Does your previous answer also apply to growing in the soil?

Question

From a Victorian hobby grower. Does your previous answer also apply to growing in the soil? 

I am a ‘back yard’ grower in Victoria and a volunteer in a larger scale ‘farm-like’ vegetation area. We grow vegetables the ‘traditional’ way in soil – not hydroponic. I am reading the hydroponic magazine out of interest – with an idea of becoming a hydroponic grower in the future (depending on the capital
investment needed).

I read your interesting answer about ‘fruiting tomato plants’ where you had mentioned the ‘number of leaves per stem’ etc. My question is a general one: is this formula true for in soil plants as well? Are there similar formulas for capsicum, eggplant, pumpkin plants? That is, should we cut off leaves and change the feed in order to increase the number of fruits and their size?

Answer

Thanks for your interesting question. For other readers, the previous question (May 2013) was about fruiting difficulties with tomatoes and mentioned changing fertiliser and leaf pruning.

Fertilisers for soil and ‘soilless’ growing
There are significant differences between growing in the soil and growing in hydroponics, which is more widely known as ‘soilless culture’.

Most soils can supply something of all the nutrients the plants require. Hence, they provide a buffer against things going wrong nutritionally. There are often limitations as to how much of an individual nutrient is available and this inhibits the potential yield compared to hydroponics. With soil there are a number of nutrients, in particular calcium and magnesium, which are usually readily available, hence even the so-called ‘complete’ fertilisers (for example, ‘Aquasol’, etc) contain little or none of these major nutrients. With hydroponics it is essential that all the required nutrients are in the feed solution. For this reason hydroponic growers should never use these fertilisers, but only use hydroponic fertilisers. If in the solution form, these come in two parts.

Most soils have relatively high cation exchange capacity (CEC), which is the ability to bind nutrients. Because of this, when you apply a fertiliser to soil many of the nutrients bind to the soil and slowly release to replace those taken up from the soil solution. This places some restriction on the uptake of nutrients. There are also interactions between different nutrients, which is far more significant in the soil than in hydroponics.

Hydroponics is basically luxury feeding. Even in the case where the hydroponic medium has a high cation exchange capacity, the medium soon ‘loads’ with the nutrients it binds and from then on the nutrients supplied are freely available.

(For hydroponic growers – a very popular growing medium is cocopeat, which has a moderately high CEC. This is not a longer term problem, but growers do need to take care during its initial period of use, while nutrients are being
bound and hence not available. Nutrients which are especially vulnerable are copper and calcium. Some cocopeat products can be supplied with additional calcium added during processing. Most inorganic hydroponic media, such as rockwool, have a very low CEC, so this is not a problem for them.)

The exception is if your soil is very sandy, as occurs in Perth (Western Australia). Unlike hydroponics, where the medium is held within a container, sandy soil has no restriction to drainage. Because of its low CEC, there are virtually no problems with interference with the fertiliser added. However, because it doesn’t bind, a high proportion of fertiliser is lost and, especially in Perth, makes its way into the ground water.

‘Fruiting’ fertilisers?
I’ll repeat what I said in my previous answer. Fertiliser formulations termed ‘bloom’ or ‘flowering’, etc, do not force the plant to produce buds then flowers. What they do is supply approximately what the plant is demanding at that stage.

There will be a change in its nutrient uptake as the plant moves from the ‘vegetative’ to the ‘generative’ or fruiting stage. The major aspect of this is a reduction in the demand for nitrogen and an increase in the demand for potassium. This effect is greatest in tomatoes and about half that intensity in the other vine crops.

This effect would be felt more strongly in hydroponics than in the soil, and in fact if not followed can lead to the hydroponic root zone solution getting too far out of balance.

In your case, it would probably help to use a fertiliser high in potassium (also known as potash, chemical symbol ‘K’), but take care not to overdo this.

Leaf pruning
There are two types of tomato plants, ‘determinate’, or ‘indeterminate’. Determinate or ‘bush’, tomatoes stop growing when fruit sets on the top bud and are usually not leaf pruned. They yield over a period of only a few weeks.

Commercial hydroponic tomato growers always grow indeterminate varieties. They take off all new side shoots to maintain a long stem strung up to a high wire. By extending the string, individual plant stems can produce for a full year, reaching a length of up to 14 metres. These plants are also leaf pruned as described in the previous answer.

Commercial growers of other vine crops sometimes leaf prune, but not as often as with tomatoes. Cucumbers are rarely leaf pruned, as the older leaves wither and return nutrients into the plant.

You need to be aware that leaf pruning may, or may not, give you more fruit, influenced by many other factors. However, if you want bigger fruit you need to prune off some small fruit, leading to a harvest of fewer but larger fruit.

PH&G June 2013 / Issue #132


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