Fertilization
Crop Requirements
Tables 4-8 give typical crop requirements for soil production. These include NC fertigation recommendations (Table 4); New York preplant recommendations for transplants in bare ground (Table 5) and for transplants in plastic mulch (Table 6); and pre-plant (Table 7) and fertigation recommendations (Table 8) for tomatoes grown in Kentucky. In all cases, it is suggested that soil pH initially be raised to 6.5-6.8 and that the soil be tested before preplant fertilizers are added. N, P, K, Mg and Ca are usually added preplant, based on soil test results, even if the crop is to be fertigated later. Also consult the field production chapter for fertilization requirements.
Under good growing conditions, nitrogen must be adequate for plants to grow rapidly. High nitrogen levels encourage vegetative growth. Under low light conditions, however, this growth can be detrimental to reproductive growth. The ammonium form of nitrogen particularly encourages vegetative growth (Papadopoulos, 1991). Typically, nitrogen levels are kept relatively low until fruitset to encourage reproductive growth, then raised. In soilless culture, the ratio of nitrogen to other nutrients is closely controlled to encourage either reproductive or vegetable growth, depending on the grower's perception of crop needs. For more details on this approach, see the greenhouse production chapter.
Deficiency
Nitrogen deficiency is sometimes hard to detect without a well-fertilized
control for comparison. Growth may be reduced overall, so the plants are
stunted, but leaves may look healthy, except for being a paler green than
normal. Symptoms appear first on the lower leaves and continue to be more
pronounced there, as nitrogen is a mobile element, moving from older to
younger tissue. Symptoms at the top of the plant include flowers that
are pale, rather than deep, yellow and a main stem is thin at the top.
The whole plant has a spindly appearance. Rather than foliage being lush
or succulent, the leaves are small, erect, and "hard". Over
time, the whole plant can turn yellow, flowers drop, and fruit remain
small. Nitrogen deficiency can appear in waterlogged soils (heavy clay),
or sandy soils (after heavy leaching) or can be induced by heavy applications
of straw or other organic material with a high C:N ratio (Papadopoulos,
1991).
Toxicity
Dark green leaves, sometimes thickened and brittle, indicate excess nitrogen.
At the top of the plant, stems remain thick and new leaves may curl into
a ball, a condition sometimes referred to as "bullishness."
Clusters and flowers are large, although fruitset may be poor. Although
leaf growth is initially promoted, it is eventually restricted under excessive
nitrogen.
Nitrogen form is also an important consideration in guarding against nitrogen toxicity. Tomatoes are much more sensitive to nitrogen in the ammonium than in the nitrate form, especially under low light. The early symptoms of ammonia injury are small, chlorotic spots on the leaves which later turn necrotic (brown and dead). Spot size may increase, covering the entire inter-veinal area, giving the leaf a scorched appearance and leaf margins may roll up.
Although phosphorus is used in much smaller quantities than nitrogen and potassium, it must be provided continuously. Initially, phosphorus is important for early root growth, especially under cool soil conditions, but later it is necessary for vegetative growth and fruit set. Phosphorus is stored well in soil but easily leached in peat media, and less available at high pH.
Deficiency
Symptoms of deficiency occur first on the lower leaves and stems, as phosphorus,
like nitrogen, is phloem-mobile. Plants appear stunted, and leaves are
unusually dark green. A characteristic red or purple color on the undersides
of the leaves, including the veins, and stem, appears later. Leaves are
small and curve slightly downwards. In severe cases, leaves first develop
chlorosis, followed by necrosis. Plants become slender, with thin stems,
and cluster development is poor. Roots become brown and develop few lateral
branches.
Toxicity
Phosphorus toxicity is uncommon; concentrations of 5-200 ppm P have been
tried in NFT without significant yield response (Papadopoulos,
1991). However excess phosphorus levels may contribute to micronutrient
deficiency.
More potassium is required for best fruit quality and to regulate growth than is required for maximum yield. As a major nutrient with a positive charge, potassium balances the negative charges of organic acids produced within the cell, and those of anions such as sulfates, chlorides, and nitrates. Potassium levels are particularly important at transplanting and to prevent ripening disorders. Problems with fruit quality, such as blotchy ripening, boxy fruit, and even to some extent, greenback, are associated with low levels of potassium and in most cases can be counteracted with high-potassium feeds (Papadopoulos, 1991).
Deficiency
Yield increases with potassium additions are greatest when nitrogen is
not a limiting factor. Potassium deficiency is first expressed as dark
green foliage, which later turns purplish brown. Marginal chlorosis and
necrosis appear first on the lowest leaves, then progress up the plant.
Like nitrogen and phosphorus, potassium is phloem-mobile, and young leaves
are the last affected. Chlorosis almost always occurs first at the margins
of older leaves, which often curve downwards. Later chlorosis moves into
the interveinal areas towards the center of the leaf, and necrosis of
leaf margins follows. In advanced stages, the small veins lose their color,
older leaves become severely scorched and drop, young leaves turn yellow
and remain small, plant growth is restricted, and fruit ripening is uneven.
Interveinal chlorosis is also symptomatic of low magnesium, and the two
deficiencies are sometimes confused. Compared to magnesium deficiency,
potassium deficiency is more likely to occur along the leaf margins, and
is also more likely to develop into necrotic spots.
Toxicity
Potassium toxicity per se is
rare. However, very high rates of potassium may induce Ca or Mg deficiency
or high salts damage. Calcium deficiency can lead to BER (see below).
Reductions in yield occur at very high levels of K, when the K:N ratio
in the liquid feed is too high, or when both N and K are too high. In
this case, yield reductions are attributed to increased salinity in the
growth medium. High levels of potassium improve fruit shape, decrease
fruit size, and reduce the proportion of hollow fruit (a disorder associated
with early growth under poor light conditions) even under conditions where
total yields are somewhat reduced.
