The critical level, defined as the range of nutrient concentrations in a plant sample below which crop output is considerably decreased, differs amongst forage crops. The connection between fertilizer availability and agricultural production. The term "critical range" refers to the nutritional level below which considerable yield decline is anticipated. The precise levels of each element within this range vary depending on the crop species and individual variances within the same species.
For example, the critical nitrate concentration for most grasses is about 0.5-1.0 mg/L. At lower concentrations, the plants can absorb some phosphorus and potassium. But at these low levels of nitrogen, they cannot absorb much phosphorus or potassium. Above this range, reductions of yield have been reported.
The critical phosphate concentration for most crops is generally between 5.25 and 17.5 ppm. If the concentration dips below this range, absorption of phosphorus by the plant will be reduced. The amount of available phosphorus that can be taken up by plants is limited by the rate at which it is lost from soil through leaching or erosion. Therefore, farmers must ensure that the soil contains enough phosphorous to produce maximum yields with the input of fertilizers.
The critical sulfur concentration for wheat is about 2.5-3.0 mg/kg. If the concentration of sulfur drops below this range, the plants will not be able to use all of the minerals in the fertilizer.
A combination of performance potential and feed intake influences nutrient needs. To satisfy the needs at each stage of production, the nutrient content in the diet is modified based on feed consumption. Nutrient requirements for maximum potential growth are called optimal requirements. Optimal requirements vary depending on the type of animal and its expected use. For example, milk producers need more calcium and vitamin D than meat producers.
Animals can only absorb a certain amount of nutrients at any one time, so their nutritional requirements change as they grow and develop. The body uses some of the nutrients it takes in through food to work with enzymes produced by digestive organs such as the pancreas and intestines to maintain healthy tissue cells. The rest are used for vital functions such as producing new blood cells, muscles, and nerves.
For example, if an adult cow eats only grass and no supplemental feeds, she would be deficient in both calcium and phosphorus because her body cannot make these nutrients itself. To meet her optimal requirements for bone health, a dairy cow needs to consume about 1 to 1.5 percent calcium in the form of hay or pasture plus a calculated additional amount in the form of milk replacer or yogurt. If she did not get enough phosphorus, her bones would be thin and brittle.
A nutrient budget is a numerical (quantitative) assessment of the rates of nutrient intake and outflow to and from an ecosystem, as well as the amounts present and transported within the system. Figure 5.1 depicts the essential components of a nutrition cycle. The diagram's outside edge denotes the limitations of an ecosystem. Nutrients are the inputs into this system. They can be obtained from two sources: natural processes or human activities. For example, nitrogen is lost only when it isn't taken up by plants, so it is considered an active nutrient; phosphorus is lost when water flows over soil, so it is considered an active nutrient. Both are required in small quantities for healthy plant growth. Other nutrients such as potassium and magnesium are needed in larger quantities and thus are referred to as major nutrients. They are not lost through surface runoff or gaseous emissions.
The middle section of the diagram shows the pathways by which nutrients move through an ecosystem. Some nutrients are lost to surface runoff, but most are not. Nitrogen and phosphorus are examples of nutrients that are often found in surface waters at concentrations high enough to support plant growth. Other nutrients such as calcium, iron, and zinc are rarely found in surface waters because they are lost through weathering of rocks or deposited in groundwater systems. Still other nutrients such as sulfur are rarely found in surface waters because they react with oxygen in the atmosphere or organic matter in the sediment to form compounds such as sulfates or hydrogen sulfide that are then released into the environment.