Soil Quality and Fertility

As in the field, tunnel crop production will benefit from deep, well-drained, fertile soil that is not compacted. On most sites, soil amendments such as compost, peat moss or coir will be desirable to increase the organic matter level to optimize tunnel production. Lime and nutrients should be added based on soil tests prior to production. On sites with poor native soil, compaction and/or drainage problems, soil can be imported either into the entire tunnel, raised beds, containers or by using ‘grow-bags’ of pre-fabricated media.

Since tunnel soils are not exposed to regular leaching from rainfall, soluble salt levels can build up over time negatively affecting plant growth. Salts dissolve into ions in soil solution and come from the application of fertilizers and composts or manures. Crops remove some of these salts in their tissues, but the excess remains in tunnel soils, unlike in the field. Strawberry, green beans, and certain herbs are very sensitive to salts, but even tolerant crops such as tomato and spinach can show reduced vigor with very high levels. Salts tend to accumulate especially in the top few inches of soil, as they move upwards with evaporation. This can affect germination of winter crops while transplanted crops such as tomato may be more tolerant to high salt levels.  Deep tilling will remix those salts into the soil profile. Salt injury can be exacerbated if soils are allowed to dry out. Excessive salts can be reduced by diluting with the addition of peat moss, coir or topsoil. Irrigating with a large amount of water can move salts down in the soil profile, but is often impractical. Removing the plastic cover over winter is perhaps the easiest way to leach salts out of the root zone. Soil tests can be used monitor the buildup of salts over time.

Because high tunnels specifically and protected culture more generally increase heat in the soil and air, the season is extended and yields are increased. This leads to obvious questions about whether soil fertility information from field crop research can be used to effectively guide high tunnel crop fertilizer application. Tunnel tomato fertility recommendations have been updated based on yield goals. While similar updates for other tunnel crops are not available, soil tests, tissue tests and observations of nutrient deficiencies can help fine tune nutrient applications in tunnels.