Building Soils for Better Crops, 2nd Edition

	Let's Get Physical: Soil Tilth, Aeration, and Water Moisture, warmth, and aeration; soil texture; soil fitness; soil organisms; its tillage, drainage and irrigation; all these are quite as important factors in the make up and maintenance of the fertility of the soil as are manures, fertilizers, and soil amendments. J.L. Hills, C.H. Jones, and C. Cutler, 1908 A soil's physical condition has a lot to do with its ability to produce crops. A degraded soil usually has reduced water infiltration and percolation (drainage into the subsoil), aeration, and root growth. This reduces the ability of the soil to supply nutrients, render harmless many hazardous compounds (such as pesticides), and to maintain a wide diversity of soil organisms. Small changes in a soil's physical conditions can have a large impact on these essential processes. Creating a good physical environment, which is a critical part of building and maintaining a healthy soil, requires a certain amount of attention and care. Let's first look at the physical nature of a typical agricultural soil. It usually contains about 50 percent solid particles and 50 percent pores on a volume basis (figure 6.1). We discussed earlier how organic matter is only a small, but very important component of the soil. The rest of the soil particles are a mixture of various size minerals, ranging from fine-grained microscopic clay to easily visible large sand grains. The relative amounts of the various particle sizes defines the texture of a soil, such as a clay, clay loam, loam, sandy loam, or sand. Although management practices don't change this basic soil physical property, they may modify the effects of texture on other properties. The sizes of the spaces (pores) between the particles and between aggregates are much more important than the sizes of the particles themselves. The total amount of pore space and the relative quantity of various size pores (large, medium, small, very small) govern water movement and availability for sustaining soil organisms and plants. We are interested mostly in the pores, because that's where all the important processes, such as water and air movement, take place. Soil organisms live and function in the pores, which is also where plant roots grow. Most pores in a clay loam are small (generally less than 0.0004 inch), whereas most pores in a loamy sand are large (generally still smaller than 0.1 inch). Although soil texture doesn't change over time, the total amount of pore space and the relative amount of various size pores (called the pore size distribution) are strongly affected by management. WATER AND AERATION The soil pore space can be filled with either water or air, and their relative amounts change as the soil wets and dries (figure 6.1). When all pores are filled with water, the soil is saturated and soil gases can't exchange with the atmospheric gases. This means that carbon dioxide from respiring roots and soil organisms can't escape from the soil and oxygen can't enter, leading to undesirable anaerobic (no oxygen) conditions. On the other extreme, a soil with little water may have good gas exchange, but it can't supply sufficient water to plants and soil organisms. Figure 6.1 Distribution of solids and pores in soil. The way in which a soil holds and releases water is pretty similar to the way it works with a sponge (figure 6.2). When it's fully saturated (you take the sponge out of a bucket of water), a sponge loses water by gravity, but will stop dripping within about 30 seconds. It's only the largest pores that lose water during that rapid drainage because they are unable to hold the water against gravity. The sponge still contains a lot of water when it stops dripping. The remaining water is in the smaller pores, which hold it more tightly. The sponge's condition following drainage is almost the same as a soil reaching field capacity water content, which occurs after about two days of free drainage following saturation by a lot of rain or irrigation. If a soil contains mainly large pores, like a coarse sand, it loses a lot of water through gravitational drainage. This is good because these pores are now open for aeration, but it's also bad because little water remains for plants to use, leading to frequent drought stress. Coarse sandy soils have very small amounts of water available to plants before they reach their wilting point (figure 6.2a). On the other hand, a dense, fine-textured soil, such as a compacted clay loam, has mainly small pores, which hold on tightly to water and don't release it as free drainage (it has little gravitational water, figure 6.2b). In this case, the soil will have long periods of poor aeration, but more plant-available water than a coarse sand. Leaching of pesticides and nitrates to groundwater is also controlled by the relative amounts of different size pores. The rapidly draining sands lose these chemicals