Nutrient Management

Of the eighteen elements needed by plants, only three—nitrogen (N), phosphorus (P), and potassium

(K)—are commonly deficient in soils. Deficiencies of other nutrients, such as magnesium (Mg), sulfur (S), zinc (Zn), boron (B), and manganese (Mn), certainly occur, but they are not as widespread. Deficiencies of sulfur, magnesium, and some micronutrients may be more common in regions with highly weathered minerals, such as the southeastern states, or those with high rainfall, such as portions of the Pacific Northwest. On higher-pH calcareous soils, especially in drier regions, keep an eye out for deficiencies of iron, zinc, copper, and manganese. In contrast, in locations with relatively young soil that contains minerals that haven’t been weathered much 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, they also cause widespread environmental problems. Poor soil and crop management; overuse of fertilizers; misuse of manures, sewage sludges (biosolids), and composts; and high animal numbers on limited land area have contributed to surface and groundwater pollution in many regions of the U.S. Because both N and P are used in large quantities and their overuse has potential environmental implications, we’ll discuss them together in chapter 19. 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 

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 whole-farm management, leading you to the goals of better crop growth and better

Environmental quality. If a soil has good tilth, no subsurface compaction, good drainage, adequate water, and a good supply of organic matter, plants should be healthy and have large root systems. This enables plants to efficiently take up nutrients and water from the soil and to use those nutrients to produce higher yields.

Doing a good job of managing nutrients on the farm and in individual fields is critical to general plant health and 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. Crops do not grow properly if nutrients aren’t present at the right time of the season in sufficient quantities and in reasonable balance to one another. Plants 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 that are under nutrient stress or growing abnormally—for

example, in the presence of too low or too high N levels— are not able to emit as much of the natural chemicals that signal beneficial insects when insect pests feed on leaves or fruit. Low K levels aggravate the stalk rot of corn. On the other hand, the pod rot of peanuts is associated with excess K within the fruiting zone of peanuts (the top 2 to 3 inches of soil). Blossom-end rot of tomatoes is related to low calcium levels, often made worse by droughty, or irregular rainfall or irrigation, conditions.

When plants either don’t grow well or are more susceptible to pests, that affects the economic return. Yield and crop quality usually are reduced, lowering the amount of money received. There also may be added costs to control pests that take advantage of poor nutrient management. In addition, when nutrients are applied beyond plant needs, it’s like throwing money away. And when N and P are lost from the soil by leach-ing to groundwater or running into surface water, entire communities may suffer from poor water quality.

ORGANIC MATTER AND NUTRIENT AVAILABILITY

The best single overall strategy for nutrient management is to enhance the levels of organic matter in soils (figure 18.1). This is especially true of N and P. Soil organic matter, together with any freshly applied residues, are well-known sources of N for plants. Mineralization of

P and sulfur from organic matter is also an important source of these nutrients. As discussed earlier, organic matter helps hold on to positively charged potassium (K+), calcium (Ca++), and magnesium (Mg++) ions. It also provides natural chelates that maintain micronutrients such as zinc, copper, and manganese in forms that plants can use. In addition, the improved soil 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.

IMPROVING NUTRIENT CYCLING ON THE FARM

For economic and environmental reasons, it makes sense for plants to more efficiently utilize nutrient cycling on the farm. Goals should include a reduction in long-distance nutrient flows, as well as promoting “true” on-farm cycling, in which nutrients return in the form of crop residue or manure to the fields from which they came. There are a number of strategies to help farmers reach the goal of better nutrient cycling:

• Reduce unintended losses by promoting water infiltration and better root health through enhanced management of soil organic matter and physical properties. Ways organic matter can be built up and maintained include increased additions of a variety of sources of organic matter, plus methods for reducing losses via tillage and conservation practices. In addition, apply only the amount of irrigation water needed to refill the root zone. Applying more irrigation water than needed can cause both runoff and leaching losses of nutrients. (In arid climates occasional extra water applications will be needed to leach accumulating

salts from the irrigation below the root zone.)

• Enhance nutrient uptake efficiency by care-fully using fertilizers and amendments, as well as irrigation practices. Better placement and synchronizing the application with plant growth both improve the efficiency of fertilizer nutrients. Sometimes changing planting dates or switching to a new crop creates a

Notes:

1. Sodium (Na) is considered an essential element for some plants.

2. Although selenium (Se) is not considered an essential element for plants, it is essential for animals and so the Se content of plants is important for animal nutrition. On the other hand, plants growing on high-Se soils (such as locoweed, asters, and saltbushes) accumulate enough Se to become toxic to grazing animals.

