High Salt Levels in Garden Compost
High salt levels in compost
The higher the fertilizer level in a compost, the higher the salt concentration and the more risk of plant injury. However, different fertilizers have a different effect on the salt level. Obviously slow-release fertilizers will have little effect on it initially, but as they are acted upon by thebacteria, the breakdown products which are soluble will contribute to the salt level. However, in the pot, part of this level will be removed as food by the plants and part will be washed away during watering. So that provided the release is not excessive, there is little risk of a dangerously high level accumulating. If such compost is stored in a bag the situation is rather different, because the system is closed and the salt and ammonia levels can build up as the slow release nitrogen becomes available, especially at high temperature.
The more readily available plant foods have the greatest effect on the salt level: ammonium nitrate comes first, closely followed by potassium nitrate and potassium sulphate. Superphosphate has the least effect on the salt level. Soilless composts can be more heavily fertilized than comparable soil-based composts without dangerous build-up, possibly because the peat acts as a sponge, absorbing the dissolved fertilizer which does not then harm the plant roots, possibly because soilless compost is better aerated.
The compost components may contribute to the salt level. Peat and sand have little effect but if soil is used it may contain a high level of salts. This is one reason why, when the John Innes composts were formulated, the soil was obtained from meadowland that had only low plant foods and hence a low salt level; an old greenhouse soil is not suitable because it is likely to contain fertilizer residues which may be harmful.
The last source of salt levels in compost is the water. Sometimes this may contain dissolved salts, particularly in bore-hole water. Tap water is usually safe but rain water is the safest of all.
Effect of high salt levels
The effect of a high salt level in a compost may vary from no apparent damage to rapid plant death. If the salt level is just on the limit of danger, there will probably be only a reduction in the rate of growth. As the salt level increases, the plant will absorb considerable quantities of fertilizer which will tend to accumulate in the leaf margins or tips, causing burning as the water evaporates and leaves the fertilizer behind. Root damage also occurs.
If the salt level becomes very high, water will actually be removed from the root into the growing medium, causing the root to brown and dry up. This will aggravate the problem since the plant will be able to take up less water, with the possibility of the leaves wilting and further damage to the leaf margins and tips.
A frequent symptom is yellowing of the leaves which may be diagnosed by the inexperienced as a shortage of nitrogen and the plant may be promptly fed with liquid fertilizer, increasing the salt level of the compost still further and making the situation worse. Very often the first sign of root trouble is a yellowing of the foliage and if the plant is knocked out of its pot, the effect of the high salt level is evident by the browning of the roots. This must not be confused with damage due to over-watering or waterlogging.
If the damage is not too severe the compost can be leached out of its excess salt level by generously watering the compost. A free-draining well-aerated compost will help this. Alternatively, the plant should have as much of the damaging compost as possible removed from its roots and be repotted into a fresh, safe compost. Damaged plants should be given plenty of shade and high humidity, thus reducing the rate of transpiration of water through the plant. The damaged roots are under less stress and new roots will be able to form. However, with most things prevention is better than cure and the compost should be made correctly in the first place.