along with the percolating water, but this is much less of a problem with clays. Figure 6.2 Water storage for three soils. (Shaded area represents water stored in soil that is available for plant use.) The ideal soil is somewhere between the two extremes. This can be found in a well aggregated medium-textured loam soil (figure 6.2c, figure 6.3). Such a soil has enough large pore spaces between the aggregates to provide adequate drainage and aeration during wet periods, but also has adequate amounts of small pores and water-holding capacity to provide sufficient water to plants and soil organisms between rainfall or irrigation events. Besides holding and releasing water well, such soils also allow for good water infiltration, thereby increasing plant water availability and reducing runoff and erosion. This ideal soil condition is indicated by crumb-like aggregates, which are common in good topsoil. Figure 6.3 A well aggregated soil has a range of pore sizes. This medium size soil crumb is made up of many smaller ones. Very large pores occur between the medium size aggregates. GOOD SOIL TILTH IS GOOD AGGREGATION Good aggregation, or structure, helps to make a high quality soil. Aggregation in the surface soil is favored by organic matter and surface residue. As pointed out earlier, a continuous supply of organic materials, roots of living plants, and mycorrhizal fungi hyphae are needed to maintain good soil aggregation. Surface residues protect the soil from wind and raindrops and moderate the temperature and moisture extremes at the soil surface. An unprotected soil may reach very high soil temperatures at the surface and become very dry. Worms and insects will move deeper into the bare soil, developing a surface zone containing few active organisms. Many small microorganisms, such as bacteria and fungi that live in thin films of water, will die or become inactive, slowing the natural process of organic matter cycling. Large and small organisms function better in a soil that is well protected by crop residue cover, a mulch, or a sod, which helps maintain good soil aggregation. An absence of both erosion and the forces that cause compaction helps maintain good surface aggregation. WATER INFILTRATION, RUNOFF, AND EROSION As rainfall reaches the ground, most water either infiltrates into the soil or runs off the surface (some may stand in ruts or depressions before infiltrating or evaporating). The maximum amount of rainwater that can enter a soil in a given time, called infiltration capacity, is influenced by the soil type, soil structure, and the soil moisture at the start of the rain. Early in a storm, water usually enters a soil readily. When the soil becomes more moist, it can soak up less water. If rain continues, runoff is produced due to the soil's reduced infiltration rate. When an intense storm hits an already saturated soil, runoff occurs very rapidly, because the soil's ability to absorb water is low. Rainfall or snowmelt on frozen ground generally poses even greater runoff concerns, as pores are blocked with ice. Runoff happens more readily with poorly structured soils, because they have fewer large pores to quickly conduct water downward. As soil tilth degrades, water infiltration decreases, producing more runoff. Runoff water concentrates into tiny streams, which loosen soil particles and take them downhill. As runoff water gains more energy, it scours away more soil. Runoff also carries agricultural chemicals and nutrients, which end up in streams, lakes, and estuaries. Soil degradation in many of our agricultural and urban watersheds has resulted in increased runoff during intense rainfall and increased problems with flooding. Also, the lower infiltration capacity of degraded soils reduces the amount of water that is available to plants, as well as the amount that percolates through the soil into underground aquifers. This underground water feeds streams slowly. Watersheds with degraded soils experience lower stream flow during dry seasons due to low groundwater recharge and increased flooding during times of high rainfall due to high runoff. Soil erosion is the result of exposing the soil directly to the destructive energy of raindrops and wind. Soil erosion has long been known to decrease soil quality, and at the same time cause sedimentation in downstream or downwind areas. Soil is degraded when the best soil material -- the surface layer -- is removed by erosion. Erosion also selectively removes the more easily transported finer soil particles. Severely eroded soils, therefore, become low in organic matter and have less favorable physical properties, leading to a reduced ability to sustain crops and increased potential for harmful environmental impacts. Figures 6.4 Effects of tillage erosion on soils. Photo by NRCS. Some ancient farming civilizations recognized soil erosion as a problem and developed effective methods for runoff and erosion control. Evidence