Have you ever had a soil test done and it looked as if Nitrogen, Phosphorus, and Potassium was non-existent? Or was just one non-existent? Try as you might, fertilizer, more soil tests, more fertilizer, and more soil tests. Still your results look like this.
Believe it or not, you might be experiencing what crop scientists call the "black hole" effect. This is where there is an underlying issue with the soil conditions that are robbing the nutrients just as fast as you put them in. Soil chemistry happens the same way it happens in high school chemistry class. Certain reactions happen and when that happens, a new chemical compound is made, or sometimes biological activity is causing nutrients to be used as food rather than food for your plants.
If you are adding known sources of Nitrogen to your soil such as blood meal, kelp meal, organic fertilizers, manure, etc. but the results show up as deficient or low, here are the causes to look for.
Mulch - any form of un-broken down material should remain on the soil surface. If you disturb the soil and mix mulch into the soil, beneficial bacteria which use Nitrogen to survive will use the nitrogen as nitrate or nitrogen gas, both forms plants cannot use. This process is called nitrogen immobilization. Nitrogen levels will be restored once the organic material has broken down fully and the Nitrogen is re-converted from an unusable form to a usable one.
Over-fertilizing - If you are a habitual fertilizer, and apply large amounts of Nitrogen rich fertilizer to the soil, you can be killing off beneficial bacteria that convert nitrogen into usable forms for plants to use. This is commonly seen with synthetic fertilizers, but is also seen even with things like urine.
Do you know why the grass dies and doesn't come back where dogs pee? The nitrogen in pure urine is too strong and kills the soil biology until it can be repaired with the addition of new Nitrifying bacteria.
If you are adding known sources of Phosphorus to your soil such as rock phosphate, bone meal, or organic fertilizers, but the results show up as deficient or low, here are the causes to look for.
High Calcium levels - Is your native soil very alkaline (has pH over 8), then your soil most likely has a lot of lime in it. You most likely live on soil with dolomitic lime deposits. Or do you regularly burn and spread ashes on your garden? Ashes contain calcium oxide (lime), and in high levels can cause problems. Or what if you use tons of gypsum or dolomitic lime to soften the soil? You can have too much calcium if overused.
When calcium levels are high in the soil, it can combine with phosphorus to form insoluble compounds, such as calcium phosphate. These compounds are not readily available for plant uptake, and as a result, the plant may not be able to access the phosphorus it needs for growth and development. This means you could apply calcium until the cows come home and never see a trace of it.
If you are adding known sources of Potassium to your soil such as potash, wood ash, greensand, or organic fertilizers, but the results show up as deficient or low, here are the causes to look for.
High clay content - Is your soil heavy in clay? This could be the main cause. Clay has a tight molecular structure, and has the ability to bind larger molecules together. Imagine Potassium being your size 9 foot, but clay is a size 7 shoe. it's so snug that you can't pull your foot out if you try until you lay on your back and have a friend pull it off for you.
Clay can be very good for your garden, but in high amounts, and unamended, it can lock up potassium as fast as you're adding it. Try breaking the clay up with compost, gypsum, or a combination of the two to break up the clay's structure.
How do you know when clay's structure has been broken up? Clay is slippery because of the tight bonds of minerals such as calcium. You will know when your soil is broken up when you can no longer feel a wet slippery texture if it is formed into a ball. The soil should crumble, or at least the edges break away before that tight of a ball can be formed.