6.2: How Nutrients Flow

The purchase of plant food is an important matter, but the use of a [fertilizer] is not a cure-all, nor will it prove an adequate substitute for proper soil handling.

—J.L. Hills, C.H. Jones and C. Cutler, 1908

Most of the essential nutrients for plants, animals, and humans are derived from weathered minerals in the soil. But plants also absorb carbon, oxygen, and hydrogen from the air and water. Nitrogen is derived from the atmosphere by legumes, but other plants absorb it from the soil. Of the 17 elements needed by all plants, only three—nitrogen (N), phosphorus (P), and potassium (K)—are commonly deficient in soils. Deficiencies of sulfur (S) are less prevalent but not uncommon. Other nutrients, such as magnesium (Mg), zinc (Zn), boron (B), and manganese (Mn), can be lacking in certain regions. Deficiencies of sulfur, magnesium, and some micronutrients may be more common in regions with highly weathered minerals, such as the southeastern United States and many parts of the tropics, or those with high rainfall, such as portions of the Pacific Northwest. Sulfur deficiency is especially common on the sandy soils on the coastal plains of the Southeast and has become more common in areas with low organic matter soils with the decrease in sulfur air pollution from coal-burning power plants. Keep an eye out for deficiencies of iron, zinc, copper, and manganese on higher-pH calcareous soil, especially in drier regions. Low phosphorus availability is also common in calcareous soils. In contrast, in locations with relatively young soil that contains minerals that haven’t been extensively weathered by nature, such as glaciated areas with moderate to low rainfall like the Dakotas, K deficiencies are less common.

Environmental concerns have resulted in more emphasis on better management of N and P over the past few decades. While these nutrients are critical to soil fertility management, their mismanagement also causes widespread environmental problems. In many regions of the United States and other countries, surface and groundwater pollution has been caused by poor soil management, overuse of fertilizers, mishandling of manures, sewage sludges (biosolids) and composts, and high animal numbers on limited land areas. Because N and P are used in large quantities their overuse has potential environmental implications. Other nutrients, cation exchange, soil acidity (low pH) and liming, and arid and semiarid region problems with sodium, alkalinity (high pH), and excess salts are covered later.

The Bottom Line - Nutrients and Plant Health, Pests, Profits, and the Environment

Management practices are all related. The key is to visualize them all as part of whole-farm management, leading you to the goals of better crop growth and better quality. Plants should be healthy and have large root systems if the soil has good tilth, no subsurface compaction, good drainage, adequate water, a good supply of organic matter, and a thriving soil biological community. This enables plants to efficiently take up nutrients and water from the soil and to use those nutrients to produce higher yields. Higher yields also imply indirect benefits like more carbon capture from the atmosphere and better water cycling.

Doing a good job of managing nutrients on the farm and in individual fields is critical to general plant health and the management of plant pests. Too much available N in the early part of the growing season allows small-seeded weeds, with few nutrient reserves, to get well established. This early jump-start may then enable them to out-compete crop plants later on. Restricted plant growth may occur if nutrients aren’t present at the right time of the season in sufficient quantities and in reasonable balance with one another. Plants under nutrient stress may be stunted if nutrient levels are low, or they may grow too much foliage and not enough fruit if N is too plentiful relative to other nutrients. Plants under nutrient stress grow abnormally, for example, in the presence of too low or too high N levels, and are not able to emit as much of the natural chemicals that signal beneficial insects capable of fighting insect pests that feed on leaves or fruit. Low K levels aggravate stalk rot of corn and winter damage to bermudagrass. On the other hand, pod rot of peanuts is associated with excess K within the fruiting zone of peanuts (the top 2–3 inches of soil). Blossom-end rot of tomatoes is related to low calcium levels, often made worse by droughty conditions, irregular rainfall, or poor irrigation.

Economic returns will be reduced when plants don’t grow well. Yield and crop quality usually are lower, reducing the amount of money received. There also may be added costs to control pests that take advantage of crops with poor nutrient management. In addition, when nutrients are applied beyond plant needs, it’s like throwing money away. Entire communities may suffer from poor water quality when N and P are lost from the soil by leaching into groundwater or running into surface water.

The 4Rs Of Nutrient Stewardship

The risks of high environmental impacts and lower crop yields are reduced when fertilizer materials are properly managed. The concept of 4R nutrient stewardship is a set of principles for good nutrient management (maximizing nutrient-use efficiency and minimizing environmental impacts) that recognizes that the best practices vary by local soil, climate, and management factors. The 4Rs encapsulate the practices that we discuss in this chapter:

• Right fertilizer source at the
• Right rate, at the
• Right time, and in the
• Right place

Taking this concept even further, 4R-Plus combines the 4R management practices with conservation practices that enhance soil health and improve the environment. 4R and 4R-Plus are therefore useful concepts that summarize some of the multi-faceted concepts we discuss in this book.

Organic Matter and Nutrient Availability

The best overall strategy for nutrient management is to enhance the level of organic matter in soils (Figure \(\PageIndex{1}\)). This is especially true for N and P. Soil organic matter, together with any freshly applied residues, are well-known sources of N for plants. (However, unusual residues with high C:N ratios can reduce N availability for a period of time.)

Mineralization of P and sulfur from organic matter is an important source of these nutrients. Also, organic matter helps hold on to positively charged potassium (K⁺), calcium (Ca⁺⁺), and magnesium (Mg⁺⁺) ions and provides natural chelates that maintain micronutrients such as zinc, copper, and manganese in forms that plants can use. In addition, the improved soil structure (tilth) and the growth-promoting substances produced during organic matter decomposition help the plant develop a more extensive root system, allowing it to obtain nutrients from a larger volume of soil. A wide diversity of soil organisms helps maintain low populations of plant pathogens.

Figure \(\PageIndex{1}\): Influence of residue decomposition on nutrient availability.

Cover crop roots (living soil organic matter) also contribute to nutrient management. They provide energy material that allows soil organisms to thrive and mobilize soil nutrients, keep nutrients from being lost by leaching or runoff, add new N to the soil (if a legume), and maintain plentiful supplies of mycorrhizae spores that lead to better inoculation of the following crop, helping it to take up soil nutrients.