Warning: rather like a lecture
Saw an interesting article about compost: HortResearch Cducted a wee trial comparing 12 different composts. They were mixed one to one with sand and then planted with tomato lettuce and marigolds. As well there was a sand alone and sand with fertiliser. The composts inlcuded were from Living Earth, two from city council operations and Zoomgro.
Three of the composts had results where the plants grew better in sand alone. Four were slightly better than sand alone, and five gave reasonable results. Sand and fertiliser was as good as any of the composts.
This is because compost can have low nitrogen levels (2 mg/litre in some while the ones that grew good plants were up to 75 mg/litre). Plants need nitrogen to grow. They take it up in the form of dissoved salts, often ammonium salts. Because plants take up nitrogen as chemicals the plants can't tell where the Nitrogen came from.
So if plants grow as good in sand and fertiliser as in compost of the highest quality, why does everyone recommend compost? Compost is organic matter rotted down, and is a fantastic improver of soil structure. It breaks up clay to let in more air and improve drainage. It binds up sand and holds more water. And it hold nutrients in the soil so they don't drain away. Because it is black, it warms the soil and speeds plant growth.
If you farm and grow crops on soil you have to cultivate, and when you do you oxidise the organic matter. Soil structure is gradually destroyed. You end up with crappy hard soil, with poor air circulation and poor water drainage that plants can't grow well in. It is necessary to add organic matter if you crop, and that could be as compost or as green plants left to rot. If you crop plants and only add compost then crops will gradually decrease in productivity as you use up the available nitrogen. If you don't add organic matter you destroy the soil. The perfect farmer would add both organic matter to replace that which is destroyed, and add fertiliser to replace nutrients removed by the crops.
Composting has a great side effect of reducing our rubbish, and producing something useful. But it is not nutrient rich enough to feed us. Compost will not save the world, it is only part of the solution...
Posted by Toni at September 18, 2003 08:49 AM