Healthy soils are an important part of gardening and a variety of factors influence their composition. Soil type, organic matter, macronutrients, electrical conductivity and pH are fundamental to soil health. Understanding how these properties influence soil allows gardeners to tailor conditions to best suit the needs of their plants and restore the properties necessary to reuse soils crop after crop.
Different plants thrive in different soil types. Soil type influences many factors of soil health and understanding the different soil compositions will ensure optimal growth for any plant. Soil makeup is generally about 50% solids, minerals and organic matter, while the other half is pore space filled with either air or water. The amount of pore space between solids depends upon the ratio of minerals in the soil. This variability generates the different soil types.
Sand, silt and clay make up about 90% of soil solids and the soil triangle (see diagram above) describes all possible combinations of these minerals and their associated soil types. These soil types all have different bulk densities, which is the measurement of the mass of particles within a volume of soil.
Bulk density is influenced by both mineral composition and compaction, and is inversely related to the porosity of a soil, a measurement of pore space. Low bulk density and high porosity are generally ideal for plant growth because they allow for the infiltration of water and air through the soil, easy root propagation and the promotion of nutrient uptake. Loam is ideal soil for gardening and agriculture as it contains almost equal parts sand, silt and clay.
The remaining 10% of soil solids is made up of organic matter which, through decomposition, provides nutrients necessary for the growth of both plants and soil microbes. Organic matter is the most important indicator of soil health. Soil organic matter is comprised of freshly deposited material, decomposing organic matter, stable organic matter (humus) and living organisms in varying quantities.
Organic matter also acts as a pH buffer and carbon sink, increases water retention and soil stability, and helps resist crusting and compaction. Estimating the contents of soil organic matter can indicate the relative availability of nutrients stored within the soil, and tells the grower how much fertilizer to use.
Organic matter is usually highest where large quantities of perennial plants consistently deposit new material for decomposition and where soil compaction is minimized. No-till agriculture and trench composting both promote these conditions to allow for increased water, soil organic matter and nutrient retention as well as the reduction or elimination of erosion by promoting soil aggregation. Understanding organic matter qualities allows for better decision-making when applying fertilizers and other additives to soil.
Plants require a variety of different macro- and micronutrients to thrive. The availability of these nutrients depends upon the electrical conductivity and pH of the soil type as well as the chemical forms in which the nutrients occur. The most important macronutrients plants derive from soils are nitrogen, phosphorus and potassium.
Plants use nitrogen to produce proteins and nucleic acids, making its acquisition critical to continued growth. Nitrogen finds its way into soils via nitrogen-fixing bacteria, decomposing plant and animal waste, and fertilizers.
These processes generate a variety of different forms of nitrogen—ammonia, ammonium, nitrates and nitrites—that not only benefit plants but also soil bacteria. Nitrogen can change forms as it is used and some of these forms are more useful to plants than others.
It is a mobile nutrient and factors such as moisture levels, temperature, aeration and conductivity all affect nitrogen cycling and can decrease available stores via runoff, denitrification and leaching.
Sandy soils with large pore spaces lose nitrogen much more quickly than clay-based soils, but poorly drained, clay-based soils can also lose nitrogen through denitrification by soil bacteria.
Ensuring your soil has adequate drainage and a good mixture of minerals and organic matter will maintain sufficient levels of nitrogen throughout the life cycle of your plants with minimal fertilization.
Phosphorus is used by plants to transfer energy from photosynthesis into growth and reproduction. This crucial nutrient finds its way into soils through a variety of cycles and in many chemical forms.
Phosphorus is much more stable in soil than nitrogen and is usually lost by more extreme conditions like runoff or erosion than any other factors. The majority of phosphorus taken up by plants comes from the decomposition of organic matter.
Phosphorus is naturally available in much smaller quantities via minerals but it is much easier for plants to access phosphorus previously used by another organism than it is to get it directly from minerals.
There are a great deal of organisms ready to extract phosphorus from organic matter for use by your plants in the right soil type with supportive pH, bulk density, temperature and watering.
Potassium plays an important role in plant biology—the regulation of stomata, photosynthesis and respiration as well as the construction of cellulose.
Potassium can be stored in minerals (unavailable), within layers of clay (slowly available), or on the surface of clay deposits or dissolved in water (readily available).
Dissolved potassium is absorbed along with water through the roots and when the concentration of potassium in the water becomes low enough, more is released into the solution from the surface of clay particles.
Potash (salts containing water-soluble potassium) may be added to soils along with water to ensure adequate amounts for plant growth.
While there are other nutrients important to plants, proper applications of nitrogen, phosphorus and potassium will ensure the continued growth of plants. There are a variety of kits available to measure the quantity of these nutrients within soil.
The amount of salts within soil is directly related to its electrical conductivity (EC), which affects water and nutrient availability as well as microbial activity in the soil. EC is higher in drier climates, as the salts exposed by weathering and erosion of rocks remain within the upper layers of soil.
In wetter climates, those salts are washed out of surface layers with rainfall. Excessive use of nitrogen, phosphorus and potassium increases soil salinity, making optimal fertilizer usage dependent on immediate soil needs. Salts in soil are an important indicator of soil health, but are rarely of concern for indoor gardening.
The relative acidity or alkalinity of soil greatly influences both the biology of plants and soil organisms as well as nutrient availability, making pH optimization crucial for soil health. A pH of between 5.5 and 7 is considered ideal for the majority of plants, but certain plants grow best within an even smaller range while others have adapted to thrive outside that range.
Soil acidity is a more common issue for gardeners than alkalinity because excessive watering and fertilizer use both lower soil pH. To maintain optimal pH, the soil must act as a buffer to resist changes to pH.
Organic matter acts as an effective buffer through the biochemical interaction of humus and soil additives. Liming the soil by adding calcium and magnesium-rich materials is the most common means of combating soil acidity, as it considerably raises the pH of soils. Consistent monitoring and adjustment of pH will ensure optimal growth for plants and microbes and allow for the reuse of soil with minimal management.
Understanding the basic principles of soil types, organic matter, macronutrients, electrical conductivity and pH covers the backbone of soil science and can make all the difference for plants in soil media.
There are a variety of conventional tests to determine soil type and measure organic matter, pH, macronutrients and conductivity that, if used effectively, can inform all levels of decision-making when gardening with soil. Healthy soil makes for healthy plants, so don’t be afraid to get your hands dirty!