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本帖最后由 喵了咪大王 于 2016-11-2 18:43 编辑
土壤的物理、化学特性，说的很清楚，及空气、水、肥与植物、土壤如何相互作用，所以怎么浇水、上肥，改良土壤。颗粒是怎么与肥料作用的，所以应该怎么上肥，甚至连土壤空隙有多大，应有多少比例，怎么测量改进都说的很清楚。缅因州大学资料。
Soiland Plant Nutrition: A Gardener’s Perspective
Written by Dr. Lois Berg Stack, ExtensionProfessor (2011). Revised by Dr. Lois Berg Stack, Extension Professor, and MarkHutchinson, Extension Professor (2012). Revised by Dr. Lois Berg Stack,Extension Professor (2016)
Note to readers: This document containsmany common soil science terms. Understanding these terms, which are underlinedin the text, will help you understand soils as you read gardening books.
Soilis a dynamic three-dimensional substance that covers some of the world’s landsurface. It varies from place to place, in response to the five factors thatform it: climate, topography, organisms, the parent rock below surface, andtime. Our Maine soils developed since the last glacier moved across the region,largely in response to the parent rock (largely granite) and topography. MostMaine soils are acidic, and have a somewhat depressed ability to hold andexchange nutrients used by plants. Our native plants evolved in this system,and are well adapted to Maine soils. However, we often amend Maine soils byadding organic matter, lime and/or fertilizer, in order to increase theproductivity of our food and landscape plants.
Soil performs four major functions:
1.It provides habitat for fungi, bacteria,insects, burrowing mammals and other organisms;
2.It recycles raw materials and filterswater;
3.It provides the foundation forengineering projects such as buildings, roads and bridges; and
4.It is a medium for plant growth. Thistext focuses on this last function.
What does soil do for plants?
Soil supports plant growth by providing:
1.Anchorage: root systems extend outwardand/or downward through soil, thereby stabilizing plants.
2.Oxygen: the spaces among soil particlescontain air that provides oxygen, which living cells (including root cells) useto break down sugars and release the energy needed to live and grow.
3.Water: the spaces among soil particlesalso contain water, which moves upward through plants. This water cools plantsas it evaporates off the leaves and other tissues; carries essential nutrientsinto plants; helps maintain cell size so that plants don’t wilt; and serves asa raw material for photosynthesis, the process by which plants capture lightenergy and store it in sugars for later use.
4.Temperature modification: soil insulatesroots from drastic fluctuations in temperature. This is especially importantduring excessively hot or cold times of year.
5.Nutrients: soil supplies nutrients, andalso holds the nutrients that we add in the form of fertilizer.
Physical properties of soil
Texture: Soil is composed of both minerals(derived from the rock under the soil or transported through wind or water) andorganic matter (from decomposing plants and animals). The mineral portion ofsoil is identified by its texture. Texture refers to the relative amounts ofsand, silt and clay in the soil. These three terms refer only to particle size,not to the type of mineral that comprises them. Sand is familiar to most of us,and is the largest textural soil size. Sand grains can be seen with the nakedeye or with a hand lens. Sand provides excellent aeration and drainage. Ittills easily and warms up rapidly in spring. However, it erodes easily, and hasa low capacity for holding water and nutrients. Clay particles are so smallthat they can only be seen through an electron microscope. Clay soils containlow amounts of air, and water drains slowly through them. Clay is difficult totill, and warms up slowly in spring. But, it tends to erode less quickly thansand, and it has a high capacity for holding water and nutrients. Silt is sizedbetween sand and clay. Individual silt particles can be seen through alower-power microscope. It has intermediate characteristics compared to sandand clay.
Most soils contain all three particle sizes(sand, silt, clay). Loam is a term that is often used generally to refer tosoils that are a mixture of sand, silt and clay. Most of our topsoils areloams. However, “loam” can varyfrom a rather equal mixture of the three textural sizes, to a mixture dominatedby sand or silt or clay. As a gardener, you should inspect loam beforepurchasing it, because these variations affect management practices.
