Help! Why are my seedlings not growing?

This article is for all the frustrated gardeners out there with seedlings that just won't grow. While it can be for many reasons, it is often linked to the potting mix and the culprit is called: Nitrogen drawdown. But what is nitrogen drawdown and how can it be avoided?

Many of you might have already experienced this. You start your seeds and the germination is successful. The shoots emerge at the correct time of the year and then, nothing. No matter how well you care for them, your young seedlings remain only 2 cm long, pale and do little to develop. If that's the case, then you probably have an issue with nitrogen drawdown in your potting mix.

What is nitrogen drawdown and how does it happen?

Nitrogen drawdown occurs when microorganisms in your potting mix consume nitrogen before your plants can get to it. As we all know, soil and potting mixes are a living environment and myriads of tiny microorganisms compete with your plants for nitrogen. Unfortunately for us, microorganisms always win the battle.

Microorganisms require nutrients, such as nitrogen, to perform their functions. Many potting mixes contain high-carbon materials such as wood chips, tree bark and similar. The manufacturer add these coarse materials to provide a loose and porous environment that allows for good root development. Unfortunately, the organic materials are often not fully decomposed. And the microorganisms (bacteria, fungi) will continue to decompose it in your plant pot right next to your precious seedlings. To do this, they use nitrogen from the surrounding medium to fuel their activities. That surrounding medium is the same soil your seedlings are developing in.

Therefore, your seedlings appear to be starving in what appears to be ideal conditions. The nitrogen is, theoretically, present in the potting mix, but it is bound up within the microbial cells and the plants can't get to it. At the point where the high-carbon materials are fully decomposed and the microorganisms have stabilized, the nitrogen will once again become available to the seedlings. However, this could take weeks or months and by that point, the seedlings could have experienced poor growth or even died.

Why do commercial potting mixes have this issue?

It's mostly because time is money. In Australia, composting is defined by the Australian Standard AS 4454-2012 for compost, soil conditioners and mulches. This standard requires the following regime: three turns, with an internal temperature maintained at a minimum of 55°C for three consecutive days or more before each turn. This is essentially the hot composting phase, which ensures that the product is sanitised. This deactivates human and plant pathogens. However, this does not mean that the compost is stable, which only occurs after weeks of cold maturation. This, of course, requires more resources and time, which producers often can't spare.

Are some potting mixes better than others?

Although some companies have a better reputation than others, it is always possible to have problems with nitrogen drawdown. The nature of producing potting mixes makes it difficult to predict the final product which can affect the level of nitrogen-draw-down in a bag. After all, the producers have to work with variable organic material that is delivered to them, which makes the job very hard.

How to avoid this issue?

DIY substrate

One of the most effective ways to prevent nitrogen drawdown is to create your own substrate for your plants. This gives you complete control over the ingredients used.

You can use your own well-matured homemade compost and combine it with other materials to give it a nice, porous structure for your seedlings. For example, you could mix 10–30% homemade compost with coco coir, peat moss, vermiculite, or perlite. The internet is full of potting mix recipes of all kind.

Alternatively, you can use a completely soil-free substrate consisting only of inert materials such as perlite, vermiculite and coco coir, to which you add your fertiliser. In a soilless system, the materials used to grow your plants do not contain any organic material that can decompose, meaning there is no competition for nitrogen. However, it should be noted that soilless systems require knowledge and attention to detail, particularly with regard to providing nutrients through the water and maintaining pH levels. Soilless growing is a science, but if done correctly, it can be highly successful. It is also important to note that you need to use special hydroponic fertiliser, as seedlings require more than just NPK, but also a dozen or so micronutrients.

Aging and pre-soaking

Another option is to age and pre-soak your substrate. Before planting your seeds in store-bought bags of potting mix, do not plant directly from the bag. Open the bag and tip the contents into a large bucket or wheelbarrow. Moisten the contents (do not soak), then add any nutrients of your choice (such as bone meal fertiliser or conventional NPK) and mix completely. Leave the mixture to settle and stabilise for a few weeks before planting your seeds in the aged potting mix.

This method combines nutrient soaking and ageing to mature your compost and add nutrients. Adding nutrients helps your seedlings, regardless of how much nitrogen becomes bound to microorganisms during the microbial breakdown process. However, care must be taken not to add too many nutrients, as this may reduce seed germination or lead to plant burn.

Fixes for when nitrogen deficiency has already occurred

Although your seedlings may currently be showing symptoms of nitrogen deficiency, such as pale leaves, yellow leaves and stunted growth, there is still hope. You can apply a liquid nitrogen fertiliser to the seedlings. How much? Unfortunately, this is hard to predict. If your fertiliser is too diluted, whatever nitrogen you add will be immediately bound by microorganisms and will be unavailable to the plants. Too many nutrients, however, can cause plant burn, resulting in the death of your seedlings. This is why gardening is as much about experience as it is about science, and why some people are said to have a green thumb.

Conclusion

One of the main points to take away is that store bought compost, potting mixes and potting soils are not necessarily better than homemade things. If your seedlings aren't growing despite having enough water, sunlight and soil, it could be due to microorganisms depleting the nitrogen. The first step in resolving this issue is to understand the concept, and then gain the experience to prevent it.

Personally, I prefer to make my own seedling mix using homemade compost, vermiculite and coco coir. The ratios change depending on what I have available. If I don't have any compost, I use a liquid organic fertiliser at a third of the recommended strength.

I hope this article has helped to explain this important gardening concept, why it occurs and how to prevent it. With this in mind, happy gardening!