Chapter 8

THE SOIL AND THE PLANT

Apparatus required.

_Dry powdered soil, sand, clay, leaf mould, seeds. Six funnels, disks, stands and glass jars [3]. Six glass tubes about 1/2 in. diameter and 18 in. long [2]. Muslin, string, three beakers. Six lamp chimneys standing in tin lids [3]. Pot experiments (p. xiii), growing plant. Two test tubes fitted with split corks (Fig. 35)._

If you have ever tried to grow a plant in a pot you must have discovered that it needs much attention if it is to be kept alive. It wants water or it withers; it must be kept warm enough or it is killed by cold; it has to be fed or it gets yellow and starved; also it needs fresh air and light. These five things are necessary for the plant:

Water, Warmth, Food, Fresh air, Light.

We may add a sixth: there must be no harmful substance present in the soil.

Wild plants growing in their native haunts get no attention and yet their wants are supplied. We will try and find out how this is done.

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Water comes from the rain, but the rain does not fall every day. How do the plants manage to get water on dry days? A simple experiment will show you one way. Put about four tablespoonsful of dry soil on to the funnel shown in Fig. 29 and then pour on two tablespoonsful of water. Measure what runs through. You will find it very little; most of the water sticks to the soil. Even after several days the soil was still rather moist. Soil has the power of keeping a certain amount of water in reserve for the plant, it only allows a small part of the rain to run through. Do the experiment also with sand, powdered clay, and leaf mould. Some water always remains behind, but less in the case of sand than in the others. In one {66} experiment 30 cubic centimetres of water were poured on to 50 grains of soil but only 10 cubic centimetres passed through, but when an equal amount was poured on to 50 grains of sand no less than 20 cubic centimetres passed through. Very sandy soils, therefore, possess less power of storing water than do soils with more clay or mould in them, such as loams, clays or black soils.

Further, water has a wonderful power of passing from wet places to dry places in the soil. Tie a piece of muslin over the end of a tube and fill with dry soil, tapping it down as much as you can, then stand the tube in water as in Fig. 30. Fill another with sand {68} and place in water. Notice that the water at once begins to rise in both tubes and will go on for a long time, always passing from the wet to the dry places. It rises higher in the soil than it does in the sand. Enough water may pass up the tube in this way to supply the needs of a growing plant. Fill a glass lamp chimney with dry soil, packing it down tightly, put into water and then sow with wheat. The plants grow very well. A longer tube may be made from two chimneys fastened together by means of a tin collar stuck on with Canada balsam or sealing wax (Fig. 31). Our plants grew well in this also, but on a sandier soil, where the water could not rise so high, it might happen that they would not.

Thus we shall expect great differences in the moisture of various soils. In some districts there is much more rain than in others, and therefore the soils get a larger supply of water. Sandy soils allow water to run through while a loam holds it like a sponge, in a loam also the water readily moves from wet to dry places. Further, water runs down hills and collects in low-lying hollows or valleys; here, therefore, the soil is moister than it is somewhat higher up. What will be the effect of these moisture differences on plants?

You must find out in two ways. Visit a soil that you know is dry--a sandy, gravelly or chalky soil in a high situation--and look carefully at the plants there, then go to some moister, lower ground and see what the plants show. You cannot be quite certain, however, that anything you see is simply due to water supply, because there may be other differences in the soil as well. So you must try the second method, and that is to find out by experiments what is the effect of varying {69} quantities of water on the plant growth. Both methods must be used, but it may be more convenient to start the experiments first, and while they are going on to collect observations in your rambles.

Fill four glazed pots with dry soil: keep one dry; one only just moist; the third is to be very moist and should be watered more frequently than the second; and the fourth is to be kept flooded with water, any way out being stopped up. Sow wheat or mustard in all four and keep out of the rain. The result of one experiment with mustard is shown in Fig. 32. Where no water was supplied there was no growth and the seeds remained unaltered. Where only little water was supplied (Pot 16) the plants made some growth, but not very much: the leaves were small and showed no great vigour; {70} where sufficient water was given (Pot 3) the plants grew very well and had thick stems and large leaves; where too much water was given (Pot 15) the plants were very sickly and small.

The weights were:--

Green weight After drying

Plants with too much water 3.9 0.5

" " too little water 5.3 0.9

" " a nice quantity of water 17.7 2.6

Fig. 33 shows two pots of wheat, one kept only just sufficiently moist for growth, the other kept very moist but not too wet. You can see what a difference there is; in the drier pot the leaves are rather narrow and the plants are small, in the moister pot the leaves are wide and the plants big. But there was also another difference that the photograph does not bring out very well--the plants in the rather dry soil were, as you can see, in full ear, ripe and yellow, while those in the very moist soil were still green and growing. We see then

(1) that on moist soils there is greater growth than on dry soils, but the plants do not ripen so quickly;

(2) in very wet soils mustard--and many other plants also--will not grow.

Water is not itself harmful. It is easy to grow many plants in water containing the proper food, but _air must be blown through the water at frequent intervals_. In the water-logged soil of Pot 15 the trouble arose not from too much water but from too little air. Air is wanted because plants are living and {71} breathing in every part, in the roots as well as in the leaves.

Now turn to what you have seen in your walks. You would probably notice that on the drier, sandy or gravel ground there was nothing like as great a growth of grass or of other plants as on the moister soil. This is so much like what we found in the pot experiments that we shall not be wrong in supposing that the difference in water supply largely accounted for the difference in growth. But you may also have noticed something else. Plants in the drier soil have generally {72} narrow leaves and the grasses are rolled up and fine, whilst those on the damp soil, including the grasses, have usually broad leaves. Thus in the dry sandy soil you may find broom, spurrey, sheep's fescue, pine trees, all with narrow leaves; whilst on the moister soil you may find burdock, primroses, cocksfoot and other broad-leaved plants. Figs. 34 _a_ and _b_ show some plants we found on a dry, gravelly patch on Harpenden common, and on a moist loam in the river valley below.

