High levels of salt in the soil may cause poor seed germination, slower growth and lower production of vegetables. Manure and nitrogen fertilizers are the main contributors of salt. Be sure to add gypsum to the soil which will help leach the salts out of the soil.
To reduce the level of salt, it is suggested to apply the manure as a mulch around plants once they are large enough. This helps to retain moister in the soil and allows the salts to leach out slowly when they are less likely to affect plant growth. It can then be mixed into the soil at the end of the growing season.
Salt levels in soil can be measured with an EC (electrical conductivity) meter. Budget meters cost around $60 while higher quality models cost hundreds of dollars. An EC value under 1 is considered normal. A value between 1 and 2 will affect a few plants. Values over 4 will cause moderate damage to many plants.
The following chart shows the sensitivity to salt of some common vegetables. For example, beans will be affected if the EC value is over 1.0. Broccoli and tomatoes are more salt tolerant.
| Crop | Maximum soil salinity without yield loss (threshold) | % Decrease in yield at soil salinities above the threshold |
| Sensitive crops | dS/m | % per millimho/cm |
| Bean | 1.0 | 19 |
| Carrot | 1.0 | 14 |
| Strawberry | 1.0 | 33 |
| Onion | 1.2 | 16 |
| Moderately sensitive crops | ||
| Turnip | 0.9 | 9.0 |
| Radish | 1.2 | 13 |
| Lettuce | 1.3 | 13 |
| Pepper | 1.5 | 14 |
| Broad bean | 1.8 | 9.8 |
| Corn | 1.7 | 12 |
| Cabbage | 1.8 | 9.7 |
| Celery | 1.8 | 6.2 |
| Spinach | 2.0 | 7.6 |
| Cucumber | 2.5 | 13 |
| Tomato | 2.5 | 9.8 |
| Broccoli | 2.8 | 9.2 |
| Squash, scallop | 3.2 | 16 |
| Moderately Tolerant Crops | ||
| wild rye, beardless | 2.7 | 6.0 |
Source: University of California IPM:
http://ipm.ucanr.edu/PMG/GARDEN/ENVIRON/salttoxicity.html
Salt index information: https://www.canr.msu.edu/news/avoiding_salt_injury_from_nitrogen_fertilizers_in_orchards