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Container Growing

Plant feed for container growing

Growing lettuce in containers

Lettucve growing in a container

Hi folks, I hope everyone had a good weekend. I don’t know what the weather is like with you but we finally had a beautiful summer weekend with plenty of heat and sunshine. It has been a woefully slow start to the year but, if the forecast is correct, temperatures are set to get back to normal.

Don’t forget that, while it has been a slow start, there is still time to sow a wide range of vegetable crops if you haven’t done so already. I did another batch at the weekend including summer salads, spring onions, cabbage, broccoli, coriander and basil but there are plenty more options for a productive vegetable garden which I will include in next week’s mail.

Growing tomatoes in a growbag

Plant feed for container growing
This week, due to quite a few requests, we will looking at how to feed plants grown in pots and containers. While I am lucky enough to have plenty of outdoor and tunnel growing space, I do grow some plants in containers (e.g. bush tomatoes) and realise, for many gardeners, containers may be the only option.

Before we look at the different plant feed options, let’s look at the differences between growing in open soil or in more confined spaces as it helps us to understand both the soil or compost that we need as well as the possible plant feeds. The point here is, the more effort you put into creating the right compost mix for your containers, the less you will need to rely on liquid feeds later on.

Bush tomatoes in containers

I think the first thing we need to understand is that vegetables grow best when there are no checks in their growth. Growing interruptions will most commonly occur from lack of water or nutrients which will be more pronounced in container grown plants because the roots are drawing from a much smaller area. Open soil has a more stable moisture and nutrient reserve because the plant can draw from a wider area while everything a container grown plant needs must come from inside the pot.

A healthy garden soil is also more diverse than a potting mix and is made up from a broader range of materials including organic (made from rotted living things) or inorganic (minerals from weathered rock). A compost mix used in pots or containers is almost exclusively made from organic material so has a more limited nutrient base.

Patio vegetable garden in containers

If soil is so great, what can’t I just use it in my pots and containers?
The reason straight soil is not suitable for pots and containers is it doesn’t hold air, moisture or nutrients as efficiently as compost and neither does it drain as well. It sounds a bit of a contradiction but what we want from a potting material is good moisture holding capacity but also good drainage. If pots dry too quickly they will need to be watered very frequently but if they don’t drain well the roots, which need air, will suffocate in the saturated soil.

Tips on watering pot plants

In open soil, excess water usually drains below the root zone but, in a pot or container, it tends to pool in the base. The level at which water collects in the bottom of a pot is the balance point between the wicking effect of the compost drawing it up and gravity pushing it down. In relative terms pots are shallow with the root zone often filling the entire pot so if the base of the pot is saturated, so are the roots. Also, and maybe I’m going on a bit here, the more dense (less air spaces) the potting mix is, the higher the water level rises in the pot. Soil is more dense than compost so when watered, roots will stay saturated, and at risk of suffocation, for longer.

Good quality garden soil

Compost has more air spaces
Compost, whether it is peat, coir or green waste based has more air spaces than soil so will drain better but can be further improved by adding a mineral aggregate like perlite or vermiculite. The choice on use will depend on the needs of the plant in the pot.

A mix of soil and compost, up to 50/50 is better for large containers, especially for taller plants as the added weight will prevent them blowing over in strong winds. If you have access to good garden soil, I find most crops grow better with some soil added to the mix, just use less for small pots and increase the ratio with the pot size.

Most commercial composts don’t contain soil (referred to as loam which is roughly equal proportions of sand, silt and clay) but this is to do with the practicality of maintaining a stable supply (soil differs so much) rather than whether it it is an effective ingredient. John Innes (a set of compost recipes rather than a brand name) compost mixes do contain soil at varying concentrations depending on the intended use of the compost, less for sowing seed, more for final growing in pots.

How to make a potting mix for container growing

Making your own container growing soil
As we have said, good garden soil has a broader range of nutrients than most shop bought composts. Also, most multipurpose composts will only have enough feed to sustain plants for 5-6 weeks. Organic composts tend to feed for longer as the nutrients used are released more slowly while compost with artificial NPK feed usually provide faster growth for a shorter period of time.

If we want to grow vegetables in containers which are, in the main, greedy plants we need to start with a broad mix of nutrients which will sustain them for as long as possible. I like to make up my own mix using soil and compost as a base and mixing in perlite for drainage for plants that don’t like wet feet. I also mix in ‘Rockdust’ which is ground basalt for extra minerals and a seaweed and poultry manure pellet which adds slow release NPK from the poultry manure and growth boosting hormones and micronutrients from the seaweed.

I am not too strict with ratios but in general I mix equal parts of good sieved garden soil, bagged compost (or sieved garden compost) and Perlite for the base. I then add approximately 5% of seaweed and poultry manure pellets and 1 or 2% Rockdust. If I have some to hand I often add extra seaweed meal as I know what a difference seaweed makes in my beds, again 1 or 2%.

Mycorrhizal fungi on seed roots

Mycorrhizal Fungi
Unless the multipurpose fertilizer you are using contains chemical NPK (nitrogen, phosphorous and potassium), the container mix is all comprised of slow release nutrients. This is good because we are looking for sustained, even growth but we do need to make sure plants are able to access these nutrients as efficiently as possible and for this they may need a helping hand.

Undisturbed garden soil is full of fungal networks that feed minerals to plant roots in exchange for sugars produced by the plant. This was a deal struck millions of years ago and is a key part of healthy plant growth, it is the reason for the ‘No Dig’ method of gardening championed by Charles Dowding and other organic growers.

