📚 9 More about Soil | 第9课 深入了解土壤
Soil is not just dirt. It is a complex mixture of weathered rock particles, humus, water, air and living organisms. In this lesson we look more closely at the chemistry and structure of soil, how it forms, why its pH matters, and how we can conserve it for farming and the environment.
土壤不只是泥土。它是风化岩石颗粒、腐殖质、水、空气和活生物体的复杂混合物。本课我们将更深入地研究土壤的化学性质和结构、土壤如何形成、为什么土壤的酸碱度很重要，以及我们如何为农业和环境保护土壤。
1. What Is Soil Made Of? | 土壤由什么组成？
Soil contains four main parts: mineral particles from broken rocks, organic matter called humus, water with dissolved ions, and air held in pore spaces. The mineral part provides structure and stores some nutrients, while humus supplies nitrogen and helps hold water.
土壤包含四个主要部分：来自岩石破碎的矿物颗粒、被称为腐殖质的有机质、含有溶解离子的水，以及存在于孔隙中的空气。矿物部分提供结构并储存一些养分，而腐殖质提供氮并有助于保持水分。
A typical fertile soil is about 45% mineral particles, 25% water, 25% air and 5% organic matter by volume. These percentages change with weather, compaction and land use.
典型的肥沃土壤按体积计算大约含有 45% 的矿物颗粒、25% 的水、25% 的空气和 5% 的有机质。这些比例会随天气、压实程度和土地利用方式而变化。
2. Weathering: How Rocks Become Soil | 风化：岩石如何变成土壤
Soil begins with weathering, the breakdown of rocks into smaller pieces. Physical weathering includes freeze-thaw action, where water enters cracks, freezes and expands, splitting the rock. Chemical weathering changes rock minerals into new substances.
土壤的形成始于风化，即岩石破碎成较小颗粒的过程。物理风化包括冻融作用，水进入岩石裂缝后结冰膨胀，使岩石裂开。化学风化则将岩石矿物转变为新的物质。
For example, rainwater containing carbon dioxide forms weak carbonic acid, H₂CO₃. This acid reacts with limestone, CaCO₃, to form soluble calcium hydrogencarbonate, so the rock slowly dissolves. Oxidation can also break down iron minerals into rust-like iron oxides, Fe₂O₃.
例如，含有二氧化碳的雨水会形成弱碳酸 H₂CO₃。这种酸与石灰石 CaCO₃ 反应，生成可溶的碳酸氢钙，因此岩石会缓慢溶解。氧化作用还能将含铁矿物分解成类似铁锈的氧化铁 Fe₂O₃。
- Physical weathering: freeze-thaw, heating and cooling, wind abrasion | 物理风化：冻融、热胀冷缩、风蚀
- Chemical weathering: acid rain, oxidation, hydrolysis | 化学风化：酸雨、氧化、水解
- Biological weathering: roots widening cracks, lichen acids | 生物风化：根系扩大裂缝、地衣分泌酸
3. Mineral Particles: Sand, Silt and Clay | 矿物颗粒：沙粒、粉粒和黏粒
Soil minerals are classified by particle size. Sand particles are the largest, from 0.05 mm to 2.0 mm. Silt particles are medium, from 0.002 mm to 0.05 mm. Clay particles are the smallest, less than 0.002 mm across.
土壤矿物按颗粒大小分类。沙粒最大，粒径为 0.05 毫米至 2.0 毫米。粉粒中等，粒径为 0.002 毫米至 0.05 毫米。黏粒最小，直径小于 0.002 毫米。
Clay particles have a very large surface area compared with their mass. This gives clay a negative surface charge, so it attracts positive ions such as Ca²⁺, Mg²⁺, K⁺ and NH₄⁺. This helps soil hold plant nutrients, but clay can also hold water too tightly for roots to use.
黏粒相对于其质量具有非常大的表面积。这使黏粒表面带负电荷，因此它能吸引 Ca²⁺、Mg²⁺、K⁺ 和 NH₄⁺ 等正离子。这有助于土壤保持植物养分，但黏土也可能把水吸得太紧，使根系无法利用。
4. Humus and Organic Matter | 腐殖质与有机质
Humus is dark, spongy organic matter formed when dead plants and animals decay. Decomposers such as bacteria and fungi break down complex molecules like cellulose and proteins into simpler substances.
