There are three main soil and management factors that can impact on legume nodulation: soil pH, nutrient availability and use of herbicides. Identifying and addressing these factors can improve legume performance, nodulation and nitrogen fixation.

1. Nutrient availability

Legume plants and their associated rhizobia both have specific nutrient requirements for function and growth. Ensuring sufficient availability of key macronutrients, such as phosphorus (P), sulphur (S) and potassium (K), will support legume growth and rhizobia function. Micronutrients, such as molybdenum (Mo) are also critical to nodule formation and function.

High levels of available aluminium (Al) can impede the nodulation process by affecting legume root development and impacting on rhizobia survival.

2. Herbicide use

Residues of some herbicides can adversely impact root development and rhizobia survival. It is critical to observe plant-back periods on herbicide labels to lessen the potential impact.

3. Soil pH

The pH of soil affects not only the plant, but also its associated rhizobia. Plants often tolerate variations in soil pH better than their associated rhizobia (see Table 1 below). For example, sub-clover can grow quite effectively down to a pHCa of 4.2, while the Group C inoculant responsible for nodulating sub-clover has a higher pH requirement for optimal performance. Plants and their associated inoculant group differ in terms of pH tolerance and species. For example, serradella (and its associated rhizobia) have a greater tolerance to lower pH (Table 1), however, plant tolerance is still greater than rhizobia tolerance.

In some cases, it may be necessary to consider applying lime to improve plant growth, rhizobia survival and legume nodulation. Where liming is not possible, consider using a more tolerant legume and its associated rhizobia. Nodulation and nitrogen fixation can also be affected under alkaline conditions, particularly where soil pHCa exceeds 8. Take care when sowing pastures to match pasture species and varieties with their associated rhizobia.

(Note: When choosing species, it is also critical to consider other stressors in addition to soil pH, including availability of key nutrients, presence of toxic plant elements such as aluminium and tolerance to waterlogging)

Table 1: Sensitivity of key rhizobia and their host plant to soil pH(CaCl2)*

*In-paddock assessments were carried out by: Janelle Jenkins (Riverina LLS), Belinda Hackney (Central West LLS/Graham Centre for Agricultural Innovation), Jo Powells (South-East LLS), Clare Edwards (Central Tablelands LLS), methodology was developed by Ron Yates (DAFWA) and nodule occupancy assessment by Sofie De Meyer (Murdoch University).