of ancient terracing methods are apparent in various parts of the world, notably in the Andean region of South America and in Southeast Asia. Other cultures effectively controlled erosion using mulching and intercropping, thereby protecting the soil surface from the elements. (Some ancient desert civilizations, such as the Anasazi in the Southwestern U.S. and the Nabateans in the Middle East, took advantage of surface runoff to harvest water to grow crops in downhill depressions. Their methods, however, were specific to very dry conditions.) For most agricultural areas of the world today, water and wind erosion still cause extensive damage (including the spread of deserts) and remain the greatest threat to agricultural sustainability. Tillage degrades land even beyond promoting water and wind erosion by exposing soil to the elements. It can also cause erosion by directly moving soil down the slope to lower areas of the field. In complex topographies such as seen in figure 6.4 this ultimately results in the removal of surface soil from the knolls and its deposition in depressions (swales) at the bottom of the slopes. What causes this type of tillage erosion? Gravity causes more soil to be moved by the plow or harrow down slope than up slope. Soil is thrown farther down slope when tilling in the down slope direction than is thrown uphill when tilling in the up slope direction (figure 6.5a). Down slope tillage typically occurs at greater speed than when traveling uphill, making the situation even worse. Tillage along the contour also results in down slope soil movement. Soil lifted by a tillage tool comes to rest at a slightly lower position on the slope (figure 6.5b). A more serious situation occurs when using a moldboard plow along the contour. This is typically performed by throwing the soil down the slope, as better inversion is obtained, than by trying to turn the furrow up the slope (figure 6.5c). Figure 6.5 Three causes of erosion resulting from tilling soils on slopes. Soil loss from slopes due to tillage erosion can far exceed losses from water or wind erosion. On the other hand, tillage erosion does not generally result in off-site damage, because the soil is merely moved from higher to lower positions within a field. However, it is another reason to reduce tillage on sloping fields! Soil Compaction A soil becomes more compact, or dense, when aggregates or individual particles of soil are forced closer together. Soil compaction has various causes and different visible effects. Three types of soil compaction may occur (figure 6.6): surface crusting plow layer compaction subsoil compaction Surface crusting occurs when soil is unprotected by surface residue or a plant canopy and the energy of raindrops disperses wet aggregates, pounding them together into a thin, but dense, surface layer. The sealing of the soil reduces water infiltration and the surface forms a hard crust when dried. If the crusting occurs soon after planting, it may delay or, in some cases, prevent seedling emergence. Even when the crust is not severe enough to limit germination, it can reduce water infiltration. Soils with surface crusts are prone to high rates of runoff and erosion (see figure 4.4 in chapter 4). You can reduce surface crusting by leaving more residue on the surface and maintaining strong soil aggregation. Figure 6.6 Plants growing in a) soil with good tilth and b) soil with all three types of compaction. Plow layer compaction compaction of the entire surface layer has probably occurred to some extent in all intensively worked agricultural soils. There are three primary causes for such compaction erosion, reduced organic matter levels, and forces exerted by field equipment. The first two result in a reduced supply of sticky binding materials and a subsequent loss of aggregation. Compaction of soils by heavy equipment and tillage tools is especially damaging when soils are wet. To understand this, we need to know a little about soil consistence, or how soil reacts to external forces. At very high water content, a soil may behave like a liquid (figure 6.7) and simply flow as a result of the force of gravity as with mudslides during excessively wet periods. At slightly lower water contents, soil can be easily molded and is said to be plastic. Upon further drying, the soil will become friable it will break apart rather than mold under pressure. Figure 6.7 Soil consistency states for a sand and a clay soil (friable soil is best for tillage). The point between plastic and friable soil, the plastic limit, has important agricultural implications. When a soil is wetter than the plastic limit, it is seriously compacted if tilled or traveled on, because soil aggregates are pushed together into a smeared, dense mass. This is why you often see smeared cloddy furrows or deep tire ruts in a field (figure 6.8). When the soil is friable (the water content is below the plastic limit) it