Legume crop residues can deliver nitrogen (N), one, two or even three years after they are harvested.
That was a message at a recent GRDC-funded More Profit from Crop Nutrition II initiative advisor workshop in Griffith.
This news comes at a time when Australian farmers potentially have the most ‘free’ N in their soil in over three years after approximately two million hectares was planted to legumes in 2016 (ABARES).
Soil mineralisation basics
This N is delivered via mineralisation – the process where N is made plant available as ammonium (NH4+) from organic matter – and legumes have the potential to offer ‘bigger and faster gains’ to the system contributing up to 6% of total N per year.
The dynamics of soil mineralisation are sensitive and achieving significant amounts of ammonium is dependent on many variables. As a general rule, warm temperatures, moist soil and a good supply of organic matter generate conditions conducive to mineralisation. This is why rates can increase after significant summer rain – and another reason why summer weeds should be controlled.
Generally, mineralisation:
- Is rapid in ‘warm conditions’ between 20-35oC and slows down above or below that temperature range
- Is rapid when the soil is wetter at 10 – 100kPa soil conditions or
- More N is available for mineralisation from legume residues than other crop types
‘Recycled’ and new N soil supply from legume residues
“Legumes don’t just fix N from the atmosphere” said Mark Conyers, NSW DPI. “The majority of N they take up is from soil mineralisation” so N that has been fixed by the legume is actually the smaller proportion of the N in a pulse plant and can be considered ‘new’ to the system.
How much N will mineralise during the growing season is hard to measure but Mr Conyers said amounts to 1-6% of the total N are possible which could amount to 13 to 78kg mineral N/ha/year based on a soil test of 0.1% total N in a soil test.
It is possible some legume residues will mineralise two or even three years after the legume crop was grown. “By this stage” says Mr Conyers, “the N supply becomes soil organic N and the distinction between this and plant residue N becomes ‘fuzzy’”. By then, more N could be supplied to the system from subsequent crop residues.
Soil testing – a key tool to monitor soil mineral N
“The trick in N management” said Mr Conyers “is that the weather determines soil N losses and N gains and the weather determines plant demand for N”.
Soil testing prior to sowing captures how much nitrogen is in the soil after the current summer period and before crops start growing. “Deep N soil testing is a ‘point in time’ measurement so we know, roughly, the initial mineral N in the soil” Mr Conyers said. “We also know how much N is in starter fertiliser” he added.
Crop models, such as Yield Prophet®, are useful options for managing in-crop N applications. Other methods include using the French and Shultz equation to estimate crop yield potential and therefore crop demand. It is then a matter of matching crop supply with crop demand to assess the need for additional nitrogen.
