In general, garlic prefers light loamy soils that drain well during wet periods, hold moisture during dry periods, have lots of organic matter and provide lots of fertility - basically the type of soil that everyone wishes they had, but few are lucky enough to be blessed with. Garlic does not perform well in wet conditions and can rot easily if the soil remains saturated. It also does not perform well in very dry conditions found in sandy soils with low organic matter. Low fertility, moisture deficiencies or excesses can all cause weak plants that form small stunted bulbs. That being said, garlic is a versatile plant that can be grown in almost any soil if fertility is added when necessary and if the soil moisture conditions are managed properly.
In clay soils that drain poorly, adding very large amounts of organic matter (peat moss, manure, compost, etc.) and growing the garlic on raised beds, garden boxes or on a slope can make a huge difference. Also growing deep rooted cover crops prior to the garlic crop can help with internal drainage of the soil. Garlic does not tolerate wet conditions for very long, so a lot of planning must take place before planting to ensure that good drainage exists. Garlic can do very well on clay soils as long as excess moisture conditions are managed during wet periods.
In Sandy soils that drain quickly and have poor fertility, adding large amounts of organic matter to help with water retention and fertility can improve plant growth tremendously. Garlic can do very well on sandy soils as long as it receives enough supplemental fertility and it receives enough water through rain and/or irrigation.
On our farm, we are located in a river valley floodplain with soils that range from river silt with high organic matter, to pure clay with medium organic matter. Although this means that our soils are naturally high in fertility, it also means that they have poor internal drainage and can become water logged very easily if not managed properly. In order for us to grow good garlic crops, we grow all our garlic on raised beds in fields that have a slight slope and have had a large amount of organic matter added. Although the heavier soils are a challenge in wet years, they have amazing fertility and allow us to grow large, healthy bulbs consistently.
In general, garlic is not a very demanding crop when it comes to fertility requirements, however will perform best when nutrients are at optimum levels. Soil testing is always recommended no matter how large or small your garlic garden or field is. Often growers who use organic or natural methods feel that they don't need soil tests. This is not a good philosophy and can often lead to deficiencies in certain nutrients or even over fertilization without realizing it.
These are the guidelines we use on our farm when determining the fertility levels for new fields. They could be lower or higher for you depending on where you live and the type of soils you have, but they can serve as a good starting point. The following amounts are the recommended levels of nutrients in the 0 to 24 inch depth portion of the soil:
Nitrogen - 120 lbs/ac
Phosphorus - 160 lbs/ac
Potassium - 150 lbs/ac
Sulfur - 30 lbs/ac
Soils that have low nutrient levels can have their fertility increased by adding either organic materials such as manure and compost or commercial fertilizer (although it is best to use natural sources if possible). When using organic sources to correct fertility deficiencies, they should be added in the late summer or early fall before planting the garlic cloves. This will ensure that the organic materials will have a chance to be incorporated into the soil and begin releasing their nutrients before the garlic starts growing.
It's important to note that all soils, regardless of fertility levels, benefit from adding large amounts of organic matter, especially when growing garlic. Organic matter holds onto excess nutrients and slowly releases them over a number of growing seasons. As well, the organic matter is extremely effective at improving soil structure and tilth. Garlic tends to respond to high organic levels in soil better than most vegetable crops.
On our farm, the fertility levels in our soils tend to be on the high side to begin with, however we do sometimes have issues with low nitrogen. This is a common problem for a lot of growers as nitrogen is not naturally high in most soils and has often been depleted over previous years when the farming practices were not replenishing the nutrient levels. When nitrogen is needed, we use chicken manure or alfalfa materials to try and raise the fertility levels. We notice a huge difference in plant size (and ultimately bulb size) in areas that are nitrogen deficient versus nitrogen sufficient, especially on the heavier soils in wet or cold years.
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