When do you start fertilizing seedlings? The most common advice is that newly germinated seedlings do not need to be fertilized because they get initial nutrients from the seed and the soil/media. If that is true, why does the horticulture industry start fertilizing as soon as they seed? Do they know something gardeners don’t?
How Do Seedlings Grow?
Seeds contain everything that a seedling needs to start life, except water. The water a seed absorbs starts a series of complex chemical reactions that trigger the embryo to start growing. For most plants the seed also contains enough food to form the first set of leaves. In flowering plants these first leaves are called cotyledons or “seed leaves”. Monocots, including daylilies, orchids, palms, bananas, pineapples, and corn produce one cotyledon leaf, while dicots produce two (most common garden plants and most vegetables).
Cotyledons usually look different than regular leaves and they can be very transient, or fairly long lived. They get all their nutrients from the food stored in the seed so the plant does not need added nutrients to develop them. Cotyledons also photosynthesize producing food to help develop more roots and leaves, but they can’t produce enough food to grow large seedlings.
At a minimum, seedlings need to get nutrients through the roots once the true leaves start to form.
Do Seeds Need Fertilizer to Germinate?
The common advice given to gardeners is, “seeds do not need fertilizer to germinate. Since seeds contain all the nutrients a seedling needs to emerge and grow its first few leaves, they can germinate without any additional nutrients.” The statement is actually correct – they don’t “need” fertilizer, but would they “benefit” from earlier fertilizing?
Available Nutrients vs Fertilizing
It is important to distinguish between fertilizing and available nutrients. They are not the same thing. If the seed is planted in very nutritious soil, the seedling may never need to be fertilized because it gets all the required nutrients from soil. Fertilizing is the process of adding extra nutrients to a growing media.
Consider native seed that lands on soil. Its seedlings germinate and grow just fine with no added fertilizer. Vegetable seeds that are sowed directly in soil, may benefit from fertilizing. In fact in agriculture it is common practice to add fertilizer while sowing.
In this blog we are really dealing with a different situation where seed is started in a pot containing soilless mix like peat moss or coir. These do need fertilizing at some point because neither of these media provide nutrients on their own.
Fertilizer in Potting Media
Commercial potting media may or may not contain added fertilizer and it can be difficult to find out how much fertilizer has been added. One product I looked at had an NPK of 0.03-0.03-0.03. I use PRO-MIX (affiliate link) and the only listing I found for it said it had “no fertilizer”, although it is promoted as having “added fertilizer”. Sunshine mix, which is also popular, is known to have less fertilizer than PRO-MIX.
Even with an NPK of 0.03-0.03-0.03, the nitrogen and potassium will wash out very quickly. I think it is best that gardeners assume their seedling mix, or potting mix does not contain enough fertilizer to make a difference, unless the product contains slow release fertilizer.
If the potting media is not providing nutrients, then seedlings should be fertilized once the cotyledon forms. But should you fertilize even sooner?
What is the best seedling mix? Read all the details here Best Seed Starting Mix, Including DIY.
Should You Fertilize the Seed?
There are numerous studies that compare seedling growth with and without adding fertilizer at the time seeds are planted. All the ones I looked at have better growth with fertilizer. However, it was not clear if this growth is a result of fertilizing at the seeding stage or because the fertilizer was there once true leaves started to grow because measurements were only made at the end of the experiment.
Soaking seed in fertilizer, called priming the seed, has been studied for sorghum and millet. Priming with fertilizer increased the percent germination and speed of germination but it did not affect the size of seedlings after 15 days. A more detailed study on sorghum seed found that organic fertilizer was better than synthetic, which was better than water, in terms of germination rate, speed of germination and seedling wet weight. Oddly, the dry weight of seedlings grown with synthetic fertilizer were significantly lower than with water, which does not make much sense.
Commercial greenhouses that raise seedlings usually add fertilizer (low levels) to the seeding soil mix. They also add water soluble fertilizer (low levels) to the irrigation water starting with the first watering after seeding. They would not do that if it didn’t result in either better germination or better plant growth.
Nitrogen is the nutrient that has the greatest effect on plant growth. The following is advice from the University of Massachusetts Extension Greenhouse Crops and Floriculture Program and it’s a good guide for most flowers and vegetables grown by gardeners. Fertilizer is applied before cotyledons are fully developed.
Ball Seed Company, a major supplier of seed to commercial growers, recommends applying 50-75 ppm N during stage 2 (radical emergence to cotyledon expansion) and 100-150 ppm N during stage 3 (cotyledon emergence until true leave emergence).
The advice from commercial operations is to add fertilizer before the cotyledons are fully developed.
Too Much Fertilizer Harms Seedlings
A nutrient level that is too high will harm seedlings in the same way that over fertilizing a lawn will kill your grass.
When Do You Fertilize Seedlings?
The advice that seedlings don’t need to be fertilized because the potting media already contains fertilizer is wrong unless the media contains enough slow release fertilizer. Fertilizer should certainly be added when the first true leaf starts to develop. I’d do this even if the media contains some slow release fertilizer.
Based on the above studies and the methods used by commercial growers, fertilizing should be started as soon as seeds are put in soil.
What about germinating seeds using the baggy method which normally uses water. Some preliminary citizen science experiments reported in our FaceBook Group suggest that low levels of fertilizer might be better than just water when using the baggy method. I am doing some testing right now and will report results in a couple of weeks.