The residue after hops have been extracted with water in the brewery is usually sold to haulage contractors who supply commercial growers with the entire output from many breweries and in consequence, the amounts available for gardeners are somewhat limited. Much of the output is purchased by fertilizer manufacturers who reinforce the spent hops with concentrated fertilizers in order to raise the food content.
In the fresh wet state straight from the brewery, spent hops contain about 75 percent water, about 1/2 percent of nitrogen, 1/4 percent phosphates and traces of potash. But since the moisture content varies considerably so does the plant food content. So an analysis based on the dry matter is the best figure. On this basis, the nitrogen content ranges from 2-3 percent and the phosphate content is about 1 percent. If you are able to get a good supply of spent hops locally you can improve the plant food content by adding cwt of National Growmore fertilizer to every ton of hops. The fertilizer should be sprinkled over the hops and the heap turned over twice to ensure even mixing.
You can not, as a rule, get small quantities of spent hops; they are usually sold by the lorry-load which may weigh several tons. If you have large areas of shrub borders to mulch or a very large vegetable garden you will need a lorry load to do any real good.
Hop manures are proprietary products prepared from spent hops and reinforced with fertilizers. They are sold in bags together with instructions for use in the garden.
Spent hops are regarded mainly as humus suppliers and are used in the preparation of the ground for planting and also for mulching established plants. The best results are obtained when they are incorporated thoroughly with the top 15cm (6in) of soil at the rate of 5kg (101b) per square meter (yard) during the winter. For mulching, purposes spent hops are very effective in keeping down weeds and retaining soil moisture in shrub borders and soft fruit plots, provided the ground is covered really thickly. If you apply a layer 10-15cm (4-6in) thick it will last for two years before rotting noticeably. The material gives off an objectionable odour after application but this usually disappears after 2-3 weeks. Although spent hops are slightly more acid than most soils they are used with great success for practically all trees and shrubs except for some of the outstanding lime-requiring plants. The fire hazard from burning cigarette ends is low since spent hops do not burn readily when used as mulches.
Spent mushroom compost
The material left after a mushroom crop has been cleared from the beds in which the spawn was planted usually consists of a mixture of well-rotted horse manure and the soil which was used for covering the eds before planting the mushroom spawn. Peat and chalk are often used in place of soil in which case the compost will have a proportion of lime in it.
So the quality of the product depends very much upon the proportion of casing soil which it contains. The organic matter of spent mushroom compost is generally more decomposed than in strawy farmyard manure and hence it may be less useful for improving heavy soils.
Being fibrous and well-rotted the compost is an ideal material for mixing in with the topsoil of all soils. The nitrogen content of fresh spent horse manure compost including soil is usually lower than farmyard manure and the phosphate and potash content is just slightly lower.
The lime-rich composts are unsuitable for rhododendrons and other lime-haters and may contain excessive amounts of lime for fruit crops.
The normal rate of application is 2.5-5kg (5-101b) per square meter (yard).
Fertilizers and manures must, as a general rule, be bought and must, therefore, add to the overall cost of your gardening. It is possible to a great extent to make your own by using waste products. Well-made, home-produced compost can be most helpful.