Structure: Sand is often found asindividual particles in a soil, but silt and clay are almost always clumpedinto larger units called aggregates. The manner of this aggregation defines asoil’s structure. Soil structure is described by terms such as blocky, platy,prismatic and angular. Productive topsoils often have a granular soilstructure. The size and shape of aggregates is influenced by mineral type,particle size, wetting and drying, freeze/thaw cycles, and root and animalactivity. Decomposed organic matter, plant sugars excreted from roots, wasteproducts of soil microbes, and added soil conditioners all act to cementparticles into aggregates. However, aggregates can break apart from tilling,compaction, and loss of organic matter in the soil. Soil structure is a verydynamic process. Good soil structure increases the pore space (see below) thatsupports root penetration, water availability and aeration.
Pore space: Soil particles rarely fittogether tightly; they are separated by spaces called pores. Pores are filledwith water and/or air. Just after a heavy rainfall or irrigation event, porespaces are nearly 100% filled with water. As time goes by, the water passesthrough the soil due to gravity, or evaporates into the air, or is used byplant roots, and more of the pore spaces are filled by air. Particles of clayfit tightly, and have very little pore space to hold air and water. On theother hand, sand on a beach has such a large amount of large pores that itdrains too quickly to grow most plants in.
Pore space generally occupies 30-60% oftotal soil volume. A well-structured soil has both large pores (macropores) andtiny pores (micropores); this provides a balance of the air and water thatplants need. Macropores provide for good drainage, and micropores hold waterthat plants can access. This helps explain how you can achieve a “well-drained but moist soil”.
Organic matter (OM) is previously livingmaterial. On the soil surface, there is usually rather un-decomposed OM knownas litter or duff (or, mulch in a landscape). This surface layer reduces theimpact of raindrops on the soil structure, prevents erosion, and eventuallybreaks down to supply nutrients that leach into the soil with rainfall orirrigation. In the soil, OM decomposes further until it becomes humus, a stableand highly decomposed residue. Humus is an important nutrient source forplants, and it is important in aggregating soil particles.
OM is always in the process of decomposing,until it becomes humus. OM levels are reduced through cropping and can bereplenished by adding compost or manure, or crop residues, or green manure(crops such as buckwheat, clover or ryegrass that are grown as cover crops andthen tilled into the soil). Soil OM can be conserved with reduced tillagepractices, such as no-till. OM improves water retention, making it a goodaddition to sandy soil. OM is also added to clay or silt soils to increaseaggregation and thereby improve drainage. OM provides nutrients as itdecomposes, buffers the pH of the soil solution (see below) against rapidchemical changes, and improves soils’ cation exchange capacity (see below).
Good horticultural soil: Most soils aredominated by mineral particles; some are dominated by organic matter. Somesoils have a high percentage by volume of pore space, while others have littlepore space. Your soil might vary from one part of your land to another.Ideally, a “good horticultural soil” contains 50% solid material (mostly mineral soil plus 5-10% organicmatter) and 50% pore space. At any given time, that pore space is occupied byboth air and water. You can assess your soil by irrigating heavily, thenallowing it to drain for a day. After a day of drainage, the pore space shouldcontain about 50% water and 50% air. If the soil is very dry after a day ofdrainage, it is likely dominated by sand, and you could amend it over time byadding OM. If the soil remains very wet, it is likely dominated by clay or itis not well aggregated; you could amend such a soil over time by adding OM tosupport aggregation.
Chemical properties of soil
Soil chemical activity is related toparticle size, because chemical reactions take place on particle surfaces.Small particles have much more surface area than large particles. Small soilparticles play a big role in two chemistry-related processes: managing soilacidity (pH), and supporting the soil’s ability to hold nutrients (CEC).
First, it’s important to know thatfertilizers are salts. When salts dissolve into the soil solution, theyseparate into a cation (a positively charged ion) and an anion (a negativelycharged ion). For example, when we dissolve table salt (sodium chloride) inwater, it separates into positively charged sodium and negatively chargedchloride ions. When we add sodium nitrate fertilizer to the soil, it dissolvesinto the soil solution as sodium cations and nitrate anions.
Tiny particles (humus and clay) are veryimportant for holding plant nutrients in the soil. Clay and humus particleshave a negative surface charge. Cations are positively charged. Becauseopposites attract, the clay and humus hold cations, and prevent them from beingleached out of the soil by water movement. Negatively charged anions remain dissolvedin the soil solution, and are very susceptible to leaching downward.
