The Most Common Mistakes When Fertilising Plants
Fertilising is a complex topic, so it is no wonder that even experienced growers occasionally make mistakes when it comes to plant nutrition. While plants may forgive some errors, others can cost you a significant portion of your harvest. In today’s article, we will look at the most common fertilising mistakes, how to avoid them and what to do when you encounter problems.
Those who fertilise plants without considering their needs or use the wrong fertiliser will usually not achieve a good harvest. Among our customers, the most common issue we encounter is over-fertilisation, which is particularly dangerous when using mineral fertilisers. Another frequent mistake is following dosage tables too literally, even when the plants clearly need less or more fertiliser. Last but not least, we often see some growers preparing plant feed like a dog and cat and mixing fertilisers from different brands and types, which often does not end well.
Over-fertilisation
There is a reason people say that less is sometimes more. Plants do need nutrients, but an excess can harm them just as much as a deficiency. If you use mineral fertilisers, the main risk of over-fertilisation is that large amounts of dissolved salts accumulate in the substrate and damage the plant roots. This is manifested by burnt leaf tips curled into a claw-like shape and slower growth.
A frequently mentioned advantage of organic fertilisers is the “impossibility” of over-fertilisation. However, this is not entirely accurate. Organic fertilisers do not contain fast-acting mineral salts and therefore do not burn the roots. Nevertheless, an excess can affect soil pH and nutrient availability. When growing with organic fertilisers, the most common issue we encounter is excess nitrogen, which manifests itself in plants through dark green leaves.
Regardless of which type of fertiliser you use, pay attention not only to nutrient deficiencies but also to signs that you are fertilising too heavily. When dosing mineral fertilisers, check the EC and pH of the feed every time you water. If plants have been over-fertilised with mineral fertilisers, flush the substrate with clean water adjusted to the correct pH and do not add fertilisers or nutritional supplements to the feed until the plants’ condition improves. Because organic fertilisers work more slowly than mineral fertilisers, it is not always easy to identify the cause of over-fertilisation immediately. If problems occur, stop fertilising the plants and measure the soil pH, which is often the most common cause of nutritional problems when growing in soil.
You may also be interested in: A guide to choosing a fertiliser for your growing
Fertilising according to a table instead of plant needs
Most fertiliser manufacturers provide detailed dosage tables with their nutritional programmes, often specifying requirements for each week of the growing cycle. It is important to realise that dosage tables are only a guide, and blindly following them in all circumstances is not a good idea. Fertiliser manufacturers often assume ideal growing conditions, which cannot always be guaranteed. Climate conditions, variety, light intensity and sometimes the individual needs of the plant can affect fertiliser consumption and dosage.
Air temperature and humidity affect transpiration (the evaporation of water through the leaves) and the amount of water that plants consume. Dry air and high temperatures cause the substrate to dry out quickly, while fertiliser residues may accumulate in it. This is primarily a problem when growing in pots. It is therefore important to reduce fertiliser concentrations in the feed at higher temperatures, especially if you use mineral fertilisers.
You may also be interested in: How to measure and manage substrate EC
Genetic predispositions play an important role in the nutrient consumption of each plant. Some varieties have been bred to tolerate high doses of fertiliser and, under the right conditions, will reward you with high yields. This applies particularly to modern hybrids intended for indoor growing. Landrace varieties or cultivars for outdoor growing are often more sensitive to over-fertilisation, and high doses of mineral fertilisers can stress them. The same applies to fast-growing varieties, which we recommend fertilising less than photoperiod varieties.
Light intensity is another important factor affecting how many nutrients plants are able to consume. The most powerful LED grow lights emit light at an intensity up to half as high again as on a sunny summer day at true midday. Plants grown under such powerful light can utilise very large amounts of fertiliser. On the other hand, under lower-powered lights or during prolonged periods of overcast weather, plant metabolism is slower and plants naturally consume fewer nutrients. In such cases, the amount of fertiliser in the feed must also be reduced accordingly.
Choosing or combining fertilisers incorrectly
Always choose fertilisers according to the substrate in which you are growing. In soil and soil-based substrates, you have the widest choice. You can use organic or mineral fertilisers, liquid fertilisers or slow-release fertilisers, as well as various combinations. When growing outdoors in the ground (beds or greenhouses), we generally recommend using organic fertilisers, which are gentler on the soil. Mineral fertilisers are better suited to growing in pots. Coco substrates are distinctive because coco naturally binds and retains calcium ions, so in addition to fertilisers, you need to add a liquid calcium and magnesium mixture (CalMag) to the feed. Hydroponics requires special water-soluble mineral fertilisers.
Regardless of which type of substrate and nutritional programme you choose, try to stick to one brand and type of fertiliser. Although it is possible under certain circumstances to mix different fertilisers from different manufacturers, you are more likely to harm the plants than help them. The same applies to additives and nutritional supplements. Use only those recommended by the manufacturer or intended for your growing technique. There is no point adding random nutritional supplements to your feed simply because they have been sitting on your shelf since the last horticultural trade fair.
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