Introduction to integrated methods in the vegetable garden
Chapter : Fertilization
Previous or next articles ; click on a title to go to the page
⇒ Can you force a vegetable?
How often do we hear about the "forcing" of vegetables and fruits that farmers would undertake to increase their yields at the expense of quality! This forcing is said to be undertaken with the help of an overdose of industrial fertilisers. This idea has been known for a long time, but it is wrong. It is rather the choice of variety, the cultivation method and the local conditions that influence quality. For example, anyone who grows tomatoes in the field has found that the taste of tomatoes is far superior to those found in supermarkets. Most tomatoes sold in supermarkets are grown out of the above ground and are fed daily with nutrient solutions that are supposed to replace the complex work of microorganisms in living soil. The vast majority of French people, especially those who live in large urban centres, are unknowingly formatted to the insipid taste of these greenhouse-grown tomatoes that have invaded all distribution structures. The varieties are chosen to meet profitability objectives and to adapt to growing out of the ground in greenhouses to resist disease.
As far as fertiliser application is concerned, it has been found that the effect of nitrogen is reinforced by potassium and vice versa; this is the so-called positive interaction phenomenon in which the greater the nitrogen fertilisation, the greater the need for potassium. But there is a ceiling. Mitscherlich's law (law of less than proportional increases) states that when increasing doses of a fertiliser are applied to the soil, yields evolve according to a curve whose peak represents the maximum possible yield in relation to fertiliser doses. Beyond this peak, any additional fertiliser input is useless and becomes an unnecessary cost for the farmer. Note that in the positive interaction, the effect of two nutrients is greater than the sum of the effects of the nutrients separately. For example, meeting potassium requirements ensures greater efficiency of nitrogen inputs (1).
The case of nitrogen.
A plant cannot be forced to take up more minerals than it needs, except for nitrogen, which is still taken up, although there is a ceiling on profitability. Excess nitrogen is noticed quite quickly because of the problems caused to the crops. If major elements such as potash or phosphates are applied in excess of requirements, the surplus is left behind. On the other hand, the surplus of nitrogen absorbed by the plants alters their development. This results in a longer vegetative period and delayed maturity of the plants, leading to problems such as foliage degradation resembling burns and the appearance of diseases. Dark and abundant foliage with reduced flowers in tomatoes accompanied by blight or alternaria indicates a possible excess of nitrogen. In general, all plants that receive too much nitrogen in relation to their needs become very susceptible to fungal diseases.
A deficiency of one important element can lead to yield loss even if the other elements are sufficient. The law of interaction states that the yield of a crop is determined by the element that is in the lowest quantity in relation to the crop's needs. Any additional supply of the other elements will not produce a yield gain. Thus, it is useless to saturate a garden soil with potash and phosphate if, for example, nitrogen is deficient. Vegetable plants will neglect all other elements until the nitrogen deficiency is also corrected. Of course, a limiting factor is always difficult to determine without laboratory analysis. One or more limiting factors are not uncommon in vegetable gardens, and they result in crops that are often stunted and affected by disease.