腐殖质是由死亡的动植物腐烂后形成的深色海绵状有机质。细菌和真菌等分解者将纤维素和蛋白质等复杂分子分解成较简单的物质。
Humus improves soil chemistry by releasing nitrate ions, NO₃⁻, phosphate ions, PO₄³⁻, and ammonium ions, NH₄⁺. It also acts like a sponge, absorbing water, and it sticks mineral particles together to form crumbs that improve aeration.
腐殖质通过释放硝酸根离子 NO₃⁻、磷酸根离子 PO₄³⁻ 和铵根离子 NH₄⁺ 来改善土壤化学性质。它还能像海绵一样吸水，并把矿物颗粒黏结在一起形成团粒结构，改善通气性。
5. Soil Texture and Soil Types | 土壤质地与土壤类型
Soil texture describes the proportions of sand, silt and clay. The main types are sandy soil, clay soil and loam. Loam is a balanced mixture often considered best for farming because it holds moisture and nutrients but also drains well.
土壤质地描述沙粒、粉粒和黏粒的比例。主要类型有沙土、黏土和壤土。壤土是一种均衡的混合物，通常被认为最适合农业，因为它既能保持水分和养分，又能良好排水。
| Soil type 土壤类型 | Drainage 排水 | Water holding 保水 | Air spaces 空气空间 | Fertility 肥力 |
|---|---|---|---|---|
| Sandy soil 沙土 | Very fast 很快 | Low 低 | Large 大 | Low, nutrients wash out 低，养分易流失 |
| Clay soil 黏土 | Slow 慢 | High 高 | Small 小 | High but waterlogged 高但易积水 |
| Loam 壤土 | Moderate 中等 | Good 良好 | Balanced 均衡 | High 高 |
6. Water and Air in Soil | 土壤中的水与空气
Water fills some pore spaces in soil, while air fills the rest. Roots need both water for photosynthesis and oxygen for respiration, so a healthy soil must balance the two.
水填满土壤中的部分孔隙，而空气填满其余孔隙。根系既需要水进行光合作用，也需要氧气进行呼吸作用，因此健康的土壤必须平衡这两者。
Sandy soils have large pores, so water drains quickly and air enters easily. Clay soils have tiny pores that hold water strongly, leaving little room for air. Waterlogged clay can become anaerobic, meaning it lacks oxygen, which harms most plant roots.
沙土孔隙大，所以排水快，空气容易进入。黏土孔隙微小，强力保持水分，留给空气的空间很少。积水的黏土会变成缺氧状态，这会伤害大多数植物根系。
7. Soil pH and Its Chemical Effects | 土壤酸碱度及其化学影响
Soil pH measures how acidic or alkaline the soil solution is. It is recorded on the pH scale from 0 to 14. A pH below 7 is acidic, pH 7 is neutral, and a pH above 7 is alkaline.
土壤 pH 衡量土壤溶液的酸性或碱性程度。它用 0 到 14 的 pH 标度记录。pH 低于 7 为酸性，pH 等于 7 为中性，pH 高于 7 为碱性。
Most crops grow best in a slightly acidic to neutral range, about pH 6.0 to 7.5. If soil becomes too acidic, farmers add lime, such as calcium carbonate CaCO₃ or calcium hydroxide Ca(OH)₂, to raise the pH. Acidic conditions can release toxic aluminium ions, Al³⁺, which damage roots.
大多数作物在最适的微酸性至中性范围内生长最好，约为 pH 6.0 至 7.5。如果土壤变得过酸，农民会添加石灰，如碳酸钙 CaCO₃ 或氢氧化钙 Ca(OH)₂，来提高 pH。酸性条件会释放出有毒的铝离子 Al³⁺，损害根系。
- Acidic soil: pH below 7, often found in rainy areas | 酸性土壤：pH 低于 7，常见于多雨地区
- Neutral soil: pH around 7, good for many crops | 中性土壤：pH 约为 7，适合许多作物
- Alkaline soil: pH above 7, common in dry limestone areas | 碱性土壤：pH 高于 7，常见于干燥的石灰岩地区
8. Plant Nutrients and Ion Exchange | 植物养分与离子交换
Plants absorb nutrients as dissolved ions from soil water. The three main nutrients are nitrogen, phosphorus and potassium, often called NPK. Nitrogen is absorbed mainly as nitrate ions, NO₃⁻, and ammonium ions, NH₄⁺; phosphorus as phosphate ions, PO₄³⁻; and potassium as K⁺.
