150 AERATION AND AIR-CONTENT. 



to air, and also why soil taken from grass-grown ground should be more 

 favourable to plant-growth than that from tilled ground. All this is in full 

 accordance with what has been estabHshed as to the behaviour of heated 

 soils towards plants, where toxic matter is formed by the heating, and in- 

 creased fertihty follows its destruction, and is in accordance, also, with the 

 results obtained with the germination of seeds in soil from grassed and un- 

 grassed ground, the time elapsing between, the drawing of the samples and 

 the germination of the seeds being sufficient for the conversion of any toxic 

 substance present into a beneficial substance." 



Bedford and Pickering (1914) stated that every growing crop re- 

 sults in the formation of a substance toxic to the growth of other 

 plants, and still more so to itself. By oxidation this toxin loses its 

 properties and increases the fertility of the soil. There is no reason 

 for assuming that the toxin is excreted by the plant. The root 

 debris from the growing roots is probably sufficient to account for its 

 formation, or an alteration in the bacterial contents of the soil due 

 to the growth of the grass. In heated soils a toxin is formed by the 

 action of heat alone, and the subsequent oxidation of the toxin can 

 occur without the agency of bacteria. There is no reason to suppose 

 that changes in the organic debris of a growing crop may not equally 

 occur w'ithout the action of bacteria, though in all probability they 

 may be materially aided by them. 



Pickering (1917 : 181) has carried out experiments for the purpose 

 of securing direct proof of the production of toxic substances by grow- 

 ing plants. Three flow^er-pots with mustard plants were fitted with 

 trays containing 5 inches of soil and with an aperture for the plants. 

 One tray contained a crop of mustard and had a perforated bottom, 

 so that water could pass through to the pot; a second had the 

 perforations closed, so that no water could reach the plants below; 

 and the third contained soil, but no plants. The mustard plants 

 below the last two trays grew normally, but those below the first 

 were reduced to a hundredth of the normal growth. It was regarded 

 as obvious that the leachings from the plants in the trays contained a 

 substance toxic to other plant-growth. By means of this method, 

 apples, cherries, plums, pears, 6 species of forest trees, mustard, to- 

 bacco, tomatoes, barley, clover, and 2 kinds of grasses, were found 

 to be susceptible to toxins, and apple seedlings, mustard, tobacco, 

 tomatoes, 2 kinds of clover, and 16 of grasses were found to produce 

 toxic effects. In pot experiments the effect varied from a reduction 

 in growth of 6 to 97 per cent, while in field experiments with trees 

 the effect ranged from slight to fatal. The possible factors elimi- 

 nated were protection and moisture, variations of temperature, alka- 

 linity and physical condition of the soil, carbon dioxid, and bacteria, 

 but it is disappointing not to have the details of these experiments. 

 As to the source of the toxin, it is said that while excretion from 

 the roots is possible, the dejecta left by the roots in the soil may 

 account for the toxic properties just as well as exudates. 



