Fertilizers and Manures

Fertilizers and manures are materials that are used to improve soil. The words are often used synonymously, but in fact a fertilizer is generally considered to be a substance that supplies only nitrogen, phosphorus or potassium (the three nutrients) to the soil, and a manure a material supplying it with both nutrients and humus.

Fertilizers are applied at the rate of a few ounces per square yard, manures at the rate of several pounds per square yard.

Both organic and inorganic fertilizers-should be used carefully as they can produce unwanted effects. While they supply nutrients that improve crop growth and quality, they also increase the strength of the soil solution, making it more difficult for the roots to absorb the water and nutrients they need. It is therefore important to remember that overdoses can cause serious wilting, and even the death of a plant. So do not give more than the recommended dressing.


Inorganic fertilizers, often referred to as chemical, synthetic, or artificial manures, are manufactured or derived from mineral deposits. They may either be used by themselves to supply a specific nutrient, or mixed together to give a compound, or so-called complete fertilizer.

Fertilizers sold by seeds companies or nurseries, are obtainable in the form of crystalline powders (the form in which they are generally sold), or in pellet or granular form, the latter being easier to store and spread.


Once the results of a soil test and the needs of the plants to be grown are known, buy the fertilizers that are required to keep the soil account balanced.

Although fertilizers can be obtained at home if animals are kept and there is a good supply of soot and wood ashes, it is generally more convenient to buy the necessary fertilizers and manures from a seed companies or nurseries. The analyses shown on the container labels are useful guides as to the effectiveness of some fertilizers, and are the guarantee of the minimum percentages of nitrogen, phosphorus and potassium that they contain. The phosphorus content is expressed as P2O5 (phosphoric acid), potassium as K2O (potash) and nitrogen as N. For example, nitrate of soda has an analysis of 16 per cent nitrogen, which means that 100 lb. of this fertilizer has 16 lb. of nitrogen in it.

Urea, however, has 46 per cent nitrogen, so only one-third of the amount of urea is needed to give the same effect as nitrate of soda.


Inorganic fertilizers are not simply nutrients, but are salts having two portions, namely, the nutrient or wanted portion, and the unwanted portion. One cannot be had without the other. In nitrate of soda the unwanted portion is sodium, which can easily spoil the tilth of heavy soils if this fertilizer is applied too heavily. A reasonable amount of sodium benefits beetroot, but is apt to spoil the cooking quality of potatoes. To avoid giving plants indigestion it is vital to select the right fertilizer and, when using branded products, to follow the manufacturer-’s directions for use.

Some advocates of the use of only organic fertilizers maintain that inorganic fertilizers are harmful to plants and to man who eats the plants, since the nutrient elements are derived from purely mineral substances, often obtained by the combination of strong acids and alkalis. But there is no scientific evidence to show that crops grown with inorganic fertilizers are inferior in quality or lower in nutritional value than those grown with organic fertilizers.

Inorganic fertilizers can be grouped into three classes: nitrogenous, phosphatic and potassic.


Nitrate of Soda (16 per cent N). This is a quick-acting stimulant for applying to growing crops in spring and summer, particularly when plants have been checked by bad weather or pest attacks. Provided the soil is moist, the effect can usually be seen within a few days of application. Nitrate of soda is good for acid and peaty soils, but may easily spoil the tilth of clay soils if applied too freely. The usual rate of application is 1/2 to 1 oz. per sq. yd.

Nitro-chalk (15-5 per cent N). This has a quick and sustained effect and does not tend to make the soil sour. Use only as a top dressing at the rate of 1/2 to 1 oz. per sq. yd., when the crops are well up.

Sulphate of Ammonia (21 per cent N). This is slightly slower-acting than nitrate of soda, usually taking effect within 10 to 14 days in the summer, but taking rather longer in cold weather. Although suitable for use as a top dressing on a wide range of crops and lawns, it is best used in combination with other fertilizers to make up a compound dressing. It tends to make the soil acid and thus works best on well-limed or chalky soils.

Chilean Potash Nitrate (15 per cent N, 10 per cent K2O). This is a useful top dressing for supplying both nitrogen and potash, and is applied at l to 2 oz. per sq. yd. In spring and summer. It is very quick acting and often recommended for use when preparing plants for show. It also makes a useful liquid feed when l oz. is dissolved in 2 gal. water.

Superphosphate of Lime (18 per cent soluble P205) (generally called ‘super’). This is the most popular fertilizer for supplying readily-available phosphates before sowing or planting, and is applied at the rate of 2 to 4 oz. per sq. yd. And usually in combination with other fertilizers. It can be used at any time of the year without fear of it being washed out, but is most usually given during the spring and summer.

Basic Slag (14 to 18 per cent P2O5). This is a slow-acting phosphatic fertilizer. It gives best results on acid soils in wet districts when applied in the autumn at the rate of 4 oz. per sq. yd. Its lime content helps to correct acidity.

Sulphate of Potash (48 per cent K2O). This is the most popular potassic fertilizer, and is safe for all plants. It can be applied at any time of the year at l to 2 oz. per sq. yd. without fear of loss by drainage. It is generally mixed with other fertilizers to give a complete feed before sowing or planting. It is also used as a top dressing for crops that have to occupy the ground for a long time.

Muriate of Potash (60 per cent K2O). This fertilizer is more concentrated than the sulphate form, and although suitable for a wide range of plants may damage strawberries, red currants, gooseberries and tomatoes. Normal rates of application are 1 to 2 oz. per sq. yd.


Organic fertilizers are usually of animal or vegetable origin, often being derived from slaughterhouse refuse and vegetable wastes such as seed residues. They are mainly used for supplying nitrogen, but some animal products contain bone residue which gives phosphates as well.

The nitrogen in these fertilizers is in the form of protein, which is not accessible to plants. Soil organisms change it into exactly the same nitrates as are available in inorganic fertilizers. This class of fertilizer — dried blood and bone meal, for example — does produce some humus, but not a sufficient amount at normal rates of application.

If the soil is sour, too wet or too cold, organic fertilizers (organics) will not work properly, since these are the conditions that are detested by bacteria, and unless bacteria are working properly, the nitrogen in the fertilizer will not be released.

Although organics are generally thought to be slow acting, dried blood, fish meal and several others may work quite quickly in warm, moist and well-limed soils. But the speed of action also depends upon how finely ground they are. Dusty forms of hoof and horn work almost as quickly as dried blood or even sulphate of ammonia, whereas coarse forms may take years to become exhausted. Among the advantages claimed for organics are, first, that they are longer lasting than inorganics and, second, that they give a steady supply of nitrogen rather than sudden rushes, and so avoid rank growth if applied at the wrong time.

A fairly heavy dressing, say 4 oz. per sq. yd., of nitrogen in the form of hoof and horn can safely be given before sowing, whereas the same amount of nitrate of soda might cause damage.

Nitrogen in organic form is less likely to be washed out of heavily watered soils.

Organics are often preferred to in-organics because they do not scorch foliage — a decided advantage when a top dressing has to be given to crops with creeping stems and rosette leaves which cover the tops of their pots.

But organics are comparatively expensive and have no fixed composition. While sulphate of ammonia always has about 20-6 per cent of nitrogen, the nitro-gen content of dried blood may vary from 7 to 14 per cent. Some waste materials may show quite a high percentage of nitrogen, but this may not be of any value in the year of application because of very slow breakdown. Leather wastes and some plastic wastes are of this nature.

05. September 2013 by admin
Categories: Gardening History, Plant Biology, Top Tips | Tags: , , , , , , , , , , , , | Comments Off on Fertilizers and Manures


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