Soils - Soil and Plant Nutrition: A Gardener’s Perspective
Soil and Plant Nutrition: A Gardener’s Perspective
Lois Berg Stack, Ornamental Horticulture Specialist, University of Maine Cooperative Extension
Soil is a complex three-dimensional substance that covers some of the world’s land surface. It is a dynamic material that changes over time. It is composed of solids, liquids and gases. And, it is affected by organisms, both while they are alive and after they die and decompose.
Soil performs four major functions:
- It provides habitat for fungi, bacteria, insects, burrowing mammals and other organisms;
- It recycles raw materials;
- It provides the foundation for engineering projects such as buildings, roads and bridges; and
- It is a medium for plant growth. This text focuses on this last function.
What does soil do for plants?
Soil supports plant growth by providing:
- Anchorage: root systems extend outward and/or downward through soil, thereby stabilizing plants.
- Oxygen: the spaces among soil particles contain air that provides oxygen, which living cells (including root cells) use to break down sugars and release the energy needed to live and grow.
- Water: the spaces among soil particles also contain water, which moves upward through plants. This water cools plants as it evaporates off the leaves and other tissues; carries essential nutrients into plants; helps maintain cell size so that plants don’t wilt; and serves as a raw material in the process of photosynthesis, the process by which plants capture light energy and store it in sugars for later use.
- Temperature modification: soil insulates roots from drastic fluctuations in temperature. This is especially important during excessively hot or cold times of year.
- Nutrients: soil supplies nutrients, and also 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) and organic matter (from decomposing plants and animals). The mineral portion of soil is identified by its texture, which is the proportion of sand, silt and clay in the soil. These three terms refer only to particle size: sand is familiar to most of us, and is the largest textural soil size. Sand grains can be seen with the naked eye or with a hand lens. Clay particles are so small that they can only be seen through an electron microscope. Silt is sized between sand and clay; individual silt particles can be seen through a lower-power microscope.
Sand has excellent aeration and drainage. It tills easily and warms up rapidly in spring. However, it erodes easily, and has a low capacity for holding water and nutrients.
Clay has poor aeration and slow drainage. It is difficult to till and warms up slowly in spring. However, it tends to erode less quickly, and has a high capacity for holding water and nutrients.
Silt has intermediate characteristics compared to sand and clay.
Most soils contain all three particle sizes. Loam is a specific textural class of soil that contains somewhat equal amounts of sand, silt and clay. Most of our topsoils are loams. Loam can vary from a rather equal mixture of the three textural sizes, to a mixture dominated by one particle size or another. As a gardener, you should inspect loam before purchasing it, because these variations affect your management practices.
Structure: Sand is often found as individual particles in a soil, but silt and clay are almost always clumped into larger units called aggregates. The manner of this aggregation defines a soil’s structure. Soil structure is described by terms such as blocky, platy, prismatic and angular. Good topsoils often have a granular soil structure. The size and shape of aggregates is influenced by particle size, wetting and drying, freeze/thaw cycles, and root and animal activity. Organic matter, especially cells and waste products of soil microbes, cement aggregates together. However, aggregates can break apart due to overtilling, compaction and loss of organic matter in the soil. Soil structure is a very dynamic process. Good soil structure increases the pore space (see below) that supports root penetration, water availability and aeration.
Pore space: Soil particles rarely fit together tightly; they are separated by spaces called pores. Pores are filled with water and/or air. Just after a heavy rainfall or irrigation event, pore spaces are nearly 100% filled with water. As time goes by, the water passes through the soil, or evaporates into the air, or is used by plant roots, and more of the pore spaces are filled by air.
Pore space generally occupies 30-60% of total soil volume. A well-structured soil with both large pores (macropores) and tiny pores (micropores) provides a balance of air and water, both of which plants need. Macropores provide for good drainage, and micropores hold water that plants can access.
Organic matter: Organic matter (OM) is previously living material. On the surface of the soil, there is usually rather un-decomposed OM known as litter or duff (or, mulch in a landscape). This surface layer reduces the impact of raindrops, prevents erosion, and eventually breaks down to supply nutrients that leach into the soil. In the soil, OM decomposes further until it becomes humus, a stable and highly decomposed residue that serves as an important nutrient source for plants.
OM is always in the process of decomposing, until it becomes humus. OM levels are reduced through cropping and must be replenished by adding compost or manure, or by tilling in crop residues or green manure (crops such as buckwheat, clover or ryegrass that are grown as cover crops, with the intention of tilling them into the soil to increase OM levels). OM improves water retention, making it a good addition to sandy soil. OM also improves soil structure, so it is added to clay or silt soils to increase aggregation. OM provides nutrients as it decomposes, buffers the pH of the soil solution against rapid chemical changes, and improves soils’ cation exchange capacity (see below).
