Trans. of the Horticultural Society of London 1: 187-193 (1820)
XXXV. On Potatoes.
By Thomas Andrew Knight, Esq. F. R. S. &c.

Read February 6, 1810.

IN the Horticultural Transactions, page 58, I have described a method of cultivating early varieties of the Potatoe, by which any of those, which do not usually blossom, may be made to produce seeds, and thus afford the means of obtaining many other early varieties. I also offered a conjecture, that varieties of moderately early habits, and luxuriant growth, might be formed, which would be found well adapted to field-culture, and be ready to be taken from the soil in the end of August, or the beginning of September; so that the farmer might be allowed ample time to prepare the same ground for a crop of wheat. I am now enabled to state, that the success of the experiment has in both cases fully answered every expectation that I had formed.

The facts that I have stated in the paper above referred to, and more fully in the Philosophical Transactions, are, I believe, sufficient to prove, that the same fluid, or sap, gives existence alike to the tuber, and the blossom and seeds, and that whenever a plant of the Potaloe affords either seeds or blossoms, a diminution of the crop of tubers, or an increased expenditure of the riches of the soil, must necessarily take place. It has also been proved by others, as well as myself, that the crop of tubers is increased by destroying the fruit-stalks and immature blossoms as soon as they appear, and I therefore conceived that considerable advantages would arise, if varieties of sufficiently luxuriant growth, and large produce, for general culture, could be formed, which would never produce blossoms.

I have since had the gratification to find that such are readily obtained, by the means which I have detailed, and I am disposed to annex more importance to the improvement of our most useful plants, than any writer on agriculture has hitherto done; because whatever increased value is thus added to the produce of the soil, is obtained without any increased expense or labour, and therefore is just so much added to individual, and national wealth. I formerly supposed that all varieties of the Potatoe, which ripened early in the autumn, would necessarily vegetate early in the ensuing spring, and could therefore be fit for use only during winter; but I have found that the habit of acquiring maturity early in the autumn, is by no means necessarily connected with the habit of vegetating early in the spring; and therefore by a proper selection of varieties, the season of planting crops, for all purposes, may be extended from the beginning of March, nearly to the middle of May, and each variety be committed to the soil exactly at the most advantageous period.

A variety, however, which does not vegetate till late in the spring, and which ripens early in the autumn, cannot, I conclude, particularly in dry soils and seasons, afford so large a produce as one which vegetates more early: I, nevertheless, obtained so large a crop from one which vegetates remarkably late in the spring, and ripens rather early in the autumn, that I was induced to ascertain, by weighing, to what the produce would have amounted had the crop extended over an acre, and I found that it would have been 21 tons, 11 cwt. 80 lb. or 48,352 lbs.

In this calculation the external rows, which derived superior advantage from air and light, were excluded. No more manure, or culture, than is usually given, had been employed, for the crop was not planted with any intention of having it weighed: the wet summer was, however, very favourable.

I am not acquainted with the ordinary amount of the weight of a good crop of Potatoes, upon an acre of ground in a favourable soil, when well-manured and cultivated; but I am confident, that it may generally be made to exceed twenty tons, by a proper selection of varieties: and if four pounds of good Potatoes afford, as is generally supposed, at least as much nutriment as one pound of wheat, the produce of an acre of Potatoes, such as I have described, is capable of supporting as large a population, as eight acres of wheat, admitting the calculation of Mr. Arthur Young, that the average produce of an acre of wheat is 22 1/2 bushels or 1440 lbs.; and as an acre of wheat will certainly support as large a number of people as five acres of permanent pasture, it follows, that an acre of Potatoes affords as much food for mankind, as forty acres of permanent pasture: an important subject for consideration, in a country where provisions are scarce and dear, and where so high bounties, on pasture, are paid in the form of taxes on tillage, that the extent of permanent pasture is certainly and consequently increasing: and it must increase, under existing circumstances; for it pays a higher rent to the landlord, and relieves the farmer from much labour, anxiety, and vexation.