Nitrogen is an interesting nutrient,because one nitrogen fertilizer might be positively charged ammonium that isheld by soil particles, while another nitrogen fertilizer might containnegatively charged nitrates that remain dissolved in the soil solution. Thisexplains why nitrates, which are anions, leach readily out of our topsoil andsometimes into our water supply. It also explains why “slow-releasefertilizers” usually contain ammonium, which can beheld by the soil particles and gradually converted to the nitrate form thatmost plants use readily.
Cation exchange capacity (CEC) is anexpression of the soil’s ability to hold and exchange cations. Ions areconstantly exchanged among the soil solution, CEC sites on clay and humusparticles, and plant roots. This is not a random process, but is dependent onelectron charge. Clay and humus have high CECs because they are tiny particleswith very large surface-to-volume ratio, with many negative sites that canattract cations. Sand has very low CEC because sand particles are large, withlow surface-to-volume ratio and hence fewer negative sites. A gardener can addhigher rates of fertilizer less frequently when gardening in a soil with a highlevel of clay or humus, compared to a sandy soil, because cations (potassium,calcium, magnesium and others) are held by soil particles. Because a sandy soilcannot hold the same amount of cations, fertilizing them more frequently withsmaller amounts of fertilizer is a better option.
pH: pH is a description of the soil’sacid/alkaline reaction. The pH scale ranges from 0 (very acid) to 14 (veryalkaline). Soils generally range from pH 4.0 to pH 8.0. Northeastern forestsoils can be very acid (pH 3.5), while Western soils can be very alkaline (pH9). pH is important because it regulates the availability of individualnutrients in the soil solution.
The pH scale is logarithmic; each unit is10 times more acid or alkaline than the next. For example, a soil with pH 4.0is ten times more acid than a soil with pH 5.0, and 100 times more acid than asoil with pH 6.0. A soil’s pH depends on the parent rock (limestone isalkaline, granite is acidic), rainfall, plant materials, and other factors.Individual plants perform best within specific pH ranges. It is just asimportant to manage pH as fertility. Most garden plants perform well in a soilwith pH 6.0 – 7.0. Acid-loving plants such as rhododendron and blueberryperform well in a soil with pH below 5.0.
Living organisms in soil
Many organisms inhabit soil: bacteria,fungi, algae, invertebrates (insects, nematodes, slugs, earthworms) andvertebrates (moles, mice, gophers). These organisms play many physical andchemical roles that affect plants. For example, their secretions help dissolveminerals, making them available to plants; some organisms convert inorganicsubstances into other forms that are more or less available to plants;organisms add OM to the soil; organisms help decompose OM; many organismsaerate the soil. Some living organisms in the soil cause diseases, some feed onplant tissue, and many compete with plants for nutrients and water.
Rhizosphere: The very thin zone of soiljust around roots is called the rhizosphere. This zone is different from therest of the soil, and it sometimes supports specific and unique organisms. Forexample, some fungi live together with roots, to their mutual benefit; thesemycorrhizal relationships provide the fungi with a place to live, and the fungiassist in the plant’s water and nutrient uptake. Similarly, somenitrogen-fixing bacteria grow together with some plants, including many legumes(members of the bean family). The bacteria convert atmospheric nitrogen intoforms that can be used by their host plants. When the host plant dies, thenitrogen compounds released during decomposition are available to the nextcrop. Any mutually beneficial relationship between two dissimilar organisms iscalled a symbiosis.
Soil water
Water is an amazing substance. It is calledthe universal solvent because it dissolves more substances than any otherliquid. It is a renewable natural resource. It exists in nature as a solid,liquid and gas. Its molecules cohere (stick together) and adhere (stick to) toother surfaces; this accounts for its ability to reach the top of tall trees.It has a high latent heat, which means that it releases a large burst of energywhen it passes from solid to liquid and from liquid to gas. And, when it passesfrom gas to liquid and from liquid to solid, it absorbs a large burst ofenergy. Gardeners reap the benefits all of these attributes of water.
