How to Compost Grass?Tuesday, 4 February 2014 | HOTBIN Composting Composting Grass/Lawn Mowings in the HOTBIN
Grass is so quick, we recommend it to 'accelerate' and raise the temperature when setting up your HOTBIN. This post focuses on composting regular weekly grass cuttings. If you want to use grass to get to 60°C, follow the link to 'How to get to 60°C. Composting grass successfully requires a little bit of knowledge but the real secret is matching your method to the amount of grass you generate and the time and effort you have available. Grass lawn mowings are a paradox. On one hand it is one of the easiest materials to compost (hence why the HOTBIN team recommends it to help increase temperature quickly); on the other hand, in traditional heaps, it is one of the most troublesome materials to compost. It often generates a lot of heat and produces a very distinctive whiff (ammonia/urine) for 2 days, before it 'collapses' into a cold 'wet slimy black' mass. The odour is caused because grass has an excess of nitrogen which the bacteria are unable to use as fast as it is released. So it forms ammonia gas and evaporates away. You are most likely to notice this when composting and/or turning large quantities of 1-2 day old grass lawn mowing. After 3 days things slow down and the nitrogen is no longer in excess. Turning grass heaps does not prevent the odour - in this instance, turning enables the gas to escape 'all in one go' so it is stinky after turning. (if you have done this job, you may well come back inside the house and realise your clothes wreak of ammonia!). The HOTBIN does have a odour filter in the lid that does remove ammonia odour. But, when you add a whole box in one go without anything else, the filter gets temporarily overload for 2-3 days. To prevent the odour during the initial 2-3 days you need to balance the carbon/nitrogen ratio. You achieve this by adding a dry high carbon waste. The key here is has to be 'easy to digest carbon' such as corrugated cardboard or paper shredding. Woody items like sawdust, shavings, wood chips are high carbon - but they are not easy to digest, so will not balance the C/N during the critical 2-days of intense activity. Here is the challenge - you need a lot of dry carbon! A 40L grass box (a typical mower box), needs 20L of paper - that's a whole carrier bag full. It also needs to be mixed with the grass. Not everyone wants to do this, especially after cutting the grass. Below we outline a few options about different methods you might want to follow. The traditional heap that generates a 'black slim' is due to excess water and too little airflow. Grass lawn mowings have a high water content (>80%) and are low in lignin (i.e. no woody stalk). As it decomposes, the cells breakdown, becomes soft and water is produced. The grass collapses and forms thick layers reducing airflow. This in turn means the water is trapped, the process slows and a viscous circle is created which water is not removed, the heaps turns anaerobic and the 'black slim' is created. Thankfully 'black slim' is a rarity in the HOTBIN. The excess water is removed as steam when hot composting and ensuing good airflow in the HOTBIN is a breeze - just add 3-5 hands of wood chip bulking agent for every box of grass clippings. (The wood chip helps aeration - the fact it is high carbon does not solve the ammonia odour issue). So we can compost small and large amounts of grass in the HOTBIN. We know large amounts need extra effort to avoid odour. Is it worth it? We think so you get great compost and lots of it. However below are we outline six options/choices that you might like to consider. Often you can 'mix n match' routines at different times of the year to cater for the grass you generate in spring versus summer.
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