (for example poor growth, often reddish, yellowish) a good starting point is to check soil fertility, rhizobia nodule presence and health, legume species and soil pH. These factors can have a big impact on legume production and ability to build soil nitrogen.
Some species for example can tolerate low pH and associated issues like aluminium toxicity far better than other species. Often it’s the lighter soils of a property more devoted to pastures whereas heavier ones are used more extensively for cropping. And it is the lighter soils in many areas (not all areas) that tend to be more acidic than the loamy clay ones.
It is good practise to check soil for acidity both in the top (0-10cm) as well as the sub soil. For example test subsoil at initially around 30 cm for an indication of likely sub-soil acidity. Top-soil acidity can in the short and long term be corrected by lime application but it is more difficult and for many situations near impossible to change already present sub-soil acidity. When testing for soil acidity be aware that some laboratories and self-testing kits purchased from stores use the water-based pH test as opposed to the calcium chloride pH method used by most NSW accredited laboratories. Readings from the latter are commonly 0.9 to 0.6 lower than from the water based tests, a considerable difference from an interpretation basis.
Solutions to poor legumes on acid soils include species tolerant to such conditions. For example serradella and biserrula have excellent tolerance to soil acidity. Species like Gland clover, even sub clover, rose clover and white clover have useful tolerance to moderate soil acidity. So does arrowleaf clover and to a limited degree woolly pod vetch. Always check legumes for presence of rhizobia nodules. They should be easily found if plants are carefully dug up and healthy nodules are generally pink. Research has found widespread poor nodulation of many legume pastures and if this is not corrected they will never contribute to building soil nitrogen or achieving their productivity potential.
One of the most common causes of poor legume growth is soil nutrient deficiencies. No research has scientifically documented them responding to anything other than correcting existing soil deficiencies via fertilisers or other products that actually supplies them with sufficient quantities of these elements in available forms. Sulphur and phosphorus are the most common deficiencies but others like molybdenum can also be an issue. Legume presence in a pasture is also very much related to variety choice within a species. Varieties with high levels of hard seed are essential for most pastures. Early maturing ones generally set seed more reliably through tough seasons. Now increasingly appreciated is the need for good grazing management and care to ensure regular seeding occurs. This can be especially challenging in dry springs especially with aerial seeding types. Heavy grazing, especially in drier seasons also leads to poor seed set.
Next week. Working out crop nitrogen rate. One of biggest costs but commonly the biggest return.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.