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Crofting Agriculture
Chapter II. Plant Foods

9. The Work of Phosphates in the Soil

Lime and phosphates, as I have said, are the main constituents of bone. Without a sufficiency of these two plant foods in the soil it is impossible to farm ground efficiently and well. Both elements are sadly short in our West Highland soils and the shortage imposes a limit on our agriculture.

Phosphorus is very firmly held by the soil, so there is not the fear of losing a dressing of phosphates by heavy rain as there is if a nitrogenous manure like nitrate of soda is put on ground before a crop is ready to take it up. This tendency of phosphates to be relatively insoluble means that we must give fairly heavy dressings of a comparatively soluble form so that this plant food can spread evenly through the soil before it again takes up a slowly soluble form by chemical action. It is rather like lime in that a reserve of it needs to be present, upon which the soil acids can keep on working. If you wish to assure yourself of the constant action of these soil acids in our sour soils, let a crowbar stand upright in the peaty ground for a night or two and see how bright will be the portion that has been below ground.

Phosphates are important for plant growth in that they encourage root growth and the development of the young plant. This is particularly helpful with shallow-rooted crops like turnips which tend to dry out in their early stages. Rapid root growth takes them down to the water and makes more soil space available on which each plant may draw for food. All crops of the same family as the turnip, such as kales and cabbage, respond well to phosphates. Cereal crops like oats and barley tiller better and ripen earlier if the phosphate content of the soil is right. A freely-tillering crop means a higher yield of grain and straw, and anything which helps early ripening is useful in our climate where we are often late in seeding and have to put up with a heavy summer rainfall which in itself retards ripening.

Perhaps the most spectacular results of applying a phosphatic manure is on grassland, where it greatly encourages the growth of the clovers and vetches. These plants are of high feeding value and, as I shall point out later, they have a particular value in building up fertility in the soil.

The world supply of phosphates suitable for application to the soil is limited, and some scientists have suggested that the continued wastage of phosphatic material will in some far-off day raise such an acute shortage that life as we know it on the world's surface will become impossible. At least I can say this—that since I have dressed my own ground on Tanera, there has been much more growth and activity of living things.

10. Forms of Phosphatic Manures

I said that the essential plant food supplied by phosphates was very firmly held in the soil, which is another way of saying that they are relatively difficult to dissolve. There are several ways of making phosphatic manure more available, and the resulting substances have different trade names and different values one to another. A short descriptive reference to these fertilizers may be useful.

Basig Slag. This substance is a residue from steel manufacture, so finely ground that most of it will pass through a sieve of 8000 holes to the square inch. Qualities vary and unfortunately the more modern processes in steel making are producing poorer slags. The best slags contain 36 per cent. of phosphate and the poorest only 10 to 12 per cent. Basic slag is an exceedingly heavy, grey-blue powder packed in sacks to weigh one boll each. Obviously, in a place like the West Highlands where transport is difficult, one wishes to buy the best grade of slag possible, though at present we have to take what we can get, which is a quality of about 20 per cent. phosphates. Basic slag shows its best results on grassland where clover is present, because all plants of the clover family are stimulated to better growth by phosphates. It should be sown at the rate of about 15 cwt. per acre. Text books often say 10 cwt. per acre, but on our starved soils this is not enough. The value of slag lasts three to five years. Slag can be sown only on a calm day or it will blow away, as well as fill the eyes and throat of the sower, to his great discomfort. One of the advantages of slag is that it has some lime value, but this means also that it should not be sown direct on to a young growing crop. It may quite safely be sown on the soil during the final preparation for turnip sowing.

Bone Meal. This is just bones ground as finely as possible. The fat in the bones prevents very fine grinding and for that reason this form of phosphatic manure lasts many years in the soil.

Steamed bone flour has had the fat removed by steam heat and is a fine powder. This is one of the most valuable forms of phosphate and is in high favour among gardeners. Unfortunately it is in short supply. It is also rather expensive, though its low weight in relation to phosphate content makes it worth consideration in the Highlands.

Rock Phosphate. There are some deposits of highly phosphatic rock in some parts of the world such as North Africa and the South Sea island of Nauru. The rock is ground so fine as to go through a sieve of 10,000 to the inch and forms a most suitable fertilizer for grassland. The same amount should be sown as of slag. Now that North Africa is in Allied hands we may hope to see rock phosphate in the market again soon. This fertilizer lasts three to five years in the soil. Moroccan rock phosphate dissolves very slowly and is commonly used for the manufacture of superphosphates rather than as a direct dressing to grassland. Tunisian rock phosphate is much easier dissolved and should be the kind chosen for grassland improvement.