To what extent, a crop of Potatoes will generally be increased by the total prevention of all disposition to blossom, the soil and variety being, in all other respects, the same, it is difficult to conjecture; but I imagine that the expenditure of sap in the production of fruit-stalks and blossoms alone would be sufficient to occasion an addition, of at least an ounce, to the weight of the tubers of each plant; and if each square yard were to contain eight plants, as in the crop I have mentioned, the increased produce of an acre would considerably exceed a ton, and of course be sufficient, in almost all cases, to pay the rent of the ground.

I do not know how far other parts of England are well supplied with good varieties of Potatoes; but those cultivated in my neighbourhood in Herefordshire and Shropshire, are generally very bad. Many of them have been introduced from Ireland, and to that climate they are probably well adapted; for the Irish planter is secure from frost from the end of April nearly to the end of November: but in England, the Potatoe is never safe from frost till near the end of May; indeed I have seen the leaves and stems of a crop, in a very low situation, completely destroyed as late as the 13th of June, and they are generally injured before the middle, and sometimes in the first week of September.

The Irish varieties, being excessively late, are almost always killed by the frost whilst in full blossom; when omitting all consideration of the useless expenditure of manure, it may justly be questioned whether the tubers of such plants, being immature, can afford as nutritive, or as wholesome food, as others which have acquired a state of perfect maturity.

The preceding statement will, I trust, point out to the Horticultural Society the importance of obtaining improved varieties of the Potatoe, and I believe no plant existing to be more extensively capable of improvement, relatively to the climate of England; and if practical evidence were wanted to prove the extent, to which the culture of the Potatoe is calculated to ncrease and support the population of a country, Ireland most amply affords it; where population has increased amongst the Catholic poor, with almost unprecedented rapidity, within the last twenty years, under the pressure of more distress and misery, than has perhaps been felt in any other spot in Europe.

I shall conclude my present communication with some remarks upon the origin and cure of a disease, the Curl, which a few years ago destroyed many of our best varieties of the Potatoe; and to the attacks of which every good variety will probably be subject.

I observed that several kinds of Potatoes, dry and farinaceous in their nature, which I cultivated, produced curled leaves, whilst those of other kinds, which were soft and aqueous, were perfectly well formed; whence I was led to suspect, that the disease originated in the preternaturally inspissated stale of the sap in the dry and farinaceous varieties. I conceived that the sap, if not sufficiently fluid, might stagnate in, and close, the fine vessels of the leaf during its growth and extension, and thus occasion the irregular contractions which constitute this disease; and this conclusion, which I drew many years ago, is perfectly consistent with the opinions I have subsequently entertained, respecting the formation of leaves. I therefore suffered a quantity of Potatoes, the produce almost wholly of diseased plants, to remain in the heap, where they had been preserved during winter, till each tuber had emitted shoots of three or four inches long. These were then carefully detached, with their fibrous roots, from the tubers, and were committed to the soil; where having little to subsist upon, except water, I concluded the cause of the disease, if it were the too great thickness of the sap, would be effectually removed; and I had the satisfaction to observe, that not a single curled leaf was produced; though more than nine tenths of the plants, which the same identical tubers subsequently produced, were much diseased.

In the spring of 1808, Sir JOHN SINCLAIR informed me that a gardener in Scotland, Mr. CROZER, had discovered a method of preventing the curl, by taking up the tubers before they are nearly full grown, and consequently before they became farinaceous. Mr. CROZER, therefore, and myself, appear to have arrived at the same point by very different routes; for by taking his Potatoes, whilst immature, from the parent stems, he probably retained the sap nearly in the state to which my mode of culture reduced it. I therefore conclude, that the opinions I first formed, are well founded; and that the disease may be always removed by the means I employed, and its return prevented by those adopted by Mr. CROZER.