ALL practising farmers and students of agriculture are well aware of the controlling influence of weather in the growing of crops. To the city man a sunny day in midsummer may be a thrilling event, because it provides the ideal conditions for picnicking, hiking and swimming. For thousands of farmers nearby the same day may be an occasion of disaster involving the local food supply, in which the city dweller, as well as the husbandman, has a vital stake. Rains that arrive a day too late to save potatoes, beans and lettuce affect both producer and consumer, but the producer more seriously.
Weather has always been considered in the category of " acts of God," and so it may very well be. Equally, however, it may be said that " God helps those who help themselves." There is nothing to be achieved here by bringing up once again the famous dispute between the Forest Service and the Weather Bureau as to whether forests actually increase the rainfall. Nor is it to the point, perhaps, to conjecture with the scientists concerning the effect of England's deforestation in past centuries on present-day climate in the British Isles. But it may be useful to point out that man has it in his power to disturb some of the moisture conditions essential to plant growth, and that, by extension, he partly controls some of those conditions.
Man can conserve the moisture laid down by the heavens, or he can waste it. The earth he took over originally was covered everywhere by a water-soaked, sometimes odorous sponge of humus. Nature maintained this water-catching cover through successive plant generations, wherever man did not disturb, and continues to maintain it down to this very hour. By imitating Nature, man could have enjoyed such benefits as he has never dared hope for; by disregarding the obvious example she set for him, he has courted disaster.
Irregular moisture has been regarded as the most important weather condition controlling crop growing. With respect to moisture, the absorbent mat we find everywhere in Nature serves a purpose which has not been recognized in agricultural literature. For lack of a better term, we may call this its " reservoir " purpose.
Farmers leave their hay stacked in the field exposed to all the rain that falls. They know that none of the rainfall can sink deeper than the upper few inches, because the porous tissues of the hay must first be filled. Since every inch of this top layer of hay will catch and hold nearly an inch of rainfall, the underlying hay is protected.
Knowing this, we ought to understand that if enough organic matter is disced into the soil surface it will constitute to its full capacity a reservoir in which a large proportion of the rainfall will be retained. If enough absorbent material has been provided to hold an inch or two of rainfall, then, when rain is falling, an inch or two of it will be retained in the surface. Naturally, this spongy mass will supply water—richly endowed with the minerals it takes from the decaying material in which it is held — to crops which otherwise would suffer seriously in the intervals between rains.
Not having this conception of the service of such a mantle of porous material, scientists have reasoned about water chiefly in terms of capillary movement within the soil. And strangely enough, some scientists have believed, from results of their tests, that there is little of such movement in the upper layers of soil. If anybody doubts that such conclusions have been introduced into serious scientific literature, it may be interesting to relate a brief conversation I had in September 1937 with a crop specialist I had known for nearly twenty years. It ran something like this:-
I had suggested doubt as to the propriety of ploughing. He quickly asked, " What's wrong with ploughing ? " " Interferes with capillarity," I replied. He had a ready answer: " Tests show that there isn't as much capillary movement in the soil as we used to believe existed—it's relatively unimportant in many cases." " Well," I replied, " in unploughed land there must be enough upward capillary movement between rains to keep the vegetation alive." Mine was the last word.
He was correct in his statements. Such tests have been made. They were made, like all soil experiments, on ploughed soil. The " reservoir " for water lies several inches deep in the ploughed soil; and, since it literally robs the upper soil layers of their water as well as shutting off upward movement of capillary water rising from deeper in the soil, no other results of such tests could have been expected. If such tests were made in soil where grass is growing, the story would be entirely different.
The very nakedness of ploughed land should of itself indicate a lack of capillary water in the surface. If capillary water were present, seeds would sprout and grow, for seeds are always present. Or had you noticed that the only bare soil in most landscapes is that which has recently been ploughed ? I discovered that highly significant fact only a few months ago; though I had seen it daily throughout a lifetime. Since ploughed land is always bare, and since practically all other land, save areas like the Sahara, is covered with greenery of some sort—which could not exist without a continual supply of water — it follows, even without tests, that there is no capillary water in the upper layers of freshly ploughed soil.