Good horticultural soil: Most soils are dominated by mineral particles; some are dominated by organic matter. Some soils have a high percentage by volume of pore space, while others have little pore space. Ideally, a “good horticultural soil” contains 50% solid material (mostly mineral soil plus 5-10% organic matter) and 50% pore space. That pore space is occupied by air and water, but when excess water has drained away after a heavy rainfall or irrigation, the pore space should contain about 50% water and 50% air.
Chemical properties of soil
Soil chemical activity is related to particle size. Small particles (humus and clay) are the most important sites from which nutrients are taken up by plants. Clay and humus are negatively charged, and many nutrients are positively charged. The clay and humus hold these positively charged ions (cations), preventing them from being leached out of range of plant roots. Negatively charged ions (anions) are held in the soil solution, and are very susceptible to leaching downward. This explains why nitrates, which are anions, leach readily out of our topsoil and sometimes into our water supply.
Cation exchange capacity (CEC): CEC is an expression of the soil’s ability to hold cations. Ions are constantly exchanged among the soil solution, CEC sites on clay and humus particles, and plant roots. This is not a random process, but is dependent on electron charge. Clay and humus have high CECs because they are tiny particles with very large surface-to-volume ratio, with many negative sites that can attract cations. Sand has very low CEC because sand particles are large, with low surface-to-volume ratio. A gardener can add a larger amount of fertilizer less frequently, when gardening in a soil with a high level of clay or humus, compared to a sandy soil.
pH: pH is a description of the soil’s acid/alkaline reaction. The pH scale ranges from 0 (very acid) to 14 (very alkaline). Soils generally range from pH 4 to pH 8. Northeastern forest soils can be very acid (pH 3.5), while Western soils can be very alkaline (pH 9). pH regulates the availability of nutrients in the soil solution.
The pH scale is logarithmic; each unit is 10 times more acid or alkaline than the next. For example, a soil with pH 4 is ten times more acid than a soil with pH 5, and 100 times more acid than a soil with pH 6. A soil’s pH depends on the parent rock (limestone is alkaline, granite is acid), rainfall, plant materials, and other factors. Individual plants perform best within specific pH ranges. It is just as important to manage pH as fertility.
Living organisms in soil
Many organisms inhabit soil: bacteria, fungi, algae, invertebrates (insects, nematodes, slugs, earthworms) and vertebrates (moles, mice, gophers). These organisms play many physical and chemical roles that affect plants. For example, their secretions help dissolve minerals, making them available to plants; some organisms convert inorganic substances into other forms that are more or less available to plants; organisms add OM to the soil; organisms help decompose OM; many organisms aerate the soil. Some living organisms in the soil cause diseases, some feed on plant tissue, and many compete with plants for nutrients and water.
Rhizosphere: The very thin zone just around roots is called the rhizosphere. This zone is different from the rest of the soil, and it sometimes supports specific and unique organisms. For example, some fungi live together with roots, to their mutual benefit; these mycorrhizal relationships provide the fungi with a place to live, and the fungi assist in the plant’s water and nutrient uptake. Similarly, some nitrogen-fixing bacteria grow together with some plants, including many legumes (members of the bean family). The bacteria convert atmospheric nitrogen into forms that can be used by their host plants. When the host plant dies, the nitrogen compounds released during decomposition are available to the next crop. Any mutually beneficial relationship between two dissimilar organisms is called a symbiosis.
Water is an amazing substance. It is called the universal solvent because it dissolves more substances than any other liquid. It is a renewable natural resource. It exists in nature as a solid, liquid and gas. Its molecules cohere (stick together) and adhere to other 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 energy when it passes from solid to liquid and from liquid to gas. And, when it passes from gas to liquid and from liquid to solid, it absorbs a large burst of energy. Gardeners reap the benefits all of these attributes of water.
Water-holding capacity: Plants get most of their water from the soil. A soil’s ability to hold water is called its water-holding capacity. Clay soils have high water-holding capacity, and sandy soils have low water-holding capacity. When a soil’s pore space is filled with water, the soil is saturated. After a heavy rainfall or irrigation, a clay soil tends to remain saturated longer than a sandy soil. A loamy soil reaches its field capacity 2-3 days after a heavy rainfall or irrigation. At field capacity, the soil holds as much water as it can against the force of gravity. If no additional water is added for many days, the water will move in the soil from wetter to drier areas. Plant uptake causes removal of water, and capillary action causes water to rise up through the tiny tube-like openings of a soil (formed by a “string” of small pores in the soil) and evaporate from the surface. Eventually, a soil may dry enough to reach its permanent wilting percentage, and the plant wilts and cannot recover. At this point, the available water (water that remains available to the plant) is gone, and the only water that remains is so tightly bound to soil particles that plants cannot access it.
Good soil management is critical for crop productivity. Good management must include consideration of maintaining the soil’s integrity over time. Poor management can lead to erosion, loss of fertility, deterioration of soil structure, and poor crop yields.
