The best fertilizer combination for tomatoes

Written by kim joyce
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The best fertilizer combination for tomatoes
For a bumper crop of tomatoes, go easy on the nitrogen. (Tomatoes image by Andytani from

Many gardeners have secret fertiliser recipes for growing the biggest and tastiest tomatoes. Even agriculture experts offer competing nutritional theories. Most everyone agrees on several details. First, use minimal nitrogen on tomatoes---nitrogen supports vine growth, not flowers and fruit. Second, make sure your soil pH is nearly neutral, between 6.5 and 7.0. Third, start your fertilisation program with a comprehensive soil test. Don't apply phosphate or potassium, for example, if your soil has plenty already.

Best Combinations

According to University of Missouri Extension, a standard tomato fertilisation program starts with adding fertiliser to the soil before planting. Because tomatoes are heavy feeders, supplement with regular side applications of fast-release fertiliser during the growing season. A fertiliser low in nitrogen (N), high in phosphorous (P) and medium or high in potassium (K)---the three numbers in fertiliser nutrient analysis, always in that order---is ideal for tomatoes. The best analyses for tomato fertilisers are 8-32-16 and 6-24-24, according to the University of Missour. These are high in phosphorous in order to support flower and fruit development. But Cornell University's College of Agriculture prefers a balanced fertiliser, with similar amounts of N, P and K, and just about any balanced fertiliser will do, such as 10-10-10.

Inorganic Combinations

Sometimes called synthetic or chemical fertiliser, inorganic fertiliser types are those with N, P and K immediately available for plant use. Inorganic fertilisers also come in slow-release forms, which are typically more expensive. Cornell's suggested 10-10-10 all-purpose fertiliser is easier to find than the University of Missouri's 8-32-16 and 6-24-24 recommended formulations, but both should be available in slow- and fast-release forms. Very easy to use as 60-day slow-release fertiliser "spikes" you stick in the ground near each tomato plant, Jobe's Fertilizer Spikes for Tomatoes have a nutrient analysis of 8-24-8. Other good tomato fertilisers---though they may be labelled as flower or "bloom" fertilisers---will be high in phosphorous, with nutrient ratios such as 10-20-10, 15-30-15, 12-24-12 or 8-16-8. Miracle-Gro Tomato Plant Food, by contrast, is 18-18-21--very similar, proportionately, to all-purpose 10-10-10 fertiliser, just more concentrated.

Organic Combinations

The same general trends in plant nutrient needs and proportions hold up with commercial organic fertilisers, but the process is much more complex. This is because plant nutrients in organic fertilisers are bound up in organic compounds, which are not available to plants for use until these compounds are broken down in the soil by microbes. This makes it harder for gardeners or growers to know exactly how much of any given nutrient is immediately available, although building living, dynamic soil eventually delivers high levels of all nutrients. Commercial organic products feasible for tomatoes include Dr. Earth Organic 5 Tomato, Vegetable & Herb Fertilizer, with a nutrient analysis of 5-7-3, and Fox Farms' Happy Frog Fruit & Flower Organic Fertilizer, with a nutrient analysis of 5-8-4. But organic growers experiment with many approaches, including devising their own fertiliser combinations and blends. To meet the relatively low nitrogen levels needed by tomatoes, for example, some very successful tomato farmers grow legumes or "green manure" crops during the previous season, because legumes "fix" atmospheric nitrogen and make it available in the soil. Composted chicken manure is a good source of both nitrogen and phosphorous. Bone meal is an excellent phosphorous source, and wood ashes provide quick phosphate and potash. Organic fertilisers such as liquid fish emulsion can be used in the garden and also on container-grown tomatoes.

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