Healthy Organic Plants Don’t Rely on Fertilizers

Good soil structure is essential to the efficient uptake of nutrients by plants. Just as importantly the acidity or alkalinity of a soil controls the availability of nutrients. Measured on the pH scale most garden plants including fruit and vegetables grow happily at a pH around 6.5 (slightly acidic) – pH 7.0 is neutral. A pH testing kit is a good investment.

How to Control the pH

The calcium level in the soil controls the pH. Calcium can be washed out of the soil, especially a free draining soil, thereby making it more acidic. The application of calcium carbonate (ground limestone) or calcium magnesium (dolomite limestone) will make the soil more alkaline, but be patient, this change is a slow process taking up to 12 months for the full effect to develop. Lowering the pH of a very alkaline soil is not easy, but annual applications of flowers of sulphur will bring it down as will regular dressing with farmyard manure and acidic fertilizers.

The Role of Fertilizers in Plant Nutrition

• The nutrients most frequently found in fertilizers are nitrogen, potassium and phosphorus. They are needed by plants in quite large quantities and are referred to as major nutrients.
• Put simply, nitrogen encourages leafy growth and is easily leached from the soil.
• Phosphorus is important for root growth and the ripening of fruit.
• Potassium is essential for good flower and fruit formation.
• Plants only require small quantities of minor nutrients (also known as trace elements) such as iron, manganese, boron, copper, zinc and molybdenum. Deficiencies of minor nutrients are rare in most UK soils.

The Fertilizers Gardeners’ Need to Apply

• For garden use ‘compound fertilizers’ which supply nitrogen (N) phosphorus (P) and potassium (K) are usually the only ones you need to apply.
• Plants growing in the restricted root volume of containers quickly use up all available food and so both major and minor nutrients have to be added to the compost in the form of ‘complete’ plant foods which may contain magnesium, calcium, sulphur and minor nutrients in addition to nitrogen, phosphorus and potassium.

Different Kinds of Fertilizers and Their Qualities

• Fertilizers are divided into organic and inorganic kinds.
• Organic fertilizers such as hoof and horn and bonemeal are derived from material that was once living. As the nutrients are released slowly there is a low risk of over-application, however the nutrient content is often variable resulting in unpredictable results.
• Inorganic fertilizers like Sulphate of Ammonia are predictable, uniform in their composition and usually cheaper. On the downside they are lost rapidly from soils through leaching and carry the risk of environmental pollution if used excessively. There is a higher risk of plant damage if they are over applied.

Which Fertilizer to Use and How Much to Apply

There is no place for inorganic fertilizers in the organic gardeners’ armoury. It is, however, useful to know the potency of the fertilizers you buy. The nutrient analysis of fertilizers is printed on the bag. The ever popular ‘Growmore’ (inorganic) contains 7% N, 7% P and 7% K. Pelleted chicken manure (organic) generally comes in at about 4.5% N, 3.5% P and 2.5% k. You therefore need to apply the chicken manure at a slightly heavier rate than the ‘Growmore’ in order to supply the same quantities of nutrients.

So for Good Soil Management Use fertilizers Wisely

When combined with good soil management which relies on bulky organic materials to maintain good soil structure, and the optimum pH level, the judicious use of fertilizers can help us grow better plants, but they are not miracle chemicals.