Compost is often touted as a garden essential, adding nutrients to the soil. It is, in effect, a fertilizer, with all the benefits and hazards but excessive use, especially manure-based or commercial compost, can be harmful.
Homemade compost has a nutrient level found in the plants used to make it, usually an NPK of around 3-0.5-1.5. Commercial compost can contain animal manure and usually has an NPK of around 1-1-1.
At optimal growing levels, ornamental plant use NPK in a ratio of 3-1-2. If plants need 3-1-2 for optimal growth, and home-made compost has an NPK of near that, compost should be the perfect food, right? There are two errors in that thought.
The first assumes that your soil is nutrient free. Nothing could be farther from the truth. Experts agree that most garden soil in North America has sufficient phosphorus for ornamental plants. They also agree that except for sandy soil and orchards, gardens do not require potassium either.
The second error is to assume that each nutrient acts the same in the soil. Not so! Nitrogen does not stay in soil. It is water soluble so tends to leach when it rains. It is chemically unstable, so it combines easily with ions in the soil making the nitrogen unusable to plants. And plants aren’t the only things consuming nitrogen, so are algae, fungi, and bacteria. Phosphorus is the least in demand of the nutrients. It is not water soluble, and it is chemically stable. The phosphorus you applied in April will likely all be there at the end of the year and have only moved about ¼ of an inch in the soil, assuming it hasn’t been washed away into a lake or river and caused an algae bloom.
Even with compost, to meet the nitrogen needs of your plants, you would have to add an excess of phosphorus. Commercial compost with an NPK of 1-1-1 being worse. And the consequences of adding too much phosphorus are serious – iron, magnesium and zinc deficiencies being the least of your worries. Adding too much phosphorus is toxic to microorganisms especially mycorrhizal fungi which is essential for nitrogen uptake, the one nutrient needed most by plants and one nutrient that the soil is most likely deficient in. Without mycorrhizal fungi, plants need to expend more energy making larger roots resulting in less energy for growth, flowering and fruiting. The irony is that adding excess fertilizers will cause nutrient deficiencies in your plants and stunt growth.
So what to do? If you are going to use compost, it is better to use plant-based compost than manure-based compost since the former contains relatively less phosphorus. Also consider using less compost plus a synthetic nitrogen fertilizer. Lawn fertilizer without herbicides is a good choice. This will allow you to meet the target ratio of 3-1-2, allowing for the nutrients already in your soil.
Finally, keep using compost, especially as a mulch but don’t add more than an inch or two a year. Compost is both a fertilizer and a soil conditioner.
– Allison Forbes, Spirithouse Gardens