How to feed your garden nitrogen
Nitrogen is the essential element that in terms of quantity is required most by plants. It has many functions in plant metabolism, not least having a crucial role in protein formation. As it readily becomes unavailable in the form that plants can absorb it, it is also applied the most as a plant feed. Therefore in the majority of gardening situations, it is necessary to avoid a state of nitrogen deficiency and add it to the soil. In what form should this be done? For nitrogen, while present in the atmosphere as a gas, and locked up in organic compounds in the soil humus, is only available to the plants as mineral salts dissolved in the soil water.
The short and simple answer or perhaps simplistic answer is to apply readily soluble, chemical fertilizer such as Ammonium Sulphate or Urea. However plant nutrition does not take place in isolation from the general conditions prevailing in the soil. For instance, a lack of oxygen in the soil sets in motion a process known as de-nitrification, whereby mineral nitrogen changes to a gas phase, and is consequently lost to the atmosphere.
In order to prevent this, it’s necessary to ensure adequate drainage, and also a large percentage of organic matter, in the form of humus, in the soil. A high humus content helps to form a crumbly soil structure and thus a satisfactory balance between air and moisture. It further serves as raw material for essential soil organisms such as earthworms, which by their activity both enhance the soil’s aeration and indirectly cause nitrogen to become more available to plant roots together with the other elements vital to plant growth. It follows therefore that periodically adding compost or humus to the earth is an integral and necessary part of plant nutrition.
If Nitrogen fertilizer in chemical form is to be applied, it should be done so during the plants’ growing season. For lawns, the spring and early autumn are generally the ideal seasons, at least in Mediterranean climates, whereas fertilizing in the summer often induces fungal infestations. Chemical fertilizing is a cheaper and probably more effective method of feeding lawns than the spreading of compost, but it should be noted amongst other drawbacks, that a serious environmental issue is at stake here.
The problem is that nitrogen fertilizer is easily leached out of the soil by rain or irrigation, and in the form that it rapidly turns into, (nitrates and nitrites) pollutes the water table, or any other body of water into which it eventually drains, such as lakes and rivers. It is primarily for this reason that I advocate the use of slow release fertilizer containing a balance of nutrients, as opposed to soluble types. In any case, for lawns, entirely satisfactory results can be achieved in most home garden situations by their use.
With regard to the garden plants as a whole, it’s usually necessary to add nitrogen fertilizer while planting a new garden bed. The soil should be thoroughly composted before hand, at a rate of at least 20 liters per square meter of ground, and a balanced slow release fertilizer, containing a high percentage of N and P (nitrogen and phosphorous respectively) applied at the rate indicated by the manufacturer’s instructions.
My name is Jonathan Ya’akobi.
I’ve been gardening in a professional capacity since 1984. I am the former head gardener of the Jerusalem Botanical Garden, but now concentrate on building gardens for private home owners. I also teach horticulture to students on training courses. I’d love to help you get the very best from your garden, so you’re welcome to visit me on http://www.dryclimategardening.com