Soil fertility is a topic that is much discussed, but poorly understood.
From the perspective of crop production, soil fertility at its core is determined by soil life. Conventional agronomy as it is taught in most formal university settings does not prioritize soil life as the central force, and it could be argued that this is causal in the drastic decrease in agricultural soil fertility worldwide. Conventional fertility and management protocols are in many ways very destructive to soil life, and due to this have effected desertification, erosion, pollution of waterways, aquifers, and the environment in general. The following analysis does not accept conventional agronomy as sufficient.
Soil fertility from the growers perspective correlates with the ability of the crop plant to get its nutritional needs met, and to flourish. To understand how this works the first central point that must be understood is that all plants in nature, and in healthy environments have well established multi-speciated symbiotic relationships with soil and leaf life. In nature, plants produce sugar through photosynthesis that is then fed by the plant to numerous species of bacteria and fungi that use that sugar to reproduce and access minerals out of the soil and air environment that are digested and then fed back into the plant and soluble organic compounds. This symbiotic relationship is at the core of soil fertility, and must be understood as the central force at work.
With this symbiotic relationship understood, then soil fertility management becomes a process of understanding what components are critical to the crop system at hand which are not present and then addressing them. In many cases key minerals that are enzyme cofactors and critical for biological system function are not present or insufficiently present and become limiting factors. Many soils are low in boron, or sulfer, or perhaps cobalt and molybdenum, and because these minerals are missing cirtical biological functions are inhibited. The BFA recommends that growers use a comprehensive soil test that tests these minerals as part of a systemic fertility plan.
Soil life species are another important limiting factor in fertility management, and due to historical environmental factors many critical species are simply not present in agricultural soils. Microbiologists estimate that there may be as many as 1,500,000 species of soil fungi, and 3,000,000 species of soil bacteria. While not all of these species are found in any one area, or needed by any one plant, the fact that most agricultural soils have no more than 5,000 species present in total in many cases means that critical biological pathways that ensure overall system health are broken. The BFA recommends that growers use biological inoculants on seed, and at planting and transplanting in an effort to address these system issues.
The next step critical to soil fertility function is to ensure that the environment that soil life need to thrive is established in the cropped area. Heavy or deep tillage is a highly destructive process when it comes to soil life establishment and should be minimized in an effort to not regularly destroy soil life populations that may be building up in soil on a seasonal basis.
Other key components that need to be attended to are sufficient carbon or orgnic matter in the soil which serves as habitat for soil life, and air and water in the soil. The species of soil life that are the critical plant symbiotes are in many cases aerobic, which means that they need air to breate, and they also need water for their systems to function. It then becomes incumbent upon growers to ensure that after the minerals and biological species are present, that they have the air, water and carbon they need to thrive. Environmental conditions where these components are limiting factors are conditions where overall fertility is threatened.