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The Dirt on Dirt - Clay

Clay soil is often cursed by gardeners but clay can be a wonderful thing. The Dirt on Dirt - Clay will teach you about clay soils, why you should love them, and how to make them even better.

Contributors: Dr. Rick Schoellhorn

Clay soil is often cursed by gardeners but clay can be a wonderful thing. The Dirt on Dirt - Clay will teach you about clay soils, why you should love them, and how to make them even better.

Soil comes in a whole array of types.  The basic categories are clay, silt, loam and sand with constant variation within each of these classes.  If you have silt or loam soils you are sitting pretty, gardening will be easy and you will love your soil.  If you have clay or sandy soils it will take a bit more input from you before you love your soil.   Trust me, you can love your clay or sandy soil, it just takes a bit of knowledge and a bit of elbow grease.  How, exactly do you learn to love clay soil?  Read on to learn more about what clay soil is, why you should be glad you have it and how you can make it even better.

First things first, how do you know you have clay soil? Clay is often reddish in color, water usually is absorbed into clay more slowly, it has a tendency to dry slowly, to clump together (and not want to break apart), and to stick like mad to shoes and gardening implements.  It will also tend to crust over and crack when it gets dry.  Does this sound like the soil in your garden?  Then you probably have clay soil.  If you aren’t completely certain take a sample of your soil to your local garden center or Cooperative Extension office, they should be able to help you determine, for sure, if your soil is clay.

What is Clay Soil?

First off what does it mean that you have clay soil?  It means that the soil in your garden is composed of many tiny plate-like soil particles that can compact with time to form a hard, solid mass that makes shoveling difficult and digging holes a bit more laborious. Clay soils come in many types and it is always hard to know which kind you have without doing a soil test. Usually your local county extension service will help you in doing a basic soil test to let you know what particular type of clay soil you have.

What’s good about clay soil & what’s bad about it?

Let’s take a look at what a clay soil does for you, both the good and the bad.

First the good parts: A clay soil provides a lot of wonderful things for your garden. First off clay soils are more fertile than many other soil types; each tiny clay particle is packed with places to hold on to water and fertilizer (soil specialists call this CEC or Cation Exchange Capacity, it just means that clay soil can hold a lot of nutrients whereas a sandy soil cannot). So clay soils can save you on watering and reduce the number of times you have to fertilize. This is very important and most gardeners in clay soil are happy to have clay, once they learn how best to garden in clay soil.

Clay soils also provide a wonderful foundation for plants to get their roots into; a lot of perennials and annuals thrive in clay soils since they can get a firm grip on the soil with their roots. This firm grip allows them to survive extremes of temperature and moisture that plants in sandy soil cannot (with clay soil there is a minimum of heaving due to cycles of freezing and thawing, the plants are less likely to "jump" out of the ground). So with a clay soil you water less, fertilize less, get a better foundation for your plants and extend their hardiness when extremes in weather or other factors occur. I bet right now you are feeling a bit better about that clay soil you’ve been cursing!

A few bad things: So, it is likely obvious that a heavy clay soil is going to be more work to till or shovel than a sandy soil.  However, when you are planting a landscape most of the digging is only done once and then you get to reap all the benefits of clay soils for the rest of the life of the landscape. No doubt, a clay soil is heavier and more likely to compact than a sandy soil, but you do get a lot of benefits for your labor.

A clay soil can get very mucky if it is too wet.  If your clay soil is sticking to your shovel, stop working.  The soil is too wet to work with and you will compound the common problems of clay soil by working with it while it is wet.  Basically, your soil will be even more compacted after you are done digging.

Clay soil has all those nooks and crannies to hold water and fertilizer, which is great.  However, clay soils will hold tight to the bad things too, like salts.  Ridding clay soil of extra salt build up or changing the pH of the soil will be more difficult due to the gripping ability of soil particles that make up clay soil.   Clay soils latch on to all minerals and this can be good (fertilizer) and bad (salt). Should you have a problem clay soil, just know that it is generally a long term process to rehabilitate the soil but in the end, you will usually prevail.   

One last thing that might be a draw back to a clay soil is when you are in a boggy area, clay soils can limit the amount of air plant roots get when they are saturated, so if you have a boggy area select plants that tolerate this condition and watch out for plants that need lots of air around their roots when planting in clay soils.

How to fertilize clay soils most effectively – One of the things we all need to learn is how to avoid wasting fertilizers as they eventually run off into our lakes, streams and groundwater if we use them thoughtlessly. Clay soils are great 'nutrient' banks, so you do not need to fertilize as much and you’ll still have a nice garden. When you are gardening in clay it is fine to use liquid fertilizers, granular fertilizer, slow release fertilizers, and organic fertilizers (like fish emulsion).  Simply make sure that whatever fertilizer you choose, you use it responsibly.  One of the biggest pollutants in our steams and lakes is the run off from lawns in urban area where gardeners are using excess fertilizer and water to try to get the perfect lawn, it is not worth the cost.

