Amending poor soil


I have tested my garden clay soil and find it has neutral PH and low phosphorous, and potash and very low nitrogen. What kind of fertilizer should I use to improver the quality and want can I use to make it more porous. Thank you


Maintaining healthy soil in the garden is a science unto itself!

By way of background, the 3 main soil nutrients are as follows:

Nitrogen (N): stimulates rapid early plant growth and gives the dark green colour to plant stems and leaves. Plants deficient in nitrogen will have light green to yellow vegetation and stunted growth. Soil can be amended with Nitrogen by the application of manure or fertilizer.

Phosphorus (P): stimulates early growth of roots and give plants a good start. As well, it is important for healthy formation of flowers and seeds and disease resistance. Where phosphorus is low, plants may flower poorly, fail to produce seed and simply fail to thrive. Leaves and stems may turn red or purple. Leaves may darken or become shiny with yellow areas, or become thick, stiff and dry. Phosphorus is often present in rocks and minerals in soil, and can derive from micro-organisms, or organic matter in soil. If needed, it can be added via manures, composts or fertilizers.

Potassium (K, Potash): helps fruit and flowers to form and also makes plants more resistant to pests and diseases. Potassium is needed for plant photosynthesis, respiration and transpiration. Lack of potassium may result in less vigorous plant growth, more diseases, thin skin, scorched leaf edges and tiny, misshapen fruit. Sources of potassium include compost, kelp meal and fertilizers.

Perhaps the wisest advice I’ve read about garden soils is in a post by a well-known plant scientist, Linda Chalker-Scott:  Why soil tests matter: lessons from my vegetable garden.  She warns that it much easier to add nutrients than to remove them – and suggests looking for evidence of deficiency in your plants prior to adding any nutrient to the soil. It’s very early in our growing season now (early May), but thinking back to last growing season, did your plants grow well? Although your soil test revealed low N, P and K — do you recall seeing evidence of deficiency in one or more of these nutrients, for example, as described above? If not, maybe fertilizer is not the first step.

Patience is key regarding garden soil. Enrich the soil by gradually amending it over the course of a few seasons. Apply organic material such as compost, leaf litter, or composted animal manure to the soil surface, as mulch. These will break down gradually and slowly release nutrients that the plants can absorb. Organic matter also improves soil structure: tiny particles loosen tight soils, thereby increasing pore space, enhancing soil drainage and enabling plant roots to penetrate soil. Work towards incorporating 20-30% organic matter to the top of the soil. Be careful not to over-work the soil, which can lead to compaction and other issues (see the link below about no-dig gardening).

It takes time for your garden soil to become healthy. If you feel it is necessary, consider judicious application of fertilizers that provide N, P and K, as you state your garden is low in all three. Fish fertilizer may be a good choice, or you could ask at a garden centre for their recommendations related to specific plants. Fertilize soil both in early spring (now, it’s early May) and late fall. In the spring, gently turn the fertilizer into the soil to prepare the garden bed for planting (again, avoid over-working). You may need a couple of applications of fertilizer in your heavy clay soil as the added fertilizer may be leached away due to poor drainage. As needed, a balanced fertilizer a few times during growing season may benefit your plants. It’s best to side dress, i.e., add the fertilizer in a band or furrow at a distance of around 10 cm (4 in) from the plants that need that a boost.

For additional information, see

  • Toronto Master Gardeners. Adding composted manure to soil.  This discusses no-dig gardening, i.e., not tilling in order to maximize soil health. The links are terrific.

All the best in achieving a nutritionally balanced garden soil that supports the healthiest plants ever!