Superphosphate. This is the most readily available form of phosphatic manure, made by treating rock phosphate with sulphuric acid. It is used for arable crops just before or just after they are sown and not for grassland. The usual dressing is 3 cwt. to the acre and is used up by the crop in the first year.

11. The Plage of Potash in Plant Growth

Much of the food value of a crop of potatoes, a crop of turnips or of mangolds is in the starch content. The storage of starch by the plant is made possible by the activity of its green leaves, for it is one of the peculiarities of green-leaved plants to have the power of breathing in carbon-dioxide gas from the air and manufacturing it into starch with water in the plant tissues. The health of the green leaf, then, is of primary importance because the plant's leaf area represents its power to do work.


These are probably new beds, dug in a virgin meadow of peat. This crop of potatoes looks fine and healthy, and when dug the flavour will be such as the rich lands of the Lowlands and England can never know. In such a place as this—Vatersay, south of Barra—there is no fear of late frosts.

Our last two talks have dealt with the plant food phosphorus, which has such a remarkable effect on the roots which form the gathering power of plants. The most obvious effect of the plant food we call potash is on the quality and efficiency of leaves. Potash is abundant in clay soils and deficient in sandy soils, but it is relatively tightly held up by the soil, so even on English clays the farmer may find it worth while to give a dressing of an easily soluble salt of potassium when planting his potatoes, a crop which reacts very favourably to potash. Sulphate and muriate of potash are examples of these highly soluble potassium salts; kainit and the manure marketed as "potash salts" are of lesser value and are more generally used on light grassland, where a dressing of potash may make all the difference to whether an application of basic slag becomes effective or not.

Unfortunately, the largest deposits of potassium salts are in France and Germany, so our supplies are severely restricted during the war. But the West Highland crofter is in the favoured position of having a manure rich in potassium on his doorstep almost, in the shape of seaweed. It is still the custom to use this valuable substance as manure, but nothing like so commonly as years ago. I have often heard it said that seaweed "poors" the ground and tends to make the potato crop soft. That is hardly correct, for seaware provides valuable nitrogen and organic matter as well. The trouble is that seaweed is deficient in phosphates and lime, which means that its continued use to the exclusion of all other manures unbalances the soil. All manuring operations should be designed towards maintaining the balance in plant foods or else we find our soil growing top heavy or unable to utilize efficiently the other plant foods present.

For myself, I use seaweed neat for part of the mangold crop, on ground that is adequately supplied with phosphates and lime. I use it composted with farmyard manure for the potatoes, and I use rotted seaweed as a mulch for all the soft fruit bushes. Sometimes in May, if the weather is dry and a large amount of weed has been thrown up, I start a fire of seaweed and keep it going all day, taking care to remove the ash fairly often, otherwise it goes to a hard clinker which takes a lot of crushing. This ash is a valuable potash manure, richer even than wood or peat ash. Three to five hundredweights to the acre of the ash of seaware would be a full dressing for potatoes.

12. Manuring with Nitrogen A Lowland farmer in a large way was once invited to give a lecture to us lads when at our agricultural college. The one thing I remember now of that lecture a quarter of a century ago is the old man shaking a finger and saying:

"Sulphate of ammonia is the alcohol of the soil." Whereupon he reached to his hip pocket and suitably refreshed himself.

That is a good saying and applicable to all the nitrogenous artificial fertilizers. A well-fed, hard-working man living out of doors can take a tot without damage to himself, but lesser men working indoors must leave it alone if they are not to suffer.

All plants need nitrogen in order to make vigorous green leaf growth. It is a gas forming four-fifths of the air, so there is plenty of it in the world, but the plant can use it only when it is chemically bound up with lime in the form of calcium nitrate. This salt is highly soluble, as are most of the nitrogenous salts used in agriculture, a fact which calls for careful control of manuring with them. In that great chemical laboratory which we call the soil, complex substances in the humus are being broken down by bacteria and nitrates formed.

The process is very slow in winter when the ground is cold; and because the salts are so easily soluble, a wet spring like that of 1943 may cause the newly formed nitrates to be washed out of the soil more rapidly than they can be taken up by the plant. I think I never saw leaves look so yellow as they did in the early spring of 1943. That was a sure sign of lack of nitrogen and indicated to me that my early potatoes needed a dash of some nitrate in addition to the heavy farmyard manuring to give them a healthier look.

Some leafy crops like potatoes, cabbage and kale, and, of course, grass, can usually do with more nitrogen than we are able to give them in farmyard manure. Also, they need it quicker than the bacteria can produce it from farmyard manure, in the early stages of growth.

