Soil is a complex part of the living landscape. Natural soil takes centuries or millennia to develop. Precipitation, temperature, plants and animals, landforms, and geologic material (bedrock, glacial deposits, river sediments, etc.) all influence soil development over time. Every plant requires different soil conditions to do well, yet we sometimes take them for granted and assume plants will grow in any soil. Soils can vary considerably, however, especially those that have been disturbed by human intervention, and this impacts a plant’s ability to survive. What is the optimal landscape soil? This will depend on the plants being grown and the conditions they require to do well. These are some of the functions that soil performs
- Reservoir for plant nutrients and water
- Habitat for bacteria and fungi that break down organic matter
- Habitat for insects and other animals that mix the soil
- Medium for anchoring plant roots
Many tree and shrub problems in urbanized areas can be traced to changes in the physical or chemical aspects of the soil. Grading and associated construction activity can quickly alter the natural soil environment and disturb roots. Understand the effects of these impacts on soil and plant health, and learning how to improve them will enable gardeners and landscapers to more wisely maintain the soil and its environment.
SOIL PHYSICAL PROPERTIES
Texture: Soil texture refers to the ratio of sand, silt, and clay-size particles in the soil. Sandy soils feel gritty, silty soils feel smooth like flour, and clayey soils feel sticky and form a ribbon when kneaded between your thumb and forefinger. Soil texture is important because it influences several other soil characteristics, including:
- Water holding capacity (the ability of a soil to hold water)
- Water, air, and nutrient availability
Soils rarely consist of only one particle size, but are typically a mix of the three. Medium-textured soils (see table) have a relatively even mixture of sand, silt, and clay. Generally, these soils are most favorable for growing plants.
Soil Texture Classes
Coarse-textured soils (sand, loamy sand, sandy loam)
- Relatively large spaces (pores) for easy air & water movement
- Good drainage but low water-holding capacity
- Relatively nutrient-poor
Medium-textured soils (loam, sandy clay loam, silt loam)
- Pore spaces provide good air & water movement
- Optimal water-holding capacity and nutrient availabilit
Fine-textured soils (sandy clay, clay loam, silty clay loam, silt, silty clay, clay)
- Tiny pore spaces restrict air and water movement
- Heavy & sticky with high water-holding capacity
- High nutrient availability
Soil structure refers to the natural bonding of soil particles into larger clumps (shapes) of varying size. Soils with good structure have plentiful natural pore spaces that allow for adequate water infiltration into the soil, good drainage and air movement, and strong root development.
Soil compaction effectively destroys soil structure. Compaction closes soil pores, interfering with the movement of water and air through the soil and limiting root growth. During wet periods, compaction at the soil surface or in the subsoil can cause water to pond on top of the soil or in the subsoil. When dry, these same soils are nearly impossible to penetrate with a trowel or shovel. Planting holes can become compacted during digging, which may allow water to collect in the bottom of the hole, much like a clay pot. In such saturated conditions, trees may drown due to lack of oxygen. Wet soils and those high in clay are more susceptible to compaction; soils higher in the sand are less susceptible. Vehicles and foot traffic are responsible for most compaction. During the development of subdivisions, large machinery (bulldozers, trucks, etc.) compact the soil as they move over it. Intense compaction on a small scale can occur in yards where walking, running, or mowing on lawns and around trees is common, particularly when the ground is wet.
Soils are poorly drained if water pools at the surface or in the subsurface for several days or more after a wet period. Compaction promotes wet, saturated zones in the soil, devoid of oxygen. Roots of sensitive plants are rapidly killed in saturated soils and cannot take up water or perform oxygen-requiring respiration. Foliage dies, ironically, from lack of water. Drowning plants show the same symptoms as those suffering from drought: drooping leaves and browning leaf margins.
Dig a 12″ x 12″ x 12″ hole. Be careful to not compact the bottom of the hole. Fill it with water, let it drain, and refill it. Water should drain within 24 hours. If not, there is a good chance that drainage is poor.