Water-holding capacity: A soil’s ability tohold water is called its water-holding capacity. Clayey soils have highwater-holding capacity, while sandy soils have low water-holding capacity. As asoil’s pore space is filled with water by heavy rainfall or irrigation, thesoil becomes saturated. Then, water gradually drains downward, and the amountof water remaining in the soil against the force of gravity is called thesoil’s field capacity. Clayey soils drain much more slowly than sandy soils.Loamy soils reach their field capacity 2-3 days after a heavy rainfall orirrigation. If no more water is added, the soil continues to dry out; plantstake up some of the water, and some water moves upward in the soil andevaporates from the surface. Eventually, a soil may dry enough to reach itspermanent wilting percentage, the point at which a plant wilts so severely thatit cannot recover. At this point, the available water (water that remainsavailable to the plant) is gone, and the only water that remains in the soil isso tightly bound to soil particles that plants cannot access it.
It’s important to understand a soil’s waterholding capacity so that we can use appropriate irrigation practices.Irrigating a heavy clayey soil and a sandy soil in the same way would result invery different results.
Soil management
Good soil management is critical for cropproductivity. Good management must include consideration of maintaining thesoil’s integrity over time. Poor management can lead to erosion, loss offertility, deterioration of soil structure, and poor crop yields.
Tilling: Mechanical manipulation of soilloosens the soil, and promotes aeration, porosity and water-holding capacity. Itallows a gardener to incorporate soil amendments such as OM and lime. On theother hand, tilling tends to decrease aggregation, causing compaction(compacted soils are dominated by few, small pores). It can take years toovercome the damage caused by overtilling.
Managing pH: Soil pH regulates theavailability of plant nutrients. pH should be managed only in response to soiltest results. Soil pH can be lowered by adding some kinds of organic matter orsulfur or sulfates; this is not often needed in Maine soils. Soil pH can beraised by adding lime or some types of fertilizer or wood ash. It is difficultto overcome the negative effects of applying excessive amounts of thesematerials. Test first!
Mulching: Mulch is a material that coversthe soil. Organic mulches such as compost, aged manure or bark chips decomposeto supply OM and nutrients in the long term. Inorganic mulches such as stone orplastic sheet materials have little effect on nutrient levels and do notcontribute OM to the soil. All mulches affect soil temperature by insulating ortransferring heat, and all mulches help soils retain moisture. Mulches may alsohelp reduce weed growth, prevent erosion and affect insect/disease presence.
Managing OM levels: In natural areas,plants and animals die, decompose and replenish OM in the soil. Each year,plant leaves deciduate and rot (compost) in place, and their nutrients and OMare added to the soil through rainfall and the freeze/thaw cycle that createscracks in the soil. On the other hand, in developed landscapes where thisnatural cycle is interrupted, gardeners must implement processes to replenishsoil OM. Leaves from deciduous trees can be left in place to decompose; plantdebris can be composted and incorporated back into gardens as OM; and plantresidue, green manures and animal manures can be incorporated directly into thesoil. Some tillage is generally required to incorporate this material into thesoil. Adding huge amounts of OM at one time can cause nutrient problems,especially if the material is not fully composted. Adding small amounts of OMperiodically can contribute to longterm soil fertility, support soilmicroflora, contribute to good soil structure, and support the soil’s abilityto hold both water and air.
Plant nutrients
Three elements, carbon, oxygen andhydrogen, are essential to plant growth and are supplied by air and water. Theother essential elements are referred to as plant nutrients, and are providedby the soil, or are added as fertilizers, and enter plants almost exclusivelythrough the roots. These plant nutrients are divided into two groups. Thoserequired by plants in large amounts are called macronutrients; these arenitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Plantmicronutrients, needed in tiny amounts include iron, chlorine, zinc,molybdenum, boron, manganese, copper, sodium and cobalt. Macronutrients andmicronutrients are all critical to normal plant growth and development; theyare simply needed in different amounts.
Organic fertilizer sources include compost,aged manure, rock phosphate, soybean meal, and fish meal. Organic fertilizercan also be “grown” by plantinga legume cover crop, which is a crop that is grown with the intention oftilling it into the soil, at which point it is referred to as a green manure.Cover crops also add OM to the soil. Inorganic fertilizer products are alsowidely available, either as single-nutrient or multi-nutrient products.
