1. INTRODUCTION TO PLANT NUTRITION
There are two ways in which soils can be improved, the physical and nutritional way. The physical way has been dealt with in the Soil Science Course and covers ploughing, cultivating, preserving hand improving the soil structure. The nutritional way of improving the soil involves plant nutrients and how they can be preserved and added to the soil.
Table 1: Shows the major plant nutrients, or major elements required for plant growth.
| CARBON HYDROGEN OXYGEN | These elements are supplied from Air and Water and do not have to be added by the farmer to the soil. |
| NITROGEN PHOSPHORUS POTASSIUM CALCIUM | These elements are taken up by growing crops in amounts ranging from a few kilograms per hectare to over 100 kilograms per hectare. These Major Elements do have to be supplied by the farmer. |
Table 2: On the next page there are some ideas of the actual amount of these major elements which are found in the seeds and stalks of the mature plants of various crops. Look at these figures as they are interesting and explain why some crops need more of a certain fertiliser than other crops. Tobacco has much less Nitrogen (57kgs) in the mature plant than Wheat (160kgs), which is why Tobacco requires less Nitrogen Fertiliser than Wheat.
When the major plant nutrients of Nitrogen, Phosphorus, Potassium and Calcium are applied as fertilisers they perform the following functions:-
- They supplement or add to the food supplies already in the soil and make sure that the crop enjoys the best possible conditions for growth;
- They raise Soil Fertility by increasing the amounts of plant food in the cycle of growth and decay which goes on all the time in the soil. Phosphorus and Potassium can only be increased in the soil by adding fertilisers. Nitrogen can be increased by other means (Lecture 5) and Calcium can sometimes be increased through using irrigation water. Otherwise Calcium has to be added to the soil as a fertiliser;
Table 2: Major plant nutrients, nitrogen, phosphorus, potassium and calcium present in the leaves, stalks and seeds of the mature plants of seven different crops.
| CROP | Yield/ha kg/ha | N kg/ha | P Kg/ha | K Kg/ha | Ca Kg/ha | |
| Irrigated | 4 000 | Grain | 83 | 33 | 25 | 1.6 |
| Wheat | 10 000 | Straw | 83 | 20 | 146 | 25 |
| TOTAL | 166 | 53 | 171 | 26.6 | ||
| Maize | 6 500 | Grain | 97.5 | 19.5 | 22.7 | 1.5 |
| 6 500 | Stover | 78 | 9.5 | 107.3 | 17 | |
| TOTAL | 175.5 | 29 | 130 | 18.5 | ||
| Virginia | 1 700 | Cured leaf | 48 | 8.5 | 88 | 42.5 |
| Tobacco | 1 500 | Stalks, tops And suckers | 17 | 4.5 | 42 | 0 |
| TOTAL | 65 | 13 | 130 | 42.5 | ||
| Potatoes | 20 000 | Tubers | 60 | 30 | 105 | 3 |
| Haulms | 40 | 13 | 36 | |||
| TOTAL | 100 | 43 | 141 | 3 | ||
| Lucerne Hay | 10 000 | Hay | 250 | 48 | 200 | 180 |
| Groundnuts | 1 620 | Kernels | 60 | 8 | 10 | 1.2 |
| 3 000 | Haulms & Shells | 68 | 18 | 60 | 40 | |
| TOTAL | 128 | 26 | 70 | 41.2 | ||
| Cotton | 1 700 | Seed cotton | 41 | 16 | 20 | 3 |
| Stalks, leaves | 32 | 10 | 41 | 32 | ||
| TOTAL | 73 | 26 | 61 | 35 |
- Organic Manures. (See Soil Science Lectures) maintain fertility by returning plant foods to the soil that has already grown a crop. This is done by ploughing in crop residues such as straw and maize stova. Soil fertility can only be increased by introducing something extra such as cattle dung or artificial fertilisers and;
- Adding the major plant nutrients to the soil reduces the costs of production per tonne of the crop being grown, because they raise the yield of the crop without a large increase in the total cost. For a good farmer the cost of fertiliser is about half (50%) the total costs of growing a crop of Maize, but makes such a difference to the yields and to the profit from the crop.
Other important Plant Nutrients are:
- Magnesium: this is added when Lime is applied to the soil as most types of lime contain some magnesium.
- Sulphur: is added in the form of Sulphate of Potash
- Sodium: is contained in most fertilisers.
The following are Minor Plant Foods and are required in small amounts by the plant, but are important. They are normally called trace elements and are: Boron, Copper, Manganese, Iron, Molybdenum, Zinc, and Chlorine.
The important thing to remember about both the major and minor plant nutrients is that they must all be present in the soil to ensure the best possible growth of the plant and crop. If one is missing or in short supply, the fact that the others are there in plenty, will make no difference. If, for instance, only Nitrogen is short and the others plentiful the plant will not grow properly.
2. THE IMPORTANCE OF LIME
Lime performs two functions and can be a form of plant nutrient and also a soil conditioner and a pH regulator.