Improving Physical Soil Properties
Adding organic matter (mulch and/or compost) is the best way to amend a soil with physical limitations. Organic matter is critical to longterm plant and soil health. It is most beneficial in the upper part of the soil where plant roots concentrate. Over time, it is incorporated into the soil by insects and other animals, and by freezing & thawing. As organic matter decomposes, it provides plant nutrients, improves soil texture, structure, and water holding capacity, and reduces the effects of compaction. These are some of the ways to improve a soil’s physical limitations:
- Plan ahead to minimize drainage, compaction, or other problems. Before landscaping, take time to understand existing soil limitations and develop a plan to amend them. If soil properties cannot be improved significantly in the long-term, select plants that will be more likely to do well in these soils
- Incorporate composted leaf mould, grass clippings, etc., into a planting bed before planting, followed by 3-4 inches of wood chips after planting. Replenish with additional mulch as needed
- To minimize compaction, never work soils when they are wet
- Till the soil to remove compacted layers. This is best done before planting. Tilling after planting will cut plant roots. Gypsum is not an effective amendment for compacted soils except in saline soils, which are only a problem in the Midwest immediately adjacent to heavily salted roadways
- Use a core aerator to improve a compacted soil surface. Core aerators remove small soil plugs to a depth of about 3 inches. In areas without sod, add organic compost or mulch after aerating
- In sites with poor drainage, drain tiles can be effective in removing water quickly. Tiles should slope downward and away from the site. Consider where the tile ends so as not to produce a problem elsewhere.
SOIL CHEMICAL PROPERTIES
The chemical properties of a soil are easily altered by the addition of fertilizers, water runoff (weathering) from concrete or limestone surfaces, road salts or other pollutants, and construction debris, such as mortar, concrete, and brick, mixed in the soil. An excellent tool for assessing soil chemistry is soil pH – the measurement of the acidity or alkalinity – which influences nutrient availability and plant growth.
Acid or Alkaline
The pH scale is 1 to 14, with 7 being neutral, below 7 being acid, and above 7 being alkaline (see pH chart below). A change of one pH unit is a 10-fold change in acidity. For example, a pH of 6 is ten times more acid than a pH of 7. Most naturally occuring soils have a pH of 5 to 8. A general measurement of soil acidity can be done with a garden pH kit, available at most garden stores. Commercial soil laboratories can provide more detailed analyses (see “Soil Testing & Interpretation” below).
Some factors to consider regarding soil pH:
- Slightly acid soils (pH of 6 – 6.5) provide the most favourable environment for nutrient availability, thus most plants perform well in these soils (see nutrient chart below)
- Urban soils are typically alkaline, a result of soil alterations already mentioned
- Although alkaline soils may actually contain adequate nutrients to support most plants, the chemistry (pH) of these soils may be such that some nutrients are not available for uptake by roots of affected plants, resulting in nutrient deficiencies. Deficiencies of some nutrients lead to reduced chlorophyll production (chlorosis) or other maladies, including stunted growth and decline. Nitrogen is the nutrient that is depleted from the soil most quickly and needs regular replenishment.
Modifying Soil pH
The addition of agricultural lime to increase pH is rarely needed for urban soils, unless for some reason the soil is too acid for the plants being grown. If alkaline-induced plant problems are suspected (chlorosis symptoms), it’s likely that the soil needs to be acidified for greater nutrient availability in conjunction with other improvements to the rooting environment. Here are some suggestions to reduce alkaline-induced plant problems:
- Add organic compost (acidic leaf mold) topped with composted woodchip mulch
- Don’t fertilize chlorotic plants with potassium or phosphorus, unless a soil test indicates a deficiency, and avoid nitrate-containing fertilizers
- Once a year, either in early spring or late fall, add granular sulfur or aluminum sulfate at a rate of no more than 3 pounds/100 square feet onto the soil beneath plants where acidification is needed. Or, break this into 1.5 pounds/100 square feet in early spring and in fall. Do not apply onto lawn or onto plant foliage. Water thoroughly after application
- Acid-loving plants should be kept away from concrete and other limestone materials that have an alkaline runoff.
