Agriculture Reference
In-Depth Information
soil before planting fruit trees. Although they require time to produce, green
manure crops cost very little in terms of money and labor compared with
purchasing, hauling, and incorporating sawdust or similar materials. Some
green manure crops add nitrogen to the soil, and none require extra nitrogen
in order to decompose.
Depending on where you live, many good green manure crops are suitable
for preparing an orchard site. Look for crops that grow quickly, form dense
stands that quickly cover the soil surface, produce large amounts of organic
matter, out-compete most weeds, are easy to grow, have few pest and disease
problems, and are easy to incorporate into the soil. The goals are to shade or
crowd out weeds while producing large amounts of organic matter that will
break down in the soil without causing nitrogen depletion.
Buckwheat, barley, clover, vetch, oats, beans, peas, and rapeseed are pop-
ular green manure crops. Avoid sod-forming grasses (hard to kill and incor-
porate) and row crops that leave bare areas of soil (poor weed competitors).
In milder climates, you can often grow two or three green manure crops in a
single season.
Green manure crops have other important advantages over other forms
of organic amendments: Some crops serve as natural soil fumigants and can
be valuable in reducing pest and disease problems. Particularly valuable are
Brassica
varieties (canola, mustard, or rapeseed) that have been selected
for high concentrations of biofumigant compounds called glucosinolates.
When glucosinolates come into contact with a group of plant enzymes called
myrosinase during mowing or cultivating, the glucosinolates break down
to form isothiocyanates, nitriles, epithionitriles, and thiocyanates. Isothiocy-
anates are the most common and abundant by-product and are considered
highly toxic to nematodes and valuable as general biocides. The isothiocy-
anates are relatively volatile and remain in the soil for a few days to a few
weeks.
Not all
Brassica
varieties are equally effective biofumigants. Canola vari-
eties for human consumption have had most of the glucosinolates bred out
of them. Two biofumigant varieties rich in glucosinolates, however, were de-
