FOOD FROM THE AIR.

Otago Witness, Issue 3467, 24 August 1920, Page 11

FOOD FROM THE AIR.

Nitrogen is by far the most costly element to purchase in the form of commercial fertilisers, and the cost of returning to the soil what crops have removed would be a serious matter, but for the simple fact that the atmosphere consists fourfifths of nitrogen, and that leguminous plants (peas, beans, vetches, cow peas, lucerne, clovers, and trefoils, etc.), have the power of extracting nitrogen from the air arid of utilising it by means of organisms contained in the nodules that may be seen on the roots of these plants. Every farmer who has grown peas, vetches, cow peas, or velvet beans (which only occupy tho ground a comparatively snort time), or lucerne and red clover (perennial or biennial legumes), has ob-

served (writes the Inverell Argus) the beneficial effect on the following crops, oven when the top growth is removed. Farmers should not, howover, bo deluded into the belief that a system of growing legumes and removing the crop as green fodder or as hay, instead of feeding off, will increase the nitrogen in tho soil. Bettor results may bo seen in a crop like maize following such annual legumes as peas or vetches which have been cut for fodder or hay as compared with maize following no legume, but this merely serves to emphasise how fast the land is being depleted of nitrogen without a leguminous crop. As a rule, the nitrogen in the roots and stubble of leguminous plants about equate tho amount of this element taken from the soil, and when tho tops are removed the nitrogen is only barely maintained in the soil, and certainly not increased.

There is an impression amongst some farmers that the nitrogen obtained from tho air by the bacteria is all present in the nodules or tubercles on the roots. Usually only about one-third or less of the total nitrogen in leguminous plants is contained in the roots and stubble, and this is reckoned to be also about the same as that taken by such plants from tho soil, the other two-thirds being taken from the air. It will be seen, therefore, that when the top growth of legumes is entirely removed from tho soil (instead of being ploughed under or fed off) there is no actual gain in nitrogen—no more than there would be if any non-leguminous crop were grown and ploughed under entirely as green manure. As regards feeding on in comparison with ploughing under the whole crop as green manure, it may be reckoned that a quarter of the nitrogen in the feed consumed_ is retained by the animal, and the remaining three-quarters is voided in the manure. Thus, if the nitrogen is just barely maintained in the soil by ploughing in only the roots and stubble of a leguminous crop—and even this method has been observed by farmers to bo beneficial to the following crop —how much more so will be the system of feeding off or ploughing in the whole crop?

The question i 3 often raised whether the nitrogen of the soil can be maintained by green manuring with non-legu-minous crops, such as rye, rape, barley, etc. It is true that nitrogen is part of the organic matter, and it might ba thought that if the organic matter of the soil is increased by green manuring with non-legumes that tho nitrogen will bo correspondingly increased. This is not the case, however, as the whole of the nitrogen of non-legumes comes from the soil. The practice of green manuring with nonlegumes, therefore, may save nitrates from being leached from the soil, as would happen on fallow land during heavy rainfall,, and it would in many cases be better than no crop at all (except in dry districts), but it does not actually increase the nitrogen in the soil, though it certainly does increase the organic matter and to some extent increases the store of nitrogen that is easily available.

Another question of interest is how much nitrogen is added to the soil by a leguminous crop that is grown for seed? An average crop of cow peas contains 1201 b nitrogen per acre in the whole plant (including the_ roots), and of this amount about 601 b is contained in the seed. As two-thirds of the 1201 b nitrogen is obtained from the air, a balance of 201 b nitrogen per aere will'be placed to the credit of the soil if the straw is returned to it and only the seed is removed. The nitrogen added to the soil per acre by different methods of dealing with an average annual leguminous crop like cow peas or field peas may be roughly expressed as follows: — Ploughing in whole crop as green manure, 801 b. Feeding off (or soiling and returning animal manure to the soil), 601 b. •Growing crop for seed (returning straw to soil) 201 b. Removing croo (ploughing in only roots and stubble), nil.