Here are six nutrients you must supply to vegetables

Fertilisation is the application of fertilisers to plants for better harvest. Fertilisers can be:

(a) Organic: They include compost, farmyard manure, blood meal, dried bones, wood ash and green manure.

They are usually applied pre-plant. They are beneficial to many vegetables but may cause quality reduction (forking in carrots and cracking in many root vegetables) if not well decomposed.

They add organic matter and nutrients to the soil but have a disadvantage in that they release nutrients slowly due to low rate of decomposition. For example, 20-50 per cent of total nitrogen is released in the first year and the rest in subsequent ones.

This may be undesirable in vegetables that need high nitrogen levels in the first year.

Phosphorous availability from organic fertilisers, especially animal manure, is mostly higher than in commercial fertilisers.

Detrimental effects of fresh organic fertilisers include locking up of nutrients during decomposition process and they may also carry some harmful micro-organisms.

(b) Inorganic: They are commercial fertilisers industrially manufactured. They are applied to the soil to supply plants with nutrients.

Advantages
i) Readily available nutrients.

ii) Contain consistently balanced nutrients according to grade not like organic fertilisers which depend on the source

iii) Less bulky and lack micro organisms.

Generally:

(a) Fertilisers with ratios 1:3:3 or 1:4:4 are useful for pre-plant application or application during planting on most vegetables.

(b) Side dressing with nitrogenous fertilisers follows soon after transplanting.

(c) Leafy vegetable crops need regular nitrogen application.

(d) Due to leaching, split application is needed for N fertilisers.

(e) Root vegetables generally respond well to phosphorous and potassium fertilisers.

Fertiliser application methods

Broadcasting: This involves random application by scattering in the field.

Side banding: Application along rows.

Circular banding: Applied around large crops like tomato, pepper and eggplant.

Fertigation.

Application of a fertiliser with irrigation water.

For crop growth and development to take place for a normal vegetable, 16 chemical elements are required.

Carbon, oxygen and hydrogen are components of organic compounds.

These elements are supplied mainly from air and water. Nitrogen, phosphorus, potassium, calcium, sulfur, magnesium, boron, iron, manganese, zinc, molybdenum, copper, and chlorine are obtained by plants from the soil.

Nitrogen

Nitrogen has the quickest and most pronounced effect.

Roles

i) Constitutes all proteins, which are the active compounds of protoplasm.

ii) Constituent of the chlorophyll molecule.

iii) Stimulates vegetative growth and encourages the development of large stems and leaves.

iv) Nitrogen tends to produce succulence, an important quality in vegetables.

Application of excess nitrogen delays fruiting and seeding. Nitrogen is easily leached from the soil and this makes it a more limiting nutrient in soils than phosphorus and potassium.

Deficiency of nitrogen is manifested as:

i) Stunted growth.

ii) Yellowing of leaves.

iii) Woodiness of plant.

On application of nitrogen, recovery is rapid. The most important fertiliser sources of nitrogen are ammonium nitrate, anhydrous ammonia, ammonium sulfate and urea

Phosphorus

Roles

i) Constituent of nucleoproteins.

ii) Plays an important role in many enzymatic reactions.

Deficiency is manifested as:

i) Slow growth of plants with small leaves of a grayish-green colour.
ii) Midribs, veins, and petioles of leaves and parts of the stem show a purple coloration in severe cases.
iii) Maturity of fruits and seeds is delayed.

The most important fertiliser sources for phosphorous are: Single Superphosphate (SSP), Double Superphosphate (DSP) and Triple Superphosphate (TSP).

Potassium (K)
Potassium is essential for the formation and translocation of carbohydrates and is needed in large amounts by most root crops.
Deficiency leads to:

i) Stunting of plants.
ii) Marginal chlorosis.
iii) Firing of the older leaves
iv) Lack of K affects secondary thickening of roots and tubers, resulting in production of slender rather than thick storage organs. Due to the fact that potassium in the plant is in soluble form, and that it is translocated from the old leaves to the young ones as shortage develops, deficiency symptoms are first noticed on the older leaves.

Most soils contain large amounts of K, but it is mainly in the form of insoluble minerals. Soils that are high in clay seem to have high available K.

Many plants absorb more K than they actually need for growth and maintenance of high yields, a condition referred to as “luxury consumption”. The common fertilisers are Muriate of Potash and Potassium Sulphate.

Calcium

Calcium plays a major role in cell-wall formation.

Deficiency symptoms

First noticed in young tissues.
i) Plant becomes woody.
ii) Leaves turn pale and brittle and the terminal bud dies.
iii) Roots are short, thickened and usually discoloured.
iv) In tomatoes, it leads to blossom end rot of the fruit, in celery blackheart and tip-burn of lettuce are physiological disorders.
Fertiliser sources of calcium are lime and single Superphosphate.

Sulfur

Sulfur is a component of some proteins and amino acids that are essential to normal plant metabolism.
It is also a constituent of some volatile compounds, which impart odour and flavour to such vegetables as onion and brassicas.
Deficiency symptoms
i) Yellowing of leaves.
ii) Stunting of the plant.
Sulfur is made available to the plants in the form of ammonium sulfate, potassium sulfate and when Single Superphosphate is applied, it is present in the form of calcium sulfate.

Magnesium
Magnesium is essential for chlorophyll formation since it is a component of the chlorophyll molecule. Deficiency symptoms include chlorosis between the veins of older leaves since it is translocated out of the old leaves and reutilised in young ones.
Magnesium is applied in the form of magnesium sulphate as foliar spray. When liming is done, Dolomitic limestone makes Mg available.
Determining fertiliser requirements
Successful production of vegetable crops requires that the produce is of high quality and that the returns are high enough to offset the cost of inputs. Nutrient deficiency symptoms develop if a soil lacks or has insufficient quantities of a given nutrient. If a grower suspects that his crop is suffering from a deficiency (leaves develop abnormal shape and colour), then it becomes necessary to take tissue and soil samples to the lab for analysis. Based on the results, the grower can know the type and amount of fertiliser to apply. Fertilisation is based on annual soil tests, soil type, crop requirements, field history, and past experience. Soil tests that are regularly done include:
(i) pH
(ii) Lime index.
(iii) Soil cation exchange capacity.
(iv) Soluble salts.
(v) Available phosphorous.
(vi) Exchangeable Potassium.
(vii) Calcium.
(viii) Magnesium.
(ix) Percent base saturation.
Samples should be collected from the top 15 to 20cm of soil when it is relatively dry. Each sample should represent 1 to 2 ha and should consist of randomly collected sub-samples. Separate tests should be collected for each area of land that differs in colour, topography, drainage, previous fertilisation, and cropping history. Soil tests that are reliable are those for phosphorous and potassium, but those for nitrogen may not provide the right picture of status. Tissue tests provide reliable indications on the nitrogen requirements.

Tomato fertiliser programme

Apply farmyard manure at a rate of 20 ton/ha before transplanting. Some 200kg/ha DSP (10g/plant) at planting
Top dress with 100kg CAN/ha applied in 2 splits; 1st split when plants are 20-25cm and 2nd split 3-5 weeks later.
Excess nitrogen application results in excessive vegetative growth at the expense of fruit production.

The writer is based at Egerton University.