March 02, 2011

Improving Problem Soils

In a previous article I discussed improving soils by adding organic matter and it is true that organic matter will improve any soil somewhat. But how can you improve the very worst of soils and turn them into productive land? It is possible but may require more physical work than just adding compost or other organic matter and in some extreme cases, it may be easier to create raised beds of organic material rather than struggle on with soil that just isn't fit to plant in.

Let's have a look at some real problem soils and conditions and some of the things that we can do to improve the situation.

Sandy Soils
Pure sand is perhaps one of the most difficult soil types to work with. Water drains through the soil profile rapidly and the size of the particles means that vital plant nutrients are often lacking. Adding some organic matter will help somewhat but the actual amounts required for any significant improvement can be enormous.
I would strongly urge raised beds for sandy soils as the organic matter in the raised bed will eventually have a lasting affect upon the soil underneath. The continual breakdown of organic matter in the raised bed will slowly improve the underlying sand but if the bed is well managed, still provide a productive growing area for immediate use.

Raised beds are good for growing most food crops but not really suitable for trees. If you wish to plant trees into a sandy soil it will require a bit more work. As much good organic compost or manure as you can get should be dug into the planting site to a reasonable depth. This will help to keep water and nutrients available to the growing tree. Once the tree is planted, the surface area around the tree should be heavily mulched with organic matter and this should be kept slightly damp at all times. Watering a tree in these conditions should be targeted at the root zone, which is usually as wide as the canopy of the tree. Watering too close to the trunk on sandy soils will just cause water to move downward through the soil profile and not outward into the root zone where it is needed. Continually adding more organic mulch and even planting green manure crops between trees will help to improve water retention in the top-soil. In some cases, gardeners will dig clay or loam soils into sands to try to improve the soil structure and if you have a source for such soils, then it will give some benefits, including increased nutrient availability. One last trick that can also help on sandy soils is mulching with rocks around trees or shrubs. This can help keep moisture in the soil longer, which is a major improvement for sandy soils.

Rocky Ground
In some places the soil can be a very thin covering over rocks of different sizes or the soil may just contain so many rocks that digging and planting is just hard work with little reward. Once again, no-dig raised beds can be the answer for growing most vegetables and I would suggest using the rocks from the top soil inside the bed as part of the raised edge if they are large enough. These conditions are sometimes a result of erosion, as the top soil has been washed or blown away. If your soil is very rocky there is little that can be done to improve it beyond adding organic matter and removing as much rock as possible from planting areas. Some trees cope well in these conditions as long as they have some fertile soil to get established in. Forming small pockets of fertility in rocky ground can greatly increase productivity and over time these areas can be expanded further. The key is probably to start small and only tackle as much area as you can handle without too much effort and expand the growing area over time. if you have sloping rocky ground then terracing may be an option to consider as well.

Heavy Clay
This can be one of the most frustrating types of soil to work with as it tends to set hard when dry and turn into sticky mud when wet. It can be stratified (layered), making water penetration poor and leading to boggy patches in some places. Once again, organic matter will help to improve the soil structure over time. Additions of gypsum will help to 'break' clay soils and allow better water penetration but it is not instant (regular applications over 3 years should see some improvement) and is slightly alkaline. Gypsum can be applied to established gardens or dug in when beds are being established. The main thing that you are trying to achieve when improving clay soils is to improve the drainage and soil structure. This is mainly about getting those air spaces into the soil and breaking up any sub-surface layers. Raised planting beds or rows and targeted watering will help to avoid forming hard-pan or compacted soils. Too much digging and working of clay soils can actually damage their structure, as can over-watering. get a clay soil right and it will reward you with one of the most fertile soils of all.

Building Sites
Many times building sites will be stripped of topsoil and if the site is sloping, this can expose different layers of sub-soil that are not good for growing anything and can cause all sorts of other problems for the gardener. Many times, the fertile topsoil can end up in landfill far removed from the property as builders see it as a problem to be removed. If you have any control over the building process then you should convince the builders to save the topsoil on the site, even if they just pile it up in one area while the build is in progress. Other problems such as compaction and contamination can also be a problem on building sites. Many builders think that they can just bury their sins and everything will be fine. This can cause problems for the home gardener further down the track. The main thing to keep in mind is what the problems may be and deal with them before trying to establish garden beds. A single block can end up with a variety of different soil conditions due to building work and that can mean a number of different techniques will be required to fix the different problems. Keep working in organic matter and building up soil fertility generally and try to deal with problem areas as required.

Boggy Areas
Quite often there may be a patch of garden that either never seems to dry out or is prone to flooding from rainfall. The soil may smell sour and somewhat rotten and nothing much seems to thrive there. This can be a naturally occurring boggy area or it may be a result of human actions that have created a low area with poor drainage. If you wish to convert these areas to productive garden, there are a few things that can be done.

Creating drainage may be possible if there is anywhere that the excess water can be drained to. This might be as easy as laying slotted agricultural drainage pipe into a trench and draining the boggy area away to lower ground that drains better, or you may need to dig out a drainage pond at the lowest point that will take the excess water out of the soil and collect it into the pit or pond. The drained bog will need to be worked over well to aerate the soil and the soil pH may need to be adjusted for that specific area. Some trees are very good at taking up excess water from boggy ground but will usually need to be planted on the edge of the boggy area, rather than right in the middle. On the other hand, there are many useful plants that thrive in boggy situations so that can also be an option to be explored.

Saline Soils
Saline or salty soils can occur naturally in some areas but are typically a result of bad land management practices. Over-clearing of trees can cause the water table to rise and push salt to the soil surface. Over-watering can have the same result in some cases, as can the continued use of mineral based fertilizers. Repairing saline soils can be a major undertaking and can involve a mixture of drainage work and planting of salt tolerant species to lower the water table. Once the water table is lowered, any remaining salt in the soil can be flushed out by a heavy watering, as long as the water table doesn't rise and push the salt to the surface again. Once again, organic matter will help improve saline soils and using organic fertilizers will reduce the problems of salt accumulation in the soil.

None of these soil types is ideal for gardening but sometimes we have to try to work with what we have. If we can turn unproductive areas into productive land then that is a real accomplishment and something to be very proud of. We humans are very inventive and if we look to different cultures in other parts of the world, we can often find solutions in use there that we may not have thought of. If all else fails, no-dig raised beds can produce food, even on top of paved areas if they are set up right.