1) Dung or compost, spread on the land to improve the soil.
Examples are quite late and the noun seems likely to have derived from the verb which had a wide range of meanings: 1555 all mucke and maner that is at the house, Knottingley
1588 Item in mayner xs, Dalton
1599 certayne meanor with a lytle stacke of strawe, Rawmarsh
1675 We present Raiph White for shovelling up great heaps of Manner Anenst his dower & Leting of it ly severall Months being in the wains way, Bridlington.
2) A verb meaning to work on by hand, used in connection with wood or stone, a form of ‘manoeuvre’ [hand labour].
1513 ‘the right to take, fell, sell or give at their pleasure from the woods growing on the farmhold and to freely maner, carry away ... without interruption’, Brimham
1521 ‘large timber which will be delivered as necessary ... any further being felled, carried and manered at John’s cost’, Malham. To manure land was to have the use of it, and then to work it through cultivation and enrichment: c.1580 grownde whiche ... is becomen very unfrewtfull and barrayne for corne and cannot be maynered witheout helppe of pasturing, Carlton in Craven
1598 did allwaies keipe the close in their owne occupacion and manneringe by ther owne cattell, Kirkby Malham. In turn this seems to have helped ‘manure’ in the sense of compost to develop its present meaning.