Flocculation: form or cause to form into small clumps or masses. AS A SOIL CONDITIONER:
- Lime controls the pH of the soil (see Soil Science Lecture 4), by replacing hydrogen ions (H+) on the clay particles with calcium ions (Ca++). Lime is also held in the soil in this way; the positive calcium ions (Ca++) being attracted and held on the surfaces of the clay colloids and neutralises and sweetens acid soils.
- It improves the structure of soils by the flocculation of the clay particles (see Soil Science Lecture 4). This makes the clay particles form into crumbs or aggregates and so a clay soil performs more like a sandy soil.
- It prevents some Phosphorus being held in the soil in an insoluble form which is unavailable to the plant.
- It is necessary to keep a pH balance for bacterial action and nitrogen fixation by bacteria in the soil.
- It is important in pastures because grazing animals get much of their calcium from grass. If calcium (lime) is deficient it is short in grass and the animal does not get an adequate amount. This is important for young growing animals that require a lot of calcium for the formation of bones and with dairy cows who use up calcium in milk production. Milk contains a lot of calcium which has to be replaced by the milking cow every day which is done by liming the pastures.
- Lime (calcium carbonate) is removed from the soil by grazing animals and the harvesting of crops. It is also leached (washed out) from the soil when the Ca++ is replaced on the soil particles by Id+ from the slightly acidic rain water. Lime in the soil is in the form of Calcium Carbonate which is only slightly soluble. The acid rainwater tends to dissolve this, turning it into Calcium Bicarbonate, which is very soluble and is washed out of the soil.
AS A PLANT NUTRIENT, IT DOES THE FOLLOWING:
- Plants take up lime in the form of calcium and magnesium which is present in the lime.
- Lime is required in the plant in the areas of cell formation; these are the root and stem tips where the cells are dividing and growing.
- Lime is required to act as a buffer in the plant cells. It controls the pH by not allowing the cell sap to become too acidic. It also regulates the osmotic pressure in the sap not allowing it to become too weak so that water flows out through the cell wall.
- Lime helps to reduce the toxicity (poisoning effect) of too much Sodium which occurs in brak soils ( see Soil Science, Lecture 9)
3. SOIL pH AND LIME REQUIREMENT
Acidity and Alkalinity are measured on the pH Scale which runs from 1 to 14. 1 is very Acid and 14 is very Alkaline. The pH of soils is measured by the conventional system where the soil sample is mixed with water and the pH read off a pH metre. Another method is called the Calcium Chloride Method which gives a more accurate reading. This pH scale is 0.6 units lower than the Water Scale.
All this gives the pH of the soil, but it does not tell you how much Lime is required in order to increase the pH to the right level for the crop to be planted. This can only be done in the laboratory.
The Lime Requirement of a soil is defined as the weight of Calcium Carbonate required to bring the top 10 inches (25cms) of soil to a pH of 6.5 (water scale) or 5.8 (Calcium chloride scale). This is the ideal pH for plant growth. The amount of Lime (Calcium Carbonate) required to do this will vary for different soil types. A Sandy Soil with a pH of 5.5 may need 2 tonnes of Lime per hectare to increase the pH to.6.5, whereas a clay soil may need 5 tonnes per hectare to bring the pH from 5.5 to 6.5. The actual amount of lime required for a soil can only be determined in the laboratory.
Figure 1: Shows a pH scale (water scale).
4. LIMING MATERIALS
THESE ARE THE COMMON LIMING MATERIALS
Ground Limestone: Chemical Name, Calcium Carbonate (CaCO3)
This is natural limestone rock which is dug out of the ground and passed through milling machines to grind it down into very fine particles. It contains small amounts of Magnesium. It has a Neutralising Value of 97.
Quicklime: Chemical Name, Calcium Oxide (CaO)
Quicklime is manufactured by taking ground limestone (calcium carbonate), mixing it with coal, and burning (825ºC) the mixture in lime Kilns. Carbon dioxide is produced by the burning process and the calcium carbonate is converted into calcium oxide. When it is cooled it reacts with the carbon dioxide in the air and over time is converted back into calcium carbonate. This is a process called calcination or lime-burning.
Quicklime is a very effective liming material with a Neutralising Value of 180 so that only half the amount will achieve the same effect as Ground Limestone. It is a very difficult material to handle as burns the skin and clothes. Once applied to the soil it absorbs water and is converted into Calcium Hydroxide which is harmless, but very wet and messy. For those who understand chemical formulae, the process can be shown like this:
CaCO3 = CO2 + CaO
Calcium Carbonate = Carbon Dioxide + Calcium Oxide
| CaO | + | H2O | = | Ca(OH)2 |
| Calcium Oxide | + | Water | = | Calcium Hydroxide |
Dolomite Limestone: This is a natural limestone which contains 12 – 14% of Magnesium and has a Neutralising Value of 100.
Gypsum: Is a soft sulphate mineral that is made up of calcium sulphate dehydrate and has a Neutralising Value of 95 and normally used on brak soils.
Figure 2: Shows a land that is being limed with a lime spreader.
Source: visualphotos