It may be repeated here that, while God, not man, controls the weather, it is nevertheless given to man to control some of the fruits of weather, and of these perhaps the most important is the natural moisture of the soil surface. The first essential in this respect is to grasp the dissimilarity of water relations in ploughed and unploughed lands. The next is to understand that the weather which kills vegetation on cultivated land may also cause vegetation to thrive, or at least to show no ill effects, on uncultivated land. The final phase is to connect logically the importance of the organic matter profile with both plant growth and the weather conditions under which plants may prosper.
For purposes of this discussion we may assume as normal any soil surface that has been left unploughed, or any ploughed soil that has had time to recover its normal capillary water movement (because of the disappearance by decay of the organic matter ploughed in). All meadow and pasture land on farms, then, as well as the land occupied by the farm fences, may be considered as part of the natural landscape, even though it is also part of the land normally subject to ploughing. It is natural landscape because in its profile there is nothing to prevent water from rising to the surface. Whatever interference may have been introduced in previous times by ploughing has been disposed of by decay.
By and large, the " voltage " of any soil depends upon the accumulations of decayable material available in its surface. By this standard it would be true almost always that wilderness soils, unploughed for many years if ever ploughed at all, would be more productive than similar soil that had been included regularly in rotation cropping. The unploughed soil has the advantage that economical use of all products of decay has been the rule for the entire period since its last ploughing. The grassland in rotation, on the other hand, has periodically had a large percentage of its accumulated material removed from the surface, resulting in the wasting of the products of decay. This deliberate (though unwitting) periodic waste of soil resources, after being repeated several times, finally results in a low-grade soil where formerly the productivity was high. The final result is erosion. And, when erosion has started, we may be sure there is not much absorbent material left in the surface of the soil. The remaining light-coloured stuff is almost identical with that which the glaciers in their time moved about from place to place.
An experienced farmer allows some of his land to lie in grass for a few years in order that its " voltage " may be stepped up. The longer the area is in turf the more productive it is when it is again put to corn. However, the period in which it accumulates a new supply of organic matter to be wasted again by ploughing is not sufficient to enable it to make the gains that would put it in its natural condition. Indeed, the progress seems always to be slightly on the down-side. No trick yet discovered has made it possible to achieve positive gains regularly on land operated in continuous three- or four-year rotation. There are probably a few exceptional cases, but this is the general rule. The wastage caused by ploughing usually more than balances the accumulations made in the interim. In fine, rotation of the type described is not a cure-all for impoverished soil, and, what is more important to the thesis of this chapter, it does not get at the water relations which are ultimately desirable.
It was shown in the last chapter that a farmer may quite abruptly step up the productiveness of his soil by simply short-circuiting the wasteful practice of ploughing. By mixing into the surface the decayable material which the plough would inter, the farmer sets the stage for biologically economical practices hitherto unknown to modern farming. Apart from questions of plant nutrition, there are several ways in which the surface mixing of organic matter brings to focus friendly forces of growth which are unable to operate when land is ploughed.
Every ton of organic matter mixed into the surface of the soil will be able to contain much more absorbed water than it could if buried at plough depth. Why ? Because, being weighted down by so much less overlying soil, its volume will be greater. And organic matter, it must be remembered, retains water volumetrically, while the mineral substances of the soil must hold it only upon the outer surfaces of the particles. Water runs into organic fragments, while it squeezes in between particles of sand, silt, and clay. We can rightly expect, then, that any absorbent material we work into the surface of the soil will retain rain-water much more effectively than would the same material if ploughed in.
Indeed, if ploughed in, organic matter gets no opportunity to catch and hold rainfall until that water has first forced its way several inches down between the mineral particles. Under most conditions it is much easier for some of the water to run off the surface than for all of it to force its way down into the soil. This means, then, that when all the organic matter is in the surface of the soil, it is able to take in water from both above and below—and in greater volume because of the greater volume of the organic matter itself.
Undoubtedly the original black soils our forefathers knew could absorb directly, and as rapidly as it fell, several inches of rainfall in a few hours. It is unlikely that very much water ever leached through the zone of surface organic matter in those highly absorbent soils. The light, fluffy leaf mould, or the springy layer of grass roots, gradually became filled with rain-water as it fell. In this connection I like to remember the story told by one of the best-known agronomists in America. He was inspecting some highly organic soil lying near the top of a mountain slope when a heavy shower developed. The slope was a little less than 45 degrees. Those familiar with geometry will recognize that this is rather steep land. This agronomist remained through the storm to observe the course of the water as it fell. He said that, so far as he could determine, none ran off. If any did so, he said, it certainly did not take any soil with it.
