The plants have a moderately high nitrogen requirement. Nitrogen promotes better growth and better flower and fruit set. A minimum application of 250 kg/ha is recommended in high-rainfall areas or for high production. Approximately a quarter of the nitrogen is applied at planting while the remainder is applied in the first 6 to 8 weeks of growth at 2 to 3-week intervals. Further light dressings are applied over the next 6 or more weeks.
Tomato also has a high potassium requirement. Adequate levels of potassium result in improved col- our, taste, firmness, and sufficient sugars, acids and solids of the fruit. Plant cells are also strengthened by adequate potassium levels.
A minimum of 100 kg/ha potassium should be applied. Phosphorus promotes root development, early flower- ing and fruit set and ensures more vigorous growth. A total of 40 to 60 kg/ha phosphorus is suggested in soils with a build-up of the nutrient. It is recommended that phosphorus be band-placed in acid soils.
Tomatoes also require micronutrients for growth and development. Deficiencies of magnesium, calcium, and molybdenum frequently occur in acid soils. Boron and copper deficiencies are not often found in toma- toes. However, boron deficiency, if it occurs, results in fruit cracking, pitted and corky areas, deformed shape/malformation and uneven fruit ripening. Iron has been found to be deficient on calcareous, alkaline soils or after heavy applications of lime. Manganese deficiencies are mainly found in calcareous soils.
The nitrogen requirement of tomatoes is generally considered to be moderately high, with some authorities suggesting only about 150 kg N per hectare, reducing this to 125 kg for certain cultivars such as Zest, which have a particularly good root system. However, in high rainfall areas, or for high production, a minimum of 250 kg N per hectare should be considered, for target yields of about 50 tons per hectare. Where higher target yields are expected, increase N by about 5 kg for every 2 tons over 50.
Nitrogen deficiencies most frequently occur in colder soils and after heavy rains which leach N. Deficiency symptoms are often seen towards the end of the season, especially under cool night temperatures, and during periods of drought.
Phosphorus promotes root development and ensures more vigorous growth, especially of young plants. It also promotes early flowering and fruit set. Where the phosphorus status of the soil has been built up over several years, 40 kg to 60 kg of applied P per hectare should be adequate. On the more acid soils, best results are obtained by banding the fertilizer.
An early symptom of phosphorus deficiency in tomatoes is the development of a purplish colour on the undersides of the leaves. The foliage eventually assumes a purplish tint, particularly at the tips of the leaves. The stems are slender and fibrous, leaves are small and the plants are late in setting and maturing fruit.
The potassium requirement of tomatoes is high. Plant analyses indicate that the plants take up about 50% more potassium than nitrogen. The major effect of high potassium is its influence on fruit quality, rather than on yield.
The colour, taste, firmness, sugars, acids and solids of the fruit are all improved with adequate levels of potassium. It also strengthens plant cells and makes the plant less susceptible to attacks by many diseases. As chloride tends to reduce fruit quality, rather use chlorine-free fertilizers. A minimum of 100 kg K should be applied per hectare. Where target yields are greater than 40 tons per hectare, apply a further 10 kg K for each 2 tons extra.
Tomatoes are not often affected by a deficiency of this element. Where it does occur, the growing point of the stem has a blackened appearance. The plant appears bushy due to the growth of new leaves below the growing point.
Iron deficiencies tend to occur only on calcareous, alkaline soils, or after heavy lime applications. The first symptoms occur in the younger leaves, and consist of a yellow mottling between the leaf veins. Tomatoes are not very prone to iron deficiency.
Copper deficiencies are very seldom found in tomatoes, partly because of the emphasis on copper-containing chemicals in a disease control programme.
As with many other plants, zinc deficiency is characterised by poor development of leaves. Leaf petioles may be full size, but the leaves tend to be small. Fertilizers containing zinc should be used in areas where zinc deficiency is common.
Manganese deficiencies occur primarily in calcareous soils. It shows as a lightening of the green colour in the leaf areas furtherest from the main veins. This yellowing becomes more marked and extensive, and leaves may later die. Growth is spindly, little or no blossoming occurs, and no fruit forms, in severe cases.
Molybdenum deficiencies are common on very acid soils, but may occur even on calcareous soils. The older leaves become mottled, with pale green veins. The mottled areas often have a puffed appearance. New leaves are at first green but become mottled with age. Leaves tend to curl inwards.