Mallee - Part 2

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Where vegetation is growing, the result is different. The growing roots absorb their water within the body of the soil, so that little of it may reach the surface for evaporation. Little of the salts therefore is brought to the surface. The main portions are left within the body of the soil at or in contact with the spot where water was abstracted by the roots. The salts in this case will not become visible at the surface, but they are present within the body of the soil just the same.

It is sometimes said that trees use up or destroy excess salts. This is not so in any particular degree; they simply prevent them rising.

The fact is often observed that salts appear on the low ground after clearing the high ground in the vicinity. The explanation is simple. Where the higher land is cleared its soil moisture is increased because there is no vegetation to use it. This extra moisture then tends to forward the salts to lower ground, in two ways: During dry weather it brings up salts to the surface, whence the next rains will carry some part of them down hill as surface drainage. The increased soil moisture after clearing may also result in a certain amount of underground soakage to lower lands, carrying the salts in solution. This second effect will naturally occur soonest under a fair rainfall and on the lighter and more porous classes of land.

                                                                               Topographical Features and the Salts.

All fertile soils produce salts, and their removal is a question of natural drainage. Such drainage is best under a good rainfall; it is next best under a limited rainfall after clearing; it is worst under a limited rainfall before clearing.

In connection with the natural removal of salts from land, another most important feature is the character of the surface. If the salts are present in important amount, a bad feature of any country is its flatness. On level country the salts, after clearing, will tend to rise in summer and sink with the winter rains, without appreciably altering their horizontal distribution. In hilly or undulating country, on the other hand, the higher lands tend to lose salts downwards towards the valleys by drainage; the salts tend to become segregated in spots or patches, and the position in regard to the salts is far more favourable. Instances of such translocation of salts to lower ground are apparent in many parts of the State, e.g., at Northam, York, and Meckering. Salt flats are produced. While a few acres of low land are temporarily ruined thereby, such appearances are to be welcomed, as showing that the higher lands are losing salts. The lower lands affected will, in turn, become practically free of salts, provided there is any drainage outflow from them. Not infrequently it would pay to run a single drain through the lowest part of such salty valleys to prevent their spread, and hasten the date of final reclamation.

                                                                                                  Soluble Salts in General.

The salts produced in all soils are non-volatile substances and will not evaporate into the air. Bush growths, orchard trees, or farm crops do not destroy nor remove excess salts in any particular degree. Excess of salts in a soil will remain where it is until removed in solution by the soil water. With a fair rainfall, clearing will greatly facilitate the removal of salts, provided the land be undulating or hilly. The aggregation of salts in spots is largely a matter of soakage and natural drainage from higher lands.

                                                                                   Selective Power of Drained Lands.

With a good rainfall and drainage, the excess of soluble salts produced in soils goes off in solution. Soils produce a variety of soluble salts under natural conditions. Some of these are essential to crop growth because a crop must obtain about five per cent. of its dry substance out of the soil in the form of soluble salts. The latter include phosphates, potash, and the useful constituents of manures. Others of the soluble salts produced in soils are either unnecessary, or are required by crops in only small amounts. Such are the chloride, sulphate, and sometimes carbonate of sodium, and to a lesser degree the soluble salts of magnesia and lime. As it happens, the soluble salts most important to crops are generally firmly held by the soils, even under leaching, while the more useless salts, including the chloride sulphate and carbonate of sodium, are easily washed away. Soils with good rainfall and drainage thus contain relatively little of the almost useless sodium salts; in soils with low rainfall or bad drainage these particular salts tend to accumulate. It is not surprising, therefore, that where soil contains an excess of soluble salts owing to deficient leaching, the salts of sodium should largely preponderate. Sodium salts are peculiarly prominent in "salty" lands. From other lands the sodium slats have washed away to the ocean, the water of which contains chiefly sodium salts, and particularly the chloride (common salt), which forms about 76 per cent. of the total salts in sea water.

Soil Alkali.

Excess of soluble salts in a soil is often referred to as "soil alkali." There are two kinds of alkali, viz.,

Black Alkali;

White Alkali.

Black alkali is chiefly carbonate of soda. It gets its name because this substance colours the moist humus matters of the soil black. It is the worst kind of alkali, because even in small amount it corrodes the root-crown of plants, which becomes brownish, and ultimately the plant dies. When present in moderate amounts the evil effects of sodium carbonate in practice have been combated by applying gypsum, (sulphate of lime), which changes sodium carbonate into sodium sulphate, which is much less harmful.

White alkali includes the salts other carbonates, but sodium chloride (common salt) and sodium sulphate (Glauber's salt) usually predominate. The white alkali may also include soluble salts of magnesia and lime. The name "white" alkali is due to the salts frequently appearing as a white incrustation on the surface of the ground. The salts of white alkali are neutral substances and have not the corrosive action of the black alkali. An important injurious action of the white alkali salts is in increasing the difficulty with which water is absorbed from the soil by the crop roots. With much white alkali present, a crop will parch on soil which still appears moist enough to support ordinary growth.

                                                                                    Crop Tolerance for Soluble Salts.

Various standards have been adopted defining the limit of tolerance by crops for the different alkali salts. These standards are principally American, and in none of the Australian States has any standard been rigidly adopted. Writing of alkali salts, Mr F. B. Guthrie, Chemist to the Department of Agriculture, New South Wales, states that carbonate there is almost exclusively associated with the use of bore water; that chloride (common salt) also occurs in bore water, but in comparison with the carbonate "its power for evil is insignificant"; that apart from bore waters he finds that common salt is hardly ever met with in New South Wales lands save as a result of flooding by salt water creeks, etc.; that "most farm crops will stand as much as from 0•1 to 0•2 per cent. common salt in the soil, but if the amount exceeds this quantity, crops are likely to be affected." in Victoria likewise no limit of tolerance has been recognised, but I have found amber cane partly blighted on soil which on analysis showed approximately 0•2 per cent. of mixed alkali reckoned as sodium salts—of this amount 24 per cent. was carbonate, 53 per cent. chloride, and 23 per cent. was sulphate of sodium.

Among American authorities the late Prof. Hilgard, of California, has received most attention, and his work is quoted by leading authorities throughout the world. He states * that the maximum percentage of the salts which can be resisted varies materially with the kind of soil. For the sandy soil of the Tulare sub-station he places the maximum percentage allowable for cereals at approximately 0•1 per cent. for sodium carbonate; about 0•45 to 0•5 per cent. for sodium sulphate. For clay soils, it is added, the tolerance, especially as regards the carbonate, is markedly less.

More recent workers find different figures. Thus Mr F. S. Harris finds that on loam soil, crops may be produced where the salts do not exceed 0•3 per cent.

  • Hilgard—Soils. the Macmillan Coy., 1912. p 464.

† Jour.of Agric. Research, U.S. Dept. Ag., Oct., 1915.