The USDA joined other global partners in celebrating 2015 as the International Year of the Soil. This initiative is meant to underscore the importance of soil as a living “ingredient” which is literally the building block of our lives.
Assessing your soils now, and amending them this fall, can provide the foundation for a healthy 2016 growing season.
Although nutrients aren’t the entire story of soil health, adding the chemicals — the macro and micro nutrients — which are needed for plant growth, is often the first step in producing a crop. Field soils may not contain the nutrients in the proper balance needed for crop growth. And, even if they already do, plant growth is going to consume some of the store of nutrients in the soil. So keeping the soil nutrients present and available for both the plant’s use, as well as the soils own needs, is important, and is what adding fertilizer should be all about.
Fertilizers are used to add nutrients needed for crop growth. Legally, fertilizers guarantee to contain a minimum amount of a nutrient or nutrients. Complete fertilizers have the three essential elements — N, P, and K — known as the primary nutrients. The secondary nutrients are calcium, magnesium and sulphur. Taken together, these two groups are known as the macro nutrients. Important micro nutrients are boron, copper, iron, chloride, manganese, molybdenum and zinc. Fertilizers can target one, some, or all of these elements.
A plant’s nutrient requirements vary throughout its life cycle. Insuring that there is no excess or lack of any particular elements during all stages of growth is key to optimal plant health and yield. Too much nitrogen early on, or not enough during maturation, and the yield and quality will be impacted, for example. Knowing which type of fertilizer is needed to support a specific crop requires both soil testing and a understanding of the nutrient needs of the crop.
Fertilizers are broken down into two categories: commercial (or synthetic) and organic. Organic here does not refer to the National Organic Program, or certified organic production, but rather means that the nutrients are derived directly from organic matter. Commercial fertilizers are often mined, so while they can be natural, and may be approved for certified organic production, they are not coming directly from living materials. In addition, many commercial mined fertilizer are processed to some degree, with the use of acid compounds being typical.
Commercial fertilizers are, typically, immediately available for the crop’s use. If synthetic fertilizers are coated with resins, they can be released over time. Organic fertilizers include: bone meal; dried blood; fish meal; kelp; manure; rock phosphate; urea and wood ash. The nutrients in organic fertilizers are typically available as the materials biodegrades. One challenge when using organic fertilizers is calculating when and at what rate to apply them, so that the nutrients are available when needed, and in the proper amounts.
Manure is an example of an organic fertilizer, used for centuries. But fresh manure has its drawbacks, particularly excessive ammonia, and the presence of pathogens. For vegetable crop production, manure is typically composted. Composting manure eliminates safety issues from fresh waste; reduces the risk of applying an excess of soluble nitrogen, which can build up in soils or runoff, causing environmental concerns; and reduces the weed concerns which come with fresh manure use.
But manure doesn’t only provide nutrients for the crop. More than fertilizing the crop, manure also amends the soil.
Compost: beyond fertility
Soil works properly and grows healthy crops when all of its elements are aligned. Unlike a fertilizer, which supplies a stated minimum amount of one or more macro nutrients, or a targeted micro nutrient, compost not only supplies nutrients, it adds organic matter and soil organisms, increasing soil health on all fronts. It enhances the soil ecology through chemistry, adds biological life, and amends the soil’s physical properties.
Compost is decayed organic matter. When mature, or fully decomposed, it becomes humus. Compost results from a proportional mix of carbon and nitrogen sources, moisture, and aerobic conditions, leading to decomposition by microbes.
Because of the wide variety and sources of raw materials, composts can vary tremendously in chemical composition. A chemical analysis of the compost may include the pH, organic matter content, moisture-holding ability, salinity, porosity, respiration rate, nitrogen availability and heavy metal concentration. Maybe you’d like to avoid animal manures, human biosolids, or grass clippings from lawns treated with chemicals. Other common compost feedstocks include paper waste, food waste, leaves, sawdust, wood chips or bark.
Having the correct chemistry for soil health is important, not only because the soil’s chemistry is a source of nutrients. The secondary elements of calcium and magnesium help to determine the soil’s porosity, along with sodium and potassium. When these elements are balanced, the water absorption abilities of the soil are optimized.
The physical properties of soil include: texture (sand or clay); structure or how the particles are arranged; color, which can indicate draining characteristics; permeability, or its ability to transmit water and air; consistency, indicating its ability to tolerate mechanical manipulation or stress; and its density, or pore space. Vermiculite, pea gravel and perlite are examples of inorganic soil amendments which are used to change physical soil properties.
The soil’s cation exchange capacity (CEC) is a measure of its ability to hold on to and release nutrients. Heavier soils have higher CECS, and will take more time to amend, than sandy soils. Amending the soil through the alteration of the soil’s physical properties can improve soil health. And adding organic matter increases CEC, too. Organic soil amendments include compost, straw, wood ash and leaves, which also have biologically active properties as well.
The biology of the soil is interrelated with its physical and chemical traits. Beneficial soil microbes need an aerobic zone in which to thrive. Plant roots, too, need this zone. If the nutrients are in the aerobic zone, which is composed of about 25 percent air and 25 percent water, the soil’s fertility will be improved, and crop yield will be optimized.
Soil health extends well beyond N-P-K and pH levels. Nitrogen, phosphorus and potassium are important soil chemicals, and they work together to help create a balanced pH level needed for growing crops. Likewise, soil pH affects the availability of these and other chemical elements, which serve as plant nutrients. But a fertile soil is so much more than these measures of its chemical composition. A healthy soil ecology encompasses the chemistry and biology, as well as the physical soil properties.