Plants need nitrogen in order to make proteins, enzymes and other nitrogen-containing substances. Plants which are starved of nitrogen fail to grow properly, and may have pale green leaves. All the plant’s supply of nitrogen must come from the soil. The soil bacteria and fungi also need nitrogen to make proteins and other substances, and they have first call on available nitrogen in the soil.
For this reason, 99 per cent of soil nitrogen is in the organic form, that is to say, it is contained in the cells of soil organisms, or it is contained in organic matter which is not fully decomposed. In humid regions, there may be as much as 3500 kg of soil nitrogen per hectare, but only about 22 kg of this is in a form which crops can use.
When the organic matter in a soil contains a small percentage of nitrogen, the soil organisms take all that is available, and turn it into the proteins of their own cells. This nitrogen is not available to plants, and is said to be immobilised, because it is locked up inside the cells of soil organisms.
When soil organic matter contains a higher percentage of nitrogen, two processes may occur. Ammonification is a process in which some soil organisms turn nitrogen-containing organic matter into ammonia. Nitrification is a process in which some soil organisms turn organic matter into nitrates.
Both nitrates and ammonia are forms of nitrogen which can be absorbed by the roots of plants. Therefore ammonification and nitrification are called mobilising processes, because they make soil nitrogen available to plants. In order that mobilisation of soil nitrogen may occur the ratio of nitrogen to carbon in organic matter must be at least 1:20.
If cereal stubble or dead cornstalks are ploughed under, soil nitrogen will be immobilised, because the ratio of nitrogen to carbon in these materials is about 1:80. In time, however, the amount of carbon will fall, because soil organisms will give out carbon dioxide as a waste product. Eventually the proportion of nitrogen will increase, and mobilisation will occur.
Most Australian soils are lacking in sufficient nitrogen. This lack can be corrected by adding nitrogen-rich organic matter to the soil. The planting of legumes, the ploughing under of green manure crops and pastures will add nitrogen to the soil.
The addition of nitrogenous fertilisers will also make nitrogen available to plants. These fertilisers are used a great deal in the cultivation of sugar cane and potatoes, but are too expensive to be used widely for crops like the cereals. Nitrogenous fertilisers, such as ammonium sulphate, may be used to raise the ratio of nitrogen to carbon in such material as cornstalks. The soil organisms will then have enough nitrogen when decomposing the cornstalks, and mobilisation of nitrogen will occur in the soil.
Nitrogen is lost to the soil:
i. By Grazing – Soil nitrogen is taken up by the plants, and passes into the bodies of animals.
ii. By Crops – Much of the nitrogen taken up by plants will not be returned to the soil, but will be removed in the form of hay or grain.
iii. By Erosion
iv. By Leaching – If soluble nitrogen compounds are formed by mobilisation, they may be leached out by water passing through the soil.
v. By Volatilisation – Some nitrogen compounds may escape from the soil in gaseous form.
Phosphorus is needed by all living things to make proteins, the nuclei of cells and to allow energy to be used. The phosphorus needs of plants and animals are high while they are young, because this is the period when their energy needs are greatest. For this reason, phosphatic fertilisers should be applied early in the growth of a crop.
A deficiency of phosphorus causes a failure of plants to develop properly. Legumes need large amounts of phosphorus, and respond strongly when it is present.
Most Australian soils are lacking in sufficient phosphorus, and this lack can only be corrected by applying phosphatic fertilisers. The commonest of these is superphosphate, but basic slag and rock phosphate also contain this element.
When superphosphate is applied to soil, some insoluble phosphorus compounds are formed. Since a compound must be soluble in the soil water before it can be absorbed by roots, some of the phosphorus is not available to plants. However this phosphorus is not entirely lost, because some of it is constantly being changed into an available form.
It is now thought that phosphorus in the soil exists in these two forms; a large reservoir of non-available phosphorus, and a small supply of the available form. Since there is a two-way change between the two forms some soluble phosphorus is slowly made available to plants.
The acidity of the soil affects the amount of phosphorus which will be made available. The low solubility of phosphorus in the soil is a blessing in one way, because it means that very little of this element is leached from soils. About half the phosphorus in the soil is in the organic form that is to say it is contained in the cells of soil organisms.
The reason why potassium is needed by plants is not at present understood, although it may be needed for photosynthesis. Most crops benefit from the application of potassium fertilisers when this element is lacking in soils, especially root crops. Potassium deficiency produces certain signs in plants. Thus lucerne leaves develop white dots near the leaf margins.
The potassium in a soil comes from the weathering of certain rock minerals which contain this element. Some of these minerals, like white mica, are very resistant to weathering, and therefore they only make potassium available to plants very slowly.
Other minerals, like the clay mineral illite, are easily weathered, but the potassium is released slowly from them. Crops take up potassium faster than it is made available from decomposing minerals. Therefore a soil which is not deficient in potassium may become so if several crops are grown in rapid succession.
The soil’s supplies of available potassium may be restored again if such a soil is rested from cultivation. Potassium-deficient soils are found in some coastal districts of eastern Australia which receive a high rainfall.
Thus some soils in eastern Victoria are known to be deficient in potassium. The chief potassium fertiliser is potassium sulphate.
Sulphur is needed by plants to make proteins, such as enzymes, and many other molecules in the cells.
Until recently, it was thought unlikely that soils would be deficient in sulphur. Then it was discovered that some of the black earths of the Darling Downs and Lockyer Valley, and volcanic red loams of northern New South Wales are deficient in this element. Sulphur deficiency causes the pasture grass Phalaris aquatica to develop a pale green colour, with yellowish tips to the leaves.
The plants are not of normal size. Legumes need more sulphur than other plants, and sulphur seems to be necessary for proper nodulation to take place. Sulphur deficiency can be corrected by spreading superphosphate, since this substance contains some calcium sulphate. Some landowners use gypsum instead of superphosphate, because of its lower cost.
These are elements needed by plants and animals in very small amounts. Most soils seem to contain sufficient amounts of these elements, but in some parts of Australia one or more of them may be deficient. Since these elements are only required in small amounts, it is usually an inexpensive matter to correct the deficiency by spreading chemicals containing them. Applications of 70 grams per hectare are usually sufficient.
Boron deficiency in certain soils causes the condition known as hollow stem in cauliflowers. The application of borax to the soil supplies boron and prevents such diseases.
The condition in sheep known as steely wool may occur in parts of northern and central New South Wales because sheep become deficient in copper.
Molybdenum, manganese and zinc deficiencies may occur in soils. Molybdenum deficiencies are fairly common, and are corrected by the use of molybdenised superphosphate. However in vegetable-growing districts a more soluble form of this element, such as sodium molybdate, must be used.
Phalaris staggers is a condition which may occur in sheep grazing on phalaris pastures. The cause of this condition is not known, but dosing sheep with pellets of cobalt compounds corrects the trouble.