It is probable that nitrogenous artificial fertilizers have done more to condemn the use of what are popularly called "artificials" than anything else. That is because—like alcohol—it is so easy to use them unwisely. The application of nitrogen can give a crop a sudden burst of growth and greenness, but unless the soil in which it is grown is well balanced and healthy, the crop will fall in and the soil will be as good as murdered. The excessive use of nitrogen on the very best of soils will result in leggy plants, roots like mangolds and carrots running to seed before they should, and straw crops which will go down or be late in ripening and be poor in weight of grain. With slower-acting phosphatic and potassic fertilizers which are firmly held in the soil it is unlikely that a dressing will be so heavy as to hurt a crop, but too much highly soluble nitrate can easily kill a plant.

If your soil is thin and poor and obviously lacking organic matter—that is, humus—do not use nitrogenous artificials. If your soil is sadly deficient in lime, you will be well advised to leave them alone and particularly to keep clear of sulphate of ammonia. But if your soil is well balanced and kept steady by the buffering effect of humus, it is like a strong man on whom you can call for that bit extra, and you can use a limited amount of nitrates with safety.

13. The Application of Nitrogen

It is worth keeping in mind that the plant takes up its nitrogenous food in the form of calcium nitrate, i.e. nitrate of lime. This means that many forms of nitrogen we apply to the soil have to undergo chemical change before they can be used by the plant.

Sulphate of ammonia is a by-product of the coal-gas industry. This salt is attacked by those bacteria of the soil I mentioned earlier—the nitrifiers. They split the salt into two chemical parts which cannot stand alone in the soil but must take up lime to become steady or stable. The sulphate part becomes calcium sulphate, commonly known as gypsum, which is not much good to plants; the nitrogenous part goes to the crop as food. All nitrogenous artificials tend to take lime from the soil, but sulphate of ammonia is undoubtedly most expensive in this respect. It may be taken as a rough measure that one hundredweight of sulphate of ammonia takes one of lime from the soil. This fertilizer contains 21 per cent. of nitrogen, a higher figure than almost any other nitrogenous manure.

Nitrate of soda is a commonly used artificial, containing 16 per cent. of nitrogen. It used to come from the nitrate deposits in Chile, but it is now being produced synthetically in Britain. It is quicker acting than sulphate of ammonia. Care must be taken to keep it dry, and even then it should be used as soon after delivery as possible as it takes up moisture from the air.

Nitro-chalk is a highly suitable fertilizer for the West Highlands if nitrogenous manuring is to be done, because it is so prepared as to contain 48 per cent. of calcium carbonate or lime as well as the 16 per cent. of nitrogen. This means that the lime-robbing action of the nitrate is exactly counteracted. It is granular in texture and easy to sow, but it should be kept dry.

Nitrate of lime used to be prepared in Norway by hydro-electric power, the source of nitrogen being the limitless air. If our own hydro-electric schemes come to fruition, it is probable that a factory for this fertilizer would come into being. It contains 13 per cent. of nitrogen and is marketed in barrels to keep out the moisture of the air. It is the quickest acting of all nitrogenous manures.

Other nitrogenous manures like calcium cyanamide are manufactured, but little of them ever gets to this part of the country.

Different crops need different amounts of nitrogen. A clover and rye-grass ley will take 1 to 1 cwt. of nitrate per acre in April with profit. A further to 1 cwt. may be given after the first hay crop is taken if it is intended to mow the aftermath. If the aftermath is to be grazed, the second application is unnecessary.

Early potatoes pay for a dressing of nitrate at planting time. The fertilizer may be applied on top of the farmyard manure in the drills at the rate of 2 cwt. per acre. If the ground is very good 3 cwt. could safely be given.

Cabbages are greedy feeders and can take a heavier dressing of nitrogen than most crops: 1 to 2 cwt. per acre can be given at planting time and a further dressing of 2 cwt. later in the season. Turnips and swedes will need 2 cwt. per acre in two dressings, the first at sowing time. A crofter has more time than a farmer to apply his manures in the most advantageous way. Thus, I like to make a V-shaped trench with the hoe about 2 inches deep and 2 to 3 inches away from the growing plants; the artificial is spread along this and the little trench covered again. The fertilizer reaches the roots and is not scattered over the drill for the weeds to use as well. (For calculations of quantities, see section 58, p. 156, and section 59, p. 158.)

We all know the shortage of feed for cattle in May before the grass comes. A dressing of 1 to 1 cwt. of nitrate during the third week of April will be a great help in bringing on the grass a fortnight earlier.

Remember always that the poorer the ground the more careful should you be in applying nitrates. Only rich ground in good heart can take heavy dressings.