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Before we can account for this observation, we must ascertain a little more closely what becomes of the water the plant takes up. It certainly does not all stay in the plant, and the only way out seems to be through the leaves. Put a test tube on the leaf of a growing plant and fix a split cork round the stem: leave in sunlight for a few hours and notice that water begins to collect in the test tube (Fig. 35). The experiment shows that water passes out of the plant through the leaves.

This experiment was first made by Stephen Hales, and described by him thus in 1727: "Having by many {74} evident proofs in the foregoing experiments seen the great quantities of liquor that were imbibed and perspired by trees, I was desirous to try if I could get any of this perspiring matter; and in order to do it, I took several glass chymical retorts, _b a p_ [Fig. 36] and put the boughs of several sorts of trees, as they were growing with their leaves on, into the retorts, stopping up the mouth _p_ of the retorts with bladder. By this means I got several ounces of the perspiring matter of vines, figtrees"--and other trees, which "matter" Hales found to be almost pure water. The test tube experiment should now be made with a narrow-leaved grass like sheep's fescue and with a wide-leaved grass like cocksfoot. You will find that wide-leaved plants pass out more water than those with narrow leaves, and hence wide-leaved plants occur in damp situations or on damp soils like loams and clays, while narrow-leaved plants can grow on dry, sandy soils.

Another thing you will notice is that fields lying at the side of a river and liable to be flooded, and fields {75} high up in wet hill districts, are covered with grass. In a clay country there is also a great deal of grass land and not much ploughed land; if you live where there is much clay you can easily discover the reason. Clay becomes very wet and sticky when rain falls, and very hard in dry weather: it is, therefore, difficult to cultivate. Farmers cannot afford to spend too much money on cultivation, and so they prefer grass, because once it is established it goes on indefinitely and does not want ploughing up and re-sowing. And besides, farmers have learned by experience that grass can tolerate more water and less warmth than most other English crops. There is much more grass land in those parts of England where the rainfall is high and the temperature rather {76} low--e.g. the northern parts of England--than in the eastern counties where the rainfall is low.

The difference in water supply, therefore, leads us to expect the following differences between sandy soils and clays or loams:--

On sandy soils (the water content being small) the wild plants and trees usually have small leaves. Cultivated plants do not give very heavy crops, but they ripen early.

On clay soils (the water content being good) wild plants and trees usually have larger leaves. Cultivated plants give good crops, but they ripen rather late. If the water content is too good or the clay is too sticky the land is generally put into grass.

Plants require to be sufficiently warm. Some like tropical heat and can only be grown in hot houses; others can withstand a certain amount of cold and will grow up on the mountains. Our common cultivated crops come in between and will not grow in too cold or exposed a situation; thus you find very little cultivated land 800 ft. above sea level, and not usually much above 500 ft. At this height it is left as grass land, and higher up as woodland, moor, or waste land. Grass requires less warmth and can therefore grow at greater heights than many other crops. If you start at the top of a hill in Derbyshire, and walk down, you will see that the top is moorland, lower down comes grass land, still lower you may find arable land, and if the valley is damp you will find more grass at the bottom. Figs. 37 and 38 show typical views of the hill slopes further south: they are taken near Harpenden. The top of the hill in each case is over 400 ft. above sea level, and has never been thought worth cultivating, but has always been left as {78} wood because it is too exposed for farm crops. On the lower slopes the arable fields are seen, while at the bottom bordering the river is rough grass land, shown in Fig. 39. The top is too cold and windy, and the bottom too wet, to be worth cultivating.

As the plant root is alive it wants air. The effect of keeping air out can be seen by sowing some barley or onion seeds in the ground and then pouring a lot of water on and plastering the soil down with a spade. Sow another row in nicely crumbled soil, not too wet, press the seeds well in, but do not plaster the soil. This second lot will generally do much better than the first. If the ground round a plant is frequently trodden so that it becomes very hard the plant makes much less growth than if the soil were kept nice and loose. A good gardener takes very great pains in preparing his ground before he sows his seeds, and he is careful that no one should walk on his beds lest his plants should suffer.

SUMMARY. We may now collect together the various things we have learnt in this chapter. Plants require water, air, warmth, food, and light, and they will not grow if harmful substances are present. The rain-water that falls remains for some time in the soil, and does not at once run away or dry off: water can also move from wet to dry places in the soil. Therefore the plant does not need rain every day, but can draw on the stock in the soil during dry weather. A sandy soil is usually drier than a loam or a clay, especially if it lies rather high: plants growing on a sandy soil make less growth and have narrower and smaller leaves than those on a moister soil.

Situations more than five or six hundred feet above sea level are, in England, as a rule, too bleak and {80} exposed for the ordinary cultivated crops. Such land is, therefore, either grass land, moorland, downland or woodland.

The roots of plants are living and require air. The soil must not be trodden too hard round them or air cannot get in, nor can it if too much water is present.

Grass can put up with more water and less warmth than most cultivated crops.

Instances of these facts may be found in going down any hill 500 ft. or more in height: the top is usually wood or waste, being too cold for crops, below this may come grass land, lower still arable land. It is both warmer and moister in the valley (since water runs down hill), and so we can account for the proverbial fertility of valleys. But just near the river, if there is one, the ground may be too wet for crops, and therefore grass is grown. Clay land that is rather too wet to plough is usually left in grass.

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