ON SALT IN SOILS.

Otago Witness, Issue 2362, 1 June 1899, Page 7

ON SALT IN SOILS.

(By George Gray, P.C.S., Canterbury Agricultural College, Lincoln, -in the Canter-

bury A. and P. Association's Journal.)

It is difficult tc decide whether common salt (chloride of sodium) is absolutely necessary for plant life or not. The opinions of scientific men on this point arc divided. While some assertthat it is a plant food, others again are of opinion that it does not in any way serve the purpose of nutrition in plants. This is probably due to the fact that any beneficial influences it may be found to exert when applied to ' crops are largely due, either to its action op the other constituents of plant food in the soil, rendering them more available, or to its action on insect ana fungoid pests, whereby the plant' is relieved from their injurious influences.

One thing, however, is certain, that an ex-cess-is injurious, and while some plants will tolerate its presence more than others, when the amount in the foil reaches a certain point none of our ordinary cultivated plants will .now ; in otber words, tho soil becomes sterile. The amount of salt present in fertile soils is 'nialJ ; thus our Canterbury soils, such as have been examined, chow aiV average of .013 per cent. Competent authorities state that when the amount reaches over .1 per cent, it ia a cause of sterility in the soil. This amount would he equivalent to a dressing of a little over one ton to the acre, distributed through tho first nine inches. Professor E. W. Hi!gard has shown that the percent-age of sugar beet is reduced when the amount exceeds 15001b of salt pei acre, 12in deep, equivalent to about .03 per cent. ; and probably the same holds good with mangels, in which sugar forms two-thirds of the total dry matter of the root. The practice adopted in England of applying common salt with highlj nitrogenous manures to corn crops for the purpose of checking the action of the nitrogen in causing the crop to run to straw ia an example showing the retarding influence of salt on vegetable growth

A good illustration, showing the eft'eefc of salt, may be found in the soils of the flats surrounding Lake Ellesmere. Here large patches occur on which only salt plants are growing. A short time ago two of these soils were submitted for analysis at the Agricultural College, one of which, produced good crops, while the other was sterile. The results obtained were as follows : — ANALYSES OF SOILS PROM LAKE ' ELLESMERE FLATS. ' 1. 2. Fertile. Sterile.

Moisture . .. „ 1.65 2.40 *Organic matter . . . . 2.81 4.41 Silica and insoluble matter .. .. 89.38 86.32 Ferric and alurnmic oxides .. „ 4.64 4.59 Calcic oxide .. .. J3.32 0.40 Magnesic oxide .. .. *0.64 0.70 . Potassic oxide .. .. 0.26 0.35 Sodic oxide . . . . 0.11 0.43 Phosphoric anhydride 0.11 0.13 Sulphuric anhydride .. 0.06 0.17 ** Chlorine .. .. 0.08 0.43 100.06 100.33 Less oxygen equivalent to chlorine .. .. 0.02 0.10 100.04 100.23 'Containing nitrogen . . 0.06 0.19 **Equal to sodmm chloride (salt) .. .. 0.13 0.70 Reaction. Alkaline. Alkaline.

The cause of infertility of No. 2 is here very apparent. Although decidedly richer m plant food than No. 1, it contains .7 per cent, of salt. No. 1 soil in its salt content is very near the limit of fertility, and would probably, not allow of any further increase without its affecting crops grown upon it. The amount of salt in any given soil is liable tc variation in different "seasons. In dry, warm weather there is a constant evaporation of water from the surface, leaving Dehind any soluble salts that niav be dissolved in it on the upper layer of soil. The salt thus gradually accumulates, and in cases where much is present forms an incrustation on the top. It is for this reason that young plants are much more aff«ctad than, thoaa of mature

growth, which have a much deeper root range. In wet seasons, especially with efficient drainage, the salt may be carried down again beyond the reach of the roots.

The only remedy for salt soils appears to be flooding the land in dry seasons with fresh water and making provision for the removal of the water by drainage. Deep ploughing, and thorough working of the soil, would also tend in some cases to prevent the rising of the salt in dry weather.

Unfortunately, an »application of gypsum (sulphate of calcium), "which has been found to act beneficially on soils containing an excess of carbonate of soda, by converting the latter into the more harmless sulphate of soda, has no effect on soils containing common salt.