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Soil fertility is one of those concepts that is often thrown about comfortably but can quickly become rather vague when you look at it closely. In this way it is much like the concept of personal health, and in fact “soil health” is a popular if somewhat dated term that is usually analogous to soil fertility. To ask if a particular location has fertile soil is much like asking if a particular person is healthy—if there are no obvious broken bones, eroded gullies, or other clear symptoms of failure, the most that you can say is that it looks good but you might be missing something important. For example, someone could have very low cholesterol but an unobserved brain tumor, just as a field could have high levels of necessary plant nutrients but toxic levels of a heavy metal. Similarly, beginning to take vitamin C daily might make you healthier, presuming of course that you don’t unknowingly have cancer, just as adding nitrogen fertilizer to a field might make it more fertile, presuming that there isn’t a serious calcium deficiency limiting plant growth. None of this ambiguity is a problem—after all, the field of medicine seems to be advancing just fine—but is simply the result of soil fertility, like personal health, being an emergent general property rather than a specific characteristic of a system.
Just as conversation with a doctor quickly moves beyond “health,” the question of whether the soil in a particular field is fertile or not inevitably leads to more specific questions about measurable soil characteristics that can serve as indicators of fertility. And just as a doctor might begin with questions of genetics, age, and lifestyle habits, some general insights into soil fertility can be inferred from knowing the geologic and climatic history and the topographic characteristics of a location. For example, soil that results from the weathering of volcanic rock is often highly fertile, whereas soil formed from weathered limestone or sand typically is not. Similarly, younger soils generally have higher soil fertility than otherwise equivalent older soils, due to the loss of some critical nutrients through leaching and excessive physical weathering of soil particles. These latter processes are accelerated under high temperature and precipitation, which define the global wet tropics. While soil fertility in a specific area can be highly variable due to numerous local factors, these underlying variables account for some broad geographic patterns and can provide some initial insights. For example, undisturbed soils on slopes in the humid tropics that are not of recent volcanic origin often consist of a nutrient rich organic surface layer on top of a nutrient poor clay layer. In this situation, plant nutrients are naturally limited, and their availability is dependent on the continual input and rapid decomposition of plant material. Removing the native vegetation and cultivating this soil can therefore be expected to quickly exhaust the locally available nutrients, which are also at high risk of loss through erosion. In contrast, a temperate river valley might have accumulated tens of feet of nutrient-rich topsoil that will remain highly fertile under continuous cultivation despite little or no additional inputs. When it comes a specific field, a farmer or agricultural scientist has an invaluable initial shortcut for estimating the local soil fertility, which is simply to look at the current vegetative growth in a field. Vigorous growth by the crop or by diverse nutrient-demanding species prior to cultivation is a good indicator that the soil is quite fertile (but also that competition from weeds will likely be a major problem). In contrast, yellowed leaves, stunted growth, or a predominance of “pioneer” species with low nutrient requirements might suggest that fertility is poor for some reason, such as low nutrient levels. A lack of any vegetation at all without any clear explanation such as recent disturbance is something like the lack of a heartbeat and is a strong indication that things aren’t looking good. This sort of visual analysis can be highly pragmatic, such as when shifting cultivators use the density of a forest stand as an indicator of the underlying soil fertility, but this approach can also be highly nuanced. For example, the nutrient analysis of vegetative growth of a crop is critical for nutrient management strategies in intensive high input agriculture, particularly for perennial crops that are highly sensitive to specific nutrient levels, such as most fruit and nut crops. Such analysis can be particularly informative as the uptake of plant nutrients can be surprisingly independent of the overall levels of those nutrients in the soil due to the complexity of the relevant biogeochemical cycles. The next level of soil fertility analysis focuses on major controlling variables within the soil ecosystem or other emergent system properties that can often be broadly associated with relative fertility levels. The most common general but quantifiable measure of soil fertility is the relative level of soil organic matter, which is a broad term for all the living and dead material in the soil and includes microbes, plant roots, gophers, etc., as well as their constituent organic compounds in various states of decomposition. This hugely diverse category typically makes up 1-5% of the dry mass of most agricultural soils, with mineral particles making up the remainder. Organic compounds can be a direct source of plant nutrients when sufficiently decomposed, but increasing the level of soil organic matter can also have numerous indirect effects on the soil, such as improving water and nutrient retention, pH buffering capacity, microbial diversity and activity, and aggregation of mineral particles and organic compounds into larger complexes. The elemental composition of organic matter in the soil is usually around 50% carbon (the rest is hydrogen, oxygen, nitrogen, and a wide range of other plant nutrients). Soil carbon can be measured more precisely than organic matter in total, so the concentration of soil organic carbon is often used as a proxy for levels of soil organic matter. As soil organic carbon is an important pool in the global carbon cycle, storing an estimated 2-3 times more carbon than the atmosphere, increasing soil organic matter is widely seen as a rare “win-win” approach that has direct agricultural and environmental benefits. Soil processes are strongly influenced by not just the level of organic matter relative to mineral particles, but also by the way that these elements are