breaks apart when tilled and aggregates resist compaction by field traffic. This is why the potential for compaction is so strongly influenced by timing of field operations. Figure 6.8 Deep tire ruts in a hay field after liquid manure was applied when soil was wet and plastic. A soil's consistency is strongly affected by its texture (figure 6.7). For example, as coarse-textured sandy soils drain, they rapidly change from being plastic to friable. Fine-textured loams and clays need longer drying periods to lose enough water to become friable. This extra drying time delays field operations. Surface crusting and plow layer compaction are especially common with intensively tilled soils. This is often part of a vicious cycle in which a compacted soil tills up very cloddy (figure 6.9a), and then requires extensive secondary tillage and packing trips to create a satisfactory seedbed (figure 6.9b). Natural aggregates break down and organic matter decomposes in the process contributing to more compaction in the future. Although the final seedbed may be ideal at the time of planting, rainfall shortly after planting may cause surface sealing and further settling (figure 6.9c), because few sturdy aggregates are present to prevent the soil from dispersing. The result is a dense plow layer with a crust at the surface. Some soils may hardset like cement, even after the slightest drying, slowing plant growth. Although the soil becomes softer when it re-wets, this provides only temporary relief to plants. Figure 6.9a ) Stage 1: Cloddy soil after tillage makes for a poor seedbed. Figure 6.9b ) Stage 2: Soil is packed and pulverized to make a fine seedbed. Figure 6.9c ) Stage 3: Raindrops disperse soil aggregates, forming a surface crust. Figure 6.9 Three tilth stages for a compacted soil. Subsoil compaction compacted soil below the normally tilled surface layer is usually called a plow pan, although it's commonly caused by more than just plowing. Subsoil is easily compacted, because it is usually wetter and tends to be naturally dense, higher in clay content, lower in organic matter, and have naturally lower aggregation than topsoil. Also, subsoil is not loosened by regular tillage and cannot easily be amended with additions of organic materials, so its compaction is more difficult to manage. Check Before Tilling To be sure that a soil is ready for equipment use, you can do the simple "ball test" by taking a handful of soil from the lower part of the plow layer and trying to make a ball out of it. If it molds easily and sticks together, the soil is too wet. If it crumbles readily, it is sufficiently dry for tillage or heavy traffic.   Figure 6.10 Tractor wheels in open furrow during plowing compacts subsoil.   Figure 6.11 Forces of heavy loads are transferred deep into the soil, especially when wet. Subsoil compaction is the result of either direct loading or the transfer of forces of compaction from the surface. Direct loading occurs by the pressure of a tillage implement, especially a plow or disk, pressing on the subsoil. It also occurs when a field is moldboard plowed and a set of tractor wheels is placed in the open furrow, thereby directly compacting the soil below the plow layer (figure 6.10). Subsoil compaction also occurs when farmers run heavy vehicles with poor weight distribution. The load exerted on the surface is transferred into the soil along a cone-shaped pattern (figure 6.11). With increasing depth, the force of compaction is distributed over a larger area, thereby reducing the pressure in deeper layers. When the loading force at the surface is small, say through foot traffic or a light tractor, the pressure exerted below the plow layer is minimal. But when the load is high, the pressures at depth are sufficient to cause considerable soil compaction. When the soil is wet, compaction forces near the surface are more easily transferred to the subsoil. Clearly, the most severe compaction damage to subsoils occurs by heavy vehicle traffic during wet conditions. CONSEQUENCES OF COMPACTION As compaction pushes soil particles closer together, the soil becomes more dense and pore space is lost. When the bulk density increases during compaction, mainly larger pores are eliminated. Loss of aggregation from compaction is particularly harmful for fine and medium-textured soils that depend on these pores for good infiltration and percolation of water, as well as air exchange with the atmosphere. Although compaction can damage coarse-textured soils, these soils depend less on aggregation, because pores between many of their particles are sufficiently large to allow good water and air movement. Compacted soil becomes hard when dried and can restrict root growth and the activity of soil organisms. The resistance to penetration, called soil strength, for a moist, high-quality soil is well-below the critical level (300 pounds per square inch (psi)), when root growth ceases for most