Avoid Using Immature Compost

A recent warning is important for all users of compost - avoid immature compost as an organic amendment - it can do more harm than good according to Pam Pittaway at USQ.

Soil health specialist Dr Pam Pittaway has warned farmers of the danger of using immature compost on crops of all kinds.

Soil health specialist Dr Pam Pittaway has warned farmers of the danger of using immature compost on crops of all kinds, and advised that either curing organic compost yourself, or testing it and the soil for nutrients, is the best way to ensure organic amendments benefit crops.

Speaking at a recent symposium about soil health at the University of Southern Queensland (USQ) in Toowoomba, Dr Pittaway said knowing the microbial stability of your amendment and its salt content were the best ways to ensure your crop got the optimum nutrients.

“If you know the salt content of your amendment, and the plant-available and total nutrient content of your amendment, you might be able to reduce the amount of NPK fertiliser you put on your crop,” Dr Pittaway said.

This could result in farmers greatly reducing their inorganic fertiliser bills, but Dr Pittaway said the age and content of the organic amendment, or fertiliser, needed to be taken into consideration, and the purchase of product of indeterminate age or properties was a risky business.

“As soon as you put on something with readily available carbon as well as nitrogen and phosphorus, you stimulate a microbial feeding frenzy which can starve your crop. If soil microbes have access to fast food, they take up luxury amounts of available N and P, out-competing plant roots,” she said.

“Adding manure or compost high in potassium can also burn plant roots, as potassium is a soluble salt,” she said. 

“And when growers continue to add fertiliser nitrogen on top of past organic amendments, they can get an uncontrolled growth flush which commits your plants to an extra load; if they can’t support that during the growth period, they can end up shedding tillers, or can lodge.”

Dr Pittaway said root disease was also a possible symptom of over-feeding crops with organic amendments.

“The best way to get root disease in a crop is nutrient imbalance.”

“My advice is if you can’t afford good cured compost, invest in more soil testing, and put immature products on your least valuable crop.”- Dr Pam Pittaway

Australian horticulture and agriculture use a vast range of organic amendments, including pig and poultry manure produced, cured and used on-farm, and commercially produced preparations, most of which incorporate straw or green waste as a source of carbon.

Dr Pittaway said the best result from using cured compost achieved organic slow release which synchronised nutrient release with plant demand, and minimised grower expenditure on nitrogen, phosphorus and potassium (NPK) inputs.

“My advice is if you can’t afford good cured compost, invest in more soil testing, and put immature products on your least valuable crop.”

Dr Pittaway said producers needed to be aware of the business model of some waste management businesses, which sought a fast turnaround on organic amendments.

“Some businesses need to push product out the gate to make space for more waste, and while Australian Standards say certain aspects of organic amendments must be tested, those results won’t necessarily tell you what’s best for your crop or soils.

“Instead of a six-week turnaround, which is what some waste-management businesses operate on, you want to use organic amendments that have been cured for 16-20 weeks, because immature compost could do more harm than good.”

She said farmers should also be aware of the environmental risks of over-application of organic amendments, which can put phosphorus surplus to the crop’s requirement into waterways.

Dr Pittaway has conducted studies on a number of organic amendments including cotton trash, feedlot manure, sugarcane waste, and sawdust in broadacre and horticultural applications.

“Fully cured compost costs more but immediately conditions your soil. Humus-like organic acids in cured compost are very good at holding moisture and buffering against acidification, and can make more phosphorus available. Adding humus-like, cured compost builds the stable, long-term organic fraction back into your soil.

“If it is fully cured, you can build a slow-release nitrogen bank [ in the soil] which will release nutrients in synch with the crop’s development.”

- Source: University of Southern Queensland.

 This advice applies to soil applied compost but may not be so important for use of above ground mulch, which also acts as a erosion protection and soil stability mechanism, while breaking down.  This is a factor with using green manure crops or crop residues on the surface.  If in doubt over using crop residuals or compost,  seek advice from a competent agronomist.