植物以溶解离子的形式从土壤水中吸收养分。三种主要养分是氮、磷和钾，通常称为 NPK。氮主要以硝酸根离子 NO₃⁻ 和铵根离子 NH₄⁺ 的形式被吸收；磷以磷酸根离子 PO₄³⁻ 的形式被吸收；钾以 K⁺ 的形式被吸收。
Clay and humus hold positive ions on their surfaces. When roots release hydrogen ions, H⁺, these positive nutrient ions are exchanged into the soil solution so the root can take them up. This process is called cation exchange.
黏粒和腐殖质在表面吸附正离子。当根系释放氢离子 H⁺ 时，这些正养分离子会被交换进入土壤溶液，使根系能够吸收它们。这个过程称为阳离子交换。
9. The Soil Profile: Layers Below Us | 土壤剖面：我们脚下的分层
A vertical cut through the ground shows layers called horizons. From the top down, the main layers are the O horizon of leaf litter, the A horizon or topsoil, the B horizon or subsoil, the C horizon of weathered parent rock, and the R horizon of solid bedrock.
从地面垂直切开可以看到被称为层次的土层。从上到下，主要层次是枯枝落叶层 O 层、表土层 A 层、底土层 B 层、风化母质层 C 层，以及坚硬基岩层 R 层。
The A horizon is usually darkest because it contains the most humus, and it is where most biological activity takes place. The B horizon often accumulates clay and minerals washed down from above. The C horizon contains broken rock fragments that have not yet formed true soil.
A 层通常颜色最深，因为其中腐殖质最多，也是大多数生物活动发生的地方。B 层常常积累从上方淋洗下来的黏粒和矿物质。C 层含有尚未形成真正土壤的破碎岩石碎块。
10. Living Organisms and Nutrient Cycling | 生物与养分循环
Soil is alive. Earthworms, insects, bacteria, fungi and protozoa form a food web. Decomposers break down dead matter into humus and release mineral ions back into the soil, making nutrients available for plants again.
土壤是有生命的。蚯蚓、昆虫、细菌、真菌和原生动物构成食物网。分解者将死亡物质分解为腐殖质，并将矿物质离子释放回土壤，使养分可再次被植物利用。
The nitrogen cycle is an important soil chemical cycle. Nitrifying bacteria convert ammonium ions to nitrite and then nitrate. Denitrifying bacteria convert nitrate back to nitrogen gas, N₂. Legume root nodules contain nitrogen-fixing bacteria that convert N₂ gas into ammonium ions.
氮循环是重要的土壤化学循环。硝化细菌将铵根离子转化为亚硝酸根，再转化为硝酸根。反硝化细菌将硝酸根转化回氮气 N₂。豆科植物根瘤中含有固氮细菌，能把 N₂ 气体转化为铵根离子。
11. Soil Erosion and Conservation | 土壤侵蚀与保护
Soil erosion is the removal of topsoil by wind and water. When plants are removed, roots no longer bind soil particles, so fertile topsoil can be blown or washed away. Chemical erosion can also occur when acid rain dissolves soil minerals and leaches nutrients.
土壤侵蚀是指表土被风和水带走。当植物被移除后，根系不再固定土壤颗粒，因此肥沃的表土可能被风吹走或被水冲走。当酸雨溶解土壤矿物质并淋失养分时，也会发生化学侵蚀。
Farmers conserve soil by planting cover crops, building terraces, using contour ploughing, and adding organic matter. These methods slow water flow, protect the surface and maintain soil structure and fertility.
农民通过种植覆盖作物、修建梯田、沿等高线耕作和添加有机质来保护土壤。这些方法可以减缓水流、保护地表并维持土壤结构和肥力。
12. Investigating Soil in the Laboratory | 在实验室中探究土壤
You can investigate soil by shaking a sample with water in a measuring cylinder and leaving it to settle. Sand settles first, then silt, then clay; humus may float. The layers show the relative proportions of each component.
你可以在量筒中用水摇动土壤样品并静置沉降，从而探究土壤。沙粒首先沉降，然后是粉粒，再是黏粒；腐殖质可能漂浮。各层显示每种成分的相对比例。
To measure soil pH, mix a soil sample with distilled water, filter it, and add universal indicator or use a pH probe. To measure water content, weigh a fresh soil sample, dry it in an oven, and record the mass loss as a percentage.
要测量土壤 pH，可将土壤样品与蒸馏水混合，过滤后加入通用指示剂或使用 pH 探头。要测量含水量，可称量新鲜土壤样品，在烘箱中烘干，并将质量损失记录为百分比。
Water content % = (mass of wet soil - mass of dry soil) ÷ mass of wet soil × 100
含水量 % =（湿土质量 - 干土质量）÷ 湿土质量 × 100
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