crops. As the soil dries, its strength increases, but may not exceed the critical level for most (or all) of the moisture range. A compacted soil has a very narrow water content range for good root growth. It's harder in the wet range where it may even be above the critical level, depending on the severity of compaction. When it dries, a compacted soil hardens quicker than a well-structured soil, rapidly reaching a hardness well above the 300 psi level that restricts root growth. Actively growing roots need pores with diameters greater than about 0.005 inch, the size of most root tips. Roots must enter the pore and anchor themselves before continuing growth. Compacted soils that have few or no large pores don't allow plants to be effectively rooted limiting growth and water and nutrient uptake. Some Crops More Sensitive Than Others Compaction doesn't affect all crops to the same extent. An experiment in New York found that direct-seeded cabbage and snap beans were more harmed by compaction than cucumbers, table beets, sweet corn, and transplanted cabbage. Much of the compaction damage was caused by secondary effects, such as prolonged soil saturation after rain, reduced nutrient availability or uptake, and greater pest problems. What happens when root growth is limited? The root system will probably have short thick roots and few fines ones or root hairs (see figure 6.6). The few existing thick roots are able to find some weak zones in the soil, often by following crooked patterns. These roots have thickened tissue and are not efficient at taking up water and nutrients. In many cases, roots in degraded soils do not grow below the tilled layer into the subsoil (see figure 6.6) it's just too dense and hard for them to grow. Deeper root penetration is especially critical under rain-fed agriculture. The limitation on deep root growth by subsoil compaction increases the probability of yield losses from drought stress. There is a more direct effect on plant growth beyond reduced root growth that limits the volume of the soil that is used for water and nutrient supply. A root system that's up against mechanical barriers sends a chemical signal to the plant shoot, which then slows down respiration and growth. This seems to be a natural survival mechanism similar to when plants experience water stress. In fact, because some of the same hormones are involved and mechanical resistance increases when the soil dries you usually can't separate the effects of compaction from those of drought. THE WATER RANGE FOR BEST PLANT GROWTH The limitations to plant growth caused by compaction and water extremes can be combined into the concept of the optimum water range for plant growth the range of water contents for which neither drought, mechanical stress, nor lack of aeration reduces plant growth (figure 6.12). This range, referred to by scientists as the least-limiting water range, is bounded on two sides when the soil is too wet and when it's too dry. Figure 6.12 The optimum water range for crop growth for two different soils. The optimum water range in a well-structured soil has its field capacity on the wet end, as water above this water content readily drains out by gravity. On the dry end is the wilting point beyond which the soil holds water too tightly to be used by plants. However, the soil water range for best growth in a compacted soil is much narrower. A severely compacted soil at field capacity is still too wet because it lacks large pores and is poorly aerated even after the soil drains. Good aeration requires about 20 percent of the pore space (about 10 percent of the volume of the whole soil) to be air-filled. On the dry end, plant growth in a compacted soil is commonly limited by soil hardness rather than by lack of available water. Plants in compacted soils experience more stress during both wet and dry periods than plants in soils with good tilth. The effects of compaction on crop yields usually depend on the length and severity of excessive wet or dry periods and when those periods occur relative to critical times for plant growth. Sources Hillel, D. 1991. Out of the Earth: Civilization and the Life of the Soil. University of California Press. Berkeley, CA. Letey, J. 1985. Relationship between soil physical properties and crop production. Advances in Soil Science 1:277­294. Ontario Ministry of Agriculture, Food, and Rural Affairs (OMFARA). 1997. Soil Management. Best Management Practices Series. Available from the Ontario Federation of Agriculture, Toronto, Ontario (Canada). da Silva, A.P., B.D. Kay, and E. Perfect. 1994. Characterization of the least limiting water range of soils. Soil Science Society of America Journal 58:1775­1781. Unger, P.W., and T.C. Kaspar. 1994. Soil compaction and root growth: a review. Agronomy Journal 86:759­766. Soehue, W. 1958. Fundamentals of pressure distribution and soil compaction under tractor tires. Agricultural Engineering 39:276­290. Top