Most landscapes and gardens need a liquid fertilizer about every 2 weeks, OR a granular fertilizer about every month, OR a slow release fertilizer 2-3 times per season. Over fertilizing is just wasting money and potentially damaging the environment. If you are one of those people who love to fertilize try using a liquid feed and just wetting the foliage of your plants, not soaking them. Plants can pick up a lot of nutrient right through their leaves and by doing this in small doses you are less likely to cause problems in your garden. Did you know that over-fertilized plants tend to be more susceptible to insect and disease problems?  This may seem counter-intuitive but over-fertilized plants tend to be really lush because they have been pushed to grow fast, this weakens the plants and makes them more susceptible to pests and disease

Because clay soils hold on to fertilizers well you should use a light hand when applying fertilizer.  Start out fertilizing at a slightly lower rate or waiting a bit longer than recommended between fertilizer applications.  If the plants remain healthy and happy you are fertilizing often enough.  If the leaves start to turn a yellow color you aren’t fertilizing quite enough.  Armed with this knowledge you should fertilize a bit more often or at a slightly stronger rate.  A bit of trial and error will tell you how often you need to fertilize with your specific soil.  That clay soil just might save you money due to lower fertilizer costs.  For more information on fertilizing click here.

How to water clay soils most effectively – Watering is the biggest challenge most gardeners’ face and most people over-water their plants, it is the single biggest cause of plants dying. Clay soil tends to hold water for long periods of time, therefore, if your garden soil is made up of clay, you should be watering less frequently. Spots in your yard that stay wet almost constantly are a sure sign you need to cut back on the amount of water you are applying. Check with your local county extension service to see what recommended watering rates are in your town.

Most landscapes and garden plants need to be watered just as plants are beginning to wilt a little, watering less frequently and more deeply will help develop deep root systems.  Frequent light watering encourages shallow roots which will make plants less drought tolerant. The best way to water is deeply and infrequently (except for recently planted flowers and landscapes, these need water frequently to get established). If you have a sprinkler system, make sure to check and see that it is not over-watering on a regular basis, plants will get used to whatever watering cycle you give them, so plants that are regularly overwatered are more likely to collapse when the water isn’t there, the reverse is also true, plants that have to work just a little bit in between watering are tougher and more likely to handle short dry periods.  For more on watering landscapes click here.

How to make clay soils better:

Incorporating compost – tired of chipping away at a clay soil that is hard and heavy? Try mixing in organic matter (compost, straw, fine wood bark, peat moss).  Adding these things to your soil will make it more difficult for the soil to clump together and harden. This is especially true around trees and shrubs.  In the garden a good compost to soil percentage can make digging a breeze and reduce the most common problems associated with clay soils.  Adding compost can also help, somewhat, with drainage,  keeps the soil for compacting which can block water flow – resulting in soils that remain wet and boggy. The compost will also act as a slow release fertilizer (it will container nitrogen and other nutrients) and as an additional way to hold water for your growing plants!  Click here to learn more about compost.

Does digging up an entire flower bed and incorporating compost sound too daunting?  While tackling an entire bed at once is the most efficient way of improving soils you can improve your soil a bit at a time.  One method for accomplishing this would be too improve each little spot where you are planting a plant.  To do this, dig a hole 2 to 3 times larger and deeper than what is necessary for the plant you are transplanting.  Incorporate a healthy dose of compost by mixing it in with the soil you dug out of the hole.  Fill some soil back into the hole, place your plant in the hole and then refill the rest of the hole with the compost enriched soil.  While the surrounding soil isn't enhanced the new plant is happily ensconced in good, compost rich soil.  Over several years you will gradually improve the soil in the entire bed.  This is also an effective way of improving soil in existing, already planted beds.

Mulching – Clay soils can tend to speed water runoff (water isn’t absorbed as quickly into clay soils as it is other soils) and certainly clay soils stick to the bottoms of your feet and make a mess when you go indoors. But you can solve these problems, and make the most of the positive properties of clay soil, by covering the exposed soil with a thick layer of tree bark, rough compost, shredded wood, or any of the other mulches available. By adding a layer of mulch to clay soil you not only can help keep the house clean, but can reduce the number of weed seeds that sprout (mulch will smother out weeds seeds that might try to germinate) and enhance the amount of moisture that your soil retains for better plant growth.  Mulch will slow down water run-off allowing clay soil more time to absorb, and store, the water.  A layer of mulch is also cooler than exposed soil so it helps to reduce temperatures overall in the garden.

In general having a clay soil can be wonderful, IF you know what the strengths and weaknesses of that clay soil are and how to garden best in this type of environment. You expend a bit more energy getting things planted and preparing garden beds, but in the long run you’ll use less water and fertilizer than folks gardening in sandy soils AND most plants prefer some clay in the soil to help them get their roots established and improve their hardiness.

For more Information on soils, read this articles:

The Dirt on Dirt - Basics

The Dirt on Dirt – Sand

The Dirt on Dirt - Potting Soil

Garden Gold

Garden Gold - Advanced

For more information on Plants for Clay Soil consider this information:

Learn about shrubs and annuals that are well suited for clay soils.

Learn about Growing Shrubs in Clay Soil.

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