Introduction

N-balance

N-fixation

Fertilizer N

Losses in hay, silage and animal products

Other components of the N balance

Introduction

	N is the most limiting nutrient in all of agriculture
	soil N is generally related to soil fertility - both are extremely difficult to measure and quantify (although N is so important, farmers have no test for soil N).
	soil N forms include NO3 and NH4, and also numerous organic-N molecules which vary in plant availability
	NO3 and NH4 are extremely soluble and don't stay in the soil for long.
	Forage production is poorly related to soil N, it is more closely related to flow through the soil N pool than the size of the soil N pool.

N-balance

	NB "balance" approach varies from a "cycling" approach
	For any site, change in soil N status = inputs - losses
	inputs = N-fixation + fertilizer N + release from soil organic matter + atmospheric deposition
	Losses = atmospheric losses + leaching + loss to plant & animal products

Legumes and N-fixation (pg 85)

symbiosis between legumes roots (nodules) and Rhizobium bacteria species specificity white clover and alfalfa can fix 150-250 kgN/ha/yr crop rotations
	rhizobia nodules on alfalfa
Key attributes of legumes N2 -fixation to grassland and subsequent crops high nutritive value (digestibility and protein), hence high animal intake
Disadvantages unreliability (pests, competition, drought) potential antiquality problems (bloat)

Fertilizer N

Fertilizer nitrogen (pg 273) Anhydrous (crops only) 82-0-0-0 Urea 46-0-0-0 NH4NO3 34-0-0-0 (NH4)2SO4 21-0-0-0 manure 0.5% N for cattle, 1% N for poultry fertilizer N suppresses N fixation "rule of thumb" - every 2 lb of N applied will prevent 1 lb of legume N split dressings (20-50 kgN per application) strategic applications (new pasture or in fall for winter stockpile) responses to applied N are 0-60 kg DM/kgN - typically 20 kg DM/kg N

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Losses in hay/silage and animal products

	Removal in hay/silage or animal products Often underestimated - but is usually the greatest loss of N from the soil
	Hay typically 25% protein (protein is 16% N = hay is 4%N) Typical alfalfa harvest 4 crops each of 1.5 ton/acre (12,000 lb/acre, 13.4 T/ha) = 480 lbN/ac (540 kgN/ha)
	Cow-calf operation Grazing 1 cow per acre, producing a 600 lb (275 kg) stocker calf per acre 60% protein (16%N) = 58 lbN/acre

Other Components of the N-balance

	Atmospheric deposition = pollution 20 kgN/ha/yr in North America 50 kgN/ha/yr in Europe
	Volatilization Release of NH3 from fertilizers - results in inefficiency of fertilizer application Unless nitrogenous fertilizers are washed into the soil by rainfall soon after application they can "evaporate" as NH3 and reduce the fertilizer response
	Leaching Not usually significant for most soils (being N deficient) Can be extremely high if total N inputs (fixation and fertilizer) exceed 250 kgN/ha Is usually high in areas with repeated applications of effluent
	Nitrification & Denitrification An environmental issue. In extreme cases can account for up 30% of N losses from soil Nitrification = oxidation of NH4 to NO3 Denitrification = reduction of NO3 to N2O, NO2 and N2 (released to the atmosphere), high in wet soils

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