Introductory soil science students have been taught, and wise farmers have observed, that productive soils are fertile soils; but not all fertile soils are productive. That is because poor soil physical and microbiological conditions can impair soil productivity and limit crop yields even in the presence of a good nutrient supply.
Today, we are hearing the term “soil health” mentioned more and more frequently. The term implies many different things to different audiences … and different scientists. For example, the Soil Science Society of America has a formal definition of soil quality, but no current formal definition for soil health in its Glossary of Soil Science terms. This is in part because indices of soil health are not easily quantified for various regions, soils, and cropping systems. The USDA NRCS currently uses an operational definition of soil health: “The capacity of the soil to function as a vital living ecosystem that sustains plants, animals, and humans.”
Although there is a lack of a uniform definition — or scientific consensus on a national or regional standard index for soil health — there appears to be general agreement among scientists and crop production practitioners that good soil physical and biological and chemical conditions are equally important for sustainable crop production.
The Risk Of Lost Productivity
There is also strong agreement that neglected soil fertility results in lost productivity; an all-too common problem in parts of Africa and some developing nations, but also a concern right here at home in the U.S. Consider for example, that summaries of more than 2.7 million soil samples in North America by the International Plant Nutrition Institute (IPNI) showed that soil test phosphorus (P) levels had declined an average of 6 parts per million (ppm) from 2005 to 2010; mainly in the Corn Belt and the Central Great Plains. Those declines in soil test P could largely be explained by the cumulative crop harvest P removal exceeding P inputs. Similarly, median soil test potassium (K) levels declined an average of 4 ppm between 2005 and 2010; and indicated that 50% or more of the sampled areas likely needed K application to avoid crop yield losses, especially east of the Mississippi River and in the provinces of eastern Canada.
When P, K and other essential plant nutrients are below agronomic optimum levels, the result is poor or impaired crop growth, limited biomass production, and reduced return of crop residues to the soil. Continued declines in soil fertility, and neglect of appropriate fertilization and liming threaten good economic returns and soil health.
Wise fertilizer use and 4R nutrient management sustain soil fertility and help stimulate greater crop biomass production, and provide for increased crop residue return to the soil. They also encourage better cover crop root system establishment, seedling vigor, and more rapid development of soil cover. Better nourishment of principal annual crops and cover crops helps improve and sustain soil organic matter levels. Sustained or improved soil organic matter levels also favor protection of desirable soil physical conditions (i.e., good tilth, soil structure, aggregation). Living organisms in the soil reflect soil biological health and diversity and require a steady supply of crop residues to help sustain and cycle soil organic matter. Proper soil fertility management, combined with other good soil conservation and cropping system management practices, leads to good soil biological activity.
Soil organic matter has long been known to be the backbone of soil fertility maintenance; especially in sandier soils. Desirable soil organic matter levels, aid infiltration of rainfall and irrigation and raise soil moisture holding capacity and timely release; which helps both in dry periods and in wet periods. With frequent tillage, and under warm, moist conditions, soil organic matter decomposes more rapidly. That is why warm, moist regions of the U.S. which may produce the same or greater amounts of crop residues than in cool and dry regions, face a more difficult struggle just to maintain soil organic matter levels.
Research in the last few decades has revealed that the soil “glue” that holds soil particles together in aggregates and forming structural units, is produced by specialized fungi that live within most crop roots (a root-fungus symbiosis called mycorrhizae) and extend into the nearby soil; essentially enlarging the root sorptive surface for nutrients and moisture. High soil fertility may reduce some of the dependence of cultivated crops on such mycorrhizal fungi, but it does not eliminate that beneficial association.
In the face of growing global food, fiber and biofuel demands, agriculture must strive to produce better harvested yields and crop quality, while also providing protective crop residues, and conditions that foster soil organic matter sustainability. Good soil organic matter levels … which vary by geographic region, soils, and climate … favor healthy soil biological conditions. Limited soil tillage, and avoidance of soil-compacting traffic in fields, helps sustain desirable soil physical and biological properties.
As with a three-legged stool, increased soil organic matter, its stability, and maintenance … are not achieved unless soil biology, soil physical conditions and soil fertility are optimized. Proper 4R soil fertility management is central to soil health, and within each farmer’s reach. Soil management knowledge and crop production technologies have increased over the past, but we have not yet achieved our very best. Plan to help farmers identify the soil health goals on their farms, and consider the choices and changes they can make. It’s through a team effort of the crop adviser and extension agent or conservationist with the farmer that the “sweet spot” of balance in soil physical, soil biological and soil chemical conditions can be achieved.
Remember — good soil health and profits are compromised without good soil fertility.