Farm Table says:
To unlock the fixed-P bank in conventional and organic farming rotations
What is the problem?
Typically, 60-80% of the phosphorus (P) fertiliser applied to crops becomes fixed in the soil and unavailable for plant uptake.
What did the research involve?
1) Glasshouse experiments
– The first experiment evaluated the performance of a break crop, the wheat rotation for four organic acid secreting pulses as a function of soil type, P application and the water regime between both crops.
2) Laboratory experiments
– Selected soil samples were fractionated to determine organic and inorganic P pools. The soils were extracted with a range of low molecular weight organic anions (LMWOAs) found in the rhizosphere of organic anion exuding plants, such as albus lupins. It was found that the LMWOAs were absorbed into the soils in the order oxalate >> malate = citrate.
3) Field experiments
– Four field trials were conducted in the cereal belt of southern and central New South Wales (NSW) on low available P soils in 2001-02 (Berthong and Greenethorpe) and 2004-05 (Boorowa and Grenfell) in which organic anion secreting crops (albus lupins and chickpeas) and non-secreting pulses (e.g. faba beans) were grown in the first season with and without P fertiliser.
What were the key findings?
No specific recommendations to growers regarding the inclusion of organic anion exuding crops resulted from the study because the field component of the project failed to show carryover P benefits from any organic anion exuding species to a following wheat crop, compared to non-secreting break crops or bare soil control plots that had no prior crop before wheat.
- while the glasshouse work showed promising results, they did not translate to the field, most likely because of adverse seasonal conditions. Consequently, it was not possible to assess economic benefits to the grains industry
- increased profits for conventional and certified organic grain growers through the improved efficiency of recently applied P fertilizer, and improved access to the large pool of fixed soil P using a break crop, such as albus lupins, that exude organic acid anions from their roots, was the project goal
- benefits to the environment could also accrue through more efficient use of P fertilizer and its extraction of P from fixed forms of soil P because these measures would reduce P loading of the soil
- the mechanism by which faba beans conferred a P benefit to the following canola crop in the glasshouse experiment should be investigated further, even though it did not show a P benefit in field trials. The large break crop effect of faba beans was somewhat unexpected, as the labile value (L-value) data indicated that faba beans did not access a significantly larger pool of soil P than wheat or canola. In follow-up experiments, designed to identify possible mechanisms by which faba beans might increase soil P availability, significant root exudation of organic anions or phosphatases in P deficient plants were not detected
- an evaluation of intercropping an organic acid exuding crops such as albus lupins and a less P uptake efficient crop (e.g. wheat) may be another research and development (R&D) opportunity, particularly for organic growers
In four field trials, there was no yield benefit of organic anion exuding crops to a following wheat crop, probably because of drought.
Faba beans improved the P nutrition and growth of canola in the glasshouse, but not wheat in the field.