Mycorrhizal fungi experiment on grass

The image above shows grass grown with and without mycorrhizal fungi, obviously the one on the left is with the fungi. The large root system is actually a mix of grass roots and fungal hyphae that are feeding each other resulting in better growth for both parties. Fungi are very efficient at mining minerals but, because they don’t photosynthesise, can’t produce sugars.

The point is that container grown plants don’t have their own fungal networks so it makes sense to add some when making your container growing mix. As our understanding of the importance of fungal networks has grown, so have the range of products available which include mycorrhizal enriched compost, root powders and granules or fungi inoculated compost ‘biscuits’. The amounts required are very small, about half a teaspoon for a tomato plant, but the key thing is the fungi are in contact with the roots. While I don’t add fungi to my compost mix, I either dip seedling roots in granules or powder before planting or sprinkle a bit in the planting hole.

Feeding vegetables with liquid plant feed

Direct plant feed for container growing
Just like growing in open soil, the organic mantra of ‘feed the soil, not the plant’ also applies to container growing. What I am saying is, build a diverse, nutrient rich growing medium and a significant proportion of your work is already done. Just like us, plants do better on a balanced diet so starting with a broad spectrum of slow release major and micro-nutrients will lead to healthier growth and better flavour.

Some heavily producing plants will need additional feed however, most commonly fruiting plants like tomatoes, cucumbers, courgettes or peppers which will likely need extra potash to produce good fruit. Additional feeds will usually be in the form of liquid fertilizers whether organic (recommended) or inorganic and have a more immediate effect on the plants.

Maxicrop tomato seaweed fertiliser

I would always advise care when applying liquid feed as the temptation to over apply is strong yet too much will do more harm than good. Apart from the waste of nutrients, excess liquid feed can lead to a build up of salts in the compost which draws moisture from the plant, a process called reverse osmosis. Always apply liquid feeds to the rate suggested on the product pack and apply little and often rather than infrequent heavy feeds which result in fast spurts of weak growth.

As a rule of thumb, heavy cropping plants should be fed every 2 weeks minimum from when they have started flowering, every week if they are in a container with a small volume of compost (e.g. a growbag). It sounds obvious but the larger the plant is relative to its container, the more feed it needs. If you look at a 6ft high cordon tomato relative to the size of a puny growbag you can easily surmise the plant will need a lot of added nutrients whereas some lettuce growing in pot will not.

Magnesium deficiency tomato leaf

Learning tell tale signs
It is also worth learning the tell tale signs of nutrient deficiencies so you know what your plant needs and can respond accordingly. The photo above is my tomato Santonio F1 from last year showing advanced signs of magnesium deficiency which is easily cured by watering with an epsom salts solution. This is quite common with tomato plants and is recognised by yellowing leaves (starting from the edges) with the veins remaining green.

If the whole leaf is yellow/green it is likely to be nitrogen deficiency particularly if the new leaves are yellow green and the older leaves are yellow shriveled. If fruits are failing to set or dropping it is likely to be a potassium issue which in almost any fruiting plant will be solved by a good tomato feed. If you are over feeding with potassium leaves will take on a bronze colour.

If you are applying a feed to a plant that is also struggling due to root damage or lack of water you will need to rehydrate the plant by watering well before adding the feed.

In most cases, if you got your compost mix right in the first place, nutrient deficiencies and the need for extra feed will only apply to the large, heavy feeding and fruiting plants ,you should not need to feed the less glamorous crops.

Simple plant deficiency guide

A quick run down in organic feeds
I know I have covered this before but just for a quick recap because it is relevant:

You all know NPK numbers refer to the amount of nitrogen, phosphorous and potassium in a plant feed. You also know that nitrogen is for root growth, phosphorous for roots and potassium is for fruit production. As you may know, I would always choose an organic plant feed over and artificial one and just wanted to make you aware that the labeling on fertilizers can be a unintentionally misleading.

NPK plant feed label

NPK numbers refer to the immediate availability of that element rather than it’s long term effect. A chemical fertilizer will usually have a higher NPK value and will supply an immediate burst of nutrients but them run out of steam. This is often the case with non organic multipurpose composts feed for a relatively short period of time.

Organic plant feeds, with lower immediately available NPK (so lower numbers) give out their nutrients over a much longer period (as the fertiliser material breaks down) so while they look like they pack less punch on paper, their total NPK benefit is often greater in the long run. A good example would be your own garden compost which, in a well rotted state, would have a very low available NPK number yet will provide all the nutrients needed for most crops over a full season of growth.

While we are on the subject, we might as well look at what benefit you will get from the main organic fertilisers.

Garden Compost/Composted Manure – Adds valuable organic matter to the soil and slowly releases nutrients as it decomposes. Good quality organic matter is the holy grail and the one you should value over all others.

Chicken manure Pellets – Still a slow release solution but in a more concentrated form. This is a very good all round fertiliser especially when composted with seaweed meal as we have in our ‘Seafeed’ product.

Bone meal – A very good source of phosphorous and very good added to soil around planting time to promote good root establishment.

Blood fish and bone – A mix of blood, fish and bonemeal to create an excellent balanced general; fertiliser. Blood provides nitrogen, fish is a good source of potassium and bonemeal provides phosphorous.

That’s it for today, I will see you next week

Andrew

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