Pasture Improvement: Fertilizing Pastures
Excerpt from Publication 19, Pasture Production, Order this publication
Table of Contents
To grow and remain productive plants require a continuous supply of water, sunlight and nutrients. Fast growing, productive pastures have high demands for mineral nutrition.
The mineral elements needed by plants are classified as:
The macronutrients are nitrogen, phosphorus (phosphate) and potassium (potash). The 10 micronutrients are calcium, magnesium, sulphur, boron, chlorine, copper, iron, manganese, molybdenum and zinc. Nutrients are supplied to plants from the soil. If the soil does not contain the proper amount and balance of elements, the nutrients can be acquired from manure, fixed from the air by legumes (nitrogen only) or applied as commercial fertilizers. All elements applied to the soil enter the nutrient cycle and may eventually become available to the plants.
Plants must have a healthy root system to use available soil nutrients. Overgrazed plants, with small, weak root systems, cannot take advantage of improvements in soil fertility.
The lack of nitrogen limits the growth of most pastures more than any other essential element. As a major constituent of plant proteins, nitrogen is necessary for plant growth and metabolism. Plants lacking nitrogen are slow growing, small and light green in colour. Pastures that are nitrogen deficient are unproductive, slow to recover after being grazed and offer a poorer quality feed to grazing animals.
Ideally, nitrogen should be supplied to a pasture from the fixation of nitrogen in the air by bacteria living in nodules on the roots of legumes. This source of nitrogen is continuous throughout the growing season, provides all the nitrogen required by the legumes and some for the grasses, leaves overwintering nitrogen in a nonleachable organic form and is low priced. Pastures with a legume content greater than 50% require no additional nitrogen for good yields. A small amount (25 kg/ha) of nitrogen can be used on legume-grass pastures in the spring in order to get the grass off to a fast start and provide early grazing.
As grass content in a pasture increases, so does the need to add nitrogen - in a readily available form, either as manure or commercial fertilizer.
You'll obtain high yields from grass pastures more efficiently if you use several small applications of nitrogen rather than one large one. Frequent applications also produce a more uniform distribution of yield over the grazing season. Even in seasons with a dry period, production is better when nitrogen is applied 2 or 3 times, rather than in one large application. Nitrate poisoning is a potential hazard when grasses receive nitrogen prior to a period of poor growth. (See Chapter 7, Animal Health Problems.)
The minimum amount of nitrogen needed to produce a yield response is 50 kg/ha.
Do not apply more than 75 kg/ha per application. Above this rate increases the chance of nitrate poisoning, particularly if the nitrogen is applied in summer or fall.
Fast-growing plants make the best use of applied nitrogen. Demand for nitrogen is highest in the spring and immediately after grazing. Apply nitrogen within 1 week of grazing. In rotations with short rest periods (less than 30 days), use smaller nitrogen applications since the grass does not have enough time to efficiently make use of large amounts of nitrogen.
Manure can be used on pastures. Its effectiveness depends on:
Apply manure when plants are small and ready to grow quickly. Early spring is best, but fall is usually most convenient. Spread manure thinly. Clumps of manure, like manure pads, smother small plants or shoots. Animals may refuse to graze the treated area if manure is applied during growing season. To prevent selective grazing, treat the whole pasture with manure. Over-application of manure, as with other sources of nitrogen, can cause nitrate poisoning or grass tetany.
Do not use hog manure on sheep pastures because of the possibility of copper toxicity.
Plants require phosphorus to support new growth and good root development. Requirements for phosphorus are highest during seedling development and initiation of growth. Established plants with phosphorus deficiency show purpling of upper leaves. This is often seen in grasses in early spring. The cool, wet soil retards root growth and the plant is incapable of extracting sufficient phosphorus from the soil. The leaves turn purple along the margins and tips.
Deficiency symptoms at this time may be more of an indication of the weather than an actual deficiency in the soil. An annual application of phosphorus to an established pasture is an efficient method of overcoming a phosphorus deficiency. Supply phosphorus in an inorganic form, such as commercial fertilizer, or from slow-releasing ground rock phosphate. Manure is not a good source of phosphorus. Grasses are unable to make use of added phosphorus unless there is an adequate supply of nitrogen.
Seedlings deficient in phosphorus may exhibit a purple colour on new growth. Leaves fail to expand and the seedlings are not competitive. This can allow other species to dominate the sward at the expense of the planted species. For this reason, it is important to ensure the presence of sufficient phosphorus in the soil prior to establishing a new pasture or when renovating a pasture. Best results are obtained when phosphorus is incorporated at time of seeding, as phosphorus moves slowly in the soil. Banding phosphorus gives small plants ready access to a good supply of this essential nutrient.
Most Ontario soils are low in phosphorus. Improving soil phosphorus levels results in better plant growth, increased palatability, feed intake and digestibility.
Potassium is referred to as the regulating element in plants. It is essential for plant growth and reproduction. The major symptoms of plants grown on low potassium soils are slower growth and reduced root reserves. On pastures, this results in lower production as plants do not grow as rapidly in the spring or after grazing. Legumes require high potassium levels; low potassium levels can contribute to legume loss from the pasture.
Some Ontario soils have high levels of potassium and pastures grown on them require no additional sources. Animal manures are a good source of potassium and regular applications reduce the need for fertilizer. Apply potassium as part of a spring fertilizer (i.e. 18-6-18) or as muriate of potassium (i.e. 0-0-60) in late summer, to stimulate storage of root reserves.
Legumes (especially alfalfa) can be luxury consumers of potassium. If excessive levels of potassium are available in the soil, plants extract potassium in preference to elements such as calcium and magnesium. Plants with elevated potassium levels and low levels of magnesium can cause grass tetany. If large amounts of potassium fertilizer are required, apply them in the fall to reduce the chance of grass tetany occurring.
Magnesium is a minor element that is sometimes deficient in pasture forage. It is essential to plant and animal metabolism and deficiency in the plants can result in grass tetany in the animals. This is a concern in spring when grass pastures are growing rapidly and their roots cannot take up sufficient magnesium to maintain normal levels in the shoots. Pastures growing on soils formed from calcitic or acid parent materials are most at risk, but grass tetany should not be discounted during rapid spring growth in pastures on dolomitic limestone. Magnesium is sometimes added to pastures as a fertilizer, but is usually applied as dolomitic lime.
Sulphur is a minor element required for plant growth and is deficient in Ontario soils. Plants deficient in sulphur are a lighter green to yellow, with leaf chlorosis. Legumes, especially clovers, are most sensitive to low levels of sulphur. While Ontario soils are deficient in sulphur, plants rarely show deficiency symptoms as fields receive sufficient sulphur from acid precipitation. Areas of northwestern Ontario (districts of Thunder Bay, Rainy River and Kenora) do not receive sufficient sulphur in this form for good plant growth. It must be added as a supplement to lime or fertilizer.
Selenium is not necessary for plant growth, but is required by animals. Ontario soils have very low levels of selenium and animals eating feeds grown on these soils need more selenium to prevent white muscle disease.
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