Discing heavy green manure crops into the surface of the soil, then, is an excellent way to create, precisely in the surface of the soil, a reserve of water upon which crop roots can draw continuously until it is used up. Such an arrangement is obviously superior to the principle of permitting the water to run down through the soil and hoping it will be brought back by capillarity. Apart from holding a plentiful reserve of water in the root zone, the mass of organic matter receives capillary water continually from below, which replaces, at least in part, the reserve from which plants are drawing. This reserve supply of water serves to tide crops over extended periods of drought which otherwise would damage them seriously. From such a source water can be made available during many more days of the growing season than could possibly be the case when surface conditions are such as to let some of the rainfall run off and be wasted. Here is " conservation of natural resources."
This, however, is only part of the story. The water stored in surface organic matter is constantly being used to assist in the decomposition of the material which holds it. It not only assists in this decay, but it dissolves and in turn holds the products released. Thus, as long as water is retained in the organic tissues, it is constantly being enriched by the cast-off substances of which the organic matter was composed. And all of this enrichment is in addition to the minerals which the capillary water picks up and dissolves in the soil depths before the water is absorbed by the organic matter. It can readily be seen that under these conditions many influences are working together effectively which could not do so if the organic matter were located six to eight inches deep, where relatively few plant roots reach.
At this point the reader should recall that, in the ploughed soil, carbon dioxide is released into the upper layer of soil; and that this gas is prevented from becoming carbonic acid because of the necessary dryness of the upper layers. In the newer situation, with all of the organic matter just in the surface, there is provided an abundance of water in the vicinity in which the carbon dioxide can be dissolved. And, since carbonic acid is one of the most efficient of known natural solvents of minerals, its work in the surrounding rock particles serves to release for plant use quantities of phosphorus, potash, and other needed plant nutrients which would not otherwise be available.
The extent to which this release of mineral substances from the rock itself can take the place of applications of mineral fertilizers is something I am not prepared to discuss. It is an interesting and a very important question. Every farmer will want to know, and is entitled to know, the answer. If it is possible that the carbonic acid released in the soil will supply enough fresh minerals to supplement adequately the minerals drawn from organic sources, then the purchase of mineral fertilizers will be unnecessary. Only this much can be said safely: If a farmer succeeds in working into his soil enough organic matter to equal the supply held when the land was first opened for cropping, then he may reasonably hope to grow maximum crops without fertilizers. An easy way to test this principle is to leave unfertilized strips in all such fields. When it becomes impossible to find those unfertilized strips at harvest time, because the crop is equally good everywhere, then the necessity for fertilizers has vanished. Within a few years, no doubt, we shall have official information on this point.
And how may we expect the plant itself to react to the optimum conditions described ? Just as any other being reacts to a constant supply of food. Plants will establish most of their millions of roots in the organic fragments. There is not the slightest chance here for plant food to be lost. The instant it is released, the water that contains it is moved into a plant root and sent upward into the plant. The matter of deep rooting of plants, which has been widely discussed in past years, becomes a dead issue. There ceases to be any need for roots to penetrate soil depths. Their food supply is in the surface. The water in this organic matter is busily engaged in wrecking the dead tissues in order to provide materials to be built into the new growth. Bacteria, too, are involved, and without them this process could not occur. The point is that " all things work together for good " in this instance; so close knit is the process that no opportunity is left anywhere for the loss of nutrient materials. Plant roots that go deep, other than for anchorage, in such a situation are working to the disadvantage of the plant they represent.
It will now be apparent that man can control to a very considerable extent the rainfall with which his land is endowed from season to season. The reasons sustaining this conclusion may be summarized as follows:
Under proper management, the soil may be caused to hold natural precipitation at just the level where plant roots normally seek the essential nutrients. The presence of an organic mass in the surface so enriches the water by solution that, volume for volume, the water thus treated will produce more plant growth than water held by the mineral particles alone. Water thus held in the organic mass becomes available to plants without the opportunity for essential plant nutrients to be wasted in any way.
Considering these important factors, it is not too much to suppose that ten inches of rainfall might accomplish as much as is ordinarily expected of twenty. Again, with ample rainfall, it may easily be possible to produce several times the average production figure for the country as a whole.
The truth about the weather is that man can indeed make the best of it—if he will.
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