Reducing soil pH to an acceptable level can take many years and much patience, so planting alkaline-tolerant trees, shrubs, and other plants is another approach.
Salinity is one of the most brutal environmental factors limiting the productivity of crop plants because most of the crop plants are sensitive to salinity caused by high concentrations of salts in the soil, and the area of land affected by it is increasing day by day. For all-important crops, average yields are only a fraction – somewhere between 20% and 50% of record yields; these losses are mostly due to drought and high soil salinity, environmental conditions which will worsen in many regions because of global climate change. A wide range of adaptations and mitigation strategies are required to cope with such impacts. Efficient resource management and crop/livestock improvement for evolving better breeds can help to overcome salinity stress. However, such strategies being long-drawn and cost-intensive, there is a need to develop simple and low-cost biological methods for salinity stress management, which can be used on a short term basis. Microorganisms could play a significant role in this respect, if we exploit their unique properties such as tolerance to saline conditions, genetic diversity, synthesis of compatible solutes, production of plant growth-promoting hormones, bio-control potential, and their interaction with crop plants.
Generally, soil salts are not a problem where the region receives adequate precipitation to leach salts through the soil system. Sometimes, however, there are concerns. Primary source of soil salts is fertilizers. Fertilizer salts become a problem when too much fertilizer is used. The application of any fertilizer should be minimal in summer because hot, dry spells reduce moisture available to plants and subsequent fertilizer uptake can increase salt concentrations in plant tissues, leading to desiccation or burn of these plant parts. Desiccation or “drying out” of leaves is seen as browning of leaf edges or the entire leaf during the growing season.
Minimizing Damage from Soil Salts
Watering plants, in addition to normal precipitation, will help flush salts through the soil and below the root zone if a soil is adequately drained. Do not overwater
Fertilize plants only when necessary. If a nutrient deficiency is suspected from off-color foliage or abnormal growth, perform a soil test to help determine what nutrients or fertilization routine might be needed. A layer of organic mulch and compost around plants will provide a natural “slow-release” fertilizer
SOIL TESTING & INTERPRETATION
Assessing the Problem
It is known that agricultural intensification can have negative effects at different scales, such as, increased soil erosion, soil fertility decline and reduced biodiversity at the local level, depletion and pollution of groundwater and eutrophication of surface waters at the regional level, and changes in atmospheric composition and climate on a global scale. Enhancing sustainable food production will require integrated strategies for the use of land and water resources:
- a) agricultural intensification on the best arable land, b) rational utilization of marginal lands for agriculture, and c) prevention and restoration of soil degradation. Soil degradation is a severe problem in countries like India with high demographic pressure. For preventing and restoring soil degradation, the main issues will be controlling soil erosion and sedimentation with the associated risks of eutrophication of surface water and contamination of groundwater, combating desertification and enhancing soil carbon sequestration to improve soil quality/productivity and mitigate the greenhouse effect.
For sustaining better soil quality under intensive systems of agriculture, the emphasis should be on developing of workable soil quality indicators and methods to assess and monitor soil quality, assessment of soil quality under different land use management systems (cropping system, tillage, water and nutrient use practices) and to identify the effect of management practices aggrading/ degrading/ sustaining soil quality. Also strategies need to be formulated to decide the amount of organic matter to be in the form of organic manures/organic waste/residue each year to maintain or build up the soil organic matter (SOM) level in various soil types.
Interpretation of Testing
After collection and analysis of soil samples, it is the responsibility of the analyst/scientist to interpret the results and properly give the fertilizer recommendations to farmers. Many people have an impression that soil testing is a simple procedure to determine the plant nutrients that are deficient and recommend those nutrients in fertilizer form so that harvests are assured. Nevertheless, soil testing is much more than determining nutrient availability.