- ORGANIC MANURES
The two main ways of adding nutrients to the soil are by means of organic manures and artificial fertilisers. The main organic manures are:
Artificial: made or produced by human beings rather than occurring naturally, especially as a copy of something natural.
FARMYARD MANURE OR KRAAL MANURE
This is a mixture of dung and straw produced at places where cattle are brought into a Kraal at night or when they are being intensively fed in pens. The best manure is made in pens where straw is added every day. Urine from the cattle is absorbed by the straw and excessive bacterial action is prevented by the straw being compacted by the cattle thus keeping out excessive air.
An animal will produce one ton of manure in pens per month.
Straw or stover makes the best manure although sawdust or wood shavings can be used as bedding for cattle. The Manure produced from a kraal or pen can be applied to the land before a crop is planted. Where pigs are housed and bedded with straw, the manure produced is also valuable if applied straight to the land. Manure stored in the open and rained on will lose much of its value as many nutrients are washed away.
ADVANTAGES OF FARMYARD MANURE
- It supplies plant nutrients to the soil;
- It supplies Carbon for the soil micro-organisms, which use this for food;
- It aerates clay and compacted soils;
- It assists drainage and encourages root growth in the plant; and
- It maintains soil structure by adding humus to the soil.
DISADVANTAGES OF FARMYARD MANURE
- It is bulky to handle; and
- It spreads both weeds and diseases of plants and animals.
Farmyard manure is normally spread on the surface of the soil and then ploughed into the soil. Poultry manure from hens housed in batteries or on deep litter is very rich in plant nutrients, especially Nitrogen and also contains Calcium and Magnesium.
Figures 1 and 2: Show farmyard manure that has had straw or hay mixed in or used as animal
bedding which is then put into piles.
Source: gordonanne.blogspot Source: vera-labartere.blogspot
STRAW OR STOVER
The average crop of wheat straw will be 3 tons per hectare and will contain 7kg’s Nitrogen, 1.5kg Phosphate and 12kg’s Potash. These are small amounts of plant nutrients and ploughing the straw into the soil will make very little difference to the crop. However, the addition of straw to the soil will help the soil structure, particularly on sandy soil. Where straw or stover is burned on the land, all the organic matter and nitrogen is lost as the phosphate and potash is returned to the soil. The weed seeds and diseased stalks and leaves are destroyed which does not happen when the straw or stover is ploughed in.
This is the ploughing in of a crop that has been especially grown for the purpose of fertilizing the soil
e.g. Jackbeans. Where a legume is used, both organic matter and nitrogen are added to the soil, the nitrogen being slowly released to the following crop, as the organic matter decomposes. The crop is ploughed into the ground while it is still green or shortly after flowering which is a great improvement to the soil structure. Where a crop which is not a legume is ploughed in, no nitrogen is added to the soil. This happens where pasture or veld is ploughed in, however organic matter is added. This system is commonly used in organic farming but will work just as well on a conventional commercial basis. Some green manure crops include: millet, sorghum, clovers, vetch, cowpeas, soya beans etc.
Figure 3: Shows a green manure that is being incorporated into the soil.
- 2. FERTILISER
Inorganic fertilisers may be grouped into four categories:
Straights, or single nutrients, containing nitrogen (N), phosphorous (P) or potassium (K).
Nitrogen is applied mainly to crops as urea or LAN but sometimes as ammonium sulphate (AS) or ammonium sulphate nitrate (ASN). Urea is the cheapest form of N but may be lost to the atmosphere if applied to the surface of sandy, low organic matter, soils. LAN has a lower N concentration and is not as volatile. Ammonia gas is the most concentrated form of N but specialised equipment is required to inject it into the soil. AS and ASN have the advantage of also supplying sulphur (S), used for crop growth. The disadvantage of these fertilisers is that they cause acidity in the soils and are more costly per unit of nitrogen.
Superphosphate has been widely used in the past to supply crop phosphorous needs but the newer compound fertilisers MAP and DAP are now more cost effective forms of P application.
Potassium fertilisers include potassium chloride (KCl) and potassium sulphate.
Compounds consist of chemical compounds containing more than one nutrient. Three important P fertilisers are available as compounds with nitrogen, namely; mono-ammonium phosphate (MAP), di-ammonium phosphate (DAP) and ammoniated supers (AMP).
Blends are mixtures of single-nutrient or compound fertilisers. Blends supply two or more of the macronutrients and often include small amounts of zinc. Blends do not have specific names but are simply identified by three numbers which refer to the ratios of N, P and K contained.
The figure in brackets after the ratio indicates the total concentration of nutrients in the product, for example:
Blend 2:3:4(38) contains 2 parts N: 3 parts P: 4 parts K and these three nutrients in total make up 38% of the entire fertiliser bag contents.
