Fertilization strategies – which to adopt?

Farming strategies and nutrient supply determine yield, quality and earnings.

And there are as many fertilization plans as there are farms. Which fertilization strategy is best adapted to the specific needs of each farm?

Manure

Best in early spring 

Nutrient supply on dairy farms can be covered to a great extent by manure, especially in case of concentrate feeding. Nutrient contents need to be monitored closely. In case of regular manure application, nitrogen availability from manure can be accounted for at 70-80& (60% from the current year and up to 20% from the preceeding year). Remaining nutrient needs, especially nitrogen, are to be compensated by the right mix of mineral fertilizers. Manure spreading is best started in early spring, 2-3 weeks prior to vegetation start. Usual application rates are 20-25m3. Less manure must be applied to pastures since it can lead to excessive potash levels.

Nitrogen 

Needed early 

Nitrogen drives crop yield. Requirements for potassium, phosphate, sulfur, calcium and magnesium are therefore related to the amount of applied nitrogen. Nitrogen needs are high from start of growth and well before emergence of the leaf system. At this point in time, nitrogen supply from the soil is insufficient and high yield strategies require application of nitrogen by manure or mineral fertilizer. 

Nitrogen is taken up by grass quickly, but incorporated into proteins and other structures with delay. Sufficient time must therefore pass between application and cutting or grazing. Split applications, adjusted to cut frequency, provide the highest results. Spring fertilization is most efficient and important to compensate for low soil supply.

Different forms of nitrogen were applied to grassland on loamy sandy soils at a rate of 190 kg/ha under controlled conditions in a German field trial. Significant differences in dry matter (DM) yield were observed. Ammonium nitrate (applied as CAN) offers the highest yield and UAN solution the lowest (-26%) mainly due to volatilization losses.

The Nitrate Directive restricts the amount of nitrogen input. Shifting application to the time when it is most needed enhances uptake efficiency and thus reduces overall nitrogen input while preserving yield.

High losses with urea and UAN

Nitrogen can be applied as mineral fertilizer or manure. When applying mineral fertilizer, not all forms of nitrogen are equal on grassland. High volatilization losses occur with urea and UAN, as shown here.

Phosphorus

Fueling plant metabolism 

Phosphorus is essential for plant metabolism and enzyme activity. Even though phosphorus demand is low compared to nitrogen, its availability accelerates grass growth. The older the sward, the more important is phosphorus fertilization. 

Phosphorus availability is limited on alkaline and acid soils, with an optimum pH of 6.5. Phosphorus is also immobile in the soil, with crop roots taking it up from the soil solution only within 2-4mm of the root. Low soil temperature further reduces phosphorus availability in spring.

For all these reasons, transient phosphorus deficiencies are common. Spring applied phosphorus improves response to applied nitrogen even on soils with good P status.

Potash

High extraction 

Potash is taken up in greatest quantities by grassland, exceeding those of nitrogen. It has an impact on nutrient uptake, photosynthesis, growth rate and feed value. It is particularly important for stem strength as well as drought and cold resistance. Lack of potash can reduce nitrogen uptake and protein content. Mowing regimes extract very large amounts of potash from the field that need to be replaced by manure, whenever possible, or mineral fertilizer. Under high yield mowing regimes, potash needs can exceed 600kg/ha. Grazing regimes have much lower needs due to direct returns by animals, rejecting most of the absorbed potash.

Sulfur

Decreasing depositions

Sulfur is essential for protein formation. Sulfur deficiency decreases nitrogen efficiency and reduces yield. Historically, sufficient sulfur was supplied by atmospheric deposition. Today, cleaner air and higher yield expectations create a need for sulfur fertilization. Sulfur exportation can reach 45kg/ha. Sulfur from manure is not immediately available to plants and can be taken into account only on a long term basis. 

Sulfur deficiency is detected by the N:S ratio in crops. A ratio of 12:1 is sufficient. Above a ratio of 15:1 deficiency becomes significant. 20-40kg of sulfur prior to the 1st and 2nd cut is recommended since sulfur mineralization sets in only at higher temperatures.