3. Silica (Si) is considered essential for the normal growth and health of rice.

cycles, the removal of agriculturally usable nutrients from the “waste stream” makes sense and helps develop more environmentally sound nutrient flows.

• Promote consumption of locally produced foods by supporting local markets as well as return-ing local food wastes to farmland. When people purchase locally-produced foods, there are more possibilities for true nutrient cycling to occur. Some community-supported agriculture (CSA) farms, where subscriptions for produce are paid before the start of the growing season, encourage their members to return produce waste to the farm for composting, completing a true cycle.

• Reduce exports of nutrients in farm products by adding animal enterprises to crop farms. The best way to reduce nutrient exports per acre, as well as to make more use of forage legumes in rotations, is to add an animal (especially a ruminant) enterprise to a crop farm. Compared with selling crops, feeding crops to animals and exporting animal products result in far fewer nutrients leaving the farm. (Keep in mind that, on the other hand, raising animals with mainly purchased feed overloads a farm with nutrients.)

• Bring animal densities in line with the land base of the farm. This can be accomplished by renting or purchasing more land—to grow a higher percentage of animal feeds and for manure application—or by limiting animal numbers.

better match between the timing of nutrient availability and crop needs.

• Tap local nutrient sources by seeking local sources of organic materials, such as leaves or grass clippings from towns, aquatic weeds harvested from lakes, produce waste from markets and restaurants, food processing wastes, and clean sewage sludges (see discussion on sewage sludge in chapter 9). Although some of these do not contribute to true nutrient

• Develop local partnerships to balance flows among different types of farms. As pointed out in chapter 9 when we discussed organic matter management, sometimes neighboring farmers cooperate with both nutrient management and crop rotations.

This is especially beneficial when a livestock farmer has too many animals and imports a high percentage of feed and a neighboring vegetable or grain farm has a need for nutrients and an inadequate land base for allowing a rotation that includes a forage legume. By cooperating on nutrient management and rotations, both farms win, sometimes in ways that were not anticipated (see “Win-Win Cooperation” box).

Encouragement and coordination from an extension agent may help neighboring farmers work out cooperative agreements. It is more of a challenge as the distances become greater

Some livestock farms that are overloaded with nutrients are finding that composting is an attractive alternative way to handle manure. During the composting process, volume and weight are greatly reduced (see chapter 13), resulting in less material to transport. Organic farmers are always on the lookout for reasonably priced animal manures and composts. The landscape industry also uses a fair amount of compost. Local or regional compost exchanges can help remove nutrients from overburdened animal operations and place them on nutrient-deficient soils.

 USING FERTILIZERS AND AMENDMENTS

There are four main questions when applying nutrients:

• How much is needed?

• What source(s) should be used?

• When should the fertilizer or amendment be applied?

• How should the fertilizer or amendment be applied? Chapter 21 details the use of soil tests to help you decide how much fertilizer or organic nutrient sources to apply. Here we will go over how to approach the other three issues.

Nutrient Sources: Commercial Fertilizers vs.

Organic Materials

There are numerous fertilizers and amendments that are normally used in agriculture (some are listed in Table 18.1). Fertilizers such as urea, triple superphosphate, and muriate of potash (potassium chloride) are convenient to store and use. They are also easy to blend

to meet nutrient needs in specific fields and provide predictable effects. Their behavior in soils and the ready availability of the nutrients are well established. The timing, rate, and uniformity of nutrient applications are easy to control when using commercial fertilizers. However, there also are drawbacks to using commercial fertilizers. All of the commonly used N materials (those containing urea, ammonia, and ammonium) are acid-forming, and their use in humid regions, where native lime has been weathered out, requires more frequent lime additions. The production of nitrogen fertilizers is also very energy-intensive—it’s estimated that N 

fertilizers account for 25% to 30% of the energy that goes into growing a corn crop. Also, the high nutrient solubility can result in salt damage to seedlings when excess fertilizer is applied close to seeds or plants. 

Because nutrients in commercial fertilizers are readily available, under some circumstances more may leach to groundwater than when using organic nutrient sources when both are used properly. For example, high rainfall events on a sandy soil soon after ammonium nitrate fertilizer application will probably cause more nitrate loss than if compost had been applied. (On the other hand, high rainfall events on a recently plowed-down alfalfa