Optimal plant performance depends on good soil health, which is determined by the interaction of a soil’s key components, including its structure, chemistry and biology. When these components are in balance, a soil is healthy, which will become evident in the performance of your crops. In this article, I will discuss amendments that can boost soil health by improving its tilth, fertility and biology.
Soil structure (tilth), involves the soil’s physical characteristics. Soil is composed of mineral and organic components, but the remaining pore space must contain moisture and air to have good tilth. Soil should have 5% organic matter, 45% minerals, 25% water and 25% air. Cultivation will break up compacted soil, but soil amendments containing gypsum or organic matter will also improve tilth. Cover cropping, or just adding organic matter, will improve soil aggregation and therefore aeration, and will also improve its water infiltration and water-holding capacities.
Microbial mixtures have also been shown to aggregate soil and thereby improve tilth. Mycorrhizae also cause soil aggregation by binding smaller particles into larger ones. Improving tilth allows roots to penetrate further into the soil profile and receive the necessary amounts of air to be healthy and functional. Amendments to improve tilth will also improve many biological aspects of soil by increasing populations of beneficial microbes such as mycorrhizal fungi, antagonists to pathogens and specific plant growth-promoting bacteria.
Soil fertility (chemistry) involves more than just supplying the necessary levels of N-P-K (nitrogen, phosphorus and potassium) and other minor elements. Soluble inorganic fertilizers are ions that either bind to soil particles or stay in the soil solution. Those that stay in solution may leach out by rain or irrigation and may need to be replaced.
Soils that have lost important elements by leaching need to be re-mineralized to correct any deficiencies. This is accomplished by adding amendments with nutrient-rich materials, such as rock dust or kelp meal—materials known to enhance the activities of beneficial microbes in the soil. Organic fertilizers require microbial activity to break them down in the soil, which means they are released for plant use slower than chemical fertilizers are.
Nutrients must be in the soil solution to flow to the roots for absorption. Elements such as phosphorus, copper and zinc are quickly bound to clay or form precipitates, making them immobile in soil. Some microbial amendments can solubilize bound elements, usually by producing acids. If microbial activity in the root zone is high, leachable nutrients will be retained in the root zone, meaning less fertilizer will be lost in the run-off. Overall, the soil becomes become more efficient at doing its job.
Soil nutrient availability to plants is also affected by pH. Some soils have pH levels that make some essential nutrients unavailable for plant uptake. Modifying the pH by adding lime to raise it or sulfur to lower it might be required to optimize soil health and enhance plant growth.
Some soils are also naturally high in salts that can be toxic to plants. In addition, application of fertilizers over time can lead to an accumulation of fertilizer salts that can inhibit plant growth when they become excessive. Opening up the soil by adding amendments with gypsum, organic matter or some microbial products, followed by leaching the excess salts, can help. Salt concentrations can be monitored by a conductivity probe (EC meter). Some of the benefits of reducing salinity also come as a result of changes in microbial populations and their activities.
Soil contains many kinds of organisms, such as bacteria, fungi, insects, nematodes, protozoans and earthworms. Some microbes are deleterious (pathogens), while others are beneficial (pathogen antagonists and mycorrhizal fungi).
Bacteria – Bacteria are the most numerous of the soil microbial components, and many affect plant growth and health. They cycle nutrients in the rhizosphere soil around plant roots, and they affect soil chemistry and structure. Some capture (fix) atmospheric nitrogen and convert it for plant use.
Others are plant growth-promoting rhizobacteria, and many are antagonistic to soil-borne pathogens, a function that may enhance plant growth. These rhizobacteria can enhance plant growth on their own by various mechanisms, but they also function in tandem with mycorrhizae. Mycorrhizal fungi also team up with nitrogen-fixing bacteria, such as rhizobium, resulting in more root nodules that capture atmospheric nitrogen that becomes available for plant use.
Some bacteria can inhibit soil-borne pathogens, thereby reducing the incidence or severity of root diseases. Root-infecting pathogens must initiate root disease by out-competing other microbes in the rhizosphere soil next to the root.
Beneficial bacteria are known to produce pathogen-inhibiting antibiotics of various chemistries, or out-compete the pathogen for space on the roots or for nutrients needed by the pathogen to infect the roots. Some may induce the plant to produce its own inhibitory chemicals that can prevent root infections. The amount and activity of these bacterial antagonists increases in the soil around roots that are mycorrhizal (the mycorrhizosphere soil).
Soil fauna – Soil is alive with various animals, including nematodes, insects, protozoans and earthworms, many of which are part of the soil food web. Nematodes can damage roots, impairing their function. Earthworms in soil indicate a healthy soil because of their well-documented function in improving soil tilth. Amending soil with worm castings can enhance plant growth as a result of improved fertility and the inclusion of a range of beneficial microbes.
Fungi – Soil fungi are diverse and can be antagonists, nutrient cyclers and pathogens. Antagonistic fungi such as trichoderma can suppress fungal infections of roots. Mycorrhizal fungi are key components in the rhizosphere of soil biology. They form a symbiotic relationship with plant roots, called mycorrhizae, that becomes the interface between roots and soil and profoundly affects plant growth and health. Among many benefits to plants, mycorrhizae enhance nutrient uptake and fertilizer-use efficiency; increase plant tolerance to soil toxicities (salinity), soil drought and diseases; improve soil tilth and increase the quality of the produce.
Some plants are highly dependent on mycorrhizae to aid in the acquisition of water and nutrients, while others benefit from mycorrhizae the most under stressful environmental conditions. Mycorrhizal fungi should be established early in the crop production cycle either by inoculating in-furrow with granular forms of inoculum, drenching inoculum into the root zone, or inoculating transplants.
Pre-inoculation of transplants with mycorrhizal fungi improves their chances of establishment and growth compared to plants that aren’t given these beneficial fungi. Two points of caution:
- Formation of mycorrhizae can be reduced by some pesticide drenches and by applications of high levels of phosphorus fertilizers
- Soil fumigation kills these fungi, along with many other useful microbes.
The actions required to improve soil health should be based on what crops are being grown, and the results of an assessment of soil tilth, fertility and biology, all of which can be improved with the steps outlined here. To re-cap, soil tilth can be improved by cultivation, cover cropping and amending with organic matter, gypsum and microbial solutions.
Soil fertility can be improved by mineral and organic matter amendments, microbial inoculations and adjustments in fertilizer applications, that is, by applying fertilizers when the plant needs nutrition.
Fertility also can be improved by making pH adjustments to assure nutrient availability, and by amending with materials to re-mineralize the soil. Cover cropping with a legume (clover, vetch, etc.) can also increase the available nitrogen in the soil due to fixation of atmospheric nitrogen.
Soil biology can be improved by the practices above on improving soil tilth and fertility, but also by organic matter amendments and by inoculations with beneficial bacteria and mycorrhizal fungi. The benefits to plants with mycorrhizae are numerous, especially if applied early in the plant’s life, but there are also many reasons why organic matter is important, too, if not essential.
Organic matter improves soil texture and its water-holding capacity. It is a source of nutrients, and it improves the capacity for beneficial microbes to function. Soil microbes can capture and hold nutrients in the root zone and help suppress soil-borne pathogens.
Amending with composts, worm castings and even organic fertilizers will increase the microbial diversity in the soil, and that increased diversity will likely include some plant growth-enhancing microbes. The key is to add these microbes to the roots as early as possible.
The key to successfully improving soil health is employing multiple tactics to achieve the balance needed between soil tilth, fertility and biology. What comes next is healthier soil that will lead to increased crop productivity and economic sustainability.