in the case of chlorides, 0•5 per cent. of carbonates, and 1 per cent. of sulphates. Harris gives higher limits of tolerance than does Hilgard. His figures, however, are based on the amounts of soluble salts which he had added to the soil, not on the amounts which could be recovered again by extraction with water, which should form the main if not the sole criterion. As a matter of fact a very large percentage of added sodium carbonate is absorbed by a loam or clay soil and is not recoverable by washing; sodium sulphate is also, but in a minor degree, held up in soil; the chloride is most easily washed out. Harris found that less of the salts could be tolerated in a course sand than in a loam. This is contrary to practical experience, the real explanation being that in the sand the added salts were not absorbed to the same extent by the soil, and therefore remained operative on the crop—in the loam they were more fully absorbed, and to that extent became inoperative. Tolerance for alkali salts must be measured not by the amount of salts added to a soil but rather by the amounts which can be abstracted by ordinary washing with water.

Another series of investigations by Messrs. Headley, Curtis, and Scofield are reported in the same Journal, * and they on the other hand indicate a tolerance which is lower than that indicated by Hilgard. In these later tests the amount of salts considered was not the amount added, but was the amount which could be recovered on digestion with water. In their results they found that, with wheat, growth was diminished by one-half when carbonate of soda amounted to 0•04 per cent. of the dry soil, or the chloride to 0•16 per cent., or the sulphate to 0•35 per cent. The soil was a sandy loam. It is important to notice, however, that the crops were harvested as seedlings when 15 to 18 days old, and it has been demonstrated in other researches that young plants are more susceptible to saline solutions than the same plants in later life. The experiments are suggestive in regard to the relative toxicity of the salts, but they have scarcely a quantitative applicability to ordinary cropping conditions.

                                                                               No maximum Limit for Salt permissible.

Standards laid down by a number of other investigators could be quoted, but there is nothing to be gained by it. The figures of Hilgard can be accepted as giving a general indication of the amount of salts permissible; they were never intended to do more. Some other standards which have been laid down, including two which have just been discussed, are inapplicable even for general purposes. The difficulty about adopting any fixed standard for the salts is that the injury arising from any given amount of them depends upon the conditions. Attention may be briefly directed to the main factors which determine the amount of injury resulting from the presence of soluble salts in soils:—

1. the Wetness of the Soil.—Far more of alkali salts can be tolerated in a wet soil than in a dry one. As has already been stated one of the chief means by which an excess of salts damages a crop is by increasing the difficulty with which its roots absorb water from the soil by osmosis. With much of the salts present the solution in the soil water may have an osmotic value equal to that of the cell-sop in the roots, and then no water will pass inwards. When the soil solution is still stronger, plasmolysis will ensure, some water will pass outwards, and the crop is still more quickly parched. As the salts act because they are in a solution, it is not so much the percentage of salts in the soil that counts as the percentage in the soil water. By a heavy rainfall, or by irrigation, the solution in the soil may be diluted by one-half, and then the crop revives. A stringent adherence to any standard for salt is therefore inadmissible, because it ignores the water-holding capacity of the soils, the local rainfall, and the other climactic conditions.

2. The Class of Land.—Hilgard and others have remarked that the same amount of salts is more injurious in clay than in sandy soils. The reason generally given is that in the sandy soils the roots can penetrate more freely, and thus get further away from the salts which during dry weather tend to accumulate at the surface. Where the salts considered are the amounts which can be extracted by conventional methods of washing, there is no doubt that a given amount of salts is actually more harmful on clay than on sand. For practical purposes this is the important point. It is doubtful, however, whether the greater root distribution in sandy soils forms the sole or even chief reason for the results. We have seen that clay soils have a greater absorptive power for the salts than have sandy soils, thus rendering extraction by water less complete, so that clay soils actually contain more effective salt than appears in their analysis. The greater toxicity of the salts on clay soils would thus be apparent rather than real. As a matter of fact Harris, * basing conclusions on added salts instead of extracted salts, found that considerably more salt could be tolerated on clay than on sand. For analytical control work, however, it is only the amount of salts extracted which can be considered, and taking this as the measure there is no doubt that a given amount of the salts is more injurious on clay than on sand.

3. character of the Salts.—It has been seen that the principal alkali salts occurring in soils are the carbonates, chlorides, and sulphates, which are toxic in the order named. These, however, may exist potentially in the soil water as salts of different bases. Thus the sulphates might include sulphate of soda, sulphate of potash, sulphate of magnesia, and sulphate of lime, and indeed they would do so in varying proportions. In the same way there would be chlorides of all the bases potentially present, and to some extent also carbonates of the bases. In soil slat investigation it has become customary, however, to state each of the salts in terms of its sodium compound. There are reasons for this. The character of the base present in the salts is of secondary importance to the character of the acid. Then again, soda is usually the predominant base where soluble salts accumulate. Then again it has been found that when present by themselves there are no salts more toxic than those of sodium. Statement as sodium salts has thus become conventional, and it furnishes a common standard for comparison. At the same time it should be remembered that a mixture of bases in the soil salts is somewhat less toxic than where these include only the salts of sodium, and the presence of lime particularly will modify the action of the salts stated as salts of sodium.

4. Vertical Distribution.—If the salts are distributed evenly through the soil they are less harmful than if collected near the surface where the main part of the root system is located.

5. Kind of Crop.—Some crops can withstand more than others. Lucerne can grow where vines are killed, and wheat and barley where Lucerne is killed. Oats in the Meckering district appear to be more resistant than wheat. Barley is probably still more resistant, and the barley grass (Hordeum murinum) can tolerate much salt. It is well known that various salt-bushes grow where cereals and other grasses fail. Among fruit trees, figs are highly resistant to alkali salts.

6. Age of Crop.—Young crops are more sensitive than old established crops of the same kind.

7. Manuring.—with cereals, superphosphate will help to overcome for that season moderate amounts of the salts by specially encouraging the growth of the root system so that moisture is drawn from a greater depth within the soil. Stable manure assists in combating salts by increasing their absorption, and by its effect in improving the water-holding capacity of the soil.

                                                                        B.—SOLUBLE SALTS IN THE ESPERANCE LANDS.

                                                                                                  General Considerations.

It is apparent from the preceding paragraphs that no hard and fast line can be laid down in any instance as to what amount of soluble salts constitutes a barrier to cultivation and settlement. Not only is it necessary to consider the amount of salts, but their nature, the extent of local rainfall, the character of the land, and the methods of cultivation and cropping have each a bearing upon the question. It is fairly certain also that our drought-resistant wheats can tolerate considerably more salts than wheats adapted to wetter climates. This assumption is based on the fact that the chief action of the white alkali salts is to render water ab-

• Loc. cit.