Feeding Greenhouse Tomatoes

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POTASH AND NITROGEN FEEDS

When you search for liquid feeds at your local garden centre you will see a bewildering array of different products. The two you are interested in, as far as greenhouse tomato feeding is concerned, are "general purpose" liquid feed (higher in nitrogen) and "tomato plant" feed (higher in potash / potassium).

Nitrogen rich feeds encourage growth of the foliage at the expense of fruit growth. Potash (or potassium) rich feeds encourage the formation and growth of fruits at the expense of foliage growth. For good tomato plant growth you need both types of feed, but they will be applied at different stages of growth. If expense is a problem and you can only buy one type of liquid feed then go for the potash / tomato plant feed. It is high in potash but also contains some nitrogen.

For the inquisitive amongst us, I set out below the typical nitrogen and potash contents of the two types of liquid feed. Note that the analysis below also includes Phosphorus proportions because that is how feeds are normally analysed (N (nitrogen) P (phosphorus) K (potassium)). The figures below show the proportions of each chemical. Click the picture on the left to enlarge it.

Without getting too technical, the above shows that a general purpose feed has equal amounts of Nitrogen, Phosphorus and Potassium. The tomato feed however has much lower levels of Nitrogen and Phosphorus compared to Potassium.

As far as greenhouse tomatoes are concerned, feed them liquid tomato fertiliser (read pack instructions) up until mid-July time in the UK. Then feed a mixture (half amounts of tomato and general purpose fertiliser) up until the end of August. From then on, feed with general purpose fertiliser for the remainder of the growing season.

When feeding tomato plants, never feed a sick plant. Starve it of food until it begins to recover. Don't feed plants where the soil / compost has dried out. This will lead to a quick build up of salts (see paragraphs below).

ROOT SYSTEM OF TOMATO PLANTS

Tomato plants have two types of roots; fibrous ones near the soil surface and one or more tap roots which reach much deeper into the soil. The fibrous roots seek out mainly nutrients whereas the tap roots seek out mainly water.

These two types of roots explain the success of the bottomless pot system of tomato plant growing. The compost in the pot is fed with liquid fertiliser for the higher fibrous roots. The lower tap roots below the pot seek out water.

BUILD UP OF SALTS IN THE COMPOST

When growing greenhouse tomato plants in grow bags or pots, the liquid fertiliser will deposit salts in the compost. If the level of salts accumulates to a high level it will damage the plant. To avoid this, avoid feeding for a week twice in the growing season and apply lots of water to wash out the salts.

FOLIAR FEEDS

Foliar feeds are applied to the foliage of plants and are a useful way to feed them. The feed is quickly absorbed. f you wish to do this then buy specific foliar feeds. A home made alternative is to let nettles soak in water for a week or two and then spray the strained liquid on the tomato plants. Good fun and free! Don't apply foliar feeds in full sun.

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