Tony Dickerson answers your questions
Q
As an organic gardener, what types of chemicalfree fertiliser are best to use? Leigh Parra , Ipswich
A
The simplest way of feeding plants is to apply generous amounts of organic matter to nourish the soil. Garden compost and well-rotted animal manures break down to form humus, which provides the major nutrients for plants, together with a range of trace elements. It’s also beneficial to soil structure and the microorganisms that live in there.
For those without access to farms or stables, bagged processed manures from garden centres are a great alternative. The percentage of nitrogen (N), phosphorus (P) and potassium (K) in garden compost and manure is relatively low compared with artificial fertilisers. They release nutrients, however, at a slow enough pace that plants can use them and they’re not lost by being washed (leached) out of the soil by rain. Nitrogen is used in the largest quantities by plants, especially in early spring, and is the nutrient most likely to be in short supply. But as temperatures rise, microbes in the soil break down organic matter more rapidly, providing plants with nitrogen and other nutrients. An easy way for organic gardeners to add extra nitrogen is with pelleted chicken manure that has a NPK ratio of about 4: 2:1. Dried blood has an NPK of 10:0:0 but it’s not always available in garden centres. Liquid feed made by steeping nettles in water and diluting the liquor provides tiny amounts of nitrogen, and even this is beneficial, especially if the weather is dry.
Comfrey provides similarly small amounts of potassium, but potassium is relatively immobile in the soil and not readily washed out. Dressings of garden compost or manure will provide sufficient potassium in most cases to replace that removed in harvested fruit and vegetables.
Wood ash from shoot tips contains some potassium (potash), but wood ash is highly alkaline and harmful to organisms in the soil.
Phosphorus is rarely deficient in cultivated soils and adequate amounts can, once again, be provided by applying garden compost or manure. Alternatively, blood, fish and bone is phosphorus-rich with an NPK ratio of 3:8:5. Phosphorus is also available as rock phosphate, but most organic gardeners would be concerned about the environmental impact of quarrying and long-distance transport involved.