Supplementing pastures with nitrogen

Nitrogen (mineral element N) is the main mineral component of plants. Nitrogenous nutrition aims to provide the necessary nutritional elements to ensure multiplication of plant cells and tissues which constitute plant dry matter (DM) (DM= total amount of matter – water content). Nitrogen also plays a part in favouring absorption by the plant of other nutritional elements in the soil.

• For non-leguminous plants, nitrogen is absobed from the soil in two different forms:
  • Ammonium nitrate in the form of ammonium NH₄⁺ ion, fixed by the soil. It is only available to the plant when near the roots.
  • Nitrogen in a nitric form, or nitrate (NO₃-) : more advantageous to the plant, it is mobile in the soil because soluble in water. It is therefore more readily available to the plant’s roots.
     
• N₂ (from the atmosphere) is absorbed by bacteria on the root knots of leguminous plants. This type of plant does not need a mineral form of nitrogen fertiliser.

Moreover, maximum efficiency of nitrogen fertiliser is only obtained when other nutritional elements (P -K) are not restrictive.

Why fertilise ?

The action is threefold:                                             

• Soil enrichement: The effect is to add structure to the soil. It increases the Capacity of Exchange of Cations (CEC) and the useful reservoir (Capacity of the soil to retain water). It contributes to soil fertility.
• Fertilising: It provides the elements N, P and K, as well as the micro-nutrients the plant needs to develop and grow.
• Living matter: it is composed of micro-organisms (bacteria, fungus,….) which will contribute by mineralising organic matter, i.e transforming it into nutritive ions  (NO3-, HPO42-) for the plant.

Manure is best spread on mowed  fields used for hay, as fodder exports mineral elements, whereas grazing animals will restitute organic matter in the form of droppings and urine.

For grass (Poaceae) species fertilising increases:

• Precocity : faster production of dry matter
• Productivity : Greater total production per plant life cycle.

For grazing

• Nitrogen fertilising is recommended when grazed intensively in the Spring. E.g  rotational grazing, when grazing load is  < 1 acre (30 – 40 ares)/ LSU (large livestock unit).
• Nitrogen fertilising is unneccessary when during spring grazing is more extensive. E.g continuous grazing with a grazing load > 1 acre /LSU

For fodder production

To produce high fiber fodder, harvested later in the season (end of June, July), for animals with lesser needs, nitrogen fertilising is unnecessary.

Fertilising can be done by spreading organic matter (liquid manure, compost) or with a chemical compound fertiliser. We favour recycling organic matter fromthe farm, rather than buying chemical fertilisers.

Nitrogen fertilising by spreading compost on the field

To cover the needs to maintain a natural pasture, producing around 6 tonnes of dry matter/ ha, a supplement of 15 to 20 tonnes of compost/ha is necessary.

The N units in organic form are freed progressively. Mineralisation (deterioration of organic matter into nutritive ions (NO₃^-, HPO₄^2- … ) is slow for the plant, and reaches a maximum when the temperatures are hot in Summer.

Thus for 78 units of N in the compost, 10-20 will be available to the plant in the first year, then a further 10 the following year.

Thus, the nitrogen available is low in the first year, due to the fact that it is being modified through the compost process to become stable, with an organic fraction mineralising progressively. When compost is spread regularly (every year), the deferred effects are substantial. Nitrogen from the previous spreads becomes available.

In the first two years, it can therefore be beneficial to provide a supplement of ammonitrates (chemical N) to alleviate the lack of available N for the plant.

Using chemical nitrogen fertilisers

In an extensive grazing situation, where the grazing load is <1 LSU/ha (approximately 1 horse for 1 ha of field for both grazing and fodder production), mineralisation of organic matter provided through droppings and manure or compost spreading, and the natural fixing process of atmospheric N by leguminous plants, will suffice to produce the grass necessary to feed the horses.

Example of chemical nitrogen fertiliser:

• Simple (N) : ammonitrates, urea, liquid nitrogen solutions
• Binary : (ammonia-phosphates N-P, potassic- nitrates (N-K))
• Tertiary : (N-P-K : 17-17-17 fertiliser for example)

Simple nitrogen fertilisers : these are manufactures from ammonia, obtained by combining nitrogen in the air with hydrogen from natural gas.

Ammonitrates : nitrogen fertilisers which are the most widespread in use in France and Europe, are obtained from ammonium nitrate, and the addition of a variable inert load (calcium carbonate or dolomite). They contain a total of 21 % to 33,5 % of nitrogen, of which 50 % ammonium nitrate and 50 % nitric nitrogen. Nitrogen fertilising using ammonitrates, which is the most widely used fertiliser (50%  NH₄⁺ and 50%  NO₃^-), is a source of nitrogen which the plant can absorb directly (Source UNIFA).

Subsequent soil enrichments

After a grazing cycle, or after mowing, wait for new green shoots to emerge (at least 48h) before spreading a nitrogen fertiliser. The young shoots are actively growing and need high levels of nutrients.

To be efficient, 15 to 20mm of rain in the two weeks following fertilisation prevents loss through volatilisation, and favours transfer of nutrients to the roots.

For fields used for grazing and /or forage crops, the quantity of nitrogen fertiliser should take into account:

• Total « exports »: quantity of DM ingested or harvested. This depends on whether the  grass is used intensively : fast rotational grazing, or extensive : late harvesting of hay.
• The supplies in the soil: organic matter added, frequency of soil-enrichment, and ratio of leguminous plants.
• Restitutions to the fields: quantities of fertilising droppings, or the number of days and number of animals grazing.

There is a preconceived idea that nitrogen enriched grass is bad for horses, and induces metabolic diseases.

The horse is adapted to ingesting grass with a high nitrogen content. However, it is a good idea to respect a transition period towards rich grass in the spring. Fertilised grass (leafy stage) is adapted for 100 % feeding, without any need for supplements, for animals with high needs (lactating mares, growing foals).