Soil Microbes Impacted by Fertilizer
Fertilizer generally enhance plant growth but it also may change soil microbial communities, leading to positive or negative changes. Chemical fertilizers are higher in salts (positive or negative ion charges). High salt content attracts water, ties it up, leading to both soil and plants becoming desiccated. If a nutrient is lacking, then fertilizer lets both microbes and the plant thrive. However, excess fertilizer generally leads to the opposite result. Also, chemical fertilizers tend to acidify or lower soil pH, generally disturb or change microbe diversity, and may lead to degraded soils.
Organic fertilizers add more carbon and are commonly found in nature. They enhance soil microbial communities and soil fertility. They improve soil structure so that plant roots, water, and air move freely in the soil. They provide nutrients in a sustainable but slower time frame which enhances both microbes and plants, leading to more diversity and growth. However, sometimes for higher yields, since nutrients are released more slowly with organic fertilizer; plant growth may not be optimal. That’s when a little commercial fertilizer may be helpful.
Another way to fertilize crops is with biofertilization. Biofertilization is when beneficial microbes are added to the soil to enhance soil fertility and soil productivity long-term. A good example is vermicomposting or “worm poop”, which has over 10,000 beneficial bacteria. Also, some biological products (Example Holganix Bio 800+) has a numerous diversity of beneficial microbes that enhance plant growth.
Often, a combination of all three fertilizers can be used in a balanced nutrient program to improve nutrient use efficiency and reduce nutrient losses to air and water. When used appropriately, soil health and total microbial biomass may increase 20-30+%, reducing the need for commercial fertilizer while improving water quality.
Overuse of nitrogen (N) commercial fertilizer has several effects. On legumes like hay and soybeans, overuse causes the rhizobia to get lazy so they produce less natural N. Soybean yields then suffer. Ideally, each soybean root segment should be packed with soil nodules. The abundance of soybean nodules is directly correlated to soybean yields. Too much commercial phosphorus (P) causes beneficial soil fungi (mycorrhizal fungi, MF) to also get lazy. The MF bring back mainly P but also water and many micronutrients to the plant in exchange for plant sugars. Often these deficiencies shows up in late summer when crops are trying to put on grain.
Plants require essential macronutrients (e.g., nitrogen, phosphorus, potassium) as well as secondary nutrients (calcium, magnesium, sulfur), and micronutrients (e.g., zinc, iron, manganese, copper, boron). The amounts of nutrients required by different plants and crops vary. Keeping soils supplied with diverse microbial species (biofertilization) allows for a continual flow of all nutrients for optimal plant growth and crop yield.
The problem today is that many of our soils have degraded. Originally our soils had 6-8-even 10-12% soil organic matter (SOM). Today 2-3% SOM is normal. With high higher crop yields, many nutrients are removed from the soil. However, the soil has vast reservoirs of many of these nutrients, it is just not plant available. That is where keeping a diversity of soil microbes thriving can greatly increase nutrient availability for higher crop productivity. In today’s high priced commercial fertilizer markets, it makes economic sense to rely on both organic and biofertilization to help reduce input costs.
Pre-World War II, crop nutrients were supplied mainly from organic sources such as manure or composted plant matter. Post World War II, chemical fertilizers became a major source of plant available nutrients (about 35%) for crops. Efficient use of nutrients is essential to avoid waste and reduce nutrient runoff loss to streams, rivers, lakes, and oceans.
Fertilizer applications need to be adjusted to soil type because soils differ in their capacity to hold onto nutrients. Sandy soils have less SOM while clay soils have higher levels of SOM. SOM has electrical charges that hold both soil nutrients and water, so they do not runoff the land. For improving water quality, N is easily leached or flows in runoff. For P, it can be absorbed to clay particles but is easily lost when it is soluble. Organic and biofertilization minimizes these losses. Both N & P are tied up by live roots, so cover crops improve nutrient use efficiency.
To improve soil health, you need diverse microbial populations. “Commercial fertilizers alter bacteria, fungi, protozoa, nematodes, earthworms, other soil fauna and plant pathogens. These organisms respond in different ways to the changed conditions; some increase in abundance, some decrease in abundance, and others are unaffected.” (Source: Australian Study On Fertilizer effects on Microbes) In general, the long-term effect is mostly negative because commercial fertilizer is too often over applied.