Guide on How to Determine Fertilizer Needs

To determine fertilizer needs for crops and soils in your locality, farm or place of work, you must know two things: first is the status of nutrients in the soil and secondly, how much of each nutrient is needed to get the highest or most profitable (optimum) yield?

There are several approaches to finding the answers to these questions;

Fertilizer recommendation of crops.

Nutrient hunger signs on growing crops (deficiency symptoms).

Soil tests or analyses to determine the fertilizer nutrients and amount needed.

Plant or plant tissue test in the field.

Fertilizer field trials.

Hunger signs in plants (nutrient deficiency symptoms).

If plants do not get enough of a particular nutrient they need, the symptoms show in the general appearance as well as in the colour of the plant.

Very typical symptom are: the nutrient deficient plants are stunted , the leaves have a pale green colour or a very dark bluish green colour, yellowish or have reddish spotting or striping. At harvest, yield is reduced, sometimes severely.

Nitrogen deficiency symptoms

The nitrogen deficiency symptoms include but not limited to the following;

Stunted growth.

Loss of green color, yellow discoloration of leaves from tip backward, older leaves brown.

Lower leaves may die premature while the top of the plant remains green.

Phosphorus deficiency symptoms include;

Stunted growth.

Leaves turn dark bluish green, purpling and browning from tip backward.

Plants slow to ripen, remaining green.

Fruits may be misshapen, grain is poorly filled.

Potassium deficiency symptoms include;

Stunted growth.

Leaves show discoloration along outer margin from tip to base.

Outer edges of leaves yellow or reddish, becoming brownish or scorched and dead; leaves wilted.

Lodging.

Tree leaves are yellowish, reddish, pinched, cupped or curved.

Fruit is small, may have lesions or injured sport, poor storage and keeping quality.

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Magnesium deficiency symptoms include;

Yellowish discolouration between Green leaf veins, followed by blotching and necrosis (death of tissue), starting at lower older leaves.

Sulphur deficiency symptoms include;

Whole plant is yellowish (often mistaken as N deficiency).

Yellowish of upper leaves, even on newest growth.

Delayed crop maturity.

Calcium deficiency symptoms include;

Young leaves turn yellowish to black and curved or cupped (brown spot).

Plants appear to wilt.

Fruits may appear rotten.

Roots are malformed.

Boron deficiency symptoms include;

Leaves frequently misshapen and crinkled, thick and brittle, white, irregular spots between veins.

Growing tips of buds die, with bushy growth near tips extension growth inhibited with shortened internodes.

Water-soaked, necrotic spots or cavities in beet and other root crops and in the pith of stems.

Fruit small and poorly formed, often with corky nodules and lesions.

Low seed production due to incomplete fertilization.

Zinc deficiency symptoms include;

Stunted growth of leaves.

Fruit trees with typical shortened bushy shoots.

Chlorotic stripes (white bleached bands) between the leaf veins in lower part of leaf.

In some cases leaves have an olive green or grayish green colour (very similar to P deficiency).

Iron deficiency symptom include;

Young leaves with typical chlorosis between green veins, along the entire length of leaves (usually on calcareous soils).

Soil Amendments

Soil amendments are the substances used for correcting the acidity or alkalinity of the soil.

In high rainfall areas, there is considerable leaching of bases leading to the formation of acidic soil, while in low land areas, saline and alkaline soils occur.

Soil reaction or soil pH

Soils vary considerably in degree of acidity or alkalinity or in reaction. The normal range is expressed in pH values as given in table below as follows;

Table: Normal Soil Reaction Range with Corresponding pH Values

Liming of Acidic Soils

Liming as the term applies to agriculture is the addition of any calcium or calcium and magnesium-containing compound to the soil for reducing acidity.

For this purpose, calcium oxide, calcium hydroxide, calcium carbonate, calcium magnesium carbonate and calcium silicate slags are used.

Methods of Applying Lime

Lime should be applied before ploughing or applied on ploughed land and then disked or mixed into the soil. When large quantities of lime are required, it is advisable to apply the required amount in two or three splits rather than in one heavy application.

On strongly acidic soils, where 7-15 tonnes of lime are required per hectare, half the quantity should be applied before ploughing and the remaining half applied and disked after ploughing. When the lime required is below 4-5 tonnes per hectare, the entire amount should be applied and disked in at one time, 8-10 days before sowing.

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