Forage Crops

HCS412 Forage Crops
Soil fertility and fertilizer- Chapter 12 (pg 276-283)

Introduction to soil anions

sulfur

phosphate

	NO3; SO4; PO3
	in order of decreasing solubility
	in order of decreasing abundance in forage, N averages 1-4%, S averages 0.25%, P averages 0.2%
	in order of decreasing nutrient flow rate	relative pool sizes and flow rates of anions click to enlarge (101kb)
	in order of increasing nutrient pool size

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Sulfur

	most USA soils are adequate, only leached and sandy soils are deficient
	many fertilizers contain S, and correct marginal situations for "free"
	acid rain contributes 6-22 kgS/ha/yr
	soil test <7 ppm is deficient; >12 ppm is adequate
	critical levels in plants are 0.16%, average levels are 0.2-0.3%

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Phosphate

· Essential for plants and animals: ATP, DNA, pentose sugars, nodulation

· More important for legumes than grasses (grasses have a finer root system, resulting in higher root surface area, hence more competitive for most nutrients)

· soil concentrations
5-300 ppm (2.5-150 lbP/ac)
forage responses up to 50 ppm (Law of diminishing returns)

pg 278 (Ludwick and Rumberg 1976 Agron. J. 68:933-37)

· in fertilizer is measured as P2O5

	Molecular Weight
P	30.97	61.94	43.6%
O	15.9994	80.00
		141.94

· in forage 0.2-0.3% is adequate for plant and animal requirements

· Grazing removes P approx. uniformly (not totally) from pasture and concentrates it as solid manure in piles comprising less than 5% of the land area. This can result in a lower efficiency of P cycling with grazing.