Tilling: Mechanical manipulation of soil loosens the soil, and promotes aeration, porosity and water-holding capacity. It allows incorporation of soil amendments. On the other hand, overtilling tends to decrease aggregation, causing compaction (compacted soils are dominated by few, small pores).
Managing pH: Soil pH regulates availability of nutrients. pH should be managed only in response to soil test results. Soils can be acidified by adding organic matter or sulfur or sulfates. Soil can be alkalinized by adding lime or fertilizer or wood ash.
Mulching: Mulch is a material that covers the soil. Organic mulches such as compost, aged manure or bark chips decompose to supply OM and nutrients in the long term. Inorganic mulches such as stone or plastic sheet materials have little effect on nutrient levels and do not contribute OM to the soil. All mulches affect soil temperature by insulating or transferring heat, and all mulches help soils retain moisture. Mulches may also help reduce weed growth, prevent erosion and affect insect/disease prevalence.
Managing OM levels: In natural areas, plants and animals die and remain in place, decompose and replenish OM in the soil. In places where this natural cycle is undesirable, gardeners must implement alternative methods of replenishing OM. Leaves from deciduous trees can be left in place to decompose, plant residue can be tilled into gardens, plant debris can be composted in a bin and incorporated into gardens, green manures can be grown and tilled into the soil, and animal manures can spread and tilled into soil. Adding huge amounts of OM at one time can cause nutrient problems, but adding small amounts periodically can contribute to soil fertility, support soil microflora, and maintain water-holding capacity.
Three elements, carbon, oxygen and hydrogen, are essential to plant growth and are supplied by air and water. The other essential elements are referred to as plant nutrients, and are provided by the soil, or are added as fertilizers, and enter plants almost exclusively through the roots. These plant nutrients are divided into two groups. Those required by plants in large amounts are called macronutrients; these are nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The others are called micronutrients; these include iron, chlorine, zinc, molybdenum, boron, manganese, copper, sodium and cobalt. Macronutrients and micronutrients are all critical to normal plant growth and development; they are simply needed in different amounts.
Organic fertilizer sources include compost, aged manure and compost tea. Organic fertilizer can be “grown” by planting a cover crop, which is a crop that is grown with the intention of tilling 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 also widely available, either as single-nutrient or multi-nutrient products.
Fertilizers are labeled as slow-release or soluble. Slow-release fertilizers provide nutrients over a period of time, as they break down or decompose. Soluble fertilizers are fast-release, and many are dissolved into water and then irrigated onto crops.
Nutrients can be provided by many products and practices. Price, availability, ease of use, needed equipment, time and philosophy all play a role in selecting the best fertilizer and application method for any situation. Occasionally, in severe nutrient deficiency situations, some micronutrients are sprayed onto the foliage of crops. In hydroponic production systems, nutrients are dissolved in water and washed over the exposed roots of plants.
Most soils have at least some residual nutrients. Only a soil test can assess this. Fertilizing without the results of a soil test leads to a waste of money and product, and can exacerbate an existing nutrient imbalance. In addition, sometimes nutrients are present in sufficient supply but are unavailable because of too high or too low pH. A soil test can reveal this, and a soil lab professional or crop consultant can recommend resolution of such problems.
Soil and fertilizer management tips for home gardeners
Many gardeners do not say that they garden, but rather that they work the soil. This reveals an understanding that good soil conditions are essential to support productive crop growth. Here are a few gardening tips related to soil management:
To amend a heavy (clay) soil, add OM, not sand. As OM decomposes to humus, it “glues” particles together, and improves drainage.
To amend a light (sandy) soil, add OM, not clay. OM increases sand’s ability to hold water and nutrients.
Most ornamental plants (woody trees and shrubs, and herbaceous perennials and annuals) are best fertilized in spring. Fertilizing late in the season can lead to a late-season flush of growth that does not adequately harden off before winter.
Most houseplants are best fertilized at the rate recommended on the product label in spring and summer, and at half that rate in fall and winter.
Fertilize vegetable gardens by broadcasting and incorporating fertilizer in spring, and supplementing that rate with a side-dressing applied next to growing plants later in the season. Manage the pH of garden soil to ensure good nutrient availability. Rotate crops with cover crops to maintain good levels of organic matter, which helps the soil retain nutrients for plant use.
When fertilizing a lawn, determine the level of growth desired. If a low-maintenance lawn is desirable, no fertilizer may be needed. Do not fertilize before spring green-up or after September 15. Avoid fertilizing in midsummer. Slow-release products are preferred over soluble fast-release formulations. Apply a maximum of 2 pounds nitrogen per 1000 square feet per year on established lawns. Leave an unfertilized buffer strip of at least 25 feet adjacent to lakes, streams, rivers, bays, vernal pools and wetlands. Avoid using phosphorus fertilizer if a soil test reveals phosphorus is not necessary, as phosphorus caused freshwater quality problems. Reduce the amount of fertilizer needed by 1/3 to 1/2 each year by mowing with a mulching mower.