On class I land, the deep, well-drained loams with 0-2 per cent slope, the joint recommendation of the agricultural experiment station and the soil conservation service for rotations on potato soils are:
1. Potatoes, potatoes, green manure
2. Potatoes 3 to 4 years, green manure
3. Potatoes every year
Such recommendations were made only on class I land, whereas on the class II and class III land, green manure or hay was included for longer periods of oftener in the rotation.
The use of green manure crops varies, in the united states, from their complete absence as in rotation 3 above to areas where the growing and management of such crops as the legumes and forage grasses, buckwheat, rye, and others may use 3 years out of every 4, with potatoes being grown the fourth year. This is particularly true on the sandier potato soils of Michigan, where the growing and proper management of he green manure crops are highly essential to efficient potato production.
Where moisture conditions and temperatures are less favorable for potatoes than in northern Maine, such large fertilizer applications can’t be used effectively. Under fairly favorable conditions, 600 to 1,000 lb. of such analyses as 3-9-18, 3-12-12, and 4-16-8 may be used, whereas on the sandier soils inclined to droughtiness not over 500 lb. an acre is considered safe. The most effective placement appears to be in bands 2 in. on either side and level with the bottom of the seed pieces. Fertilizer in direct contact with the seed piece may injure the sprouts.
Stable manure is best applied during the summer or fall previous to planting potatoes. If lime must be applied in order to grow alfafa or the clovers in a potato rotation, light applications are usually recommended; if heavier amounts are used, they are preferably applied 4 or 5 years in advance of the growing of the potato crop. Fresh stable manure and excessive lime both are conducive to the development of the organisms that cause scabby potatoes. On the other hand, potatoes can be grown effectively in acid soil, and many growers supply organic matter with green manure crops that don’t require so much lime as do alfafa and sweet clover. Rye, rye and vetch, and buckwheat are among the organic-matter-producing crops that can be used effectively on acid soils.