Zinc (Zn) is the most widely deficient micro-nutrient in South Africa and for this reason most fertiliser compounds and blends will contain 0.5% or 1.0% zinc.
Deficiencies of other important micro-nutrients will best be detected by leaf analysis. The most common are iron (Fe), copper (Cu), manganese (Mn), boron (B), and molybdenum (Mo).
Table 1: Shows straights, compound and blend fertilisers
|LAN (Limestone ammoniuim nitrate)||Y||28|
|ASN (ammonium sulphate nitrate)||27||13|
|As (ammonium sulphate)||21||24|
|Potassium chloride (potash)||50|
|MAP+ 0.5% Zn (mono-ammonium phosphate)||11||22|
|DAP+ 0.5% Zn (di-ammonium phosphate)||18||20|
|AMP+ 0.5% Zn (ammoniated superphosphate)||6||14||±5|
EXAMPLES OF THE ACTUAL CONTENT OF RAW FERTILISER IN A SPECIFIC FERTILISER:
Table 2: Actual contents of raw fertilisers
|Sulphate of Ammonia||21% Nitrogen|
|Ammonium Nitrate||34.5% Nitrogen|
|Nitrate of Soda||16% Nitrogen|
|Single Superphosphate||18.5% Phosphoric Acid|
|Muriate of Potash||60% Potash|
|Ground Limestone||97 Neutralizing Value|
|Gypsum||95 Neutralizing Value|
When deciding how much fertiliser to apply to a land the requirements for the crop being grown must be looked at. This will come either from the result of a soil analysis or from fertiliser recommendations made by fertiliser companies or crop production modules.
EXAMPLE OF CALCULATING THE AMOUNT OF FERTILISER NEEDED BY A CROP
Assuming a crop will require:
Nitrogen…………………………………. 70kg per hectare
Phosphate………………………………. 30kg per hectare
Potash…………………………………… 30kg per hectare
One can use any blend of fertiliser or straight but should be chosen according to a soil analysis or whichever one best suits the cropping situation. For this exercise we will use a 2:3:4 (38). One should always meet the Phosphorus and Potassium requirements first and this should go in at planting as a basal application. The Nitrogen is put in as a basal application but should be split over the growing season and remaining requirements top dressed at a later stage. This is done because nitrogen leaches out of the soil easily.
A 50kg bag of 2:3:4 (38) contains 38% of usable fertiliser (NPK). Therefore
50kg x 38 / 100 = 19kg of NPK
The next step would be to work out the ratios of NPK (2:3:4) in the 19kg of fertiliser. One should add the ratios together to work out how much of each element is in the 19kg.
Nitrogen: 2 / 9 (add the ratios together ie: 2+3+4 = 9) x 19kg
= 4.22kg of N in a 50kg bag of fertiliser.
Phosphorus: 3 / 9 x 19kg
= 6.33 kg of P in a 50 kg bag of fertiliser.
Potassium: 4 / 9 x 19kg
= 8.44kg of K in a 50kg bag of fertiliser.
This means that in every 50kg of 2:3:4 (38) there is 4.22kg of N, 6.33kg of Phosphorus and 8.44kg of Potassium. From this one can work out how much of 2:3:4 (38) to apply and is worked out as follows:
Phosphorus requirements are 30kg/ha therefore:
30 / 6.33kg = 4.74 bags (50kg) 4.74bags x 50kg = 237.3kg of 2:3:4 (38)
Potassium requirements are 30kg/ha therefore:
30 / 8.44 = 3.55 bags (50kg)
3.55 bags x 50kg = 177kg of 2:3:4 (38)
From the answers above one can see that there is a difference in the amount of Phosphorus and Potassium to apply. One should use the phosphorus application which is 237.3kg/ha which is more than the Potassium, although adding more Potassium will not harm the plants. The Nitrogen supplied by applying 237.3kg of 2:3:4 (38) is: 237.3kg x 38 / 100 = 90.19kg multiplied by the ratio: 2 / 9 x 90.19kg = 20kg of N in 237.3 kg of 2:3:4 (38). The Nitrogen requirements that remain are 70kg – 20kg = 50kg
Urea can be used to top dress a crop. It contains 46 % Nitrogen therefore in a 50kg bag of urea 46% is usable to the crop. 50 x 46 / 100 = 23kg
Therefore requires a further 50kg / 23kg = 2.2bags x 50kg = 110kg of Urea
Therefore 237.3kg of 2:3:4 (38) will supply enough Phosphorus and Potassium, leaving 50kg of Nitrogen to be supplied in addition as topdressing. This can be done by applying 110kg of Urea as a top dressing to the growing crop.