Fertilizers are labeled as slow-release orsoluble. Slow-release fertilizers provide nutrients over a period of time, asthey break down or decompose. Soluble fertilizers are fast-release, and manyare dissolved into water and then irrigated onto crops.
Nutrients can be provided by many productsand practices. Price, availability, ease of use, needed equipment, time andphilosophy should be considered when selecting the best fertilizer andapplication method for any situation. Occasionally, in severe nutrientdeficiency situations, some micronutrients are sprayed onto the foliage ofcrops, but most are applied to the soil and taken up by roots. In hydroponicproduction systems, nutrients are dissolved in water and washed over theexposed roots of plants.
Most soils have at least some residualnutrients. Only a soil test can assess this. Fertilizing without the results ofa soil test leads to a waste of money and product, and can exacerbate anexisting nutrient imbalance. In addition, sometimes nutrients are present insufficient supply but are unavailable because of too high or too low pH. A soiltest can reveal this, and a soil lab professional or crop consultant canrecommend practices to resolve such problems.
Soil and fertilizer management tips forhome gardeners
Some gardeners do not say that they garden,but rather that they work the soil. This reveals an understanding that goodsoil conditions are essential to support productive plant growth. Here are afew gardening tips related to soil management:
To amend a heavy (clayey) soil, add OM, notsand. As OM decomposes to humus, it “glues” particles together into aggregates, and improves drainage.
To amend a light (sandy) soil, add OM, notclay. OM increases sand’s ability to hold water and nutrients.
Most ornamental landscape plants (woodytrees and shrubs, and herbaceous perennials and annuals) are best fertilized inspring. Fertilizing late in the season can lead to a late-season flush ofgrowth that does not adequately harden off before winter.
Most houseplants are best fertilized at therate recommended on the product label in spring and summer, and at half thatrate in fall and winter.
Fertilize vegetable gardens by banding(place fertilizer alongside the crop row, 2” away and 2” deep in the soil) and/or by incorporating fertilizer into the soilin spring. Side-dressing supplemental nitrogen fertilizer next to growingplants later in the season may be necessary. Manage the pH of garden soil toensure good nutrient availability. Rotate vegetable crops with cover crops tomaintain good levels of organic matter, which helps the soil retain nutrientsfor plant use.
When fertilizing a lawn, determine thelevel of growth desired. If a low-maintenance lawn is desirable, no fertilizermay be needed. Slow-release fertilizers are preferred over soluble fast-releaseformulations. Apply a maximum of 2 pounds nitrogen per 1000 square feet peryear on established lawns; in most cases, apply half at spring green-up andhalf in fall (before September 15). Avoid fertilizing in midsummer. Leave anunfertilized buffer strip of at least 25 feet adjacent to lakes, streams,rivers, bays, vernal pools and wetlands. Avoid using phosphorus fertilizer if asoil test reveals phosphorus is not necessary, as phosphorus can causefreshwater quality problems. Reduce the amount of fertilizer needed by 1/3 to1/2 each year by mowing with a mulching mower. Avoid weed-and-feed products,which do not allow the option to adjust the fertilize rate.
Avoid compacting soils. Walk on paths, keepgarden carts on paths, park in the driveway rather than on the lawn, and avoidwalking on one path across a lawn when it is frozen. Never walk on saturatedsoil. Wait until the garden dries out in spring before planting.
Avoid bare soil in your vegetable garden.When a crop is harvested, replant the area with another crop or plant a covercrop. Bare ground is prone to erosion and surface compaction by raindrops.
To assess whether a soil is adequatelydrained for many landscape plants, dig a hole 6” wideand 12” deep. Fill it to the top with water and let thewater drain. Refill the hole with water, and time how long it takes to draincompletely. If it drains within 3 hours, the soil is likely sandy. If it drainsin 4-6 hours, drainage is adequate for a wide variety of plants. If some waterremains after 8 hours, the soil is likely high in clay content and the site mayretain too much moisture for some plants to thrive. | 
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发表于 2016-11-2 18:27
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