Have you grown potatoes in the past and they turned out very small?
If the answer is yes, then there will likely be a few reasons why this happened –
The main mistake which many people make is not knowing the N,P,K breakdown of the soil. Not knowing these figures means the soil may not contain the right ratios of these nutrients to grow a crop of large potatoes.
Potatoes need fertile soil to grow successfully – if you skip this your crop will suffer in size and development.
What is even more important is getting the ratio of N P and K correct for growing potatoes. If you add large amounts of the wrong nutrient like nitrogen (potatoes do not need large amounts of nitrogen) then you will be doing more harm than good)- more on this later.
How to test soil for nutrients
You should carry out a soil analysis by either buying a kit online or by sending a sample of your soil to an agricultural testing lab. I have found many of the kits online are very hard to interperet and are infact a waste of money. A better way to do a soil analysis is to get your local agri supplies stores to arrange it.
You put your soil sample into bag and leave it at your local agri store- where an accredited lab tests it and sends you the results. The accredited lab should email you with the results.
What type of fertilizer?
Choosing the type of fertilizer for growing your potatoes is a matter of personal preference. There are many different ways you can help your potatoes to grow, you can choose the organic route or standard chemical fertilizer.
Whichever way you choose to fertilize your crop (and you will need to apply some type of fertilizer to grow a good crop of potatoes) you need to decide if you are going to apply your fertilizer in a general way or if you want to add it in a specific way.
Adding farmyard manure/ compost for potatoes
For those who want to add fertilizer in a general way I would recommend adding a wheelbarrow of farmyard manure or compost per m2 in autumn and digging it in well and let it sit until spring when you can rototill it to a fine tilth before planting.
I have an article which shows you how you can make your own homemade fertilizer for potatoes.
How to grow potatoes
For those looking for an in depth article covering all aspects of potato growing please visit my article How to grow potatoes.
More Specific Knowledge
For those who want to apply more specific knowledge to adding chemical fertilizer, the first thing you should do is to carry out a soil analysis.
This begins by collecting a soil sample and either testing it yourself using a kit bought online, or by sending in your sample to your local Department of Agriculture for them to analyse.
Collecting your soil sample
To take your sample in a small area (less than a couple of m2) you simply dig down 4-6 inches and grab a handful of soil.
For larger areas you make an imaginary x from corner to corner of your plot and take a handful every few steps along the two lines.
Mix these sub samples together and take one sample from it. This will mean your small sample is made up of lots of small parts from all over your plot.
Reading Your Soil Analysis Report
When you receive your soil analysis results or complete your test you will have figures which relate to the amounts of Nitrogen (N) Phosphate (P) and Potash (K) which are present in your soil.
Now you have real data on how much of these nutrients are present, you should then consult the fertilizer manual i have linked to below to find out how much more of these nutrients you need to add to your soil for the crop you have chosen to grow.
These fertiliser articles are very useful reading. https://www.ers.usda.gov/topics/farm-practices-management/crop-livestock-practices/
Here is some information on a soil analysis test: https://efotg.sc.egov.usda.gov/references/public/WI/Soil_Quality_Test_Kit_Guide.pdf
This is typical of the layout of the soil analysis report you receive.
Using this report you will be able accurately apply the correct amount of nutrients for the crop you have chosen.
Recommended potato fertilizer
The fertilizer i have listed below is higher in P and K and lower in Nitrogen. This is essential for the proper development of your potatoes – especially for growing larger potatoes and keeping the potato tops size in check.
Simply scatter the fertilizer over the tilled ground at planting time, or lay a trickle of fertilizer in the bottom of the trench you are planting into.