organized in the soil, which is known as the soil structure. This organization is heavily dependent on the soil texture, which is the relative proportion of particles in various size classes (clay, silt, and sand), but is also influenced by the soil organic matter and the specific history of a field, particularly with regards to cultivation, precipitation, and previous vegetation. This organization of the soil matrix determines the size and distribution of pore spaces and thereby the movement and retention of air and water in the soil, which are two critical variables driving chemical and biological soil processes. For example, clay soils consist of primarily very fine mineral particles, and therefore have the potential to become highly compacted in a way that sandy soils never could. When they do, it is difficult for water and air to move into and through the soil matrix, leading to increased flooding and erosion at the surface and anaerobic and saturated conditions at depth. This is particularly a problem when organic matter levels are low and there has been no soil disturbance, and the same clay soil can become well structured through physical cultivation and the addition of organic amendments. Soil structure, like soil fertility, is often discussed as an emergent system property but then measured through more specific soil characteristics such as bulk density, pore volume, water holding capacity, and water infiltration rate. The traditional concept of soil tilth is essentially a tacit and qualitative measure of soil structure. The third primary controlling variable is the soil pH, which is the measure of the concentration of hydrogen ions (H+) in the water within pore spaces and on the surfaces of soil particles. This water is known as the soil solution and these aqueous environments are the site of most biological and chemical activity in the soil. Hydrogen ions (which are actually found in the form of hydronium ions, H3O+) play an important role in many chemical reactions so changes in this concentration drive these reactions towards new equilibriums. This changes the relative abundance of different chemical forms of plant nutrients, which can radically change the bioavailability of those nutrients despite the overall levels in the soil remaining constant. For example, at both high and low pH, soil phosphorus is chemically "fixed" into forms that cannot be taken up by plants. Hydrogen ions also play an important role in biological processes, such as the movement of water and nutrients across cell membranes, and soil pH has recently been identified is the most important global environmental variable controlling microbial communities. In addition to the static pH of a soil, it is also important for the soil to be able to buffer rapid changes in pH, such as the slightly acidic inputs that come with precipitation and organic matter decomposition. This pH buffering capacity is influenced by the level of organic matter, the soil texture, and specific chemical composition of the soil, such as the amount of aluminum and carbonates. Soil fertility can be assessed through numerous other more specific measures, the most obvious being levels of specific nutrients in the soil. Plants require a balance of nutrients to support growth, and these necessary nutrients are commonly divided into those that are required in relative abundance, such as nitrogen, phosphorus, and potassium, and those that are required only in trace amounts, such as manganese, iron, and zinc. The first are called “macronutrients” and the second “micronutrients” and the relative overall requirement varies widely among plant types, growth stages, and environmental conditions. While it is something of a conceptual simplification, plant growth is limited by the first of the necessary nutrients to become scarce, just as construction of a building stops once you run out of nails no matter how much lumber you might have available. As a result, while the field might appear fertile by most measures—high organic matter, good structure, and sufficiently neutral pH—it might be relatively short of some critical nutrient. In most agricultural systems, nitrogen is the most limiting nutrient, followed by phosphorus and potassium, but in some cases missing micronutrients can dramatically reduce yield, such as is again often the case with perennial tree crops such as fruits and nuts. This can also result from the underlying soil type, such as if the geologic parent material is naturally low in some critical nutrients, or if certain nutrients are biologically unavailable due to the specific chemical composition of the soil. |
Selina Wale, an agricultural pioneer in the Solomon Islands, in the garden that she built on a solid rock outcropping. She established this garden using some soil that she brought in from off-site, but she maintains soil fertility sufficient for continuous production through amendment with composted kitchen scraps and other plant materials.
Silas Kere, another pioneering farmer in the Solomon Islands, standing in a field of cassava under no-input "slash-and-remove" management with less than one year fallow periods (above). Cassava planted at the same time in his garden (below) benefits from nearly twenty years of compost amendment and legume cover crop rotations. These fields are ten feet apart.
Casper and myself discussing soil texture, structure, and pH, along with mechanisms of nutrient retention and release, pathways of water and air movement, and the physical benefits of increased aggregation--all courtesy of betel nut props.
Dissecting one of Casper's early experiments at developing an intensive soil management system (above). Three months previously he had removed roughly two feet of clay soil and added layers of plant residue at two feet, one foot, and on the surface. We found that the upper and middle (below, top) layers had decomposed nicely, but the bottom layer was too sealed off by the clay to allow for significant aerobic decomposition (below, bottom).
Organic materials tend to break down slowly and continue to release nutrients and improve the soil structure long after their incorporation into the soil. This picture is taken of the garden bed shown at the bottom of the page, two years after the initial preparation.
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Getting a sense of the relative soil fertility of a particular field is of course only the first step—the next is to make some management decisions to improve it. There is a huge diversity of specific practices to manage soil fertility, but they can be broadly understood as falling into four general approaches.