14. Farmyard Manure

Farmyard manure is the basis of almost all systems of manuring at the present time, just as it has been for thousands of years. There is no need nowadays, when we have mineral chemical fertilizers available, to think of any controversy—farmyard manure versus artificials— because the man of sense knows the two are complementary and not in opposition. But where we cannot get artificial fertilizers easily, we should pay much more attention to conserving dung and learn as much about it as we can.

The popularity of farmyard manure is well founded, because dung contains all the plant nutrients—nitrogen, phosphorus, potash and a little lime, and in addition it now seems probable that it contains traces of substances called plant hormones which activate growth in young plants as chemical fertilizers alone cannot do. Another valuable part of farmyard manure is the large spongy mass of organic matter which turns to humus in the soil and helps both to lighten it and hold it together.

First of all, what kinds of farm animals give the best manure ? That of sheep is most concentrated, apart from poultry manure, which, of course, should always be carefully preserved when taken out of the hen-house. Sheep manure is easily broken down. It is valuable, obviously, on our West Highland ground, but I should feel it was still more so if it did not represent just what the sheep puts back after it has taken its own requirements wholly from that ground. Where sheep are folded or enclosed on a piece of ground and given extra feeding, the ground is receiving more, probably, than is being taken from it, and then sheep manure is almost the best that could be applied. Its moisture content is slightly less than two-thirds of the total weight.

Horse manure is the next most valuable. Its distinguishing feature is the readiness with which it ferments and gets hot. Thus it is helpful in getting a heap of mixed manure to "work" and break down into humus. By itself, horse manure does not last long in the soil and its nitrogen content is apt to diminish rapidly through loss as ammonia.

Pig manure comes next in value, but as we do not keep pigs in the West Highlands as a general rule, there is little point in saying much about it here.

Cattle dung contains over three-quarters of its weight of water and is the poorest in soil nutrients. Also, it is slow to ferment and is a cold manure. It is most necessary for it to be mixed with straw, bracken or some bedding that will soak up its moisture and help it to ferment.

There are also differences in the value of dung from various classes of animals of the same kind. For example, growing beasts will take more out of their food and leave less in the dung than adult ones: fattening cattle receiving oil cake give the best quality manure, and cows of high milk yield give the poorest.

The most readily available plant nutrients in animal excreta are contained in the urine, yet this is often wasted when cattle beasts are tied by the neck in stalls. This brings us to the whole large question of the care of farmyard manure as our principle aid in maintaining fertility. Those arable farmers of the south and east who keep bullocks in winter for the sake of converting straw into manure follow the excellent method of keeping the animals in covered yards with plenty of straw under them. There is no loss by rain, and not only is the manure compacted by the constant treading, but all the urine is soaked up by the straw, which itself is broken down gradually to humus. The greatest waste of urine is in a cow byre with an open drain which does not run into a tank. Recent shortages of potassic manures have brought about some easing of the official conditions for milking sheds in some parts of the country, so that straw or chaff may be spread in the drain before milking to soak up the urine. A crofter would do well to put down in the drain behind his beasts a few shovelfulls of that crumbly peat which collects where the peat stacks are made. Peat has great absorbent powers and some of its own plant nutrient properties can be liberated by mixing it in this way with farmyard manure and a little lime.

Finally, a few words about the manure heap itself. If a man loses a stirk, he is very conscious of his loss and complains, "There goes 16," or whatever he thought it was worth. But he may watch the rain falling on the midden the whole winter through and see a stream of black ooze making its way to the burn. A good third of the value of the manure is being lost, yet you will not hear that man complaining of this loss as he leans against the jamb of his byre door. He never thinks of it, for it is a sight to which he has got accustomed. It is certainly an annual loss which the crofter cannot rightly afford. A covered manure pit with concrete floor would be a profitable investment on every croft. The roof would not only keep off the rain, but the walls would help to exclude the air which is necessary to the bacteria which break down the nitrogenous portion of the manure and cause loss. Manure should always be well firmed and made into a compact heap.

When dung is spread on the land it begins to lose some of its nitrogen into the air as ammonia, so it is best ploughed in as soon as possible. When it is being put in drills for the roots in the hot weather of May, it becomes particularly necessary to get the drills split and the manure covered quickly, for bacterial action is very rapid under such conditions.

There is always controversy and misunderstanding as to the wisdom of letting muck rot or ripen, or applying it fresh. Well-rotted manure is undoubtedly better for the plants, and pound for pound it is richer than fresh manure. But remember that a heap of rotted manure will have suffered some loss in the making. A man has to decide for himself which plan will pay him better.

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