If you have fed a bed and nothing changed, adding more fertiliser should not be the automatic next step. Yieryi makes soil meters, and there are four things worth checking first: whether soil pH is limiting nutrient availability, whether salt has built up in the root zone, whether you added the nutrient that was actually short, and whether the problem was a nutrient problem at all.
Feeding again is the instinctive response. It can also worsen salt accumulation, and it may add nutrients the soil does not need, so it is worth checking these conditions before spending money on another bag.
Is soil pH limiting nutrient availability?
This is a useful first check, because pH affects how available several nutrients are to plants.
A bed can hold nitrogen, phosphorus and potassium and still produce pale or stunted growth if the pH sits outside the range the crop prefers. Extension guidance on soil pH for vegetables puts the preferred range for most crops in mildly acidic to neutral soil, and our guide to the best soil pH for vegetables lists where individual crops sit. If the reading is well outside the crop's preferred range, investigate the pH issue before adding more fertiliser.
Two things to keep in mind. Correcting pH is gradual, so it is not a fix for this weekend. And the amount of lime or sulphur needed depends on soil texture and buffering capacity as well as on pH, so a handheld reading tells you that something is off rather than how much to add. If the reading is outside the range, see how to raise soil pH or how to lower soil pH.
Has salt built up in the root zone?
Not all applied fertiliser is taken up by the plant. Depending on the nutrient, the soil and the drainage, some may remain as soluble salts, become held by the soil, undergo chemical or biological changes, or move out of the root zone.
EC readings taken under comparable conditions can help show whether soluble salt levels are rising over time. USDA guidance on soil electrical conductivity describes EC primarily as an indicator of salinity, and notes that it is also affected by soil moisture and other soil properties, so read it under conditions you keep consistent. If you have not used the reading before, our guide to what soil EC is explains what the number represents.
This deserves particular attention in containers and intensively fed beds, especially where drainage is limited or watering does not produce enough leaching. If EC readings continue to rise after feeding when measured under comparable moisture, temperature and sampling conditions, investigate salt accumulation before adding more fertiliser. Our guide on how to lower high soil EC covers what to do about it.
Did you add the nutrient that was actually short?
A complete fertiliser supplies all three primary nutrients, even when the soil may not need all three in the same proportion.
Adding a balanced fertiliser when only one nutrient is limiting can also add nutrients the soil may not need. Our guide to what the NPK numbers mean explains what each of the three drives in a plant, and how to read the ratio printed on a bag.
One limitation worth stating plainly. On our meters, the displayed NPK figures are conductivity-derived estimates. They should not be used on their own to identify a specific nutrient deficiency or to set an application rate. For a rate, use a laboratory soil test, and for how far to trust the figures on screen, see whether NPK soil testers are accurate.
Was it a nutrient problem at all?
Some symptoms that people treat with fertiliser have other causes.
Cold soil slows uptake even when nutrients are present, which is why a spring feed can appear to do nothing until the ground warms. Waterlogged soil restricts roots. In dry soil, dissolved nutrients move less readily toward the roots. Light, root damage, pests and disease can all produce symptoms that resemble nutrient deficiency.
Soil readings help you investigate several soil-related conditions. They do not diagnose light, pests, disease or root damage, and if the soil numbers come back reasonable, those are the next places to look.
What order should you check these in?
For a general nutrient-related investigation, a practical order is pH, then EC, then the nutrient channels, then non-soil causes such as light, pests, disease and root damage. Adjust the order when an obvious water, temperature, pest or disease problem is already visible.
The reasoning behind that sequence is straightforward. pH helps you interpret whether nutrients may be available to the plant. EC tells you whether feeding has already left salts behind. The nutrient channels are conductivity-derived estimates, so it makes sense to interpret them last rather than to act on them first.
If you would rather take these readings in one insertion instead of carrying separate meters, our 9 in 1 digital soil tester covers pH, EC, moisture, temperature and the nutrient channels on one screen.
Before you feed again
Record the current readings under conditions you can repeat, and note the date.
That record is what allows you to tell later whether anything changed. Our guide on how to tell whether your soil is improving explains how to build and compare it.
Quick reference
Check soil pH first in a general nutrient investigation, because pH affects how available several nutrients are to plants.
Check EC if you feed regularly, particularly in containers or beds with limited drainage, and compare readings taken under similar moisture, temperature and sampling conditions.
Read the nutrient channels last. On our meters these are conductivity-derived estimates, so do not use them alone to diagnose a specific deficiency or to set a fertiliser rate.
If the soil readings look reasonable, investigate light, water, root health, pests and disease instead.
Record a reading before you feed, so that a later reading has something comparable to be judged against.
About Yieryi
Yieryi has built pH, EC and TDS instruments for more than ten years, for applications including brewing and water treatment. On our soil meters we separate the values that are measured directly from the figures that are calculated, so that each reading can be taken for what it is.
*Written by Sunny Feng, Yieryi.*
FAQ
Can too much fertiliser stop plants growing? Repeated feeding can leave soluble salts in the root zone. Repeated EC readings taken under comparable conditions can help show whether soluble salt levels are rising over time. Adding more fertiliser in that situation may raise soluble salt levels further.
How long should I wait before deciding a feed did not work? That depends on the product and on soil temperature. Follow the timing on the label, and compare a reading taken before feeding with one taken afterwards under comparable conditions rather than judging by appearance alone.
Should I correct pH or feed first? If the reading is well outside the range your crop prefers, investigate pH first, because nutrient availability depends on it. Bear in mind that pH corrections act gradually.
Do I need a laboratory test to work this out? Not for the initial checks. A laboratory test is worth it when you need an application rate rather than a direction, when you are amending a whole plot, or when a problem has survived everything you have already tried.