1) The soil can be amended with material from off-site. These inputs are typically nutrient-rich, such as compost or synthetic fertilizers, but can also be added primarily to change the chemical or physical nature of the soil, such as would be the case with adding lime or fire ash to raise the pH or mulch to reduce evaporation and erosion.
2) The fertility can be improved through short-term in-field management practices, particularly through physical cultivation. While tillage alone doesn’t directly increase the nutrient levels of the field, the physical implications of cultivation radically improves the biological growing conditions for roots and microorganisms, which can also indirectly increase the availability of some nutrients.
3) Long term management practices, such as crop rotations, are often a critical part of agricultural management plans. The reason for these rotations is sometimes simply to break up pest and pathogen life-cycles, but the previous use of a field also has implications for soil fertility and can be managed accordingly. For example, improved shifting cultivation practices include managing the fallow vegetation for nitrogen-fixing legumes or species that are known to provide good mulch or erosion control. When livestock are spatially integrated with crop production, rotating them annually through fields can provide a nutrient input through the addition of manure and urine. In more intensive cropping systems, some species can be planted as cover crops that will not be directly harvested but rather will provide benefits to a subsequent crop. Examples of these include grass and herbaceous species, which can reduce the loss of soil nutrients and increase soil organic matter, and legumes, which can also fix atmospheric nitrogen. Other cover crops serve more specific functions, such as deep-rooted radishes that penetrate compacted subsurface layers and leave channels for air and water to follow when they are left to rot in the field.
4) The soil fertility of a particular field can be influenced by management of the surrounding landscape. While it would be unusual perhaps to say that drainage ditches were dug, windbreaks planted, and terraced carved to improve soil fertility, these common practices can have important implications for fertility management plans and are often done with such effects in mind. On a very broad scale, landscapes themselves can be managed to control the flow of nutrients and water. This is most obvious in agricultural watersheds, but full landscape management is typically more of a concept or a recommendation than a practice.
1) The soil can be amended with material from off-site. These inputs are typically nutrient-rich, such as compost or synthetic fertilizers, but can also be added primarily to change the chemical or physical nature of the soil, such as would be the case with adding lime or fire ash to raise the pH or mulch to reduce evaporation and erosion.
2) The fertility can be improved through short-term in-field management practices, particularly through physical cultivation. While tillage alone doesn’t directly increase the nutrient levels of the field, the physical implications of cultivation radically improves the biological growing conditions for roots and microorganisms, which can also indirectly increase the availability of some nutrients.
3) Long term management practices, such as crop rotations, are often a critical part of agricultural management plans. The reason for these rotations is sometimes simply to break up pest and pathogen life-cycles, but the previous use of a field also has implications for soil fertility and can be managed accordingly. For example, improved shifting cultivation practices include managing the fallow vegetation for nitrogen-fixing legumes or species that are known to provide good mulch or erosion control. When livestock are spatially integrated with crop production, rotating them annually through fields can provide a nutrient input through the addition of manure and urine. In more intensive cropping systems, some species can be planted as cover crops that will not be directly harvested but rather will provide benefits to a subsequent crop. Examples of these include grass and herbaceous species, which can reduce the loss of soil nutrients and increase soil organic matter, and legumes, which can also fix atmospheric nitrogen. Other cover crops serve more specific functions, such as deep-rooted radishes that penetrate compacted subsurface layers and leave channels for air and water to follow when they are left to rot in the field.
4) The soil fertility of a particular field can be influenced by management of the surrounding landscape. While it would be unusual perhaps to say that drainage ditches were dug, windbreaks planted, and terraced carved to improve soil fertility, these common practices can have important implications for fertility management plans and are often done with such effects in mind. On a very broad scale, landscapes themselves can be managed to control the flow of nutrients and water. This is most obvious in agricultural watersheds, but full landscape management is typically more of a concept or a recommendation than a practice.
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The general method that Casper settled on for dealing with the acidic clay hard pack of the old playing field is as follows (clockwise from above). First, since improving the compaction will increase the volume of the soil, logs are brought in to create borders for what will become raised beds. This also greatly reduces the risk of water-logging or erosion. A fork, hoe, or even a stick is then used to break up the clay hard pack. Organic material--cacao pods, leaves, cut grass, charcoal, or whatever else is available--is then added to the soil and worked in with a hoe (an improvement on the earlier layering method). Fire ash may also be added at this time. The bed is then covered with palm fronds to maintain a moist soil environment and left for 2-3 months for the organic material to decompose. The finished bed can then either be mixed again by hand, or directly planted into. In the latter case, additional fire ash, which can damage plants if it is applied in high concentrations, may then be sprinkled on top of the fronds, where it slowly reaches the soil in beneficial doses. With dramatically improved soil organic matter levels, soil structure, and soil pH, the finished bed is extremely fertile and will remain so for numerous plantings, with the fertility maintained in the long run through additional nutrient rich amendment, mulching, and the use of legume cover crops.
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