Sweet potatoes will grow in clay soil but obtaining a worthwhile harvest from this soil type can be a challenge. Clay will make it harder for them to grow because the ground is always hard and unforgiving when dry, and when wet has the tendency to rot any plant that dares to try and grow there.
Good surface drainage is paramount on this kind of ground.
We have one patch of clay soil on site and have discovered a method of growing sweet potatoes in clay that takes very little effort and just a bit of time. More on our method below.
How to prepare clay soil for growing sweet potato.
Applying dolomite to clay is a good first step to prepare the ground so sweet potatoes can be grown with reasonable expectations of a crop. Tilling the ground and applying organic matter like compost or mulch and mixing it will also help break the clay bonds.
Over a few years this method can turn clay into good garden soil.
Organic matter, or better yet, compost, and then mulch should be applied each year for several years as the organic matter will break down and disappear, leaving the clay bonds to return. It is an ongoing process that can bring great results over time.
In many cases the clay holds enough nutrients to grow plants without fertilizers however, the need to apply organic matter to assist unlocking them is always present.
How we grow sweet potatoes in clay soil.
To begin this section, we need context.
We live in the wet tropics of northeast Australia, where sweet potato thrives all year, and we have several locations in our yard where we have sweet potatoes growing.
These locations vary in soil type and fertility, with one area near a mango tree being quite fertile dark soil, and the opposite side of the yard is heavy clay.
We placed the clay in this spot many years ago when we dug an in-ground swimming pool for the family, and we wanted to keep the soil on site for landscaping.
That was almost twenty years ago, and fast forward to today (late 2022) the ground is still quite hard and problematic. The clay is a layer that is eight inches thick on average and sits on good black loamy soil.
We initially had a row of shade and fruit trees on the property boundary that were able to push roots through the layer into the good ground below, but during some garden re-design work these trees just happened to be in all the wrong locations.
The re-designing work in this area was to fit two metal raised garden beds, with a pipe and mesh structure over the top to take vines. Around these beds we placed a deep heavy mulch obtained from the local rubbish tip, where they mulch all green waste from gardens around the shire.
As we mentioned above, this is a tropical area where palm trees grow like weeds so the mulch contained a lot of shredded palm fronds and similar; this is notable as you will discover.
This garden area is in full sun for most of the day, and we allowed a few wild sweet potato vines to establish themselves over time. There is a chicken pen next to this garden and we often pick and feed some sweet potato leaves to the chickens, so the sweet potatoes we have growing are primarily for fresh leaves.
We paid little attention to the tubers as we thought the clay was far too hard for them.
We placed the thick mulch over the ground to help control weeds and also because it is softer under-foot when we tend the garden beds.
Not long ago, we happened to be chatting with our neighbor over the fence in this area, and he asked if we had picked any potatoes from the vines yet. We replied that we hadn’t, and as we spoke we stepped back a pace and looked down at a lump in the mulch.
We scratched the mulch aside and there was a sweet potato waiting to be picked. It was growing half out of the ground with the mulch covering the tuber. This got us thinking about what we were seeing. We have since harvested this area and replaced the mulch with a good thick layer.
The image below shows what was harvested.
We are aware that sweet potatoes grow best in well drained sandy loam soil types, as we have sweet potatoes growing in this soil type in another section of the yard.
Are there enough nutrients in clay soil?
Ground that is heavy in clay is often very mineral rich but it also locks these minerals up in tight bonds. We can break these bonds with humus and organic matter allowing plants to access the nutrients more easily.
When this kind of ground is dry, it can be like rock, and when wet is slippery and sticks to everything. Organic matter helps keep the fine clay particle apart allowing plant roots to access the nutrients more easily. A healthy garden relies on this process.
Will manure help when planting out sweet potato slips?
Animal manures will be a benefit if organic matter is introduced at the same time. Applying manures on top of clayey soil can create pools of nutrient soup that can burn plants. We recommend breaking up the ground and mixing in mulch at a minimum.
How do you plant sweet potatoes in clay soil?
There are a few easy steps to follow when planting sweet potato slips. The list below is a how-to guide, and it reflects how we go about it here in our yard.
- Break up the ground to 8 inches depth minimum.
- Apply gypsum if available.
- Add organic fertilizers like chicken and cow manure, blood and bone mix.
- Cover with deep layer of silica heavy mulch like shredded palm fronds.
- Plant sweet potato slips and tubers that have sprouted.
- Water in well.
We have just the one patch of clay to deal with, but when you push to be self-sufficient, every square inch of garden becomes important.
A very important note: If your clay is deep, and where you choose to plant sweet potatoes has poor drainage, we strongly recommend using raised garden beds instead of digging down into the ground.
The reason being there is a high probability of creating sub-surface ponds after digging and filling with organic matter that can become anaerobic, very acidic, and can stop any sweet potatoes from growing.
We have a dedicated article on growing sweet potato in metal raised garden beds to get you going. The raised beds are great for getting your food garden out of problem soils.
We are fortunate that our location sits atop a slight ridge so surface drainage is not an issue. Our clay is also a relatively thin layer over good fertile soil. A few seasons of applying organic fertilizer and thick mulch should be enough to break the mineral bonds, creating more fertile conditions.
Is any organic matter ok to use?
This is where things become interesting. Any plant matter that is added to clay soils will be of benefit, however there are certain kinds of plant matter that help when growing sweet potatoes.
Two pieces of information came together recently about our patch of clay soil with the sweet potato harvest we gained accidentally.
The first was a video that we came across where rice husks were used in the growing medium for sweet potatoes. The second piece of information was really an understanding of why rice husk is such a benefit, and how that translated to our situation here in our clay patch.
Rice husk can hold as much as 20% silica in it. Silica is a vital mineral that helps with root growth and stem strength. Plant cells are more resilient when silica is present in appropriate quantities.
Sweet potatoes are a root crop essentially, so it stands to reason that silica is an important component to be aware of, especially when growing sweet potatoes.
We mentioned the mulch earlier that the local tip creates from shredded plant matter, with palm fronds being a large component of this mulch. There is scientific research pointing to palm fronds having 3.8% silica content.
The common denominator between the video that showed good results with rice husk and planting sweet potatoes in it and our observations with palm frond mulch suggests to us that silica plays a large role in sweet potato quality and quantity.
The test is to see if we can repeat the process, with the express intent to maximize the yield of sweet potatoes in this area.
This is how we plan to approach the planting task.
When we harvested the small sweet potatoes, we turned over the ground with a garden fork. This allowed last season’s mulch residues to mix with the dirt to fork tine depth, or six inches approx.
We plan to now spread organic fertilizer on the surface followed by a deep layer of silica mulch. There are plenty of small sweet potatoes left to sprout, as are lots of slips available for planting.
We will then leave the sweet potatoes growing over the wet season without any interference to see how the theory stands up to reality.
We will only water this area of the garden to stabilize the slips so the roots don’t dry out, then we will leave it be over the wet season.
This area will get plenty of water then, and we know how sweet potatoes do during this time of the year. If you are unsure about how to handle sweet potato slips, or how to make your own, this article is recommended.
We strive to be self-sufficient in as many things as we possibly can be, and food security is important to us. We are constantly working on our garden fertility, and the soils we depend on to supply our needs.
Growing sweet potatoes in good ground makes sense, and clay soil will make it harder for them to grow. In this light, it is logical to do what we can to maximize the yield when growing sweet potatoes in clay soil.
Don’t know where to start with growing sweet potatoes? This article is a good start for you.
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.