ARCHIVE: Fertiliser Manual (RB209)

Miscanthus

Typical offtake of nutrients in harvested biomass (excluding the first two years after planting when yields are much lower than in later years) are:

Nutrient

per tonne of dry biomass

In a typical crop yielding 14
tonne dry biomass per ha

 

kg/t

kg/ha

Nitrogen (N)

6

84

Phosphate (P2O5)

1

14

Potash (K2O)

8.5a

120a

a. Potash offtake in Miscanthus is very variable being affected by weather and time of harvesting. These values refer to crops harvested in January; it is now common for crops to be harvested later (e.g. April-May) and the offtake of potash then is generally less because rainfall leaches out potash from the standing crop and returns it to the soil.

There have been no published studies to test crop responses to different applications of phosphate or potash, and only a few with nitrogen. Nitrogen and phosphate offtakes are small compared with may other crops because much nitrogen and phosphate is transferred from stems and leaves to the rhizomes before harvest; these nutrients can be re-used in future years.

In addition to nutrients removed in the harvested parts of the crop, an additional amount is required for the growth of rhizomes in the first few years after planting. On the basis of (a) a relatively small number of measured nutrient offtakes by Miscanthus, (b) comparisons with offtakes by other crops on soils at different levels of crop-available phosphate or potash, and (c) experience gained by those growing the crop the following is proposed:

Phosphate

Maintain soil at P Index 1. Check every 3-5 years by soil testing (see Section 1 and Appendices 3 and 4).

Potash

Maintain soil at K Index 1- 2. Check every 3-5 years by soil testing (see Section 1 and Appendices 3 and 4).

Nitrogen

In some cases a biomass yield response to 50-100 kg N/ha has been observed, but rarely if ever to higher rates and sometimes no response for many years (up to 15 years at one UK experiment). For Miscanthus growing on soil previously under arable crops, and with little previous organic manure (i.e. in SNS Index 2 or below), the soil is likely to supply about 40 kg N/ha (depending on soil type and management history). On the basis of current information, annual fertiliser applications in the range of 60-80 kg N/ha (or organic applications estimated to supply this quantity of nitrogen) are likely to provide sufficient nitrogen for maximum production. In soils starting at a higher SNS Index, nitrogen applications are probably not required for some years.

In the first 2-3 years after planting, some nitrogen is required for the growth of rhizomes in addition to that removed in harvested biomass. However, the quantity of nitrogen removed in these years is less than in subsequent years so it is likely that no additional nitrogen for rhizome development is required. Thus, in contrast to some earlier suggestions, it is recommended that very little nitrogen (perhaps none) will generally be required in the first 2 years; nitrogen applications, as inorganic fertiliser or organic manures only should start for the third year’s crop. There is some evidence that nitrogen applications applied in the first year after planting are subject to large losses and also encourage weed growth.

Applying nitrogen in late May, just before rapid growth begins, is common practice and this seems appropriate – though there has been no work to test the best time to apply nitrogen.

Sulphur

There is currently no evidence of sulphur applications being required by Miscanthus in the UK.

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