The Effect of Fertilizers on Courgettes

zucchini image by Olga Shelego from

Fertilisers affect different plants in different ways. Some types of fertiliser are better for plants grown for their leaves such as spinach, basil and celery. Plants grown for their fruit or seeds benefit from different fertiliser formulas.

Knowing which type to use and when may help maximise yields of courgettes---or zucchini, as they are also known.

Purpose of Fertilizers

Many people mistakenly believe that fertilisers are plant food. In fact, plants manufacture their own food from carbon dioxide and water through the process of photosynthesis, using sunlight for energy and the green pigment chlorophyll as a catalyst. Fertilisers are nutrients that allow the plants to grow and thrive. Think of these nutrients as vitamins for plants. All plants require varying amounts of about 16 basic nutrients. Carbon, oxygen and hydrogen are derived from the air and water. Nitrogen, phosphorous and potassium are the three most important soil nutrients and used by most plants in the largest amounts. Along with calcium, magnesium and sulphur, these are called macronutrients. Manganese, boron, iron, chlorine, zinc, molybdenum and copper are used in small amounts and are called micronutrients. Most fertilisers contain only nitrogen, phosphorous and potassium as the other nutrients are found in sufficient amounts in almost any soil.


Courgettes, or zucchini, are vegetables known as summer squash. Summer squashes are characterised by a shorter growing season than winter squashes---and by relatively tender skins. They are best when picked young and do not keep as well as winter squashes. They are members of the curcurbit family and are related to other types of squash, including various winter squashes such as acorn squash and pumpkins. They are also related to cucumbers and melons.

Nutrient Requirements for Courgettes

Courgettes benefit from a fertiliser containing a ratio of nitrogen, phosphorous and potassium in the range of approximately 1-2-1. Fertilisers with this ratio will provide the best growth and production. Fertilisers higher in nitrogen may be used early in the season to promote the formation of vines and leaves, but should be discontinued after the plants are well-established. Squash are relatively heavy users of nutrients and so will require some supplemental nitrogen; but over-fertilising with high nitrogen fertilisers will result in plants that produce lots of leaves but fewer flowers and fruit. As long as the proper balance of nitrogen, phosphorous and potassium is maintained, the plants will remain productive throughout the growing season.

Maximising Fertilizer's Benefits

In order for the plant to get the maximum benefit from fertilisers, certain other factors must be taken into consideration. Squash need lots of light to grow well. Plants which do not get enough light will not have the energy for strong growth and will not be able to take best advantage of fertiliser. Water is also a consideration. If the soil is allowed to remain too dry, the plant will not take up the fertiliser, as nutrients are absorbed through the roots dissolved in groundwater. Soil pH or relative acidity is also important. Courgettes will grow best in soil with a pH of 6.0 to 7.5. Soil pH outside of this range will restrict the plant's ability to absorb nutrients.

Organic Fertilizers or Inorganic?

Organic fertilisers are fertilisers derived from natural sources. There are many types on the market. Inorganic fertilisers are chemical fertilisers that are entirely man-made. The main difference from the plant's perspective is that organic fertilisers tend to present nutrients in a more controlled slow-release form, while inorganic fertilisers make nutrients more readily available. This means the plant will absorb more nutrients more quickly from inorganic fertilisers, but it will become depleted of them at a faster rate.