Fertilising tomatoes after planting

When it comes to fertilising tomatoes after planting, much will depend on what fertiliser you used in the pre-planting stages and the condition of the soil.

The soil at my facility, for instance, is substantially better now than when I started many years ago. The macro-elements are sufficient and the organic content ensures good crops with virtually no additional fertilisation. In other words, there’s no standard recipe when it comes to a fertiliser programme.

In poor soil, low in organic matter, you may have to apply a side dressing of nitrogen fairly early on, as this element can easily leach with irrigation in these conditions. Nitrogen is used as an accelerator to control growth and ensure a good balance between leaves and fruiting. Too much nitrogen at this stage will cause less fruit development and ultimately affect the firmness and even shape of the fruit in some cases. Each variety will have a slightly different requirement. As the plant develops and sets more fruit, more nitrogen is required to keep foliage healthy and fill the fruit which is forming.

Potassium

Potassium is often given as a side-dressing later on to increase fruit size and quality. This is a well-known general principle which is sometimes misunderstood. In other words, application will depend on how much potash is in the soil in the first place. Adding more will only be of benefit if this is low or marginal. Furthermore, the potash has to be in a balance with calcium, magnesium and sodium for the plant to gain maximum benefit.

Bear in mind that tomatoes grown in the soil are usually drip irrigated, which means that the active area where the plant extracts water and nutrients is in the area wetted by the drippers, and this area is actually a small area relative to the land area. So, while we may base our fertiliser programme on a soil analysis and know a little extra potash can be added later, we need to remember that, when we apply potash through the drippers, we are in fact applying it to a small area of the land.

Magnesium

The soil analysis may show that the potassium/magnesium ratio is satisfactory, and the extra concentration of potash in this area may knock it out of balance and cause a magnesium deficiency. This is very common. Magnesium deficiency is seen on the older leaves. These will be dark, thick, large and have a yellow mottled chlorosis.

I once visited a farmer who told me he had a potassium deficiency. However, when we approached his land, the plants were yellow and chloritic almost to the top. I said this was the worst magnesium deficiency I’d ever come across. The farmer replied that a seed representative had told him it was a potash deficiency. And so, the more potash he applied, the worse the problem became as the imbalance increased. In most cases, a slight magnesium imbalance isn’t serious. Rather than dispense with the potash if you feel you need it, you can apply a magnesium sulphate foliar spray or other propriety products for the purpose.

Iron

Another consequence of minerals building up in the confined area ‘created’ by drip irrigation is an iron deficiency. This is especially the case when you use borehole water of dolomitic origin. The calcium in the water becomes concentrated in this area towards the end of the crop. This may cause the soil pH to rise and result in iron becoming unavailable to the plant.

In turn, this causes the new growth to become a yellow-white colour. Iron deficiency manifests on the new growth first and some farmers initially think it’s a sign of a lack of nitrogen. You will find a fairly sharp contrast between the relatively dark leaves lower down compared to the chloritic top growth. Nitrogen also tends to show up lower down on the older leaves. Apply iron chelate as a foliar spray to correct the deficiency. When trying to diagnose deficiencies, always start by finding out where the relevant symptoms manifest themselves.